Using the virtual-case principle, the plasma boundary, the plasma current center, and the x-point are identified for the HL-2A tokamak. The plasma current is represented by the current center and the virtual multipole moments which produce a magnetic flux in a form of polynomial. Adaptive parameters in the polynomial are determined by the least-square fit of the poloidal magnetic fields. The measurement of the magnetic field is performed using pick-up coils. The virtual-case principle is applied outside the plasma boundary. The virtual-case currents decide the position of the current center and produce a negative confinement magnetic field inside the plasma and the magnetic field generated by the plasma current outside the plasma boundary. The convergence is fast enough to get a picture between the sequent shots. The configuration reconstructed is in good agreement with the TV image taken by camera with a tangential view.
1 Introduction Internal kink mode driven by barely trapped suprakthermal electrons is reported on D Ⅲ-D tokama . This instability is most active when electron cyclotron current drive （ ECCD ） is applied on the high field side of the flux surface. It has a bursting behavior with poloidal/toroidal mode number m/n = 1/1. In positive magnetic shear plasma this mode becomes fishbone instability.
WANG ZhongtianLONG YongxingDONG JiaqiWANG LongZonca F.
