2011 Theses Doctoral

Plasmas of Arbitrary Neutrality

Sarasola Martin, Xabier

The physics of partially neutralized plasmas is largely unexplored, partly because of the difficulty of confining such plasmas. Plasmas are confined in a stellarator without the need for a plasma current, and regardless of the degree of neutralization. The Columbia Non-neutral Torus (CNT) is a stellarator dedicated to the study of non-neutral, and partially neutralized plasmas. This thesis describes the first systematic studies of plasmas of arbitrary neutrality. The degree of neutralization of the plasma can be parameterized through the quantity η ≡ |n_e - Z n_i|/|n_e + Z n_i|. In CNT, η can be varied continuously from pure electron (η = 1) to quasi-neutral (η ≈ 0) by adjusting the neutral pressure in the chamber, which controls the volumetric ionization rate. Pure electron plasmas are in macroscopically stable equilibria, and have strong self electric potentials dictated by the emitter filament bias voltage on the magnetic axis. As η decreases, the plasma potential decouples from the emitter, and spontaneous fluctuations begin to appear. Partially neutralized plasmas (10^-3 < η < 10^-1) generally exhibit multi-mode oscillations in CNT. However, when magnetized ions are present, the electron-rich plasma oscillates at a single dominant mode (20 - 100 kHz). As the plasma approaches quasi-neutrality (η < 10^-5), it also reverts to single mode behavior (1 - 20 kHz). A parametric characterization of the single mode fluctuations detected in plasmas of arbitrary neutrality is presented in this thesis along with measurements of the spatial structure of the oscillations. The single mode fluctuations observed for η ≈ 0.01 to 0.8 are identified as an ion resonant instability propagating close to the E × B velocity of the plasma. The experiments also show that these oscillations present a poloidal mode number m = 1, and a toroidal number n = 0, which is identical to the spatial structure of the diocotron instability in pure-toroidal traps, and implies that the ion-driven instability breaks parallel force balance and the conservation of poloidal flux in CNT. The low frequency oscillations detected in the quasi-neutral regime are a global instability convected by the E × B flow of the plasma. In this case, the mode aligns almost perfectly with the field lines, and presents a resonant m = 3 poloidal structure.