Optical Transition Radiation (OTR) Measurements of an Intense Pulsed Electron Beam

By using the OTR angular distribution property [1], it is possible to extract the energy and the angular dispersion of several kinds of beam [2]. Recently, Le Sage et al. [3] succeeded in producing a transverse phase-space mapping of a 100 MeV electron beam by coupling an interferometric measurement and a ’’mask’’ technique. We present the first OTR time resolved angular distribution measurements made on an intense pulsed electron beam with energies of 3.8 and 20 MeV. The DARHT accelerator produces an intense pulsed electron beam (1.7 kA, 20 MeV, 60 ns) that impinges on a high-Z target. The quality of the X-ray source is determined by spot size and dose. The spot size is effected by magnet focal length, emittance, energy spread and beam motion. The dose is determined by beam energy, total charge, target material, and convergence angle. We present results of OTR angular distribution experiments performed at 3.8 and 20 MeV electron beam energies on the first axis of DARHT and compare these results to a 3D Ray-Tracing program which is able to calculate the effect of each electron beam parameter. Comparison shows that the data is most sensitive to the electron beam energy and the divergence/convergence angle. The maximum collection angle of the optical system limits results at low energy by mixing angular and spatial information. The layout of the paper is as follows. The first section describes OTR angular distribution properties. This is followed by a brief description of the 3D Ray-Tracing program. The third section describes the experimental set-up. Section 4 shows initial OTR observations. Comparison between results and simulations are made in the fifth section. Time resolved measurements are described in section 6. Limitations and planned improvements of this diagnostic are discussed in the last section of this part. 2 OTR ANGULAR MEASUREMENTS