Contributions to ERC Plasma Source Dynamics: Diasnostic Development and Experimental Results

Texto completo

(1)

(2)

(3) Doctoral Thesis. Contributions to ECR Plasma Source Dynamics: Diagnostics Development and Experimental Results. Author: Ana Marı́a Megı́a Macı́as. Supervisor: Prof. Osvaldo Daniel Cortázar. University of Castilla-La Mancha Doctoral International School Superior Technical School of Industrial Engineering Department of Applied Mechanics and Project Engineering. A thesis submitted in fulfillment of the requirements for the degree of Doctor of Science and Technology Applied to Industrial Engineering. July 2014.

(4)

(5) Tesis Doctoral. Aportaciones a la Dinámica de Fuentes de Plasma ECR: Desarrollo de Diagnósticas y Resultados Experimentales. Autora: Ana Marı́a Megı́a Macı́as. Director: Prof. Osvaldo Daniel Cortázar. Universidad de Castilla-La Mancha Escuela Internacional de Doctorado Escuela Técnica Superior de Ingenieros Industriales Departamento de Mecánica Aplicada e Ingenierı́a de Proyectos. Tesis presentada como requisito para acceder al tı́tulo de Doctora en Ciencias y Tecnologı́as Aplicadas a la Ingenierı́a Industrial. Julio 2014.

(6)

(7) A mis abuelas, Bienve y Pepita.

(8)

(9) Acknowledgments This thesis is the result of four years of work in which I have been accompanied and encouraged by many people, to all of them, thank you very much. In particular I would like to thank Daniel Cortázar for having helped me so much and above all, for making our work so enjoyable over the years. The time that my PhD work has lasted has been a period full of emotions and good times, it has been an exciting and rewarding challenge that has made me grow professionally. All of this is largely thanks to him, and I must say that the work presented here is as much ours as mine. Thank you very much, Daniel. I also want to thank my family, who have always been part of my success and have provided my main support. Special thanks go to my grandmothers, to whom I dedicate this thesis. They have been present in every important moment of my life ever since I can remember. I am equally grateful to my parents and my brother; without them, this would not have been possible . There have been many people without whose support it would have been impossible to carry out the experiments that form the basis of this thesis, I want to express my gratitude to all of them: to Joan Bordas and Javier Bermejo, for the support they have given us, for believing in us and for giving us the opportunity to undertake this journey; to José Alonso for his support and advice, which was always so useful; to Olli Tarvainen, for first listening to our ideas that hot afternoon in Sicily; to Hannu Koivisto, for giving us the opportunity to work on his team, which was an unforgettable experience; and to Janni Komppula, who came to work with us and shared such good times. I also want to thank everyone who has encouraged me every day: Sira Cordon, for always supporting me; Carmen Abaitua, for that optimism that was so necessary sometimes; Iker Etxebarria, for being so genuine; Maider Camarero, for her sincerity and enthusiasm, Tomaso for his useful suggestions and Roberto Martinez, who was always ready with a good idea. I can not forget Pedro Jimenez, who dedicated so much of his time to us, or Pedro Hungrı́a, who was always so kind and has been the life and soul of the INEI during this past two years. To both of them, thank you very much. Finally I want to thank Kieron Spackman who has read and reread each page of this thesis with me. Thanks, Kieron. Although these years of work have been fantastic, they have not been without obstacles. Each obstacle has meant a need to go a little further and that, too, has been uplifting. To all who have ever put an obstacle in our way, thank you very much..

(10)

(11) Agradecimientos Esta tesis es el fruto de cuatro de años de trabajo en los que he estado acompañada y alentada por mucha gente, a todos ellos, muchas gracias. De un modo particular me gustarı́a agradecerle a Daniel Cortázar cuánto me ha ayudado y, sobre todo, cuánto hemos disfrutado del trabajo durante estos años. El tiempo que ha durado mi doctorado ha sido un periodo lleno de emociones y de buenos momentos, ha sido un reto emocionante y enriquecedor que me ha hecho crecer profesionalmente. Todo ello es, en gran parte, mérito suyo y he de decir que el trabajo que aquı́ se presenta es tan mı́o como nuestro. Muchas gracias, Daniel. También quiero dar las gracias a mi familia que siempre forma parte de mis éxitos y que es mi principal apoyo. En especial a mis abuelas, a quienes dedico esta tesis, que han estado desde que recuerdo en cada momento importante de mi vida. A mis padres y a mi hermano, sin vosotros nada esto habrı́a sido posible. Muchas han sido las personas sin cuyo apoyo habrı́a sido imposible llevar a cabo los experimentos que componen esta tesis, a todos ellos quiero darles las gracias. A Joan Bordas y a Javier Bermejo por el apoyo que nos han prestado, por haber creı́do en nosotros y por habernos dado la oportunidad de emprender este camino. A José Alonso por su apoyo y sus consejos, siempre tan acertados. A Olli Tarvainen, por prestar por primera vez oı́dos a nuestras ideas aquella tarde calurosa de Sicilia. A Hannu Koivisto, por darnos la oportunidad de trabajar en su equipo, fue una experiencia inolvidable. A Janni Komppula, que vino a trabajar con nosotros y nos hizo pasar tan buenos momentos. También quiero dar las gracias a toda la gente que me ha animado cada dı́a, a Sira Cordón por apoyarme siempre, a Carmen Abaitua por ese optimismo que tan necesario se ha hecho a veces, a Iker Etxebarria por ser tan auténtico, a Maider Camarero por su sinceridad, a Tomaso por sus sugerencias y a Roberto Martı́nez que siempre ha estado dispuesto a aportar una buena idea. No puedo olvidarme de Pedro Jiménez que tantas horas nos ha dedicado ni de Pedro Hungrı́a, siempre tan amable, que ha sido el espı́ritu del INEI durante estos dos últimos año. A los dos, muchas gracias. Por último quiero dar las gracias a Kieron Spackman que ha leı́do y reléido conmigo cada página de está tésis. Gracias, Kieron. Estos años de trabajo, si bien han sido estupendos, no han estado exentos de obstáculos. Cada obstáculo ha sido una necesidad de llegar un poco más lejos y eso,.

(12) también, ha sido edificante. A todos los que alguna vez han puesto un obstáculo en el camino, muchas gracias..

(13) Abstract Presented herein is a record of experimental research work focused on ECR plasma dynamics with applications in ion source engineering. The results were obtained using a series of novel diagnostics that have revealed the existence of phenomena that have never before been observed and that can be used to acquire a deeper understanding of both ion source physics and its applications for engineering. A wide-ranging and systematic study of breakdown times; original time-resolved measurements of plasma parameters at breakdown/decay, including visible and ultraviolet spectroscopy; and the discovery of eight plasma density distribution modes, all combine to contribute to the state of the art. Moreover, the new tools developed offer the possibility to control plasma parameters in real time, which can lead to major improvements in ECR ion source performance..

(14)

(15) Resumen Se presenta un trabajo de investigación experimental enfocado a la dinámica de plasmas ECR con aplicación en la ingenierı́a de fuentes de iones. Se muestran los resultados obtenidos en el desarrollo de novedosas técnicas de diagnóstico que han revelado fenómenos no observados hasta el momento y que pueden ser utilizados tanto para un mejor entendimiento de fı́sica de las fuentes de iones ECR como para su ingenierı́a. Un amplio y sistemático estudio de los tiempos de encendido, mediciones originales con resolución temporal de los parámetros del plasma incluyendo espectroscopia visible y ultravioleta y el descubrimiento de modos de distribución de densidad no observados anteriormente son parte de los principales aportes realizados al estado del arte. Además, las nuevas herramientas desarrolladas ofrecen posibilidades de control en tiempo real de los parámetros del plasma que permitirán mejoras sustanciales en el funcionamiento de fuentes de iones ECR..

(16)

(17) Contents Acknowledgments. v. Agradecimientos. vii. Abstract. ix. Resumen. xi. Contents. xiii. List of Figures. xvii. List of Tables. xxi. Abbreviations. xxiii. Physical Constants. xxv. Symbols. xxvii. 1 Introduction 1.1 ECR Ion Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 ECR Ion Source Architecture . . . . . . . . . . . . . . . . . . . . . 1.1.2 Operating Regimes . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1 6 7 9. 2 TIPS: Test Bench for Ion Source Plasma Studies 15 2.1 TIPS Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3 Study of Plasma Breakdown Times 3.1 Experimental Set-Up . . . . . . . . . . . . . . . 3.1.1 Diagnostic Port . . . . . . . . . . . . . . 3.1.2 Magnetic Field Profiles . . . . . . . . . 3.2 Measurement Procedure . . . . . . . . . . . . . 3.3 Results . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Breakdown Time Measurements with Bz xiii. . . . . . . . . . . . . . . . . . . . . . . . . . > ECR. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. 21 22 22 23 26 29 30.

