Evaluation of methods to increase beam pulse width on the DARHT Axis-II accelerator

Summary form only given. The second axis (Axis II) of the Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility at Los Alamos National Laboratory (LANL) is an electron linear induction accelerator (LIA) using 74 induction cells, each driven by a separate pulse-forming Marx (PFM). The ability to perform precise multipulse radiography is heavily influenced by the temporal beam energy spread, beam pulse width, related beam motion, and other focusing and target factors. The nominal beam-pulse flattop width is about 1.6 μs. A wider beam pulse would allow for increased spacing between kicked pulses and hence more information in radiographic experiments. A flattop pulse-width increase of even 75 to 100 ns would be of significant value. In this paper, we present a few recently evaluated methods to widen the existing beam pulse without significant changes to the accelerator or infrastructure. The first method is a constrained optimization between total beam energy (sum of all the cell and injector voltages) and maximum cell pulse width. It determines the voltage set point of each of the PFMs that drive the cells such that their respective pulse widths are nearly identical while being subject to a constraint of minimum acceptable total beam energy. Each cell has a unique magnetic core V-s capability, and properly setting pulse voltages allows for maximum pulse width based on individual core properties. The second method involves retiming the cell pulses such that their trailing edges align instead of their leading edges. The leading edge of the cell pulses are of less importance than the trailing, hence, if the end of the beam pulse can be improved, additional pulse width suitable for radiography can be obtained. The third method focuses on the preset current for the magnetic cores in the cells. To achieve sufficient flux swing in the cores, each is preset to a known saturation state prior to each pulse. Experiments have shown that the levels of preset current vary by more than 15% and the timing of the preset pulses can be adjusted to obtain additional improvement in available V-s capability. After adding the available increases in beam pulse width from these separate methods, we expect to be able to obtain over 100 ns in usable pulse width. The paper presents the results from simulations, experiments and describes measured and expected increases in pulse width due to each of the methods.