III. MATERIALES Y MÉTODOS
3.6. Materiales y equipos
3.6.2. Equipos, Materiales y Reactivos de Laboratorio de biología
5
Ial in which it is concluded that a population inversion may be obtained if the | free electrons are cooled by a rapidly terminated ionising pulse in a time Trp^
which is considerably shorter than the electron density relaxation time Furthermore, the current pulse must not be allowed to oscillate as the repeated cycles will re heat the plasma and destroy the population inversion. The current overshoot must be kept as low as possible. Table 1.2 summarises the modulator requirements.
Laser Cathode Voltage Peak Forward Laser Current Overshoot Current
Pulse Repetition Frequency Rate of Rise of Current Rate of Fall of Current
10 - 30 kV 100 - 1400 A Minimal 100 Hz - 30 kHz 10^ As'* (10% - 90%)
>10*0 y^g-1 (90% _ 10%)
(corresponding to <75ns) Table 1.2. Recombination laser modulator requirementsbased upon the modulator/laser system shown in Figure 1.3, A high voltage power supply resonantly charges a pulse forming network to twice the supply voltage via a charging (and a load bypass) element. A closing switch is triggered and discharges the storage capacitance through the laser load.
The power supply is a conventional transformer-isolated step-up design capable of supplying 20kV DC at half an amp. Chapter Five also reviews the high-power closing switches used to transfer energy to the discharge. The switch must be capable of switching kilovolts at high peak currents in the multi-kilohertz region. Metal vapour laser circuits almost invariably employ thyratrons due primarily to their high voltage hold-off capability and r e l i a b i l i t y ^ ^ H o w e v e r , the thyratron has limited peak current and di/dt ratings. The required pumping pulse amplitudes are typically of magnitude up to IkA with a duration of a few tens to 500ns, holding off voltages up to 30kV at pulse repetition frequencies of lOOHz to 30kHz.
The operating characteristics and test results of a discharge-heated longitudinal strontium vapour laser are presented. An average power of
approximately 0.5W is reliably obtained under sealed-off conditions. There is % good agreement between model predictions and system performance over the range
of experimental conditions encountered.
Recombination laser performance depends critically on the shape of the applied discharge current pulse. Chapter Six describes a technique of pulse shaping in which the bypass element is a saturable inductor. The magnetic switch is conventionally used to decrease the current pulse rise time. However, in the application described here, it is the fall time which is reduced. Saturation of the magnetic core provides an alternative path for the discharge
current and rapidly terminates that through the laser head. Furthermore, the point in time at which saturation occurs can be controlled by appropriate magnetic core biasing.
A detailed design example of the use of GCAP is included as an indication of the method by which the designer interacts with the program. GCAP is used to estimate the theoretical limits on the fall-time of the laser current by analysis of a flux-controlled model of the saturable inductor. The design parameters relevant to two methods of biasing the core (pulsed and DC) are outlined. This circuit generates current pulses with peak amplitudes up to 300A and fall times of less than 70ns (90%-10%).
REFERENCES FOR CHAPTER 1
1. I. S. Veselovskii, "Electron recombination coefficient with three-body collisions in a plasma", Sov. Phys. - Tech. Phys., 14, No.2, p. 193 (1969).
2. C. E. Little, M. D. Ainsworth and J. A. Piper, "On the feasibility of a ‘white-light’ Au-Cu-Sr laser", Oyo Buturi, 57, No.7, pp. 1047-1052 (1988). 3. R. W. Waynant, "Medical applications of ultraviolet lasers", SPIE vol.894,
‘Gas Laser Technology’,
Los Angeles, CA, pp.60-68 (1988).4. M. Hamza and M. Hamza, "Laser transcutaneous bilirubin meter: a new device for bilirubin monitoring in neonatal jaundice", SPIE vol.907,
‘Laser Surgery:
Characterization and Therapeutics’,
Los Angeles, CA, pp. 146-149 (1988).5. L. I. Gudzenko, L. A. Shelepin and S. I. Yakovlenko, "Amplification in recombining plasmas (plasma lasers)", Sov. Phys. - Usp., 17, No.6, p.848 (1975). 6. M, H, Key, "XUV Lasers", J. Mod. Opt., 15, No.3, p.575 (1988).
7. D. R. Bates, A. E. Kingston and R. W. P. McWhirter, "Recombination between electrons and atomic ions. I. Optically thin plasmas", Proc. Roy. Soc., A267. p.297 (1962).
8. L. I. Gudzenko and L. A, Shelepin, "Radiative enhancement in a recombining plasma", Sov. Phys. - Dokl., 10, No.2, p. 147 (1965).
