Experimental studies of a helicon plasma

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Lee, Charles Anton

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The goal of this dissertation is to provide experimental insight into the mechanism behind the efficient power absorption of helicon plasmas. This work presents evidence which is consistent with the Radially Localized Helicon (RLH) theory put forth by Breizman and Arefiev. Helicon discharges produce peaked density profiles with radial density gradients creating a potential well that sets up the RLH waves, which we believe is the major power absorption mechanism in the plasma. The experimental data presented in this dissertation must be taken in totality along with parallel efforts in theory and computer simulation. We show photographic evidence along with Langmuir probe measurements of the axial density that shows an asymmetric, right-hand circularly polarized wave being launched in a direction consistent with RLH theory. Additionally, we are able to show, through Langmuir probe measurements, that significant radial density gradients exist in the plasma which is required by the RLH dispersion but contrary to the uniform density assumption of current theory. Furthermore, using the two-dimensional density profile obtained from experiment, we are able to use that data as input into a model which confirms key features of the power absorption in terms of location and magnitude. The time-varying magnetic field is measured and analyzed against the RLH dispersion relation. Using a Fourier decomposition technique, the analysis indicates the proper scaling of the wavenumbers with the RLH dispersion. Finally, using the experimental density as input to a computer model, simulations show very good agreement with the amplitude and phase of the experimentally measured RF magnetic fields.