Moderator Simulation for the Compact Positron Source at NLCTA

TL;DR

This paper uses G4beamline simulations to optimize a positron moderator setup, incorporating an RF cavity and novel stopping distribution calculations, achieving significant efficiency improvements for the NLCTA.

Contribution

It introduces a new simulation-based moderator design with RF cavity integration and a novel method for calculating positron stopping distributions.

Findings

39.3x increase in moderated positrons with new setup

Effective use of RF cavity and tungsten target

Novel stopping distribution calculation method

Abstract

Positron moderation is a key part of the production of slow positrons. This paper examines G4beamline simulations of various moderator designs and the addition of a radiofrequency cavity to increase the efficiency of the moderator. A setup consisting of a 100 MeV electron beam incident on a 7 mm tungsten target, a 5 T adiabatic matching device, a 5-cell L-band RF cavity, and a single-crystal tungsten moderator leads to a 39.3x improvement in the number of positrons moderated over a common thin-foil polycrystalline moderator without the AMD and cavity. We also present a novel calculation method for determining the stopping distribution of low-energy positrons in a thin foil, modifying the Makhovian distribution with an exponential ramp to account for edge effects.