9. L. I. Gudzenko, A. T. Mamachun and L. A. Shelepin, "Hydrogen level populations in a pulsed recombination plasma", Sov. Phys. - Tech. Phys., 12, No.5, p.598 (1967).
10. B. F. Gordiets, L. I. Gudzenko and L. A. Shelepin, "Relaxation processes and amplification of radiation in a dense plasma, Sov. Phys. - JETP, 28, No.3, p.489 (1969).
11. B. F. Gordiets, L. I. Gudzenko and L. A. Shelepin, Zh. Prikl. Mekh. Tekh. Fiz, No.6, p. 115 (1968).
12. H. W. Drawin, "Non-equilibrium plasmas", in ‘Reactions Under Plasma Conditions: Volume 1’, ed. M. Venugopalan, Wiley-Interscience, New York, Ch. 3, Section IV, p. 149 (1971).
13. B. F. Gordiets, L. I. Gudzenko and L. A. Shelepin, "Cooling of free electrons in a plasma", Sov. Phys. - Tech. Phys., J_l, No.9, p. 1208 (1967).
14. L. I. Gudzenko, S. S. Filippov and L. A. Shelepin, "Rapid recombination of plasma jets", Sov. Phys. - JETP, 24, No.4, p.745 (1967).
15. B. E. Cherrington, "Distribution functions and the Boltzmann equation", in ‘Gaseous Electronics and Gas Lasers’, First Edition, Pergamon Ptess, Ch.4, pp.53-57 (1980).
16. V. V. Zhukov, E. L. Laïush, V. S. Mikhalevskii and M. F. Sèm, "Recombination lasers utilizing vapours of chemical elements. I. Principles of achieving stimulated emission under recombination conditions", Sov. J. Quantum Electron., 7, No.6, p.704 (1977).
17. E. L. Latush and M. F. Sèm, "Stimulated emission due to transitions in alkaline-earth metal ions", Sov. J. (Quantum Electron., 3, No.3, p.216 (1973).
18. E. L. Latush and M. F. Sèm, "Laser recombination transitions in Call and SrII ions", Sov. Phys. - JETP,
31,
No.6, p.l017 (1973).19. V. V. Zhukov, V. S. Kucherov, E. L. Latush and M. F. Sèm, "Recombination lasers utilizing vapours of chemical elements. II, Laser action due to transitions in metal ions", Sov. J. Quantum Electron., 7, No.6, p.708 (1977). 20. L. M. Bukshpun, E. L. Latush and M. F. Sèm, "Influence of the temperature of the active medium on the stimulated emission characteristics of an Sr-He recombination laser", Sov. J. Quantum Electron., 18, No.9, p. 1098 (1988).
21. V. E. Prokop’ev and V. I. Solomonov, "Investigation of a strontium vapour laser", Sov. J. Quantum Electron., f5, No.6, p.832 (1988).
22. M. S. Butler and J. A. Piper, "Optimization of excitation channels in the discharge-excited Sr recombination laser", Appl. Phys. Lett., 45, No.7, p.707 (1984).
23. M. S. Butler and J. A. Piper, "Pulse energy scaling characteristics of longitudinally excited Sr discharge recombination lasers", IEEE J. Quantum Electron., OE-21. No. 10, p. 1563 (1985).
24. C. E. Little and J. A. Piper, "Development of efficient high-power violet Sr and ultraviolet Ca recombination lasers", SPIE vol.894,
‘Gas Laser
Technology’,
Los Angeles, CA, pp.121-127 (1988).25. C. E. Little and J. A. Piper, "High-power violet Sr^ recombination lasers", SPIE vol. 1041,
‘Metal Vapour, Deep Blue, and Ultraviolet Lasers’,
Los Angeles, CA, pp. 167-174 (1989).26. C. E. Little and J. A. Piper, "Average-power scaling of self-heated Sr^ afterglow-recombination lasers", IEEE J. Quantum Electron., OE-26. No.5, p .^ 3 (1990).
27. J. A. Piper and C. E. Little, "Discharge-excited metal-atom plasma recombination lasers", Japan-Australia Workshop on Gaseous Electronics and its Applications, CSIRO Nat. Measurements Lab., Sydney, Jan 18-22, 1989.
28. C. W, McLucas and A. I. McIntosh, "Discharge heated longitudinal Sr^ recombination laser", J. Phys. D, 19, p. 1189 (1986),
29. V. V. Zhukov, V. L. Il’yusko, E. L. Latush and M. F. Sèm, "Pulse stimulated emission from beryllium vapour", Sov. J. Quantum Electron., 5, No.7, p.757 (1975).
30. B. G. Bricks, T. W. Karras and R. S. Anderson, "An investigation of a discharge-heated barium laser", J. Appl. Phys., 49, No.l, p.38 (1978).
