Recent electron cyclotron heating experiments with low field launch of the ordinary mode on the DIII-D tokamak

In recent ECH experiments on DIII-D, the H-mode regime of improved confinement has been observed using low field launch of the ordinary mode at power levels in excess of 1 MW. This result has been obtained for both the fundamental and second harmonic resonances using ECH as the sole source of auxiliary heating. The approximately linear scaling of the H-mode power threshold with toroidal field has been extended to 2.1 T. The largest stored energies, about 150 kJ for 1.5 MW total input power, occurred when the fundamental resonance was at the center of the discharge, and the stored energy decreased monotonically to the Ohmic value as the resonance location was scanned to the small major radius of the torus. The time derivative of the total energy at the start of the ECH pulse was calculated. Under the questionable assumption that the energy confinement time does not immediately degrade with application of ECH, this indicated that, for central heating, slightly less than half of the injected ECH power was absorbed by the plasma at the fundamental resonance and approximately one third was absorbed when the toroidal field was reduced so that heating was at the second harmonic resonance. Calculated singlemore » pass absorption was 85% and 12%, respectively, for these cases. In the case of the second harmonic, substantial conversion to the extraordinary mode takes place upon reflection from the wall following the first pass through the plasma. Despite the poor calculated absorption for the second harmonic, the H-mode transition was clear and showed the characteristic behavior previously observed with extraordinary mode heating. 7 refs., 10 figs.« less