(18) Contents . . . . . . . . . .. 30 31 32 32 33 34 34 35 36 41. 4 Plasma Density and Temperature Measurements during Breakdown 4.1 Experimental Set-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Diagnostic Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Magnetic Field Profiles . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Measurement Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Data Analysis and Calculations . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Difference between Plasma and Floating Potentials . . . . . . . . . 4.3.2 Slope of a Linear Fitting in a Current Logarithm-Voltage Plot . . 4.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Temperature and Density Evolution with Asymmetric Bz > ECR . 4.4.1.1 Low Pressure Regime . . . . . . . . . . . . . . . . . . . . 4.4.1.2 High Pressure Regime . . . . . . . . . . . . . . . . . . . . 4.4.2 Temperature and Density Evolution with Symmetric Bz ' ECR . 4.4.2.1 Low Pressure Regime . . . . . . . . . . . . . . . . . . . . 4.4.2.2 High Pressure Regime . . . . . . . . . . . . . . . . . . . . 4.5 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . .. 45 46 46 48 49 52 53 54 57 58 58 61 62 62 64 66. 5 Plasma Density and Temperature Measurements during Decay 5.1 Experimental Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Data Analysis and Calculations . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Temperature and Density Evolution with Asymmetric Bz > ECR . 5.3.1.1 Low Pressure Regime . . . . . . . . . . . . . . . . . . . . 5.3.1.2 High Pressure Regime . . . . . . . . . . . . . . . . . . . . 5.3.2 Temperature and Density Evolution with Symmetric Bz ' ECR . 5.3.2.1 Low Pressure Regime . . . . . . . . . . . . . . . . . . . . 5.3.2.2 High Pressure Regime . . . . . . . . . . . . . . . . . . . . 5.4 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . .. 71 72 73 75 76 76 77 78 79 80 81. 6 Plasma Vacuum Ultraviolet Emission during 6.1 Experimental Set-Up . . . . . . . . . . . . . . 6.1.1 Diagnostic Port . . . . . . . . . . . . . 6.1.2 Magnetic Field Profile . . . . . . . . . 6.2 Measurement Procedure . . . . . . . . . . . .. 83 84 84 86 86. 3.4 3.5. 3.3.1.1 Low Pressure Regime . . . . . 3.3.1.2 High Pressure Regime . . . . . 3.3.2 Breakdown Time Measurements with Bz 3.3.2.1 Low Pressure Regime . . . . . 3.3.2.2 High Pressure Regime . . . . . 3.3.3 Breakdown Time Measurements with Bz 3.3.3.1 Low Pressure Regime . . . . . 3.3.3.2 High Pressure Regime . . . . . Simple Model of Breakdown Time . . . . . . . Summary and Conclusions . . . . . . . . . . . .. . . . . . . . . . . ' ECR . . . . . . . . . . < ECR . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. Breakdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .. . . . .. . . . . . . . . . .. . . . .. . . . . . . . . . .. . . . .. . . . . . . . . . .. . . . .. . . . . . . . . . .. . . . .. . . . . . . . . . .. . . . .. . . . . . . . . . .. . . . .. . . . ..

(19) Contents 6.3 6.4. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 96. 7 Influence of Microwave Driver Coupling Design on Plasma Parameters 99 7.1 Preliminary Design Description . . . . . . . . . . . . . . . . . . . . . . . . 100 7.2 Optimized Design Description . . . . . . . . . . . . . . . . . . . . . . . . . 104 7.3 Design Comparison and Experimental Results . . . . . . . . . . . . . . . . 109 7.3.1 Electric Field Distribution . . . . . . . . . . . . . . . . . . . . . . . 109 7.3.2 Magnetic Field Distribution . . . . . . . . . . . . . . . . . . . . . . 113 7.3.3 Beta Coupling Parameters . . . . . . . . . . . . . . . . . . . . . . . 115 7.3.4 Density and Temperature Measurements . . . . . . . . . . . . . . . 116 7.4 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 8 Ultra-Fast Pictures as a Method for Ion 8.1 Experimental Set-Up . . . . . . . . . . . 8.2 Results . . . . . . . . . . . . . . . . . . . 8.2.1 General Behavior . . . . . . . . . 8.2.2 Column Mode . . . . . . . . . . 8.2.3 Hourglass Mode . . . . . . . . . 8.2.4 Slug Mode . . . . . . . . . . . . 8.2.5 Flower Mode . . . . . . . . . . . 8.2.6 Full-Chamber Mode . . . . . . . 8.2.7 Ring Mode . . . . . . . . . . . . 8.2.8 Yin-Yang Mode . . . . . . . . . . 8.2.9 Donut Mode . . . . . . . . . . . 8.3 Summary and Conclusions . . . . . . . .. Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 9 Plasma Breakdown Evolution through Ultra-Fast 9.1 Experimental Set-Up . . . . . . . . . . . . . . . . . 9.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1 Column Mode . . . . . . . . . . . . . . . . 9.2.2 Hourglass Mode . . . . . . . . . . . . . . . 9.2.3 Slug Mode . . . . . . . . . . . . . . . . . . 9.2.4 Flower Mode . . . . . . . . . . . . . . . . . 9.2.5 Full-Chamber Mode . . . . . . . . . . . . . 9.2.6 Ring Mode . . . . . . . . . . . . . . . . . . 9.2.7 Yin-Yang Mode . . . . . . . . . . . . . . . . 9.2.8 Donut Mode . . . . . . . . . . . . . . . . . 9.2.9 Rotating Plasma Configurations . . . . . . 9.2.9.1 Rotating Yin-Yang Mode . . . . . 9.2.9.2 Rotating Half-Moon Mode . . . . 9.3 Summary and Conclusions . . . . . . . . . . . . . .. Diagnosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Pictures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . .. 123 . 124 . 128 . 128 . 131 . 132 . 133 . 134 . 135 . 136 . 138 . 139 . 140. . . . . . . . . . . . . . .. 143 . 144 . 146 . 147 . 149 . 151 . 153 . 154 . 155 . 156 . 156 . 157 . 159 . 160 . 162. 10 Conclusions 165 10.1 Work Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165.

(20) Contents 10.2 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 10.3 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 11 Conclusiones 173 11.1 Trabajo realizado . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 11.2 Contribuciones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 11.3 Trabajo futuro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 Bibliography. 183.

(21) List of Figures 1.1 1.2 1.3 1.4 1.5 1.6 1.7. Simplified Diagram of an Ion Source . . . . . . . . . . Simplified Diagram of an ECR Ion Source . . . . . . . Afterglow Phenomena After the Microwave Shut-Down Ar13+ Afterglow at 16.6 GHz MINIMAFIOS . . . . . Plasma Electrostatic Potential Distribution . . . . . . Plasma Distribution Ultra-Fast Picture . . . . . . . . . Preglow Current at SMIS 37.5 GHz . . . . . . . . . . .. . . . . . . .. 2 8 10 11 12 12 13. 2.1 2.2 2.3 2.4 2.5 2.6. Photograph of TIPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section View of TIPS and Main Subsystems . . . . . . . . . . . . . . . . . Section of TIPS Magnetic Field Generation System . . . . . . . . . . . . . Photograph of TIPS with the Magnetic Field Measurement Set-Up Installed Example of 2D Magnetic Field Simulations . . . . . . . . . . . . . . . . . Experimental vs. Simulated Magnetic Field Values . . . . . . . . . . . . .. 16 17 18 18 19 20. 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15. Section View of TIPS with the Biased Probe Set-Up Installed . . . . . . . Axial Magnetic Field Profiles Used during Experiments . . . . . . . . . . 2D Representation of Simulated Magnetic Profiles used during Experiments Schematic Representation of Diagnostic Set-Up . . . . . . . . . . . . . . . Typical Oscilloscope Signal for Measuring Breakdown Times . . . . . . . Typical Relative Absorbed Power . . . . . . . . . . . . . . . . . . . . . . . Breakdown Time for Bz > ECR and 3.8 x 10−3 mb of Hydrogen Pressure Breakdown Time for Bz > ECR and 6.2 x 10−3 mb of Hydrogen Pressure Breakdown Time for Bz ' ECR and 3.8 x 10−3 mb of Hydrogen Pressure Breakdown Time for Bz ' ECR and 6.2 x 10−3 mb of Hydrogen Pressure Breakdown Time for Bz < ECR and 3.8 x 10−3 mb of Hydrogen Pressure Breakdown Time for Bz < ECR and 6.2 x 10−3 mb of Hydrogen Pressure Calculation of Temporal and Spatial Density Evolution . . . . . . . . . . Calculation of Seed Electron Temporal Density Evolution at r = 0 . . . . Calculation of Breakdown Time for 3.8 x 10−3 mb and 6.2 x 10−3 mb . .. 22 24 25 26 27 28 30 31 32 33 34 35 39 39 41. 4.1 4.2 4.3 4.4 4.5. Section View of TIPS with the Lagmuir Probe Installed . . . . Photograph of TIPS with the Langmiur Probe Set-Up Installed Langmuir Probe Diagram . . . . . . . . . . . . . . . . . . . . . Lagmuir Probe Photograph . . . . . . . . . . . . . . . . . . . . Axial Magnetic Field Profiles Used during Experiments . . . .. 46 47 47 48 48. xvii. . . . . . . . . . . . . . . . . in an ECRIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . .. . . . . .. . . . . .. . . . . . . .. . . . . .. . . . . . . .. . . . . .. . . . . ..

