Maintaining high reliability PFN Marxes on DARHT II

The Dual-Axis Radiographic Hydrodynamic Test Facility (DARHT) at Los Alamos National Laboratory consists of two linear induction accelerators at right angles to each other. The First Axis produces a nominal 20-MeV, 2 kA single pulse with 60 ns width. In contrast, the DARHT Second Axis produces up to four pulses in a variable pulse format by slicing micro-pulses out of a longer pulse (~1.6 microseconds flattop) of nominal beam energy and current of 17 MeV and 1.65 kA respectively. Bremsstrahlung x-rays are produced by focusing the electron beam-pulses onto a high-Z target. This paper focuses on the second axis accelerator and maintenance of its components. The injector Marx delivers 2.1 MV to a 6.25 inch diameter thermionic cathode producing a 1.65 kA beam. The beam is accelerated in a Linear Induction Accelerator (LIA) consisting of 74 cells operating at a total of 14.8 MV, delivering a beam of ~17 MeV and ~1.6 μs flattop at the accelerator exit. Each cell is driven by an individual Pulse Forming Network Marx (PFN Marx) tuned to provide voltage regulation of +1%. The Axis II LIA uses solenoids for focusing the beam and dipole pairs for steering. Each cell incorporates a solenoid and a pair of dipoles. Including all magnets in between blocks of cells, in the diode anode region and elsewhere, there are 91 solenoids and 80 pairs of dipoles. The ideal energizing currents in the focusing and steering magnets to optimize the electron-beam transport is very dependent on cell gap locations and potential in those gaps. Excessive PFN Marx jitter or a prefire would seriously compromise beam transport and the bremsstrahlung radiographic spot sizes. Low erection time jitter and exceptionally low prefire rates of the 74 PFN Marxes is required to ensure proper transport of the electron beam. The design and maintenance of the four-stage unipolar PFN Marxes are described here for the first time.