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(1)Study of instability formation and EUV emission in thin liners driven with a compact 250 kA, 150 ns linear transformer driver J. C. Valenzuela, G. W. Collins IV, D. Mariscal, E. S. Wyndham, and F. N. Beg Citation: Physics of Plasmas 21, 031208 (2014); doi: 10.1063/1.4865225 View online: http://dx.doi.org/10.1063/1.4865225 View Table of Contents: http://scitation.aip.org/content/aip/journal/pop/21/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Measurements and modeling of the impact of weak magnetic fields on the plasma properties of a planar slot antenna driven plasma source J. Vac. Sci. Technol. A 33, 031303 (2015); 10.1116/1.4916018 Linear study of Rayleigh-Taylor instability in a diffusive quantum plasma Phys. Plasmas 20, 082108 (2013); 10.1063/1.4817744 Rotating copper plasmoid in external magnetic field Phys. Plasmas 20, 022117 (2013); 10.1063/1.4793729 Electrothermal instability growth in magnetically driven pulsed power liners Phys. Plasmas 19, 092701 (2012); 10.1063/1.4751868 One- and two-dimensional modeling of argon K-shell emission from gas-puff Z-pinch plasmas Phys. Plasmas 14, 063301 (2007); 10.1063/1.2741251. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 146.155.94.33 On: Tue, 24 May 2016 16:27:40.

(2) PHYSICS OF PLASMAS 21, 031208 (2014). Study of instability formation and EUV emission in thin liners driven with a compact 250 kA, 150 ns linear transformer driver J. C. Valenzuela,1,a) G. W. Collins IV,1 D. Mariscal,1 E. S. Wyndham,2 and F. N. Beg1 1. Center for Energy Research, University of California San Diego, La Jolla, California 92093, USA Facultad de Fısica, Pontificia Universidad Cat olica de Chile, Ave. Vicu~ na Mackena 4860, Macul, Santiago, Chile 2. (Received 30 August 2013; accepted 30 October 2013; published online 28 February 2014) A compact linear transformer driver, capable of producing 250 kA in 150 ns, was used to study instability formation on the surface of thin liners. In the experiments, two different materials, Cu and Ni, were used to study the effect of the liner’s resistivity on formation and evolution of the instabilities. The dimensions of the liners used were 7 mm height, 1 mm radius, and 3 lm thickness. Laser probing and time resolved extreme ultraviolet (EUV) imaging were implemented to diagnose instability formation and growth. Time-integrated EUV spectroscopy was used to study plasma temperature and density. A constant expansion rate for the liners was observed, with similar values for both materials. Noticeable differences were found between the Cu and Ni instability growth rates. The most significant perturbation in Cu rapidly grows and saturates reaching a limiting wavelength of the order of the liner radius, while the most significant wavelength in Ni increases slowly before saturating, also at a wavelength close to the liner radius. Evidence suggests that the instability observed is the well-known m ¼ 0 MHD instability. However, upon comparing the instability evolution of Cu and Ni, the importance of the resistivity on the seeding mechanism becomes evident. A comparison of end-on and side-on EUV emission possible indicates the formation of precursor plasma, where it has been estimated using EUV spectroscopy that the precursor plasma temperature is approximately 40 eV with ion density of order 1019 cm3, for both C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865225] materials. V I. INTRODUCTION. Recently, 1D and 2D simulations have shown that it is possible to obtain a 100 kJ fusion yield with deuteriumtritium fuel1,2 using the Magnetized Liner Inertial Fusion (MagLIF) concept on Z, a driver with a maximum current of 27 MA and a rise time of 100 ns.3 One of the challenges for the MagLIF concept is the magneto Rayleigh–Taylor (MRT) instability. In a recent work, Sinars et al.4,5 studied the evolution of single mode MRT instabilities in aluminum liners. They seeded the MRT instability by machining sinusoidal profiles on the liner’s outer surface, finding that the instability growth agreed well with LASNEX simulations and MRT single mode theory. Also, multi-mode MRT instability growth has been studied in Be liners.6 In this case, the liners’ surfaces were not machined with a sinusoidal profile as mentioned above. Instead, they used a single-point diamondturned technique to obtain a root mean square roughness of 100 nm. On comparing their results with simulations, they concluded that the liners’ evolution is extremely sensitive to the modeling of the initial surface perturbation. In addition, they found good agreement with simulations at stagnation when the proper initial conditions were used. Several mitigation strategies for instabilities have been proposed, e.g., shockless implosions by shaping the current wave front.7 This method conserves significant material strength which prevents disruption due to the MRT a). Author to whom correspondence should be addressed. Electronic mail: jcval@ucsd.edu. 1070-664X/2014/21(3)/031208/7/$30.00. instability,8 but the lower implosion velocity—a consequence of the current shaping—may cause significant radiation loss, in addition to allowing instabilities more time to build up.9 Thus, a tradeoff exists between normal liner implosions and shockless implosions. Significant work is still needed to fully understand MRT instability physics