(22) List of Figures 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 4.21 4.22 4.23. 2D Representation of Simulated Magnetic Fields Used during Experiments I-V Curve Adquisition Conceptual Plan . . . . . . . . . . . . . . . . . . . View of a Synchronism Record During Breakdown Process . . . . . . . . . Schematic Representation of the Diagnostics Set-Up . . . . . . . . . . . . Langmuir Probe I-V Curves Obtained during Experiments . . . . . . . . . Slope of a Linear Fitting Langmuir Probe Curve Analysis method . . . . Electron Temperature Results by Two I-V Curve Analysis Methods . . . Electron Density Results by Two I-V Curve Analysis Methods . . . . . . Plasma Parameters and (Pi − Pr )/Pi Ratio Evoultion during Breakdown . Plasma Parameters for Bz > ECR and 3.8 x 10−3 mb of Hydrogen Pressure MW Coupling Time for Bz > ECR and 3.8 x 10−3 mb of Hydrogen Pressure Plasma Parameters for Bz > ECR and 6.2 x 10−3 mb of Hydrogen Pressure MW Coupling Time for Bz > ECR and 6.2 x 10−3 mb of Hydrogen Pressure Plasma Parameters for Bz ' ECR and 3.8 x 10−3 mb of Hydrogen Pressure MW Coupling Time for Bz ' ECR and 6.2 x 10−3 mb of Hydrogen Pressure Plasma Parameters for Bz ' ECR and 6.2 x 10−3 mb of Hydrogen Pressure MW Coupling Time for Bz ' ECR and 6.2 x 10−3 mb of Hydrogen Pressure Cross Section of the Main Physical Processes in a Hydrogen Ion Source .. 49 50 51 51 53 55 56 56 58 59 60 61 62 63 63 65 65 68. 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8. View of a Synchronism Record during Decay Process . . . . . . . . . . . . Langmuir Probe I-V Curves Obtained During Experiments . . . . . . . . Electron Temperature During Decay by Two I-V Curve Analysis Methods Electron Density During Decay by Two I-V Curve Analysis Methods . . . Decay Parameters for Bz > ECR and 3.8 x 10−3 mb Hydrogen Pressure . Decay parameters for Bz > ECR and 6.2 x 10−3 mb Hydrogen pressure . Decay Parameters for Bz ' ECR and 3.8 x 10−3 mb Hydrogen Pressure . Decay Parameters for Bz ' ECR and 6.2 x 10−3 mb Hydrogen Pressure .. 72 73 74 74 76 77 79 80. 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8. Section View of TIPS with VUV Diagnostics Installed . . . . . . . . . . . Photograph of the Experimental Set-Up . . . . . . . . . . . . . . . . . . . 2D Representation of the Magnetic Field Configuration Used in Experiments Schematic Representation of the Diagnostics Set-Up . . . . . . . . . . . . Time-Resolved Signals and Plasma Parameters during Breakdown . . . . Normalized Rates of Ionization and Molecular Excitation . . . . . . . . . Lyman band Light Signals Recorded for Different Powers . . . . . . . . . Lyman-alpha and Lyman band VUV-signals Saturation Time . . . . . . .. 84 85 86 87 89 91 94 95. 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8. Section View of TIPS Preliminary Design and Main Subsystems . . . . . 100 Microwave Coupler Ridged Dimension Diagram . . . . . . . . . . . . . . . 101 Thermal Picture of Waveguide Heating Due to Secondary Plasma Presence102 2D Representation of B-Field Used in Preliminary Design Experiments . . 103 Simulated Electric Field Distribution with the Preliminary Design Geometry104 Simulated Electric Field Distribution without Coupler . . . . . . . . . . . 105 Simulated Electric Field Distribution with the Optimized Design Geometry106 One-Step Ridged Coupler Design. 3D View and Ridged Section Dimensions107.

(23) List of Figures 7.9 7.10 7.11 7.12 7.13 7.14 7.15 7.16 7.17 7.18 7.19. Section View of TIPS Optimized Design . . . . . . . . . . . . . . . . . . . 108 2D Representation of B-Field Used in Optimized Design Experiments . . 108 Simulated E-Field along Plasma Chamber Axis on the Studied Designs . . 110 Picture of Plasma Sustained in Optimized Design without B-Field . . . . 111 Preliminary and optimized design Pr /Pi ratio comparision . . . . . . . . . 112 Preliminary and optimized design E-field relative change comparision . . . 113 Simulated Axial B-Field along the Plasma Chamber Axis for both Designs 113 Resonant Electric Field Distributions and Resonant Surface Positions . . 114 Typical Langmuir Probe Curves of Preliminary and Optimized Designs . 117 Electron Density Measurements in Preliminary and Optimized Designs . . 118 Electron Temperature Measurements in Preliminary and Optimized Designs118. 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12 8.13 8.14 8.15. Section View of TIPS with the Ultra-Fast Pictures Diagnostic Installed Photograph of TIPS with the Ultra-Fast Picture Diagnostic Installed . . Close view of TIPS with the Ultra-Fast Picture Diagnostic Installed . . Photograph of the Experimental Set-Up . . . . . . . . . . . . . . . . . . Typical Time Integrated Visible Spectrum . . . . . . . . . . . . . . . . . Different Types of Plasma Distribution Modes . . . . . . . . . . . . . . . Electric field Distribution Inside the Plasma Chamber . . . . . . . . . . Column Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hourglass Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slug Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flower Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Full-Chamber Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ring Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yin-Yang Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Donut Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. 124 125 126 126 127 129 130 131 133 134 135 136 137 138 139. 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 9.15 9.16 9.17. Schematic Representation of Diagnostics Set-Up . . . . . . . . . . . . View of Synchronism Record During the Breakdown . . . . . . . . . . Scope Record Showing a Peak of Visible Light during the Breakdown . Column Mode Breakdown Evolution Study . . . . . . . . . . . . . . . Normalized Photodiode Signals with Column Mode . . . . . . . . . . . Hourglass Mode Breakdown Evolution Study . . . . . . . . . . . . . . Normalized Photodiode Signals with Hourglass Mode . . . . . . . . . . Slug Mode Breakdown Evolution Study . . . . . . . . . . . . . . . . . Normalized Photodiode Signals with Slug Mode . . . . . . . . . . . . . Flower Mode Breakdown Evolution Study . . . . . . . . . . . . . . . . Full-Chamber Mode Breakdown Evolution Study . . . . . . . . . . . . Ring Mode Breakdown Evolution Study . . . . . . . . . . . . . . . . . Yin-Yang Mode Breakdown Evolution Study . . . . . . . . . . . . . . Donut Mode Breakdown Evolution Study . . . . . . . . . . . . . . . . Typical Rotating Plasma Scope Record . . . . . . . . . . . . . . . . . Rotating Yin-Yang Plasma Mode Magnetic Field Profile . . . . . . . . Rotating Yin-Yang Plasma Mode . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. 144 145 147 148 149 150 151 152 153 154 154 155 156 157 158 159 160. . . . . . . . . . . . . . . . . ..

