Three-Dimensional Multispecies Distribution Functions in a Plasma Boundary With an Oblique Magnetic Field

Abstract

The physics of a weakly magnetized boundary region are investigated by immersing a 76.2 mm diameter stainless steel disk in a helicon plasma. The velocity distributions of ions and neutral particles are mapped in the boundary region, created in 3.6 mTorr argon gas, 650 W RF power, and magnetic field B = 0.06 T, where the magnetic field lines obliquely intersect the wall (α = 16 •). These distributions are mapped using laser induced fluorescence in three spatial (3D) and three velocity (3V) dimensions, and reveal that models neglecting any one of the velocity components omit important components of the flow field. Electrostatic probe measurements are presented and establish a typical helicon plasma discharge with electron temperature T e ≈ 4.2 eV and density n ≈ 5.5 × 10 17 m −3. In addition to the observed ion temperature, T i ≈ 0.37 eV, these measurements indicate a plasma in which the electrons are highly magnetized (the ratio of the electron cyclotron frequency to the electron collision frequency is ω ce /ν e = 2.9 × 10 3) and the ions are weakly collisional (the ratio of the ion cyclotron frequency to the ion collision frequency is ω ci /ν i = 0.5 ± 0.3). Ion drift velocity measurements are presented in the E × B direction and are compared to predictions from measured plasma potential gradients. Ion flow fields are compared to predictions from collisional fluid simulations and particle-in-cell models. These models require collisions to reproduce the ion drifts well. Neutral particle distributions are observed to be in thermal equilibrium with the chamber walls (T n ≈ 0.028 eV) and essentially non-flowing on average. Charge exchange population fractions are expected to be O(10 −2), and are too small to be resolved, but are excluded to the 5% level. "O me! O life!... of the questions of these recurring; Of the endless trains of the faithless-of cities fill'd with the foolish; Of myself forever reproaching myself, (for who more foolish than I, and who more faithless?) Of eyes that vainly crave the light-of the objects mean-of the struggle ever renew'd; Of the poor results of all-of the plodding and sordid crowds I see around me; Of the empty and useless years of the rest-with the rest me intertwined; The question, O me! so sad, recurring-What good amid these, O me, O life? Answer. That you are here-that life exists, and identity; That the powerful play goes on, and you may contribute a verse." Walt Whitman, Leaves of Grass (1892) v I want to first express my gratitude for the support of my advisor, Earl Scime, who smoothed my way to WVU and who for the last two and half years has trusted me with his lab probably more than he had reason to. He has answered annoying questions at 7 am and after midnight and was available for lunch almost every day I've been in Morgantown. That kind of face-to-face relationship building is exceptional and not lost on me. He is a mentor and a friend.