as well as mitigation strategies. The seeding mechanisms for these instabilities, e.g., surface initiation and electrothermal instability, are also very important. It is believed that the electrothermal instability could seed the MRT instability.10 Electrothermal instability is produced when there is a positive feedback mechanism in material heating. In general, the resistivity of a metal in solid, liquid, or bi-phase state increases with temperature. Therefore, in regions of a liner driven by a current, where temperature is higher (higher resistivity), Joule heating will further increase the temperature and a positive feedback occurs. Initial variations or imperfections will eventually be transformed into temperature perturbations due to electrothermal instabilities. For example, alloyed metals or surface contamination could affect the metal conductivity, leading to significant variations in Joule heating along the material’s surface. Additionally, any initial metal imperfections or radial variations will introduce perturbations in the current density and, subsequently, in Joule heating. Usually, temperature perturbations are not problematic, but, in compressible fluids, they could lead to pressure perturbations that redistribute mass. The electrothermal instability appears as striations perpendicular to the current direction when the temperature dependence on the resistivity is positive.11–13. 21, 031208-1. C 2014 AIP Publishing LLC V. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 146.155.94.33 On: Tue, 24 May 2016 16:27:40.

(3) 031208-2. Valenzuela et al.. When it is negative (plasmas), filaments parallel to the current direction are found.14 It has been shown that there are three destabilizing terms that cause electrothermal instability to grow in current carrying metals:12,13 The increase of resistance with temperature, the increase of temperature with time, and the increase of the resistivity with decreasing density. In contrast, the thermal conductivity is a stabilizing term. In this work, we study the initial phase of plasmainstability formation in thin metallic liners in order to understand the growth of the instability. A small-scale university experiment allows higher shot rates and easier diagnostic access, both significant advantages over large scale devices. These small-scale experiments may benchmark simulations that will help us understand and design large scale experiments. In our experiments, we find that the instability formation and growth in Cu and Ni liners are significantly different, despite having similar atomic number. We attribute this difference principally to their respective values of resistivity. Our work has significant relevance to pulse power driven Z-pinches as radiation sources and their application to inertial confinement fusion (ICF), radiation physics, and laboratory astrophysical plasmas. An improved understanding of the instability formation at early stages of the current pulse will help in the above described applications. The paper is organized as follows: Experimental set-up is given in Sec. II, results are described in Sec. III with Sec. III A discussing instability growth and Sec. III B addressing extreme ultraviolet emission and plasma formation, and finally, in Sec. IV is presented a summary of the work. II. EXPERIMENTAL SET-UP. A series of experiments was carried out on GenASIS (Generator for Ablation Structure and Implosion Studies), a compact, low inductance Linear Transformer Driver (LTD) capable of producing 250 kA in about 150 ns in a short circuit load and a nominal voltage of 80 kV. The details of the driver can be found in Ref. 15. Figure 1(a) shows the current waveforms through a short circuit and a liner load while charging the driver to 75 kV. The current amplitude and rise time show a small variation when using the liner as a load due to an increase in inductance. The typical current obtained with liner loads was approximately 200 kA with a corresponding rise time of 160 ns. These values are 10% and 6% different from a short circuit load, respectively. The liners were constructed from copper and nickel foils that were wrapped around a rod and placed between the electrodes (see Figure 1(b)). The rod was withdrawn after placing the liners between the electrodes. The dimensions of the liners were: 1 mm radius, 7 mm height, and 3 lm foil thickness. We tested nickel and copper to study the effect of different electrical conductivities (while keeping the atomic number approximately constant) on instability formation and growth. We employed a number of diagnostics to examine the plasma produced on the surface of the liners. To optically probe the plasma, we used a 4-frame dark field schlieren laser. Phys. Plasmas 21, 031208 (2014). FIG. 1. (a) Current waveform in a short circuit and liner load. (b) A photograph of hardware used in this series of experiments. In the enlarged image, a nickel liner is shown. The thickness of the liners was 3lm.. probing system, with a 5 ns pulse width of 532 nm laser light, and 15 ns inter-frame separation. The detection limits of the optical system were 0.0048 rad and 0.096 rad, given by the schlieren stop and collection optics, respectively. These values correspond to an average electron density gradient of 3.7  1020 cm4 and 7.5  1021 cm4, respectively. The images were recorded with a charge-coupled