(24) List of Figures 9.18 9.19 9.20 9.21. Rotating Half-Moon Plasma Mode Magnetic Field Profile . . . . Rotating Half-Moon Plasma Mode . . . . . . . . . . . . . . . . . + Temporal evolution for H+ , H+ 2 and H3 . . . . . . . . . . . . . . Typical Normalized Balmer-Alpha and Fulcher Band Photodioide. . . . . . . . . . . . . . . . Signals. 161 161 162 163.

(25) List of Tables 3.1 3.2 3.3 3.4. Parameters Used during Experiments . . . . . . . . . . . . . Summary of Breakdown Time Measurements for Bz > ECR Summary of Breakdown Time Measurements for Bz ' ECR Summary of Breakdown Time Measurements for Bz < ECR. 4.1. Summary of Plasma Parameter Evolution during Breakdown . . . . . . . 66. 5.1. Parameters Used during Experiments . . . . . . . . . . . . . . . . . . . . . 75. 7.1 7.2. Calculations of Stored Electric Energy Percentages . . . . . . . . . . . . . 112 Coupling Parameters in Vacuum βv and with Plasma βp . . . . . . . . . 116. xxi. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. 29 43 43 43.

(26)

(27) Abbreviations CW. Continuous Wave mode. DAS. Data Acquisition System. DC. Duty Cycle. CCD. Charge Coupled Device. ECR. Electron Cyclotron Resonance. ECRIS. Electron Cyclotron Resonance Ion Source. EEDF. Electron Energy Distribution Function. MCP. Multi Channel Plate. MW. Microwave. OFHC. Oxigen Free High (Termal) Conductivity. TE. Transversal Electric (Resonant Mode). TIPS. Test Bench (for) Ion-sources Plasma Studies. TM. Transversal Magnetic (Resonant Mode). VUV. Vacuum Ultra Violet. xxiii.

(28)

(29) Physical Constants Boltzmann Constant. k. =. 8.617 332 24 × 10−5 eV K−1. Electron Charge. e. =. 1.602 176 57 × 10−19 C. me. =. 9.109 382 91 × 10−31 Kg. mH +. =. 1.672 621 77 × 10−27 Kg. c. =. 299 792 458 m/s. Vacuum Electrical Permittivity. εo. =. 8.854 187 81 × 10−12 F m−1. Vacuum Magnetic Permeability. µo. =. 4π × 10−7 N A−2. Electron Mass Hydrogen Ion Mass Speed of Light. xxv.

(30)

(31) Symbols A. Generic Variable to Designate Area. mm2. Ap. Langmuir Probe Area. mm2. Asheath. Langmuir Probe Sheath Area. mm2. B. Generic Variable to Designate Magnetic Field. T. Bz. Magnetic Field along Plasma Chamber Axis. T. BECR. Electron Cyclotron Resonance Magnetic Field Value. T. De. Ambipolar Diffusion Coefficient for ECR Plasmas. m2 s−1. E. Generic Variable to Designate Electric Field. V/m. Eef f. Effective Electric Field. V/m. Emax. Incident Electric Field. V/m. F. Generic Variable to Designate Force. N. I. Generic Variable to Designate Electric Current. A. Ie. Electron Current. A. Ies. Electron Saturation Current. A. Ii. Ion Current. A. Iis. Ion Saturation Current. A. P. Generic Variable to Designate Power. W. Pa. Absorbed Power. W. Pi. Incident Power. W. Ploss. Power Losses. W. Pmax. Maximum Incoming MW Power Used in our Experiments. W. Pr. Reflected Power. W. Q. Microwave Coupling Factor. a.u xxvii.

(32) Symbols R. Plasma Chamber Radius. m. T. Generic Variable to Designate Temperature. eV. Te. Electron Temperature. eV. Ti. Ion Temperature. eV. U. Generic Variable to Designate Electric energy. J. Us. Stored Electric energy. J. V. Generic Variable to Electric Potential. V. Vf. Plasma Floating Potential. V. Vp. Plasma Potential. V. W. Energy Gain of an Individual Electron. eV. f. Generic Variable to Designate Frequency. Hz. n. Generic Variable to Designate Particle Density. m−3. nc. Plasma Electron Critical Density. m−3. ne. Plasma Electron Density. m−3. ne0. Initial Plasma Electron Density. m−3. ne,critical. Minimum Electron Density Necessary to Produce Breakdown. m−3. ne,ss. Plasma Electron Density during Steady State. m−3. ne,t. Plasma Electron Density during Transients. m−3. ne,res. Residual Electron Density after MW Shut-off. m−3. nn. Neutral Gas Density. m−3. p. Generic Variable to Designate Gas Pressure. mb. pmax. Maximum Gas Pressure Used in our Experiments. mb. r. Generic Variable to Designate Radius. m. t. Generic variable to Designate Time. s. tbreak. Breakdown Time. s. v. Generic Variable to Designate Velocity. m s−1. ve. Electron Velocity. m s−1. vd. Drift Velocity. m s−1. vr. Relative Velocity (Electron and Neutral Particles). m s−1. z. Position along Plasma Chamber axis. m.

(33) Symbols β. Generic Variable to Designate Coupling Parameters. a.u. βp. Coupling Parameter with Plasma. a.u. βv. Coupling Parameter in Vacuum. a.u. . Generic Variable to Designate Electron Energy. eV. ss. Electron Energy during Steady State. eV. t. Electron Energy during Transients. eV. εr. Relative Electrical Permittivity. eV. µr. Relative Magnetic Permeability. eV. ω. Generic Variable to Designate Angular Frequency. rad−1. σ. Generic Variable to Designate Cross Section. m2. σion. Ionization Cross Section. m2. σrec. Recombination Cross Section. m2. τion. Characteristic Ionization Time. s. τionmin. Minimum Order of Magnitude of Characteristic Ionization Time. s. τof f. Off-Time between MW pulses. s. ξ. Generic Variable to Designate Light Emission Intensity. W/m2. ξss. Light Emission Intensity during Steady State. W/m2. ξt. Light Emission Intensity during Transients. W/m2.

(34)

(35) Chapter 1. Introduction Ion source technology is an important issue nowadays in both scientific and industrial fields. Over the last few decades, increasing ion source performance and reliability has become one of the main goals for ion source designers. Most ion sources are plasma based, i.e., they contain a plasma from which ions are extracted. The parameters of this plasma are a determining factor for the characteristics of the ion beam extracted from the source and they depend strongly on the design and the technical details of the ion source. For this reason, a deeper understanding of plasma characteristics, its evolution and parameters can be decisive in the design, development and operation of ion sources. The strong relationship between the characteristics of the plasma contained inside an ion source and the performance of the ion source itself have been highlighted by many well know scientists. Thus, Ian Brown starts the plasma physics chapter of his book “The Physics and Technology of Ion Sources” [1] by pointing to the influence of plasma characteristics inside the source on beam current, beam emittance and beam composition. Moreover, he states that the physics of an ion source is largely plasma physics. On the other hand, Richard Geller, known as one of the fathers of Electron Cyclotron Resonance Ion Sources (ECRIS), points out in his book “Electron Cyclotron Resonance Ion Sources and ECR Plasmas” [2] that while ion extraction and transport are aspects that seem to be well understood by engineers, ion generation is a field that is far less understood or developed. He claims that while most studies focus on engineering issues; atomic physics for ion production; surface phenomena on the electrodes and walls or the properties of electrical discharges, the relations between ion source performance and plasma performance are rarely underlined, even though ions are extracted from plasmas. After explaining how the main characteristics of the beam, such as composition, ion charge, emittance or ionization percentage are deeply related to plasma characteristics, 1.

(36) 2. Geller comes to the conclusion that “specific, high-performance ion beams are extracted from specific, high-performance plasmas”. The object of this thesis is to develop plasma diagnostics that can serve as tools for ion source design and optimization. As will be seen throughout this study, the point of view is always that of the engineer. The work has been focused on the development of plasma diagnostics which permit the exploration of ion source plasma characteristics with the aim of using them as tools for ion source development and optimization. The research presented here is part of the work developed in the Ion Source Engineering Laboratory at the University of Castilla-La Mancha in Ciudad Real (Spain) in collaboration with ESS BILBAO. The team, led by Dr. Daniel Cortázar, started working together in September 2010 with the goal of commissioning an Electron Cyclotron Resonance plasma source that could serve as a test bench for ECR ion sources. All the plasma diagnostics that will be presented in this thesis have been implemented in the plasma source that we called TIPS (Test Bench for Ion source Plasma Studies). It is important to note that the terms “plasma source” and “ion source” are, in some cases, used interchangeably. However, in a plasma source the ions embedded in the plasma usually possess little directed energy. The ion drift energy is zero or, at least, small compared with the mean thermal ion energy, while the ions in an ion source have a large drift energy compared to the mean thermal energy. In most cases, the ion source contains a plasma source as its most essential component. Plasma is formed in the heart of the plasma source and the hardware and electronics needed to produce it are key parts of the overall ion source. The beam is formed from the ions contained in the plasma by means of an electrode system consisting of specially shaped biased metal electrodes. This system is commonly known as an extractor, yet ions are not “pulled out” from the plasma at all; they rather flow from the plasma to the electrode system at a rate quite independent from the extractor voltage and are then accelerated by the extractor to form an energetic beam [1]. Thus an ion source can be considered to be formed mainly by a plasma source and an extractor.. Ion beam. Plasma source. Extractor Figure 1.1: Simplified diagram of the main elements of an ion source: plasma source, extractor and ion beam..