31. V. V. Kazakov, S. V. Markova and G. G. Petrash, "Investigation of physical processes in a pulsed barium vapour laser", Sov. J. Quantum Electron., 14, No.5,
p.642 (1984).
32. P. A. Bokhan, "Mechanism limiting the pulse repetition frequency of a barium vapour laser", Sov. J. Quantum Electron., 16, No.8, p. 1041 (1986).
33. A. M. Bogus, V. L. Dzhikiya and A. A. Chemov, "Apparatus for investigating stimulated emission from transverse discharges in pure metal vapours", Sov. J. Quantum Electron., 8, No.2, p.259 (1978).
34. M. Brandt, "Transversely excited Sr^ recombination laser", Appl. Phys. Lett., 42, No.2, p. 127 (1983).
35. M. Brandt, "Repetitively pulsed transversely excited Sr^ recombination laser", IEEE J. Quantum Electron., OE-20. No.9, p. 1006 (1984).
36. M. S^ Butler and J. A. Piper, "High-pressure, high-current transversely excited Sr recombination laser", Appl. Phys. Lett., 42, No. 12, p. 1008 (1983). 37. M. S. Butler and J. A. Piper, "High-power transverse-discharge Ca^ recombination laser", Appl. Phys. Lett., 43, No.9, p.823 (1983).
38. A. G. Molchanov, "Lasers in the vacuum ultraviolet and in the X-ray regions of the spectrum", Sov. Phys. - Usp., 15, pp. 124-129 (1972).
39. S. Slutz, G. Zimmerman, W. Lokke, G. Chapline and L. Wood, "Concerning the creation and utilisation of population inversions in spatially anisotropic, dense, high Z plasmas". Bull. Am. Phys. Soc., 17, p.972 (1972).
40. M. H. Key, C. L. S. Lewis and M. J. Lamb, "Transient population inversion at 18.2nm in a laser produced CVI plasma". Optics Commun., 28, No.3, pp.331-335 (1979).
41. D. Jacoby, G, J. Pert, S. A. Ramsden, L. D. Shorrock and G. J. Tallents, "Observation of gain in a possible extreme ultraviolet lasing system". Optics Commun., 37, No.3, pp. 193-196 (1981).
42. H. Daido, P. R. Herman, T. Jitsuno, Y. Kato, E. Miura, S. Nakai, H. Nishimura, H. Shiraga, T. Tachi, H. Takabe, M. Takagi, C. Yamanaka, M. Yamanaka, G. J. Pert, M. H. Key, P. T. Rumsby, S. J. Rose and G. J. Tallents, "ILE/UK joint experiment on X-ray lasers", Rutherford Appleton Laboratory report no.
RAL-88-042-A1-2 (1988).
43. N. Aoki, H. Kimura, C. Konagai, S. Shirayama, T. Miyazawa and T. Takahashi, "High-power copper vapor laser development", SPIE vol. 1412,
‘Gas and Metal Vapor
Lasers and Applications',
Los Angeles, CA, pp.2-11 (1991).44. C. E. Little and J. A. Piper, "Average-power limitations of large-aperture self-heated Ca afterglow-recombination lasers", Optics Commun., No.4, p.282 (1988).
45. H. Menown and C. V. Neale, "Thyratrons for short pulse laser circuits", IEEE Thirteenth Pulse Power Modulator Symposium, Buffalo, NY, pp. 125-128, June 20-22,
1976.
46. P. D. Culling, H. Menown, C. A. Pirrie and C. A. Roberts, "Recent developments in high repetition rate thyratrons for copper vapour lasers". Sixth IEEE Pulsed Power Conference, Arlington, VA, pp.598-603, June 29-July 1, 1987.
FIGURES FOR CHAPTER 1 Energy 25 “ (eV) He^(He+)+Sr > Sr+++He+e' He, Sr+e'+e' -> Sr+ +e‘ (Recombination) Sr+ Laser 430.5nm 416.2nm Sri.i'+ Sr+ Electron impact He Helium i Strontium i 25 — 20 -1 5 10 ^ 0 Strontium i i
Energy - j (eV) 16 14 — 12 10 — 8 6 Sr++ Recombination pumping
S
: Group 2 • 6S Laser 430.5nm416.2nm from GrouD 2 and higherPumping due to transitions
r"1B 16 —14 - 1 2 10
j
5P Group 1 4D ' Sr+ (5S) .Sr ^ - 61,
Strontium i Strontium i i Level population,Nj / gj
Figure 1.2. Energy level and term diagram of SrII illustrating Boltzmann distributions within the two groups of levels and a population inversion between these groups
TIMING
CIRCUIT CIRCUITDRIVER
SWITCH RESONANT CHARGING PFN LOAD HIGH-VOLTAGE POWER SUPPLY