MINIMIZING BEAM MOTION IN A LONG-PULSE LINEAR INDUCTION ACCELERATOR

The Dual Axis Radiography for Hydrodynamic Testing (DARHT) Facility at Los Alamos uses two linear induction accelerators (LIAs) for flash radiography of explosively driven experiments from orthogonal viewpoints. The DARHT Axis-II long-pulse 1.8-kA, 16.5-MeV LIA is unique. It has a beam pulse with a 1.6s current flattop during which the kinetic energy varies by less than 2%. During this flattop, a kicker cleaves out four short micro-pulses, which are focused onto a high-Z target and converted to bremsstrahlung for multipulse flash radiography of the experiments. Asymmetric injection of the beam into the solenoidal focusing field, small temporal variations in accelerating potentials, and slight cell misalignments combine to cause the beam position to wander during the flattop. This is undesirable for radiography. The slow beam motion in the LIA causes a displacement of the four radiographic source spots. Moreover, since the specific energy deposition from each micro-pulse is sufficient to vaporize target material, succeeding pulses impact an asymmetric object causing a distortion of the source spot. Therefore, we have spent some effort to tune out the beam motion at the exit of the LIA.