(37) Chapter 1. Introduction. 3. Fig. 1.1 shows a simple diagram of an ion source formed by a plasma source and a extraction system placed in a such way that some of the ions contained in the plasma flow toward it. These ions are then accelerated by the extractor to form the ion beam. There are many types of ion sources and usually, they are classified depending on the type of energy used by the plasma source (laser, radiofrequency, electric discharges and so on). Our device, TIPS, is a 2.45 GHz microwave discharge plasma source designed for positive ion production. Many 2.45 GHz plasma sources like TIPS are used as the main part of ion sources around the world, both for industrial and scientific applications. As an example, some of the scientific facilities that use 2.45 GHz ion sources are listed below: • PKUNIFTY, Peking University Neutron Imagining Facility (China). • LEDA, Low-Energy Demonstration Accelerator (USA) • CPHS, Compact Pulsed Hadron Source (China). • TRASCO, High-Current Proton Source (Italy) • FAIR, Facility for Antiproton and Ion Research (Germany). • RCNP, Research Centre for Nuclear Physics (Japan). • VECC, Variable Energy Cyclotron Center (India). • ESS, European Spallation Source (under development in Sweden). In this chapter, ECR ion sources will be presented starting with an overview of their operating principles and a brief summary of its main architectural features. The most important operational regimes; preglow, steady state and afterglow; are detailed in the final section. A study of the transient regimes (preglow and afterglow) requires time-resolved diagnostics, and providing our own diagnostics for time resolution was one of the key points in the design of the plasma diagnostics described in this thesis. Chapter 2 describes the device called TIPS in which all the diagnostics presented here have been implemented. The main parts of the device are described including the magnetic field generation system in more detail. 2D simulations of the magnetic field generated are presented together with the corresponding experimental measurements for validation. Chapter 3 is the first chapter completely dedicated to plasma diagnostics development. The diagnostics described in this chapter are designed to determine plasma breakdown time. In pulsed operation ion sources, the synchronization between pulses.

(38) 4. and extraction is an important issue, and for this reason determining plasma evolution times could result in a valuable tool for ion source users. Knowing breakdown times and their evolution within the range of working parameters is of special importance for ECR ion sources operating during afterglow and steady state regimes. Time-resolved measurement of the current circulating through a biased probe was combined with signals of incoming and reflected power and visible light emission to determine breakdown times. Measurements were taken under a wide range of working parameters (power, duty cycle, magnetic field profile and pressure). A simple model of the breakdown process was also developed and compared with experimental results. Calculated and experimentally measured breakdown times are demonstrated to be in well agreement. Chapter 4 is focused on the development of a diagnostic to determine the evolution of plasma parameters, electron density and temperature during breakdown and determine how such evolution was related to the breakdown times measured in chapter 3. A Langmuir probe was introduced into the center of the plasma chamber to acquire I-V curves. Taking advantage of the high pulse-to-pulse plasma reproducibility, the system was designed to take each point of the I-V curve at a different pulse. Using this strategy, it was possible to take a set of curves during breakdown evolution with 1 μs time resolution. The experimental set-up and the curve analysis methods are described in the first part of the chapter. Results obtained under a wide range of working parameters are shown and compared in the second part. Unexpected transient temperature peaks that reach 18 eV during 20 μs were observed at the beginning of plasma breakdown. Decays of such peaks reach final stable steady state temperatures of around 5 eV at the flat top of the microwave excitation pulse. These peaks suggest a possible connection with the preglow process observed in ECR ion sources. Under some of the explored working conditions an electron density peak was also observed during breakdown. In chapter 5 the set-up developed in previous chapter is used to study the evolution of plasma parameters during decay. The same working condition range explored in chapter 4 was chosen on this occasion. This study shows the capability of the diagnostic during decay transient process. An interesting structure was found on the reflected MW power signal. It reaches a peak just after the incoming power shut-off and a second smaller rebound approximately 10 μs later. The electron temperature also shows a rebound coinciding with the second reflected power peak. Chapter 6 presents a new diagnostic designed to measure the temporal evolution of microwave-plasma coupling, vacuum ultraviolet light emission and plasma electron temperature in TIPS at the same time. The aim of this diagnostic is to confirm the.

(39) Chapter 1. Introduction. 5. existence of the electron temperature peak during breakdown reported in chapter 4. A 5-10 μs transient peak of light emission exceeding the steady state by a factor of 3.3, was observed to coincide with an abrupt drop of the MW electric field. Observed light emission intensities combined with cross section data indicate that electron temperature during breakdown should exceed the steady state value of 4-6 eV by a factor ≥ 3 which is in line with the Langmuir probe data shown in chapter 4. Chapter 7 is dedicated to the study of the microwave coupling driver system. The influence on plasma characteristics of stationary electric field distribution throughout the microwave excitation system just before breakdown is analyzed in this chapter. 3D simulations of resonant stationary electric field distributions, 2D simulations of external magnetic field mapping, experimental measurements of incoming and reflected power and of electron temperature and density are all used to compare the performance of both designs. By using these tools, an optimized set formed of plasma chamber and microwave coupler has been designed and built paying special attention to the optimization of the stationary electric field value in the center of the plasma chamber just before breakdown. This optimized system shows a strong stability level for plasma behavior, allowing a wide range of working parameters and even sustaining plasma formation without any external magnetic field. In addition, it is capable of producing electron density values four times higher than the preliminary system. In chapter 8 a new diagnostic is presented. The new optimized set of plasma chamber and MW coupler is used in TIPS from this point on. The plasma reactor was modified to include a transparent double-shielded quartz window allowing the full plasma volume to be viewed. An Image Intensified CCD frame camera, based on a combination of multichannel plate (MCP) light intensifiers and CCD cameras, was used to take plasma pictures. Several different plasma stable modes were discovered, depending on the working parameters. The distribution of these modes also depends on the working parameters: pressure, MW power and magnetic field distribution, with the last of these being the most critical. Visible time-integrated spectra were also taken for each of the plasma modes. The possibility to use these signals as a tool to find information about the plasma ion species fractions was explored. Throughout chapter 9, the breakdown process is studied using the diagnostic previously described in chapter 8. Pictures showing the breakdown process of the plasma distributions described in the previous chapter were obtained for integrated full visible light, Balmer-alpha and Balmer-beta lines and Fulcher band emissions. Photodiode signals of the temporal evolution of such emissions for the high-brightness plasma modes were also recorded. The similarities between these signals and the cluster ion H+ 2 and.

(40) 6. H+ 3 currents measured by Y. Xu et al. in an ECR plasma source at Pekin University is discussed [3]. Finally chapter 10, is a conclusions chapter. It gives an overview of the work described in this thesis and shows the conclusions that can be extracted from the work. It contains also a contributions section where the publications and conferences generated by this work are listed. The last section is dedicated to the group future plans.. 1.1. ECR Ion Sources. Essentially, microwave ion sources consist of a chamber filled with gas or vapor with a superimposed magnetic field and a means of introducing microwaves. In the absence of a magnetic field, microwaves can not propagate in a plasma with an electron density higher than a critical value nc given by [4]:. nc =. εo mω 2 e2. (1.1). where ω is the microwave frequency, e is the electron charge and me is the electron mass. The critical density for a plasma generated in a 2.45 GHz plasma source like TIPS is 7.45 x 1016 m−3 . The introduction of a magnetic field offers the possibility of acquiring high plasma densities by Electron Cyclotron Resonance Heating. Electrons immersed in a magnetic field travel in circles in a plane perpendicular to the field due to Lorentz force. The angular frequency of this movement is known as the ECR frequency and can be obtained from Eq. 1.2 where B is the magnetic field.. ωECR =. eB me. (1.2). In ECR ion sources, a magnetic field is generated to match the value, making the ECR frequency equal to the microwave frequency. For a 2.45 GHz microwave frequency plasma reactor like TIPS, the value of the ECR magnetic field (BECR ) is 87.5 mT. Microwaves injected into the plasma chamber should show circular polarization in the direction of the electrons’ rotation, so that the wave can be transferring energy to them continuously. This is a major advantage of ECR ion sources compared to microwave ion sources operated using linearly polarized waves, where the energy transfer depends on collisions since the energy gained by an electron in one semi-period is lost in the next.

(41) Chapter 1. Introduction. 7. unless it collides against another particle. This energy transfer dependence on collisions makes it necessary to work with high pressure, whereas ECR sources can operate with low pressure, where the low collision ratio allows the electrons to be accelerated by the microwave resulting in an increment in its temperature [2]. On high-frequency ion sources the magnetic field profile is designed in such a way that it provides electron confinement. The ions and electrons are tied in their orbital motion to the field lines and that provides a means of confining the plasma in the direction transverse to the magnetic field lines. In some cases a configuration called magnetic mirror is used, it consist on the increment on the strength of the magnetic at the ends of the confinement region. In this case the plasma is confined, albeit imperfectly, both longitudinally and transversely. Two groups of 2.45 GHz operating frequency ion sources can be found, depending on the shape and value of the magnetic field with respect to the BECR [5]. In some cases, the sources operate at a magnetic field below the ECR: the incoming wave is no longer a pure electromagnetic mode, but interactions with dense plasma trigger non linear phenomena and electromagnetic oscillations. The incident electromagnetic wave absorption can be described by means of a collective approach: the electron gyromotion is disturbed by fluctuating fields and by the plasma “effervescence” with high frequencies [2]. Under this circumstances, Bernstein waves (BWs) can be excited. These waves penetrate the warm plasma core without any cut-off and they can be absorbed at cyclotron harmonic fields BECR /2 or BECR /3. This absorption is useful, but plasma created in such a way is turbulent and non-uniform. The other group is formed by sources operating with magnetic field profiles above ECR. If a first ignition due to single particle ECR heating occurs, resonances between electromagnetic waves and the plasma electrons can occur even for high plasma densities [6, 7]. ECR regions inside the chamber can be understood as triggering areas where the plasma is ignited and then sustained in the whole chamber.. 1.1.1. ECR Ion Source Architecture. High current sources working at 2.45 GHz were first used about 30 years ago mostly for industrial applications. They can produce high levels of brightness and high current proton beams and present many advantages in terms of compactness, reliability, reproducibility, low transverse emittance and low maintenance. This makes them widely used for both research and industrial applications [8]. In many industrial applications, and for the low current accelerators, the ion source can be considered as a “black box” whose behavior has almost no consequences on the beam acceleration. However, for high.

(42) 8. current accelerators, where requirements in terms of reliability and low emittance are more complex, the ion source performance plays an important role.. Magnetic field generator MW generator. Ion beam Plasma chamber. Extractor Plasma source. Figure 1.2: Simplified diagram of an ion source whose plasma source is of type ECR.. Despite the fact that there are many designs for this kind of ion source there are some elements common to all of them. The main parts of an ECR ion source are shown in Fig. 1.2 and briefly described below.. • Plasma chamber: It is the reservoir where plasma is generated and sustained. Gas or vapor from the element chosen to be ionized is introduced into the chamber. Microwaves then ionize it to produce plasma and ions are extracted from it through an extraction hole producing the ion beam. Most of the ions generated inside the plasma diffuse in all directions, collide with the chamber wall and are neutralized. Only ions which happen to diffuse towards the extraction hole can be extracted and form the beam. In TIPS, as will be detailed later, the plasma chamber has cylindrical shape and the gas used to generate plasma was Hydrogen. • Microwave driver system: Microwaves need to be transferred from the microwave generator to the plasma chamber. This can be done by means of an antenna introduced into the plasma chamber [9] or by directing the microwaves to the plasma chamber through a waveguide and a matching transformer to adapt the impedance of the plasma and the waveguide [10]. This second option is generally preferred because it requires less periodic maintenance and makes the equipment more reliable for long-term operations. In most cases, microwaves are injected along the plasma chamber axis. Microwave to plasma coupling design is still an issue under discussion in the ECR ion source community. The microwave driver coupling design for the particular case of TIPS is widely discussed in chapter 7..

(43) Chapter 1. Introduction. 9. • Magnetic field generation system: A magnetic field is required for electron cyclotron resonance that corresponds with the microwave frequency. It is also used to prevent the generated plasma, especially hot electrons, from diffusing to the wall of the plasma chamber. This magnetic field can be generated by means of permanent magnets or solenoids surrounding the chamber. In some of the sources using solenoids for magnetic field generation, they can be moved axially to allow changes in the magnetic field profile distribution to try to find the one that results in the best ion source performance. This is the case of TIPS, where we have four solenoids distributed in two movable structures known as pancakes. • Extraction system: The extraction system of an ion source uses voltage differences to accelerate and focus the beam. In general, a positive ion source is polarized to a positive potential while the first electrode is grounded. The power supplies, microwave generator and control subsystems of the ion source can also be polarized at high voltage or, in some cases, a DC break is used to keep only the plasma chamber at high voltage. Different solutions have been found for isolating the microwave generation system device from the high voltage as is shown, for example, in references [10, 11]. In TIPS, no extraction system has been used; instead of extraction electrodes we have designed different diagnostic ports with the aim of giving support to the different diagnostics developed and described in this thesis.. 1.1.2. Operating Regimes. ECR ion sources can be operated in either continuous wave (CW) or pulsed mode. The choice of operating mode depends on the application of the source and the requirements of the beam. In the case of TIPS, it can be operated in CW mode or in pulsed mode from 50 Hz to 20 KHz. For sources operating in pulsed mode, the extraction and the microwave pulses can be synchronized. The instant when the extraction takes place strongly determines the source operation regime. Extraction can be done during the flat top of the MW pulse or during its transients, i.e. the rise time and the fall time, taking advantage of two interesting phenomena called preglow and afterglow, respectively. In both cases an unexpected peak of extracted current appears, associated with breakdown and decay transients. The first of these effects to be reported was the afterglow, observed for the first time by the team at DRFMC in Grenoble in 1988 on the MINIMAFIOS 16.6 GHz and described by Melin et al. in 1989 [12]. The same temporal structure has now been.

(44) 10. observed in many ECR ion sources like MINIMAFIOS 14.5 GHz at CERN, ECR4 14.5 GHz at GANIL, and CAPRICE 14.5 GHz at GSI. When the ion source is tuned to the CW mode and the microwave power is shut off, a sharp peak of extracted ion current appears just after the power switches off; such a peak is called the afterglow current pulse. This peak can be optimized by tuning the ECR ion source magnetic field, the pressure and the MW power for the specific purpose of acquiring the highest and most stable afterglow current. The afterglow peak is more noticeable in ion sources producing very high charge state ions, while in sources for low charge state ion production, this effect is weaker or even non existing. It has also been reported that the afterglow current is generally at its maximum when the source settings are those producing the minimum current in CW mode [13]. As an example, Fig. 1.3 shows the time structure of the ion currents of an ECR ion source: the CW operating mode (a); the afterglow peak after the switch-off of the MW power when the source is tuned to optimize continuous current (b) and the optimized afterglow current (c).. a.u. MW power Extracted current. t. (a). t. (b). t. (c). Figure 1.3: Afterglow phenomena after the microwave shut-down in an ECRS: (a) CW operating mode, (b) afterglow peak when the source is tuned to optimize continuous current and (c) optimized afterglow current [13].. Fig. 1.4 shows the incoming MW power and the Ar13+ extracted current signals measured by Melin et al. on the 16.6 GHz MINIMAFIOS [12]. What is notable is the complex structure that appears in the current signal just after the shut-off of the microwave power. The afterglow phenomena was first used for acceleration purposes in 1992 on the MINIMAFIOS source at CERN and, based on this and the positive results also obtained at GANIL, the new heavy ion injector at CERN was equipped with a source optimized for afterglow operation: the ECR4..

