Atlas rail gap arc plasma/switch closure character [Using a novel fiber isolated E-dot detector]

Summary form only given. The Atlas Pulse Power Experimental Facility (at Los Alamos National Laboratory) will use 152 Marx Modules, in parallel providing 20-45 Meg-Ampere current pulses to an experimental load for weapons and high energy density physics. The Marx units use the Maxwell Laboratories/DNA ´rail gap´ [a low inductance, gas filled, linear electrode spark gap]. Each Marx unit provides a nominal 300 KA pulse peaking at 4.5-57 s into a very low inductance transmission line and load assembly. At this current the gap plasma density distribution is about 12 KA/cm along the electrode [photographically estimated electrode current densities of /spl sim/60,000 A/cm/sup 2/; argon-SF/sub 6/ gas fill]. The circular symmetry of the current arrival at the experiment load is of significant importance to both the experiment and machine welfare. Models have shown potentially destructive voltage spikes will exist in the machine with system jitter of 300 ns or more. Much lower jitter envelopes (<100 ns) will produce variant current symmetry on the load that will impart unwanted instabilities to the experiment. Therefore the time domain character of the machine triggering and rail gap conduction has been a subject of study. Until recently we have not had a practical means of determining or monitoring the overall time jitter of the system due to the complexity of making high bandwidth measurements across the very tenuous environment of the rail gap Marx switches. The system current pulse is far too slow (/spl sim/55 KHz network ring frequency) to provide the necessary fast rising fiducial. An optical arc light monitor has been used for some time to monitor the gap operating character but shows a similar (slow) temporal wave shape to the Marx current. We have developed a self-powered, fiber optically isolated E-field detector with the required bandwidth. This detector can reside in the extreme fields of the Maxwell rail gap faceplate (in air at 7000 ft altitude) when holding- off 120 KV stress without distorting the field or causing coronal problems. The detector is now allowing us to delve into the nature of the plasma resistive phase of this low inductance spark gap.