Abstract
The second axis of the dual-axis radiography of hydrodynamic testing (DARHT) facility produces up to four radiographs within an interval of $1.6\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$. It does this by slicing four micropulses out of a $2\mathrm{\text{\ensuremath{-}}}\ensuremath{\mu}\mathrm{s}$ long electron beam pulse and focusing them onto a bremsstrahlung converter target. The 1.8-kA beam pulse is created by a dispenser cathode diode and accelerated to more than 16 MeV by the unique DARHT Axis-II linear induction accelerator (LIA). Beam motion in the accelerator would be a problem for multipulse flash radiography. High-frequency motion, such as from beam-breakup (BBU) instability, would blur the individual spots. Low-frequency motion, such as produced by pulsed-power variation, would produce spot-to-spot differences. In this article, we describe these sources of beam motion, and the measures we have taken to minimize it. Using the methods discussed, we have reduced beam motion at the accelerator exit to less than 2% of the beam envelope radius for the high-frequency BBU, and less than $1/3$ of the envelope radius for the low-frequency sweep.
Highlights
The dual-axis radiography for hydrodynamic testing (DARHT) facility produces flash radiographs of highexplosive driven hydrodynamic experiments
Most of the 12 beam position monitors (BPMs) in the downstream transport had eight detectors for ellipticity measurements because of the quadrupole magnets used for DST transport
The predicted helical trajectory is stationary only if the beam initial energy and the cell accelerating potentials are constant in time. If either of these vary in time, the helix phase and gyroradius vary at the linear induction accelerator (LIA) exit, causing the beamcentroid position to sweep in time, as illustrated in Fig. 10, which shows the sweep resulting from coherent variation of the accelerating-cell potentials
Summary
The dual-axis radiography for hydrodynamic testing (DARHT) facility produces flash radiographs of highexplosive driven hydrodynamic experiments. Two linear induction electron accelerators (LIAs) make the bremsstrahlung radiographic source spots for orthogonal views of each test. The Axis-II LIA has a long (1:6-s) flattop with energy variation less than Æ1:5%. It has been operated at more than 16 MeV with beam currents greater than 1.5 kA. It creates up to four short radiography pulses by kicking them out of the long flattop. The Axis-II LIA, the beam it produces, and its solenoidal focusing are further described. After the long pulse exits the accelerator it passes through a kicker, which slices out shorter pulses to be transported to the bremsstrahlung converter target. Integration of fast (ns time scale) motion over the multi-ns radiographic pulses would blur and enlarge the source spots. Such wandering would distort the spots of the last pulses in the sequence due to asymmetric target erosion by the preceding pulses
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