First Observations of Partially Neutralized and Quasineutral Plasmas in the Columbia Non-Neutral Torus

The Columbia Non-neutral Torus (CNT) is a stellarator designed to study pure electron, partially neutralized and positron-electron plasmas. CNT usually operates with electron rich plasmas (with negligible ion density) [1]. The accumulation of ions alters the equilibrium of electron plasmas in CNT and a global instability has been observed when the ion fraction exceeds 10%. A characterization of this instability is presented in [2], analyzing its parameter dependence and spatial structure (non-resonant with rational surfaces). A segmented conducting mesh has been installed to impose the electrostatic boundary condition at the plasma edge. Results after the installation of the conducting boundary confirm the presence of an instability in electron rich plasmas with a finite ion content. A new set of experiments is currently underway studying plasmas of arbitrary degree of neutralization, ranging from pure electron to quasineutral plasmas. Basic observations show that the plasma potential decouples from the emitter bias when we increase the degree of neutralization of our plasmas. Partially neutralized plasmas are also characterized by multiple mode behavior with dominant frequencies between 20 and 200 kHz. When the plasma becomes quasineutral, it reverts to single mode behavior (2- 18 kHz). The first results on partially neutralized and quasineutral plasmas in CNT are reported on here. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim