Ion acceleration and non-Maxwellian electron distributions in a low collisionality, high power Helicon plasma source

A large, time-averaged double layer-like plasma potential drop of 80 V over several hundred Debye lengths has been observed in the magnetic expansion region on the Madison Helicon Experiment (Madhex). It is operated in an inductive mode at 900 W and low operating pressures (0.12-0.20 mTorr) in the collisionless regime. The plasma space potential drop is due to the formation of a double layer-like structure in the magnetic expansion region and is much higher than the potential drop caused by Boltzmann expansion. With the plasma potential drop, a locally negative potential "ion hole" region with a higher electron density than ion density has also been observed just downstream of potential drop region. Two-temperature Maxwellian electron distributions with a hot tail component are observed just upstream of the double layer through RF compensated Langmuir probe and Optical Emission Spectroscopy (OES) diagnostics. In the expansion chamber downstream of double layer potential drop, a single hot Maxwellian electron distribution is observed via both Langmuir probe and OES diagnostics. Plasma potential fluctuations and the hot electron component in the upstream source chamber are shown to enhance the plasma potential drop and ion acceleration.