Measured Radiation and Background Levels During Transmission of Megawatt Electron Beams Through Millimeter Apertures

TL;DR

This study measures photon and neutron radiation levels during high-power electron beam transmission through millimeter apertures at JLab, demonstrating feasibility and characterizing background radiation for experiments like DarkLight.

Contribution

It provides the first detailed radiation characterization during megawatt electron beam transmission through small apertures at a major accelerator facility.

Findings

Transmission of 430 kW beam through 2 mm aperture is feasible with manageable backgrounds.

Radiation during beam-off RF-on operation is mainly due to multipactoring in cryomodules.

Background levels are characterized for experimental environments requiring high-power beam transmission.

Abstract

We report measurements of photon and neutron radiation levels observed while transmitting a 0.43 MW electron beam through millimeter-sized apertures and during beam-off, but accelerating gradient RF-on, operation. These measurements were conducted at the Free-Electron Laser (FEL) facility of the Jefferson National Accelerator Laboratory (JLab) using a 100 MeV electron beam from an energy-recovery linear accelerator. The beam was directed successively through 6 mm, 4 mm, and 2 mm diameter apertures of length 127 mm in aluminum at a maximum current of 4.3 mA (430 kW beam power). This study was conducted to characterize radiation levels for experiments that need to operate in this environment, such as the proposed DarkLight Experiment. We find that sustained transmission of a 430 kW continuous-wave (CW) beam through a 2 mm aperture is feasible with manageable beam-related backgrounds. We also find that during beam-off, RF-on operation, multipactoring inside the niobium cavities of the accelerator cryomodules is the primary source of ambient radiation when the machine is tuned for 130 MeV operation.