Electron plasmas: confinement and mode structure in a small aspect ratio toroidal experiment

Abstract

Toroidal electron plasmas have remained less explored due to their poor confinement properties. Their equilibrium, stability, and confinement properties are therefore not entirely understood and continue to remain a topic of intense ongoing research. Large aspect-ratio theory suggests poor confinement in toroidal devices can be overcome by the application of a radial electric field; this has been verified successfully in some of the recent experiments. In the present paper, we report the longest confinement time without these external forces. Increasing the toroidicity has helped us to generate these forces intrinsically. To this end, a trap to confine electron plasmas has been created in a small aspect-ratio ( ≈1.6) torus. Electrons after being injected from a thermionic source are seen to remain confined with a purely toroidal magnetic field. The confinement time is far more than known single particle drift time scales. Importantly, it is in the absence of any external electric field, additional rotational transform, and/or magnetic fields, which, although not required, in principle, may appear essential particularly due to their role in improving confinement in some of the recent large aspect-ratio traps. The successful confinement in the small aspect-ratio limit has also led to several interesting observations: the evolution of the confined plasma is marked by an interesting nonlinear (large amplitude), electrostatic wave activity. Coherent, periodic, double peak oscillations result from a low-frequency E× B motion of a toroidal vortex in a plasma that closely leans against the inner wall. As many as 16 highly phase-coherent harmonics with dominant power in m = 2 suggest that the mode is not merely a center-of-charge motion. Rather, a strong coupling of modes leads to a novel nonlinear state. The predominant energy is present in the shaping of the electron cloud (m = 2) and not in the displacement of the center of charge (m = 1) seen in large aspect-ratio traps. The absence of any power-law tail suggests absence of any turbulence, at least on time scales longer than the wall-probe resolution (40 ns). The frequency, (around 100 kHz at 200 G) shows an unusual shear in time: it reduces as the mode evolves, but later increases as the mode dies.