[Paper Review] Optical Transition Radiation (OTR) Measurements of an Intense Pulsed Electron Beam

Christophe Vermare, D.C. Moir+1 more|ArXiv.org|Aug 18, 2000

Laser-Plasma Interactions and Diagnostics2 references3 citations

TL;DR

This paper demonstrates time-resolved optical transition radiation (OTR) measurements of intense pulsed electron beams (3.8–20 MeV, 1.7 kA, 60 ns) at the DARHT facility. By analyzing the angular distribution of OTR light emitted from a 45°-tilted aluminum foil, the authors extract beam energy and divergence/convergence angle with sub-mrad precision, achieving energy resolution of 150–250 keV, validating the method against 3D ray-tracing simulations.

ABSTRACT

We present the first time resolved OTR angular distribution measurements of an intense pulsed electron beam (1.7 kA, 60 ns). These initial experiments on the first axis of the Dual Axis Radiographic Hydro-Testing (DARHT) facility and subsequent analysis, demonstrate the possibility to extract, from the data, the energy and the divergence angle of a 3.8 and 20 MeV electrons.

Motivation & Objective

• To develop a non-invasive, real-time diagnostic for intense pulsed electron beams using optical transition radiation (OTR).
• To measure beam energy and divergence/convergence angle from the angular distribution of OTR light.
• To validate the OTR diagnostic against 3D ray-tracing simulations and experimental data at 3.8 MeV and 20 MeV.
• To assess the feasibility of time-resolved beam parameter monitoring during beam pulse evolution.
• To identify limitations in angular resolution and dynamic range and propose improvements for future diagnostics.

Proposed method

• Measure OTR angular distribution using a 45°-tilted 10-μm aluminised Kapton foil as the radiator in vacuum.
• Collect OTR light with a 200 mm focal length achromatic doublet lens, forming an angular image on a screen.
• Use an 8-frame gated camera with 10 ns temporal resolution to record time-resolved OTR intensity profiles.
• Apply polarization filters to isolate OTR components and suppress Cherenkov background from secondary electrons or X-rays.
• Compare experimental OTR angular distributions with predictions from a 3D ray-tracing code that models beam phase space and optical geometry.
• Vary beam parameters via solenoid current to modulate beam divergence and use energy variation via de-energizing induction cells to probe energy sensitivity.

Experimental results

Research questions

• RQ1 Can OTR angular distribution measurements provide accurate, time-resolved estimates of electron beam energy and divergence in intense pulsed beams?
• RQ2 How sensitive is the OTR signal to beam energy and divergence angle, and can these parameters be decoupled from the measurement?
• RQ3 To what extent does optical system collection angle and dynamic range limit the accuracy of OTR-based diagnostics at low beam energies?
• RQ4 How do beam dynamics such as space-charge effects and gas neutralization influence OTR signal evolution during the beam pulse?
• RQ5 Can OTR measurements be used to infer beam emittance or convergence angle with sufficient precision for radiographic facility diagnostics?

Key findings

• The OTR angular distribution at 20 MeV shows clear sensitivity to beam divergence, with experimental data matching 3D ray-tracing simulations when solenoid current is varied.
• At 3.8 MeV, the OTR signal becomes sensitive to beam size and angular spread due to large emission angles, requiring detailed ray-tracing for accurate interpretation.
• Energy resolution of 250 keV at 20 MeV and 150 keV at 15 MeV was achieved by varying beam energy via de-energizing induction cells.
• Time-resolved measurements revealed a 5 mrad variation in beam divergence over 40 ns, attributed to beam neutralization in residual gas at 3×10⁻⁵ Torr.
• The OTR diagnostic is limited by optical system collection angle and CCD dynamic range, with current 256-level dynamic range insufficient for quantitative analysis.
• Polarization measurements confirmed that the dominant signal is OTR, not Cherenkov light, with background reduced by blocking the target with a thin Al foil.