E×B Energy Analyzer for Electrons

Abstract

A simple electron energy analyzer has been used to measure energy distribution functions of the ``hot electron plasma'' produced in a compression mirror machine. The analyzer is located in a region of a steady 1.2 kG field so that electrons which escape from the 23 kG mirror field have most of their transverse energy transformed into the parallel component. This parallel component of energy is determined by the transverse drift in a region of mutually perpendicular electric and magnetic fields; low velocity electrons drift farther as they spend more time in the E×B field region, and vice versa. Electrons are detected by being accelerated onto a plastic scintillator which is viewed through a light pipe by a photomultiplier. Good time resolution is obtained, but since only one narrow energy band can be observed on each shot, the electric field is varied between shots to measure the complete energy spectrum. The magnetic field of the analyzer joins smoothly with the plasma containment magnetic field so that the electrons do not pass through a zero field region. With the 1.2 kG field, the analyzer works well for electron energies up to 20 keV; at higher energies the energy resolution decreases, and at very low energies (<100 eV) secondary electrons cause difficulties.