Observation of vertical charge-separation current in an electron-cyclotron-heated toroidal plasma

Abstract

Suppose an axisymmetric toroidal plasma is immersed in a toroidal magnetic field Bϕ in the cylindrical coordinates of (R, ϕ, Z) with Z coordinate on the symmetric axis. The electrons drift downward while the ions drift upward due to the field gradient and curvature by the velocity, . It was conjectured that the electrons that reached the bottom wall flowed through the conducting vessel to the top where they recombined with the ions, thus a closed circuit being completed for the vertical charge-separation current. This current has for the first time been directly measured by current-collecting electrodes newly fabricated in the Low Aspect ratio Torus Experiment (LATE) device for a plasma maintained by a microwave power at the electron cyclotron range of frequency. The amount of current is found to be consistent with the above vertical drift model for electrons. The discharge terminates quickly during microwave injection when the current circuit is blocked by the artificial insertion of a large resistance.