Circuit Modeling and Component Trimming to Achieve Performance Requirements for the Darht Prototype Injector

To produce an acceptable electron beam, the Dual-Axis Radiographic Hydrodynamic Test (DARHT) Facility accelerators require high-voltage pulses that are flat to within ±1% for 65±5 ns. Achieving this accuracy and pulse duration in operation on the Integrated Test stand (a fullscale prototype of the DARHT 4-Mev injector and first eight linear induction accelerator cells) has required fine-tuning selected components in the injector pulsed power circuits. Several circuit analysis codes are used to analyze circuit performance and to predict the effects of component changes. Four cases are presented. (1) The center conductor of an ethylene glycol transmission line was tapered to compensate for a 4-5% tilt of the flat-top portion of the injector voltage waveform. (2) The mixture of water-glycol solution used in the Blumlein "peaking" section was changed to compensate for a 2% hump in the flat-top portion of the voltage waveform. (3) Confirmation was provided that observed changes in pulse shape could be caused by changes in dielectric constant of the ethylene glycol used in the Blumlein, caused by hygroscopic absorption of water vapor. ( 4) A damping resistor was added in series with the Blumlein center conductor to reduce the amplitude of low-frequency, post-pulse oscillations that were causing late-time flashovers of the vacuum insulator.