Initial results from the TST-2 spherical tokamak

A new spherical tokamak, TST-2, was constructed at the University of Tokyo and started operation in September 1999. Reliable plasma initiation is achieved with typically 1 kW of ECH power at 2.45 GHz. Plasma currents of up to 90 kA and toroidal fields of up to 0.2 T have been achieved during the initial experimental campaign. The ion temperature is typically 100 eV. Internal reconnection events are often observed. The internal magnetic field measured at r/a = 2/3 indicated growth of fluctuations up to the fourth harmonic, suggesting the existence of modes with several different mode numbers. In the presence of a toroidal field and a vertical field with positive curvature, non-inductively driven currents of the order of 1 kA were observed with 1 kW of ECH power. The driven current increased with decreasing filling pressure, down to 4 × 10^-4 Pa. A study of high harmonic fast wave excitation and propagation has begun. Initial results indicate highly efficient wave launching. The spatial distribution of the RF magnetic field was qualitatively consistent with the result of a full-wave calculation.