EXPERIMENTAL STUDIES OF MULTI-WIRE ARRAYS ON THE COBRA GENERATOR* R. D. McBride, J. D. Douglass, S. A. Pikuz, T. A. Shelkovenko, J. B. Greenly, D. A. Hammer and B. R. Kusse Laboratory of Plasma Studies, Cornell University, Ithaca, NY USA Abstract The rebuilt pulsed power generator, COBRA, 1 capable of providing variable current pulses (e.g., MA peak current; ns rise times), is currently being used to study wire array z-pinch implosions at Cornell University. Wire arrays constructed from four, eight, and twelve wires of aluminum, nickel, tungsten, Invar (Fe, Ni alloy), as well as other materials, have been tested. Various array diameters have also been tested, although typically 1.6 cm has been used, and the standard array height is 20 mm. The influence of the current waveform on the array implosion has been studied using x-ray imaging diagnostics with spatial and/or temporal resolution. Photodiodes such as XRD, PCD, and AXUV-HS were used with various filters to investigate a wide energy range that spanned from EUV up to 10-keV x rays during various stages of the array implosion process. Characteristic to this research was the use of x-ray backlighting, which was employed to focus in on the explosion processes of the individual wires during an array implosion. The source of the x-ray backlighting was two, four-wire X pinches made from either aluminum or molybdenum wires. These X pinches replaced two of the four standard electron return current posts in the anode hardware setup, and were capable of providing the desired point-projection backlighting 2. This imaging technique made it possible to determine the expansion rates and inner structure of the various wire cores, as well as to observe the development of sausage instabilities. Additionally, a spherically bent mica crystal spectrograph and a flat KAP crystal spectrograph were used to determine plasma parameters for both the X pinches and the main wire array. Pinhole cameras were also used, and provided wide view images of the entire configuration. The images consist of a full view of the main array, with an X pinch off to either side. By using various filters with these pinhole cameras, potential problems associated with the wire contacts at the electrodes were observed. These observations have been examined to understand the influence of contact points on precursor plasma formation. 1. J. D. Douglass et al., abstract submitted to the 2005 Pulsed Power Conference. 2. T. A. Shelkovenko, D. B. Sinars, S. A. Pikuz, K. M. Chandler, and D. A. Hammer, "Point-projection x-ray radiography using an X pinch as the radiation source", Rev. Sci. Instrum., vol. 72, pp , * This research was supported by the Stewardship Sciences Academic Alliances program of the National Nuclear Security Administration under DOE Cooperative agreement DE-FC03-02NA Effects of Variable Current Waveforms Four-hole pinhole camera (VPC1) Four-hole pinhole camera (VPC2) Backlighting camera for X-pinch 1 Backlighting camera for X-pinch 2 The COBRA pulsed power generator at Cornell University provides MA current pulses with rise times ranging from 100 to 150 ns Variable waveform shapes are possible due to programmable laser triggering now online X pinches placed in electron return current path provide point-projection backlighting, making it possible to image individual wire explosions during an array implosion Photo of array with two X-pinch backlighters Photo of vacuum chamber with diagnostics setup around the array and X-pinch backlighters Introduction to COBRA and Experimental Setup Pulse waveform to the left shows a larger second peak, a smaller peak current (i.e., 644 kA), and a slower rise time (i.e., maximum di/dt of 6x10 12 A/S) Pulse waveform on the right has a larger first peak, a faster rise-time (i.e., maximum di/dt of A/s), and a larger overall peak current (i.e., 800 kA) – Resulting spectra shows the addition of Hydrogen-like line Al XIII (7.17 Å; 1.73 keV) – Resulting pinhole camera image shows greater number and intensity of hotspots 1 Time (s) Current (A) 8 wires of  m Al Wavelength 7.75 Å (1.6 keV) 500 ns 8 wires of  m Al Wavelength 7.75 Å (1.6 keV) 7.17 Å (1.7 keV) 400 ns

* This research was supported by the Stewardship Sciences Academic Alliances program of the National Nuclear Security Administration under DOE Cooperative agreement DE-FC03-02NA Tungsten (W) Array Imaging Waveform shape and higher peak current of pulse 195 produces higher energy z-pinch and X pinches than pulse 194, and provides more opportunity for wire imaging X-pinch backlighting provides time resolved (<1ns) images showing corona formation with  m wavelength, and a possible dense core wavelength of ~  m – X-pinch timing determined by Si and X-ray diode (XRD) traces Precursor plasma column in center of array provides backlighting for a time-integrated image of wire core and corona, showing a wavelength of approximately  m Invar (64% Fe; 36% Ni) Array Imaging Two layers of Biomax film used in four- hole pinhole camera, along with various filters and pinhole diameters to determine energy range of array z-pinch radiation In vertical dimension,  m structure observed in wire core, as well as a mm dense core wavelength F-like and Ne-like Ni lines observed in spectrograph Aluminum (Al) Array Imaging Nickel (Ni) Array Imaging Eight-wire, 8-  m Ni array imaged by two, four-wire Mo X pinch backlighters In vertical dimension, dense core shows ~5  m structure, and  m dense core wavelength In horizontal dimension, a 5  m dense core was observed, while the outer region of the wire had blown-up to ~30  m in diameter 750  m 25  m 50  m 2.34 mm Pulse 270 Pulse 270 was eight, 17-  m Al wires, and achieved a peak current of 985 kA 5-25  m scale size for vertical structure ~  m dense core wavelength Pulse 267 was eight, 25-  m Al wires, and achieved a peak current of MA 5-25  m scale size for vertical structure ~  m dense core wavelength from X-pinch backlit image, and ~  m corona wavelength from time-integrated, z-pinch backlit, pinhole image Time-integrated open pinhole image 2.34 mm 950  m 25  m 145  m 95  m 20 mm X-pinch backlit wire core image Conclusions and Future Work The imaging and diagnostic capabilities now available for COBRA have been shown for various wire diameters and materials (W, Invar, Al, and Ni), as well as for some of the different current pulse waveforms and amplitudes These preliminary results are encouraging, and future work will apply the techniques demonstrated above to a systematic and quantitative study of the various array and current pulse parameters in order to more fully understand their effects on z-pinch implosion dynamics X-pinch backlit, wire core image Wavelength Summary Material & CurrentDense core wavelength from time-resolved, X- pinch backlit image Corona wavelength from time-resolved, X-pinch backlit image Corona wavelength from time-integrated, z-pinch backlit image  m 907 kA  m  m  m 10-  m 819 kA560-1,100  m NA 17-  m 985 kA  m NA 25-  m MA  m NA  m 8-  m 926 kA  m NA 50-  m pinhole diameter XRD, 4-  m Polypropylene filter, E > 0.6 keV Si-diode, 25-  m Be filter, E > 1 keV Si-diode, 60-  m Al filter, E > 6 keV Load current (in Amperes) vs. Time (in seconds) X-pinches Precursor Pinhole Diameters Filters Wavelength r T e > 300 eV Pulse 267