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

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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-network-based Marx (PFN). 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 50 to 100 ns would be of significant value. In this paper, we present analyses, simulations and data for three low-cost, relatively easy means of lengthening the beam pulse for multipulse radiography. The first involves optimizing the PFN charging voltages such that their respective cell pulse widths are nearly the same. The second method studies the utility of timing the cell voltage pulses such that their trailing edges are aligned instead the leading edges. And the third method, presents experimental data on the effects of increasing the cell core reset currents which increases the available volt-second (V-s) products in the cores yielding increased pulse width.