Nonuniform electron distributions in a solenoidal ioniser

Abstract

Solenoidal ionisers are a new class of highly efficient helium detectors that are increasingly important for high resolution atom scattering, molecular scattering and scanning helium microscopy. They operate via electron ionisation, where the electrons are trapped by the magnetic field of a solenoid and additional electrostatic potentials. Their ionisation efficiency scales with the electron population they contain, motivating large devices with high emission currents. However, these detectors typically become unstable at high electron densities, constraining their performance improvement. Through imaging the electron population at the exit of the ioniser, we demonstrate that these instabilities arise from non-uniformities in the electron distribution. Considering the ioniser as a non-neutral plasma leads to the proposal of the formation of a virtual cathode and a plasma instability as the origins of the non-uniformity.