Halo current study on the EAST tokamak

Disruptions is unavoidable events for tokamak. In current quench period, a halo current is generated on the surface of plasma, which produce the magnetic field distribution required to maintain plasma force balance. When the safety factor of the plasma drops nearly to 2, the current can grow as large as tenths of the plasma current, this will be a severe threat for machine components because of J x B force. For better understand halo current on EAST tokamak, we have developed a series of current diagnostics. All the sensors are Rogowski coils that surround conducting structures, and all the signals are analog integrated. Measurements from disruptions show that the halo current diagnostics are working well. The disruption experiments show that when plasma volume drift upward a significant amount of halo current flows in the top of the machine, while little is seen at the bottom. With the measurement of sensors installed on tungsten divertor, it is shown that the current flow into outer plate through cassette finally back from inward plate. The interesting thing is that litter current is observed in center column, this implies that almost all the halo current just ow through the divertor component not through center column. From the several sets of sensors, it is estimated the total halo current is up to 40% plasma current, with toroidal peaking factor less than 2. Primary halo current mitigation is observed using helium gas injection with valve response time within 0.15 ms. It is demonstrated that nearly 1/3 halo currents were mitigated compared with non-mitigation. Noble gas jet injection can effectively shorten the vertical displacement, resulting in plasma volume non-contact completely with the wall, this can be clearly seen from reconstruction using filament code. (author)