Measured radiation and background levels during transmission of megawatt electron beams through millimeter apertures

Abstract

We report measurements of photon and neutron radiation levels observed while transmitting a 0.43MW 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 100mev electron beam from an energy-recovery linear accelerator. The beam was directed successively through 6mm, 4mm, and 2mm diameter apertures of length 127mm in aluminum at a maximum current of 4.3mA (430kW 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 430kW continuous-wave (CW) beam through a 2mm 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 130mev operation.