Dose measurements and beam parameter inference from intense relativistic electron beam interactions with a high-Z, solid-density target

The Dual-Axis Radiograph Hydrodynamic Test (DARHT) facility utilizes linear induction accelerators to produce intense relativistic electron beams (IREB) with end-point energies of 16-20 MeV and currents of 1.6-2kA. The IREB interaction with a high-Z, solid-density x-ray converter target produces a broadband emission of x-rays, often described with thick-target bremsstrahlung theory. An analytic approach to this process is embodied in the LANL code DOSECALC[1] which includes the effect of finite beam emittance at the converter target. We present measurements of the x-ray emission with and without penumbral imaging optics using radiochromic films and a Pt calorimeter. The films were calibrated on a bremsstrahlung source at both 10 and 20 MeV to reproduce the correct x-ray spectrum. We find that the dose distributions allow an inference of the lab-frame transverse momentum of the beam and, in conjunction with inferred spot sizes yield the beam emittance. We further find that the uncollimated dose closely matches the analytic theory but begins to deviate when the high-Z converter is probed after it has been heated into the plasma state and allowed to expand.

[1] Hawkins, Watson, and Kauppila, “Absolute bremsstrahlung energy spectral and dose distributions – theory and experiment”, Los Alamos Technical Report LA-UR-93-3544, 1993.