Effects of Ohmic current in the TJ-II stellarator

Abstract

TJ-II plasmas respond in a very controllable way to the external induction of a loop voltage in electron cyclotron resonance heating conditions. First experiments showed that the confinement improves (degrades) when a negative (positive) electric field is induced. Combining loop voltage induction with electron cyclotron current drive, the cause is now proven to be the plasma current but not the induced electric field. It is also found that high positive currents (7 kA) invert trends in plasma density, no longer deteriorating the confinement. If, as suggested, the current acts via magnetic shear modification, then both its magnitude and sign affect the confining properties of the plasma. Several ideas are put forward based upon magneto-hydrodynamic, particle orbits and drift wave turbulence arguments.