(45) Chapter 1. Introduction. 11. In general, the current obtained during the afterglow can reach 2-3 times that which is extracted during the flat top of the pulse. However, K. Langbein reported that if one looks only at a single high ionization state of a heavy element, the relative increase in the ion current can be much higher, reaching, for example, a factor of 100 for P b27+ according to Langbein’s experiments in ECR4 ion source at CERN [14].. Figure 1.4: Ar13+ Afterglow at MINIMAFIOS: MW input power and extracted current signals [12].. Details of the afterglow mechanism are still a topic of discussion in the scientific community. Nowadays, afterglow ion pulses are used with great success, but the explanation of such a process is not unequivocal. Certainly different conditions generate different phenomena that may be superimposed. Non linear effects, as has been experimentally observed, appear to have great influence on both the process itself and the highly charged ion current in the afterglow pulse. In 1991, P. Sortais proposed that in any device containing a hot plasma, the central plasma should be isolated from the wall by an electron cloud due its higher mobility compared to ions. Such a distribution of electrons and ions would produce on the central plasma region a positive potential bias. In an ECR ion source the production of ions in the center of the plasma chamber by means of ECR heating would produce a potential depression in the axis of the plasma chamber as shown in Fig. 1.5. Thus, the sudden shut-off of the MW power would result in an increase to the ion density in the chamber axis area. This model, although it was conceived for higher frequency sources could also be valid in some cases for 2.45 GHz sources like TIPS. However, the discovery of nonhomogeneous plasma modes inside the chamber that is shown in chapter 8 makes this.

(46) 12. model valid in only a few of the plasma configurations. According to our observations in TIPS, the axis of the plasma chamber is not always the area where maximum electron density is found. In fact, it is not even the region where plasma breakdown necessarily takes place. As an example, Fig. 1.6 shows one of the plasma configurations found in TIPS where it is clear that the plasma is not concentrated in the center of the chamber. The ultra-fast pictures diagnostics presented in chapter 8 opens up an interesting new set of questions related to plasma distribution inside the chamber and its influence on the extracted beam. Wall. central hot plasma. Sheath. Sheath. Figure 1.5: Plasma electrostatic distribution according to P. Sortais [13].. Figure 1.6: Ultra-fast picture of plasma inside the chamber..

(47) Chapter 1. Introduction. 13. The relevance that afterglow phenomena has acquired over the last few decades among the accelerator community together with the interest in this process shown by the volume of research done into it, provided the motivation for the plasma decay study described later in chapter 5. This study was undertaken with the aim of trying to find some correlation between the current peak found in many ion sources just after the MW power shut-off and the plasma parameter evolution during decay in our plasma source. The second transient effect commonly used as an ion source operating regime is the so-called preglow, which consists of an ion current peak registered during the plasma breakdown transient. The preglow phenomenon was first reported by P. Sortais et al. in 2004 [15] while the team was looking for ECR ion source operating conditions to produce short operation pulses. Ion sources producing short pulses of multicharged ions are in great demand for researchers in nuclear physics and the physics of elementary particles to be carried out on new generation accelerators. The effect was first observed in both PHOENIX 28 GHz and SMIS 37.5 GHz ion sources. As an example, Fig. 1.7 shows the extracted current from SMIS 37.5 GHz where the preglow current peak at the beginning of the pulse can be observed.. Figure 1.7: 2004 [15].. Total extracted current from SMIS 37.5 GHz reported by Sortais et al. in. Many questions related to the causes of preglow, its characteristics and possible uses are still open to debate in the scientific community. This fact motivated the development of some of the plasma diagnostic tools described in this thesis, and their design was focused on obtaining information about plasma during breakdown. In 2006 V. A. Skalyga et al. proposed a theoretical model of gas breakdown in an ECR ion source. According to such model, plasma breakdown can be understood to.

(48) 14. occur in two stages. In the first step, the breakdown process is dominated by ionization of the neutral gas produced by collisions with hot electrons: plasma density grows exponentially; the degree of gas ionization is less than unity; small charge ions dominate in the distribution of ions over their charge states; and the power absorbed by the plasma is low. In the second stage, the rate of density growth is slower; the process of ion peeling goes further; the ion charge increases and the absorbed power is greater [16]. After breakdown, the plasma reaches its steady state that can be assumed to be a quasi-gas-dynamic regime. Research carried out on TIPS and described in chapters 3 and 4 gives experimental evidence of the two stages of the plasma breakdown process. We have named these two stages “MW coupling” and “plasma formation time” as detailed in section 3.2. However, while V. A. Skalyga’s theoretical model predicts that plasma breakdown time should rise when power is increased, experimental data measured on TIPS show the opposite behavior. This is probably due to fact that quasi-gas-dynamic conditions can not be assumed in TIPS. A small breakdown time model based on the influence of seed electrons is presented in section 3.4. As stated by V. G. Zorin et al. in Ref. [17] the transition from breakdown to steady state can be attended with an unexpected transient peak of multicharged ions, i. e., the + preglow. Very recently, Y. Xu et al. reported a peak of cluster ions H+ 2 and H3 during breakdown in their 2.45 GHz ECR ion source [3]. The authors suggest that this peak could be strongly related to the existence of the temperature peak found in our device and reported in October 2012. Although preglow and afterglow transients are more noticeable on high-frequency sources, the study of breakdown and decay in TIPS can provide information about plasma dynamic processes that can be useful also for high-frequency sources..

(49) Chapter 2. TIPS: Test Bench for Ion Source Plasma Studies TIPS, as stated in chapter 1, is an Electron Cyclotron Resonance 2.45 GHz Hydrogen plasma source built to be a test bench for ion source engineering. The key aim of the research carried out with TIPS as described in this thesis is to develop diagnostic tools which allow us to acquire a deeper knowledge of the plasma characteristics and plasma physics processes involved in ECRIS performance. In order to access the plasma inside the ion source with different diagnostics, a diagnostic port was placed in the position where the extraction electrodes would be positioned to use it as an ion source [18]. In this chapter, TIPS will be presented and its main subsystems described.. 2.1. TIPS Description. In TIPS, pulsed microwaves are generated in a 2.45 GHz, 3 kW adjustable power magnetron and travel through a rectangular waveguide WR340 to the plasma chamber. The chamber is surrounded by a magnetic field generation structure that is described in detail later. Fig. 2.1 shows a photograph of TIPS, the magnetron, the microwave waveguide and the magnetic field generation structure and the pumping system can be seen in the picture.. 15.

(50) 16. Figure 2.1: Photograph of TIPS.. Fig. 2.2 shows a cross section view of the device, including its main subsystems. The plasma chamber (a) is made of OFHC copper and it is 90 mm in diameter and 97 mm long. The chamber wall has four longitudinal channels for water cooling (not visible in the figure). Boron Nitride discs with a thickness of 2 mm (not present in the figure for clarity) are placed at both ends of discharge chamber. Attached to the chamber is a ridged five-step brass microwave coupler (b), designed to adapt impedance between the rectangular waveguide WR284 and the plasma chamber. It also serves for gas injection (c), for chamber pressure measurements (d) and for water cooling input (e) and output (water output channel not visible in the image). Two removable parts made of OFHC copper (f ) form the ridged steps. A tapered waveguide is used to adapt the input of microwave coupler WR284 to the WR340 section of the microwave generator system (g). It is connected to a 30 mm rectangular holder sustaining a 10 mm thick window (h) which separates the volume under vacuum from the atmospheric one. On the atmospheric side, a dual-directional coupler is fitted (i) and a time synchronization signal is obtained from it. A two-stubs tuner (j) is used for fine impedance tuning and, next to it, another dual-directional coupler (k) is used to record incoming and reflected power to and from the plasma respectively. Both dual directional couplers have a 60 dB coupling factor with an approximated coupling loss of 4 x 10−6 dB and a directivity of 25 dB. A closed loop chiller provides cold water to all subsystems..

(51) Chapter 2. TIPS: Test Bench for Ion Source Plasma Studies. 17 (j) (k). (a). (i). (g). (o) (c) (e). (l). (b). (h). (f ) (d) (m). Figure 2.2: Section view of TIPS and main subsystems: plasma chamber (a), brass coupler (b), gas inlet (c), pressure gauge flange (d), cooling water inlet and outlet (e), ridged steps (f ), tapered waveguide WR284/WR300 transition (g), vacuum break window (h), dual directional couplers (i) and (k), two-stubs tuner (j), diagnostics port (l), 7 mm pumping hole (m) and magnetic field generation system (o).. On the diagnostics side of the chamber, a diagnostic port (l) fulfills two tasks: it serves as a pumping port through a 7 mm hole in the center (m) and also holds the diagnostic systems. In the course of this thesis, different ports will be described according to the necessities of the diagnostics that have been implemented. Surrounding the plasma chamber, there is a magnetic field generation system composed of four coils (o) arranged in two axially movable pancakes. The magnetic field profile can be adjusted by regulating of the current circulating through each coil and by changing the axial position of the pancakes. The direction of the magnetic field is always toward the MW wave injection side of the chamber. Magnetic field distribution is one of the key points in ECR ion source performance. To characterize these profiles in TIPS, the magnetic field distribution in the plasma chamber volume has been measured by means of a Hall probe capable of measuring in all the three axes with a typical error of ± 1 mT. Fig. 2.3 shows a section view of the set-up used for these measurements. A diagnostics port (a) was specially designed for the purpose. The magnetic probe (b) is fixed inside a plastic tube (c) and this tube is inserted inside in a rotating piece (d). By rotating this piece and displacing the plastic tube axially, the tip of the probe can be.

