Shot reproducibility of the self-magnetic-pinch diode at 4.5 MV

Abstract

In experiments conducted at Sandia National Laboratories' RITS-6 accelerator, the self-magnetic-pinch diode exhibits significant shot-to-shot variability. Specifically, for identical hardware operated at the same voltage, some shots exhibit a catastrophic drop in diode impedance. A study is underway to identify sources of shot-to-shot variations which correlate with diode impedance collapse. The scope of this report is limited to data collected at 4.5-MV peak voltage and sources of variability which occur away from the diode, such as sheath electron emission and trajectories, variations in pulsed power, load and transmission line alignment, and different field shapers. We find no changes in the transmission line hardware, alignment, or hardware preparation methods which correlate with impedance collapse. However, in classifying good versus poor shots, we find that there is not a continuous spectrum of diode impedance behavior but that the good and poor shots can be grouped into two distinct impedance profiles. In poor shots, the sheath current in the load region falls from 16%-30% of the total current to less than 10%. This result will form the basis of a follow-up study focusing on the variability resulting from diode physics.

Current transport and loss mechanisms in the Z accelerator

“…Electrons are emitted from the cathode after the local electric field stress exceeds the tolerance of the conductor [17,31]. The emission threshold depends on the material and its preparation, with typical values ranging from 150 to 280 kV=cm as determined when the anode current exceeds the cathode boundary current [15,16]. A previously tested value of 240 kV=cm [11] is used here.…”

“…The expected early-time behavior of the Z machine is derived from studies of lower-energy accelerators [15,16], in which the pulse propagation in vacuum creates sufficient electric field stresses and Joule heating to initiate spacecharge-limited electron emission [17] from the cathode. Electrons cross the anode-cathode (AK) gap and further heat the anode surface until their flow becomes magnetically insulated.…”

Simulations of the generation and transport of a 5 MV end-point x-ray beam on a pulsed power generator

“…Because of these large gaps, the plasmas that form on the surfaces do not have time to expand significantly into the gap before the pulse is over, which limits the effect that surface plasmas have on the diode impedance. This is helpful from a simulation standpoint, because impedance collapse caused by surface plasmas is notoriously hard to simulate in high-power diodes with small gap spacings (although some progress is being made [3][4][5][6][7][8]).…”

Design and experimental research on a self-magnetic pinch diode under MV

“…Paraxial diodes are disadvantageous because of their complicated structure and large focal spots (diameter=4-6 mm), and their impedance is than that of rod pinch or self pinch diodes [9]. Self-magnetic pinch diodes (SMPD), which can achieve high radiation dosage (hundreds of rad) and small focal spots (approximately 2 mm) at high voltages (>10 MV), have been examined in current theoretical and application studies to satisfy the requirements of flash radiography for thick materials with high atomic numbers [10][11][12][13][14][15].…”

“…In 2003, the diode structure was optimized, SMPD was driven by Mogul D, and 54 rad at 1 m and 2.1 mm focal spots were obtained at 4.2 MV. Sandia National Laboratories in the United States tested SMPD on RITS-6 at high voltages (6.5 MV), achieved 350 rad at 1 m and 2.9 mm focal spots, and focused on the impedance stability of SMPD [12][13][14].…”