Helicon Mode and Xenon Operation with the Nonambipolar Electron Source

Abstract

A Radio Frequency (RF) plasma-based electron source that does not rely on electron emission at a cathode surface has been constructed. All of the random electron flux incident on an exit aperture is extracted through an electron sheath resulting in total nonambipolar flow within the device when the ratio of the ion loss area to the electron loss area is approximately equal to the square–root of the ratio of the ion mass to the electron mass, and the ion sheath potential drop at the chamber walls is much larger than Te/e. The Nonambipolar Electron Source (NES) has an axisymmetric magnetic field of up to 275 gauss at the extraction aperture that results in a uniform plasma potential across the aperture, allowing the extraction of all the incident electron flux without the use of grids. A prototype NES has produced 30 A of continuous electron current, using 2 sccm Xe, 1300 W RF power at 13.56 MHz, yielding a 180x gas utilization factor. A helicon mode transition has also been identified during NES operation with an argon feed gas, using 15 sccm Ar, 1000 W RF, and 100 gauss magnetic field.