device (CCD) camera, allowing a resolution of the optical system of 20 lm. A four-frame extreme ultraviolet (EUV) gated camera with unfiltered 50 lm pinholes was implemented. Each frame took a 5 ns exposure; the magnification of the system was approximately 1 and a spatial resolution of 100 lm was achieved. Two different delays of 10 and 20 ns between the frames were used. The cut-off energy of the photocathode is 5 eV, but diffraction effects become comparable to geometry effects at 25 eV. At lower photon energy, light is diffracted away and hardly reaches the quadrant camera.16 A flat-field grazing incidence soft X-ray spectrometer17 was used to examine the emission spectrum from the liners. Typical spectral resolution, using a 15 lm entrance slit, is 1 Å when a grating of 600 lines/mm is used. The spectrometer was operated in time-integrated mode using a Toshiba CCD, model 1304AP. Analysis of the captured spectra was carried out with PrismSpect18 atomic code. Time resolution of the EUV emission was obtained with 0.8 lm aluminum filtered diodes (model AXUV HS519).. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 146.155.94.33 On: Tue, 24 May 2016 16:27:40.

(4) 031208-3. Valenzuela et al.. Phys. Plasmas 21, 031208 (2014). FIG. 2. Time evolution of the perturbation observed with the schlieren technique, showing the results for Cu liners (a) and Ni liners (b).. These filtered diodes detected EUV photons with energy smaller than 72 eV, as no signal was observed using the same diodes filtered with 1 lm copper, which has a transmission band that overlaps the lower energy range of the aluminum K-edge window (energy greater than 600 eV). Both spectroscopy and filtered diodes were implemented side-on and end-on with respect to the axis of symmetry of the liner in order to observe emission from precursor plasma formed on axis.. III. RESULTS AND DISCUSSION A. Instability growth. One of the main motivations of this work was to study plasma instability initiation and evolution on a liner surface. We have accomplished this task by studying the plasma from the liner surface using schlieren laser probing and EUV gated imaging. The magnitude of the current and the liner dimensions were insufficient to cause the liners to implode. Figs. 2(a) and 2(b) display time sequences of instability evolution observed with schlieren laser probing in copper and nickel, respectively. The time with respect to current start is displayed under each image, and the colors (black, red, and green) indicate frames are from the same shot. The initial position of the liners is illustrated with a red dashed rectangle. Surface perturbations were found as early as 70 ns, at which time the plasma perturbations just exceed the limit of the optical resolution (20 lm). In both materials, the amplitude and the wavelength of the instability increase noticeably with time. Good correlation (symmetry) is not seen until very late in time, when the wavelength reaches the order of liner radius. Better correlation of the perturbation is observed in copper. Another difference is the very distinctive features observed in nickel liners developing at approximately 110 ns: We observe spikelike structures that persist until 175 ns. Subsequently, turbulent-like structures are seen. In copper, a sinusoidal-like profile is observed to evolve and grow in time. Neither the spikes nor the turbulent behavior is observed. Figure 3 shows the liner expansion rate, measured at the average position of the perturbation amplitude from the images shown in Fig. 2. The expansion of the materials is observed to be nearly linear. In copper, an expansion rate of (2.0 6 0.2)  105 cm/s is obtained from the linear fit, while a. value of (1.5 6 0.2)  105 cm/s is measured for nickel. Using the values found with spectroscopy (shown in Sec. III B: Te  16 eV, Z  8), we found that the sound speed for both materials is of the order of 2  106 cm/s. Thus, the surface liner plasma is sub-sonically expanding. The approximately constant liner expansion rate means that plasma pressure balances magnetic pressure. Under these conditions, MRT instabilities are not expected to grow.10 In this respect, our work provides a unique contribution to the observations of a highly imploding system, such as the one studied in Refs. 4–6. In these articles, the MRT instability was the main issue, and it was not possible to study the ablation and seeding mechanisms. A detailed analysis of the perturbation growth on schlieren data was carried out using the Lombard method,20 which gives information on the wavelength distribution spectrum of a certain function: in our case of the instability profile. The Lombard method is especially appropriate as it also measures the probability of a wavelength component being white noise, as well as being able to analyze unequally spaced data. As multiple wavelengths are present in the perturbation instability, a full analysis is not possible, so instead, we follow the growth of the most significant wavelength (the largest amplitude component). Figure 4 shows such a comparison for copper and nickel. The vertical error. FIG. 3. Liner expansion rate was estimated measuring the average radial position of the instabilities, linear expansion rate is observed in Cu and Ni liners.. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 146.155.94.33 On: Tue, 24 May 2016 16:27:40.

