First experimental studies of the physics of plasmas of arbitrary degree of neutrality

The first detailed experimental characterization of the physics of plasmas of arbitrary neutrality is reported. It is shown that the degree of neutralization of the plasma in the Columbia Non-Neutro Torus stellarator can be varied continuously from quasi-neutral to pure-electron plasma, and that the plasma can be sustained indefinitely at any degree of neutralization. The physical phenomena change significantly as the degree of neutralization is varied. Quasi-neutral plasmas exhibit a single mode low-frequency flute instability. This mode aligns almost perfectly with the magnetic field lines, presenting a resonant m = 3 poloidal structure. For weakly magnetized ions, and moderate degrees of neutralization, multi-mode behaviour is observed, in some cases fully turbulent. At the other extreme of the neutralization scale, electron-rich plasmas with a small but non-negligible amount of ions exhibit single mode fluctuations. Essentially identical phenomena were previously reported by Marksteiner et al (2008 Phys. Rev. Lett. 100 065002). In addition to the already known poloidal mode number m = 1 of these fluctuations, the toroidal mode number is now known: n = 0. This not only confirms the previous observations that the electron fluid force balance appears to be broken, but the frozen-in flux condition is also violated. (paper)