Plasma source reproducibility studies for ACE 4

ACE 4 is a 4 MJ X-ray simulator using inductive energy storage and a plasma opening switch (POS). The POS is the only switch downstream of the Marx generator and is the critical pulse compression element determining the current rise time and magnitude transferred to the load. The plasma source used for the ACE 4 POS is a set of flashboards. An off-line study has been conducted using laser interferometry to measure the electron density produced by a single free-space flashboard. Aspects of the study included a comparison of densities produced by 'dirty' flashboards and cleaned flashboards, and a comparison of cleaned flashboards to new ones. Also, the effect of more carefully controlled routing of the drive cables on the flashboards themselves to produce a more consistent inductance associated with each chain was investigated. Multi-channel measurements were taken in both longitudinal and transverse axes in a plane parallel to the surface of the flashboard at distances from one to five inches. Typical reproducibility was within 10%, although some shots deviated as much as 20%. A consistent density profile was obtained between the cleaned, used flashboards and the new flashboards, provided the new boards were first 'cleaned' with acetone and Scotch-Brite(R) pads. Cleaning used flashboards was found to restore a 'prompt' component of the electron density which arrived at our measurement location 250 ns earlier than the first measurable densities from 'dirty' flashboards. Cleaning also reduced the amplitude of the second peak in the density by 20%. Both these features are shared by new, cleaned flashboards.