(5) 031208-4. Valenzuela et al.. FIG. 4. (a) Growth of the most significant wavelength found with Lombard method. (b) Amplitude growth of the most significant wavelength.. shown in Fig. 4 is the standard error from averaging the values found either side of the liner and the horizontal error is given by the uncertainty in the measuring method. The error contribution found using the Lombard analysis from random noise was less than 1% for all the analyzed wavelengths. It may be observed from Fig. 4(a) that copper and nickel exhibit a very different behavior. The copper perturbation grows rapidly and saturates, reaching a limiting wavelength of the order of the liner radius (1 mm), while the (most significant) wavelength in nickel slowly increases with time and also saturates at a similar value to that of copper. The amplitude of the most significant wavelength is also shown in Fig. 4(b). The nickel amplitude grows approximately linearly between 80 ns and 210 ns. Copper also presents a near linear growth, but, around 200 ns, some sort of saturation is observed at a value of 150 lm. No such saturation is seen for nickel in the time-span studied here. Figure 5 shows time-gated EUV images obtained with unfiltered 50 lm pinholes for copper and nickel. The time is shown under each image and the colors indicate the frames are from the same shot. The spatial resolution was not sufficient to resolve the small structures at the same early times,. Phys. Plasmas 21, 031208 (2014). as was achieved with the schlieren images. However, the resolution is sufficient to allow measurement of the surface perturbation symmetry and correlation. Early in time, it is not possible to observe any correlation at all, but, as time advances, we do observe some darker bands crossing from one side to another, meaning that the perturbation is axially symmetric. This effect is more distinctive in copper from 142 ns up to 246 ns (Fig. 5(a)), but it is also observed in nickel around 200 ns (Fig. 5(b)). The mechanisms that determine the period of this correlated wavelength are not yet understood and require further study, but, presumably, it is related to the liner radius and the uncorrelated structures seen early in time. As pointed out above in the schlieren images with nickel, the same spike-like features around 120 ns are seen, however, these features are not as evident later in time as is the case using schlieren imaging. We infer that this is due to cooling of the plasma. The good symmetry seen in the instabilities late in time suggest that the m ¼ 0 instability may develop. The timescale for development of this instability is of the order of pffiffiffiffiffiffiffiffi r/VA, where r is the radius of the pinch and VA ¼ B= l0 q is the Alfven velocity.12 B is the magnetic field, q is the mass density, and l0 is the vacuum permeability. Considering the coronal plasma whose ion density was found to be approximately 5  1017 cm3 (see Sec. III B) and taking a magnetic field of 40 T at a radius of 1 mm, corresponding to peak current, the time for the m ¼ 0 instability development is on the order of 6 ns. This time is shorter than the discharge timescale and supports the idea that m ¼ 0 may develop. Nevertheless, if we instead estimate the diffusion time sdif  l0 rL2 (L is the characteristic length and r is the plasma conductivity), using the value of r  3  104 X1m1 and the scale length of 1 mm, we obtain sdif  40 ns. This means that during the liner dynamics timescale (100 ns), a significant magnetic field has diffused through the plasma. Therefore, one can argue that the m ¼ 0 is presumably developing due to its short characteristic timescale, but the finite value of plasma resistivity will produce some magnetic field leakage, stopping the m ¼ 0 instability from growing.. FIG. 5. Time gated pinhole camera results showing the evolution of copper liners (a) and nickel liners (b).. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 146.155.94.33 On: Tue, 24 May 2016 16:27:40.

