UCLA

Understanding the physics of shear Alfv�n waves in two-ion species plasmas, and the consequent interaction of the waves with the plasma, is important for explaining many observations in both laboratory and space plasmas. In this dissertation, the propagation and polarization of shear Alfv�n waves in two-ion species plasmas is investigated under a wide range of conditions in the Large Plasma Device (LAPD) at UCLA. The primary motivation of this thesis is two-fold: (1) to quantify and understand the propagation and polarization of shear waves in two-ion plasmas, both theoretically and experimentally, and (2) to explore the shear wave’s viability as a diagnostic tool in two-ion plasmas.

Waves are injected into a mixed helium-neon plasma using a magnetic loop antenna, for frequencies spanning the ion cyclotron regime. Two distinct propagation bands are observed, bounded by ???? <Ω???????? and ????????????< ???? <Ω????????, where ???????????? is the ion-ion hybrid frequency and Ω???????? and Ω???????? are the helium and neon cyclotron frequencies, respectively. Expanding on the work of previous authors [102], the ion-ion hybrid parallel cutoff frequency is systematically measured under a wide range of plasma conditions. A new diagnostic technique and accompanying algorithm is developed in which the measured parallel wavenumber ????‖ is numerically fit to the predicted shear Alfv�n wave dispersion in order to resolve the local ion density ratio. A major advantage of this algorithm is that it only requires a measurement of ????‖ and the background magnetic field in order to be employed.This diagnostic is tested on the Large Plasma Device (LAPD) at UCLA and is successful in yielding radially-localized measurements of the ion density ratio.

The polarization of shear waves in mixed helium-neon plasmas is investigated in detail, both theoretically and experimentally. While the lower frequency band’s (???? <Ω????????) polarization is found to be in good agreement with a dispersion-based theory, the upper band (????????????< ???? <Ω????????) is found to be significantly more left-handed than theory or simulations predict. This behavior is observed using several different antenna configurations and a wide range of plasma conditions, showing that this feature is not unique to a given antenna geometry. The possibility of asymmetrical spatial damping is explored, but measurements show that the left-handed component of the upper band damps faster than the right, eliminating damping as a possible cause.

In order to better understand the effects of antenna geometry on wave coupling and polarization, an analytic model is developed for determining the electromagnetic field of a current-driven antenna immersed in a cold plasma. The model is numerically solved for both an electric dipole antenna as well as a magnetic loop antenna, and shows excellent agreement with previously published simulation studies. The mathematical model presented here may be advantageous over other methods, as it allows the user to solve parts of the problems analytically, thereby cutting down significantly on computation time.