Tuning the DARHT Long-Pulse Linear Induction Accelerator

Flash radiography of large hydrodynamic experiments driven by high explosives is a well-known diagnostic technique in use at many laboratories. The Dual-Axis Radiography for Hydrodynamic Testing (DARHT) facility at Los Alamos produces flash radiographs of large hydrodynamic experiments. Two linear induction accelerators (LIAs) make the bremsstrahlung radiographic source spots for orthogonal views of each test. The 2-kA, 20-MeV Axis-I LIA creates a single 60-ns radiography pulse. The 1.7-kA, 16.5-MeV Axis-II LIA creates up to four radiography pulses by kicking them out of a longer pulse that has a 1.6- μs flat top. The DARHT LIAs use solenoidal focusing for transport of the beam through the accelerators. The long-pulse Axis-II LIA has 74 accelerating cells, and uses 91 solenoids and 80 pairs of dipoles for focusing, transporting, and steering the beam. The setting of the currents of these 251 magnets is called tuning the accelerator. Tuning is done in two stages. First, the solenoidal focusing magnets are set to values designed to provide a beam with minimal envelope oscillations, and little or no instability growth. Then, steering dipoles are adjusted to minimize the low-frequency motion of the beam centroid and center it at the LIA exit. The design of the focusing tune is computationally intensive. Focusing tune design methods, simulations, and validation are the main topics of this article.