(52) 18. moved to map all of the volume where the plasma chamber is usually mounted with a spatial resolution of 2 mm.. (a) (b). (c). (d). Figure 2.3: Section of TIPS magnetic field generation system with the diagnostic port (a) for B-field mapping installed: Hall probe (b), plastic tube (c) and rotating holder (d).. Figure 2.4: Photograph of TIPS with the magnetic field measurement set-up installed. Fig. 2.4 shows a photograph of TIPS with the magnetic field measurement set-up installed. Using this set-up to directly measure the magnetic field distribution requires the plasma chamber and MW coupler to be dismounted and the diagnostics port to be changed. Therefore, to provide a quick tool to calculate the magnetic field distribution.

(53) Chapter 2. TIPS: Test Bench for Ion Source Plasma Studies. 19. MW INJECTION SIDE. r z. DIAGNOSTICS PORT SIDE. obtained by any combination of coil currents and pancake positions, 2D simulations were carried out using FEMM [19].. PLASMA CHAMBER 1 cm. mT 150 146 142 138 134 130 126 122 118 114 110 106 102 98 94 90 86 82 78 74 70. Figure 2.5: Example of 2D magnetic field simulations.. As an example, Fig. 2.5 shows a 2D map of the simulated magnetic field for a particular combination of symmetric coil currents and positions. MW injection takes place on the left side and the diagnostic port is on the right side. The chamber limits are marked with a solid black line and a dotted horizontal line marks the chamber axis. Superimposed to the color map the r and z axes are shown. Simulations were validated with experimental data and the error remains below 2 %. Fig. 2.6 uses red dots to show the experimental B-field measurements along the axis of the plasma chamber (designated Bz throughout this thesis) for the same configurations of coil currents and positions used in the previous figure simulation and a solid red line to show the simulated values of Bz ..

(54) 20. ) ) ). ). Figure 2.6: B-field measurements along the axis (Bz ) of the plasma chamber and the simulated values obtained for the same conditions..

(55) Chapter 3. Study of Plasma Breakdown Times Plasma evolution times are very important parameters in pulsed ion source operation. All over the world, several different ion sources operate in pulse mode to take advantage of the preglow and afterglow transients. In these kinds of sources, the synchronization between the plasma source pulses and the extraction is critical. Knowing the duration of each plasma evolution period, i. e. breakdown, steady state and decay, helps ion source users to achieve better performances. The times associated with the plasma ignition process are of particular interest for ion source designers and users. The diagnostic described in this chapter permits the measurement of the plasma breakdown times in TIPS under a wide range of operating conditions to obtain information about MW coupling and plasma formation stages during the process. The work is useful for designers who need to extract short beam pulses from a 2.45 Hydrogen ECR plasma source for any application because the total breakdown time measured is defined as corresponding to what is required to reach the steady-state plasma parameters. A simple model considering the influence of seed electrons between pulses is proposed in section 3.4 as a first approach to estimating breakdown times. The study has been undertaken using four time-resolved simultaneous diagnostics: electrical biased probe saturation current, visible emitted light, incoming power and reflected power measurements.. 21.

(56) 22. 3.1. Experimental Set-Up. 3.1.1. Diagnostic Port. Measurements were taken by means of a specifically designed diagnostic port. It allows the biased probe to be placed in the center of the plasma chamber while pumping through the center of the port. This is a very critical point because any change in the vacuum pumping system could result in changes to the plasma breakdown dynamics. As TIPS was designed to be a close reproduction of an ECR ion source, it has been a constant in the design of all the diagnostics to try not to make any changes that could make the plasma reactor operation less comparable to that of an ion source.. (a) (b). (c) (d). Figure 3.1: Section view of TIPS where the breakdown time measuring set-up is mounted: diagnostic port (a), probe holder (b), biased probe (c) and observation window (d).. Fig. 3.1 shows a section view of TIPS with the diagnostic port (a) installed. A holder (b) sustains the probe (c) in the plasma chamber axis. An 11 mm-diameter quartz window (d) was also placed at the diagnostic port to allow the observation of the plasma inside the chamber and the connection of a set made up of a collimator, a fiber optics and a photodiode used to register the visible light emitted by the plasma. The 2 mm thickness Boron Nitride disks at both sides of the plasma chamber were kept in the design although they are not shown in the figure for clarity. The disk in the diagnostic port side was machined to fit the new port geometry. It is worth noting that emitted light has become a useful tool to check plasma stability and, above all, the reproducibility of plasma behavior between pulses. Moreover, in a repetitive phenomena like this, where data can be obtained from different pulses (as will be detailed later), keeping jitter very low (under 200 ns in TIPS’ case) is of vital.

(57) Chapter 3. Study of Plasma Breakdown Times. 23. importance and thus having a measurement of plasma emitted light has turned out in a very valuable tool.. 3.1.2. Magnetic Field Profiles. During the reactor commissioning, start up and tuning stages, different magnetic field distributions were measured and tested. Some of them caused a strong plasma tendency to be allocated at the rear part of plasma chamber (the MW injection side and MW coupler piece). Taking into account that plasma quality close to the extraction zone is an important factor in achieving good ECRIS performances, we consider this tendency as a undesirable behavior. Even in the best cases, it clearly produces low-density plasma at the extraction zone. Careful attention was paid to establish a set of parameters where the plasma shows acceptable behaviors. This chapter displays the results of the breakdown study for three of the different magnetic field profiles typically used in TIPS taking as its symmetry reference the center of plasma chamber. They have been named according to their values on the axis as Asymmetric Bz > ECR, Symmetric Bz ' ECR and Symmetric Bz < ECR. Fig. 3.2 shows three Bz magnetic field profiles measured experimentally along the chamber axis with the set-up described in the previous chapter. Plasma chamber limits are indicated by dotted vertical lines, where the left border shows the microwave injection side and the right border, the diagnostics side. The ECR magnetic field level of 87.5 mT is marked by a broken, flat black line. The magnetic field was simulated for each configuration and the corresponding 2D map is represented in Fig. 3.3. Following the pattern of chapter 2, the plasma chamber limits are marked with a solid black line and the left side corresponds with the MW injection side while the right side corresponds with the diagnostic port. Note that the case (a) corresponds with the asymmetric profile where Bz takes higher values that reach 120 mT; case (b) corresponds with a symmetric Bz field with a value coincident to ECR; and case (c) is an symmetric flat Bz magnetic field profile with the values inside the chamber always below the ECR. The surface where the value of the magnetic field is exactly that of the ECR B-field is called the ECR surface. The position of this surface inside the plasma chamber has a strong influence on ECR plasma dynamics [20]. This kind of surface position is marked in Fig. 3.3 with broken black line..

(58) 24. Figure 3.2: Axial magnetic field profiles used during experiments. (a) Asymmetric Bz > ECR magnetic profile (b) Symmetric Bz ' ECR magnetic profile and (c) Symmetric Bz < ECR magnetic profile.. These three magnetic field distributions are the B-field experimental conditions that were used during measurements as typical operation modes in order to check the influence of magnetic field on plasma breakdown dynamics. Note that none of the field topologies are magnetic mirrors because this plasma generator was originally designed for the purpose of studying Hydrogen plasma for proton generation, without trapping to enhance confinement time in order to reach high degrees of ionization..

(59) Chapter 3. Study of Plasma Breakdown Times. 25. (a). (b). r z. (c). mT 150 146 142 138 134 130 126 122 118 114 110 106 102 98 94 90 86 82 78 74 70. Figure 3.3: 2D representation of simulated magnetic profiles used during experiments: (a) Asymmetric Bz > ECR magnetic profile, (b) Symmetric Bz ' ECR profile and (c) Symmetric Bz < ECR magnetic profile. Plasma chamber limits marked with black line. Axial and radial coordinates are represented in image (b)..