Design and performance of the DARHT second axis accelerator

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, operating since 1999, 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 1.8 kA respectively. Bremsstrahlung x-rays are produced by focusing the electron beam-pulses onto a high-Z target. The DARHT Facility achieved full operational status in FY2010 with the successful execution of five dual-axis experiments, including an explosively driven flyer plate and four integrated weapon hydrotests. This paper focuses on the second axis accelerator and its components. The injector consists of an 88 stage Marx delivering 2.5 MV to a 6.25 inch diameter thermionic cathode. A crowbar switch in the Marx tank can shorten the 2 μs pulse to as little as 200 ns. The beam is accelerated in a Linear Induction Accelerator (LIA) consisting of 74 cells operating at a total of 14.5 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 (PFN) tuned to provide voltage regulation of ±1%. The design and performance of the injector, accelerator cells and PFNs are described herein.