Doppler coherence imaging of ion flows and temperatures in the MAGPIE helicon plasma

Abstract

A snapshot coherence imaging system has been designed to measure brightness, flows and temperatures for argon ions in a helicon plasma with expected ion temperatures of ~1 eV and subsonic ion flows less than 1000 m/s. This technique measures Doppler information from the 488 nm ion spectral line, and encodes this information in the phase and contrast of an interferogram. Taking into account line-of-sight effects, demodulation of the interferogram yields a 2D spatial map of the ion temperatures and flows. At these plasma conditions, large interferometric delays (greater than 10^4 waves) are necessary to resolve the Doppler features. Passive stabilisation and an automated online calibration system was used to counteract thermally-induced phase drifts in the birefringent delay plates. The design features of this device are presented here.

Measurements are presented of the ion brightness, flows and temperatures in the MAGnetised Plasma Interaction Experiment (MAGPIE) for an argon plasma at 3 mTorr gas pressure, forward power of 1 kW and a 0.08 T peak magnetic field (in a magnetic pinch configuration). Spatial scans taken longitudinally along the chamber shows the peak brightness occurs in the high magnetic field region and the radial profile is centrally peaked with secondary wing-like features. There are also high ion temperatures in the plasma edge which indicate a secondary ion heating mechanism. In the magnetic pinch region the azimuthal ion flows are largest and the axial ion flows show a flow reversal. Measurements of the brightness, ion temperature and ion flows are also examined over a range of magnetic field configurations and gas pressures and ion flow measurements are confirmed using a Mach probe.

The results of this study demonstrate that, with careful consideration to instrument design, coherence imaging can be used to study ion features in cold plasmas. This opens opportunities for measurements of ion dynamics in laboratory-based plasmas across a range of research areas. Show more