Confinement jumps in a non-neutral plasma

Measurements of confinement jumps in pure electron plasmas confined on magnetic surfaces are presented and discussed. The experiments were performed in the Columbia non-neutral torus stellarator T. S. Pedersen, J. P. Kremer, R. G. Lefrancois, Q. Marksteiner, N. Pomphrey, W. Reiersen, F. Dahlgren, and X. Sarasola, Fusion Sci. Technol. 50, 372 2006 . The jumps exhibit hysteresis and are associated with a negative differential resistance. The jumps occur at particular emission currents of the biased emissive filaments that create and sustain the electron plasmas independent of the methods used to affect the emission current. This observation, as well as other experimental evidence, supports that the jumps are caused by a cathode instability. The jumps can also be triggered by the application of a bias potential on a nearby mesh. In most circumstances, the jumps occur between two stable but measurably different equilibrium states. These different equilibrium states have substantially different confinement times. The cathode physics is important for the jumps because the cathode instability provides the perturbation that triggers the jump of the whole plasma into the other equilibrium state, but as mentioned, an external electrostatic perturbation is also capable of triggering such a jump. © 2009 American Institute of Physics. DOI: 10.1063/1.3075933