Linear-induction-accelerator beam-energy-spread minimization: Cell models and timing optimization

The second axis (Axis II) of the Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility at Los Alamos National Laboratory (LANL) is a linear induction accelerator (LIA) using 74 cells, each driven by a separate pulsed-power modulator. The summation of the injector and 74 cell voltages is the beam-energy temporal profile. The ability to perform precise multi-pulse radiography is heavily influenced by the temporal beam energy spread, related beam motion, and other focusing and target factors. Beam loading affects both the shape and magnitude of each cell's voltage during the pulse. Ideally, each pulsed-power modulator/cell pair is tuned such that the loaded-cell voltage is flat with minimal amplitude variation during the pulse. However, changes in operating parameters on Axis II (beam current, operating cell voltage) have altered the amount of flattop variation resulting in more energy spread than when commissioned. In this paper, we present an optimization and synthesis method which minimizes the beam's temporal energy spread by adjusting the timing of cell voltages, either advancing or retarding them, such that the injector voltage plus the summed cell voltages in the LIA result in a flatter energy profile. The method accepts as inputs the beam current, injector voltage, cell-voltages, and synthesizes loaded cell voltages as needed. Simulations and experimental data for both unloaded and loaded-cell timing optimizations are presented. For the unloaded cells, the pre-optimization baseline energy spread was reduced by over 30 % as compared to baseline. For the loaded-cell case, the measured energy spread was reduced by 49% compared to baseline.