The role of rf-scattering in high-energy electron losses from minimum-B ECR ion source

TL;DR

This study measures high-energy electron losses from a minimum-B ECR ion source, revealing a significant energy hump at 150-300 keV linked to rf-induced diffusion, with implications for understanding electron confinement.

Contribution

It identifies a distinct high-energy electron loss feature and links it to rf-induced momentum space diffusion, advancing knowledge of electron dynamics in ECR ion sources.

Findings

A hump at 150-300 keV contains up to 15% of lost electrons.

The energy hump's mean energy increases with magnetic field strength.

The hump appears only during microwave power application.

Abstract

The measurement of the axially lost electron energy distribution escaping from a minimum-B electron cyclotron resonance ion source in the range of 4-800 keV is reported. The experiments have revealed the existence of a hump at 150-300 keV energy, containing up to 15% of the lost electrons and carrying up to 30% of the measured energy losses. The mean energy of the hump is independent of the microwave power, frequency and neutral gas pressure but increases with the magnetic field strength, most importantly with the value of the minimum-B field. Experiments in pulsed operation mode have indicated the presence of the hump only when microwave power is applied, confirming that the origin of the hump is rf-induced momentum space diffusion. Possible mechanism of the hump formation is considered basing on the quasi-linear model of plasma-wave interaction.