(6) 031208-5. Valenzuela et al.. The time scale for m ¼ 0 instability development (r/VA) at the moment of liner explosion (near 50 ns), when the liner density is 2 g/cm3 (critical point) and B  20 T, is 2 ls. This is much larger than the dynamics timescale, meaning that the instabilities observed early in time are not m ¼ 0 in nature and another mechanism must be invoked. It may also explain the difference in instability growth and plasma dynamics observed late in time in copper and nickel. As plasma temperature, density, atomic mass, and average ionization state are similar when liner ablation is underway, we expect similar MHD parameters. From these considerations, we deduce that the instability seeding mechanism must be different early in time for the two materials. Before plasma formation, when the liners are still in the liquid-biphase state, the resistivity of copper and nickel are very different (at least by a factor of 4 at room temperature). It is at this early stage that the conditions for the development of instabilities for both materials bifurcate, for example, due to the effect of electrothermal instabilities. The characteristic electrothermal instability wavelength has been found (theoretically and with simulations) to be of the order of 10 lm for copper,12,13 but, due to the resolution of the optical system (20 lm), we are unable to measure such instability wavelengths right after the liner explosion at around 65 ns. We can certainly say that they are smaller than 50 lm. Hence, electrothermal instabilities might be seeding the instabilities seen late in time. Additional work is needed to understand the nature and evolution of the instability seen on the liner’s surface, computational simulations will be fundamental to complete this task. B. EUV emission and plasma formation. EUV spectroscopy provides a valuable diagnostic tool for coronal plasmas, such as the one observed on the liner’s surface. As mentioned previously, spectroscopy was performed side-on to analyze the coronal plasma, as well as end-on in order to study any evidence of precursor plasma. Figure 6 shows the results obtained with copper liners. First, we contrast the different emission spectra of the two directions, which clearly indicate different plasma temperatures and densities. The temperature and density were estimated by comparing the observed spectra with synthetic spectra using a non-local thermodynamics equilibrium (non-LTE) model with the PrismSpect Atomic Code.18. FIG. 6. copper spectra: a comparison is made between the synthetic spectra and the experimental data for side-on (a) and end-on emission (b).. Phys. Plasmas 21, 031208 (2014). FIG. 7. Nickel spectra: a comparison is made between the synthetic spectra and the experimental data for side-on (a) and end-on emission (b).. A good match between synthetic spectra and experimental data was found for side-on emission (see Fig. 6(a)) when a constant ion density of 5  1017 cm3 and a time dependent plasma temperature was used (16 eV peak). We found that the use of an approximate temporal profile mimicking the diode signal (shown in Fig. 8) gave a better fit, although it was not extremely sensitive to this parameter. In contrast, a good match was found for end-on emission when both ion density and plasma temperature were time independent. Fig. 6(b) shows a synthetic spectrum using 1  1019 cm3 and 40 eV together with experimental result. The predominant species found side-on are CuVII-CuX, while end-on spectroscopy shows that the predominant species are CuXII-CXV. Fig. 7 shows the results obtained with nickel liners. Interestingly, a good match was found using the same plasma properties as those found with copper. Additionally, the same ionization states were found in nickel, namely NiVIINiX and NiXII-NiXV for side-on and end-on, respectively. In both copper and nickel liners, oxygen lines were found (principally OVI at 130, 150, and 173 Å), likely due to liner surface impurities and/or oxidation. It has been shown that oxygen is a very good indicator of temperature, and, in some cases, the impurities are desired.21,22 From a non-LTE model in Ref. 22, it was shown that the relative intensity of the. FIG. 8. End-on and side-on diode signals for Cu and Ni liners, using a 0.8 lm Al filter. The current is also displayed.. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 146.155.94.33 On: Tue, 24 May 2016 16:27:40.

(7) 031208-6. Valenzuela et al.. 173 Å OVI line is dominant for low temperature plasmas (20 eV), and, for higher temperatures, the relative intensity of the 130 Å line increases. If we observe Figs. 6 and 7, the 173 Å of OVI is dominant for the low temperature, side-on plasma, but in the case of end-on, the intensity is shifted to the shorter wavelength range, which supports the idea of a higher plasma temperature. A more detailed analysis is needed in order to perfectly match the results with synthetic spectra. In Ref. 22, a two temperature plasma is implemented to fit the modeling with experimental results. In the future, we are planning to use a 1200 lines/mm grating to better resolve the emission spectra and estimate temperature from intensity ratios of the oxygen lines and models. In addition, it is not yet clear why the temperature and density of the plasma inside the liner are so different than in the external surface plasma. The formation of a precursor might explain this discrepancy. The formation of precursor plasma is discussed later. The time evolution of EUV emission was followed using 0.8 lm Al filtered diodes. This allowed a good insight into photon emission from multi-eV temperature plasmas. Consequently this diagnostic is a good indicator of plasma formation. Fig. 8 shows the results obtained with copper and nickel together with the current