Neutral dynamics in a helicon plasma

Author / Creator

Green, Jonathan, 1987-, author

Available as

Online

Physical

Summary

Helicon plasmas are high density, low temperature plasmas with applications in particle accelerators, space thrusters, and current drive in tokamaks. While efficient ion production is often the chi...

Helicon plasmas are high density, low temperature plasmas with applications in particle accelerators, space thrusters, and current drive in tokamaks. While efficient ion production is often the chief interest in helicon plasmas, no theory explaining the mechanism behind the high ionization efficiency has been demonstrated through consistent measurement. Particle balance, power balance, and radio frequency (RF) wave coupling all play a role in determining the ionization rate and source distribution. In this work, significant upgrades and diagnostic improvements on the MARIA helicon device in the 3D Plasma Surface Interaction laboratory are presented. Laser induced fluorescence (LIF) measurements are calibrated against an RF compensated Langmuir probe to produce particle flux measurements at several magnetic field strengths and axial positions. Under the assumption that parallel transport is dominant, one dimensional mass and momentum conservation arguments are used to measure the distribution and magnetic field strength scaling of an ionization source in the range of 8x1020 m-3s-1. Axial and radial LIF measurements of neutral argon reveal a region of high recycling near an on axis boundary plate, and a 90% depletion of neutral particles near the helicon antenna. Measurements of radial ion losses and neutral source terms are also measured and presented. By identifying both particle sources and sinks, an understanding of the circular refueling process by recycling neutrals begins to form. Additionally a new collisional radiative model developed in collaboration with Auburn University is presented. Comparisons between solutions to the collisional radiative model and experimental results provided valuable insight into the behavior of atomic population processes occurring in MARIA plasmas. Additionally, the collisional radiative model is used in concert with line ratio spectroscopy to establish a new passive diagnostic for neutral dominated argon plasmas. Agreement with Langmuir probe electron temperature measurements is currently achievable, but additional work is needed to improve the extraction of electron density.

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