G. Randall McKee

Multi-pulse electron diode development for flash radiography

Presently the Self Magnetic Pinch (SMP) diode is successfully utilized for flash radiography with pulsed power drivers. However, it is not capable of more than one pulse. Multi-pulse single-axis radiography is most preferred since it provides images of time-evolving dynamic targets. In an SMP diode, because the anode cathode (A-K) gap is very small (~1-2 cm), the debris from the anode converter target arrives soon after the first pulse and completely destroy the cathode electron emitter, and thus the diode cannot produce a second pulse. We propose a feasibility study to scientifically evaluate the idea of decoupling the anode converter from the cathode electron emitter. This work will be based on two successful previous works we have accomplished: first, making a very small pencil-like beam in a magnetically immersed foilless diode (M.G. Mazarakis et al., Applied Physics Letters, 7, pp. 832 (1996)); and second, successfully demonstrating the two-pulse operation of a foilless diode with the RIIM accelerator (M. G. Mazarakis et al., Applied Physics 64 part I pp. 4815, (1988) Our approach will combine the above experimentally demonstrated successful work. The generated beam of 40-50 kA will be propagated in the same diode magnetic solenoid for a sufficient distance before striking the converter target. This way the diode could be multi-pulsed before the target debris reaches the cathode. Although the above describes the option of a foilless diode and a solenoidal transport system, a similar design could be made for a non-immersed low emittance 10 kA velvet emitter foilless diode.

Z driver post-hole convolute studies utilizing MYKONOS-V voltage adder

The modern high current, high voltage pulsed accelerators utilize vacuum-post-hole convolutes to add the current of a number of parallel self Magnetic Insulated Transmission Lines (MITL) to a single one located very close to the centrally located load. The reason of course of using several parallel MITLs to transfer the current pulse from large, ~1.5 m, radii to the 1-2 cm load is to reduce the transfer inductance. For example, the vacuum chamber of the 24-26-MA Z machine has a 1.45-m radius vacuum section containing four parallel conical MITLs merging into one 6cm radial disc MITL adjacent to the centrally located load via a double post-hole convolute array located at 7.62 cm from the axis. Although special care has been taken to reduce the electrical stresses on the cathode hole surfaces in order to avoid electron emission, substantial current losses, 4-6 MA, are observed most probably due to plasma formation and the unavoidable magnetic nulls. In the proposed experiments we will study the behavior of only one convolute using the MYKONOS-V driver. MYKONOS-V is a Linear Transformer Driver (LTD) voltage adder composed of 5 nominally 1-MA cavities connected in series. The voltage adder radial A-K cavity is deionized water insulated. The experimental set-up is designed in such a way to reach conditions on the convolute very similar to those existing on Z. Most importantly, in contrast to Z, it provides full view of the convolute for optical and spectroscopic imaging and gives the flexibility and freedom to study various options in an effort to reduce the convolute losses without affecting the day-to-day Z experiments. This is going to be a dedicated convolute study experiment. The hardware design, numerical simulations and proposed diagnostics will be presented and discussed.

The ZR Final Design Pulsed Power Performance Expectations

The largest X-ray generating facility in the world (~1.8 MJ), the Z machine at Sandia National Laboratories (SNL), is presently undergoing a major upgrade. Upon completion of its refurbishment, ZR is expected to deliver an output current of 26 MA to a standard 20 mm by 40 mm diameter Z-pinch load with a 100-ns implosion time. In addition to nearly doubling the output energy and providing a useful increase in current capability to the research community, the Project criteria include providing an enhanced precision, improved timing jitter, and advanced pulse shaping flexibility needed for full parameter space assessment for the material science community. Another criteria is to increase the shot capacity by enabling the facility and diagnostics infrastructure to routinely support 400 shots per year while minimizing the impact of implementing improvements on existing experimental programs. The Projects objective of extending the life of Z in a balanced way also served to exercise SNLs pulsed power research and engineering capabilities and resources and retain the expertise that will be required for future endeavors. This paper summarizes the predicted pulsed power performance of the 36-module ZR from the 577-kJ (at 85-kV charge) Marx generators in the oil energy storage section through the water pulse forming section components and the vacuum section to the Z-pinch load. Comparisons show good agreement between the most recent models and experimental results from full-scale, single module systems which are very similar to the final design. Discussions will emphasize the laser triggered gas switches and the self-break water switches which are continuing to be optimized