waveform. The error bars in the current as well as in the diodes’ signal reflect the error obtained averaging traces from 3 shots. First, we notice that for copper and nickel both end-on and side-on emission begins at the same time. Second, we observe that emission from the nickel liner starts at approximately 45 ns from current onset compared to 65 ns in copper. Third, the side-on emission is almost three times larger than the end-on emission. Furthermore, the end-on emission falls at approximately 120 ns as compared to 210 ns in side-on emission. The signal from the diodes provided valuable information about plasma formation. As was shown in Ref. 23, the appearance of this signal is consistent with the formation of thermal plasma with a temperature above 2 eV. Since the time of plasma formation in Cu was observed to occur at 65 ns as compared to 45 ns for Ni, we may infer that liner conductivity is important in plasma formation. Since nickel’s resistivity is larger than that of copper by a factor of 4 (at solid density and room temperature), the energy required for plasma formation will be deposited by Joule heating earlier in time. The smaller diode signals observed end-on may be due to the higher ionization level of the hotter plasma shifting the emission spectra to a higher energy range not aligned with the filter transmission windows. Alternatively, the smaller signals may be due to the decrease in the skin depth of the current as the plasma heats on the outer surface, in which case current is diverted to a small portion of the surface plasma. Initially, we had expected to observe liner expansion early in time as the current increases before plasma formation. However, via the schlieren and time-gated pinhole cameras, we detect no measurable liner expansion (see Figs. 2 and 5). Despite being limited by the resolution of our respective systems, we can estimate the liner expansion if we consider that diode signal starts at the critical material density, which for copper is 2 g/cm3. Then, considering that the. Phys. Plasmas 21, 031208 (2014). initial copper density is 9 g/cm3, we have Dr0 q0 ¼ Drc qc and therefore, Drc  13 lm. Since this value is under the resolution limits of our systems, it explains why we did not detect liner expansion. Another observation from the diode signals is that both end-on and side-on emission start at the same time. From this, we deduce that plasma formation occurs at the same time both inside and outside the liner, which implies that the current skin depth is greater than the thickness of the liner. The current skin depth at solid density and room temperature for copper is of the order of 60 lm and 120 lm for nickel, and these values increase to 1 mm as the liner melts due to the decrease of material conductivity. Hence, it is possible that a precursor plasma forms from the current-ablated plasma that then propagates to the axis. In similar observations24 using axial X-ray point projection backlighting, the plasma precursor formation at peak current from copper wire arrays was compared with copper liner experiments. The authors found that a precursor plasma is formed in liners, but with densities much lower (9  1017 cm3) than in wire arrays experiments ((1  1019 cm3). They also observed a layer of high density plasma adjacent to the liner’s inner wall (1  1019 cm3). It is likely that in our experiments, a precursor plasma is formed inside the liner. This will be a topic for future publications. IV. SUMMARY. We have reported on plasma instability growth on thin liners driven by a small scale driver. We compared two different liner materials (Cu and Ni) to study the effect of material resistivity on plasma formation and evolution. It was found that, late in time, the instabilities observed with schlieren and with a time resolved EUV pinhole camera may well be m ¼ 0 MHD in nature, but it is not possible to ascertain this, as the magnetic field diffusion time is of the order of the liner dynamics timescale. However, the different growth and dynamics suggest that the seeding mechanism may play an important role in the evolution of the instability. The short exposure pinhole images show clear instability correlation (symmetry) at a late stage in the driver current pulse, but due to diagnostic spatial resolution, it was not possible to see if this correlation exists early in time. EUV emission presumably suggests the existence of a precursor plasma formation, which needs more investigation. This work studies instability formation in liners with low currents, and it could be used to benchmark computational codes. In the future, we plan to investigate instability formation with higher temporal resolution, so that we can directly link initial perturbations with fully developed instabilities. ACKNOWLEDGMENTS. The authors would like to thank Sandia National Laboratories for the loan of LTD pulser. They also appreciate technical advice of Dr. Ken Struve and Dr. Michael Mazarakis of Sandia National Laboratories. E.S.W. acknowledges support from the Chilean government research Grant FONDECYT 1120816.. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 146.155.94.33 On: Tue, 24 May 2016 16:27:40.

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Figure

FIG. 1. (a) Current waveform in a short circuit and liner load. (b) A photo- photo-graph of hardware used in this series of experiments
FIG. 2. Time evolution of the perturba- perturba-tion observed with the schlieren  tech-nique, showing the results for Cu liners (a) and Ni liners (b).
FIG. 4. (a) Growth of the most significant wavelength found with Lombard method. (b) Amplitude growth of the most significant wavelength.
Fig. 7 shows the results obtained with nickel liners.

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