Hot-electron confinement in magnetic cusp geometries

A hot electron plasma is established by ECR (6.4 GHz) in a simple cusped magnetic field. The plasma thus generated consists of hot electrons, cold electrons and helium ions. The hot-electron temperature is determined from X-ray spectra to be about 30 keV and the density of the hot electrons is of the order of 1010 cm−3, which is about 10% of the cold-electron density. The effect of the cusp field is particularly noticeable in the electron-density distribution, which has a void at the centre. Plasma decay times (10 – 60 ms) are observed to increase with the field current. There are several collision processes which cause the loss of hot electrons. The authors derive a simple equation which determines the confinement time of hot-electron plasma and study the confinement of hot electrons by ECR-wrapping. With a wrapping microwave power of 150 W, the decay time is observed to increase to more than three times that without the wrapping power.