Fast waves at frequencies far above the ion cyclotron frequency and approaching the lower hybrid frequency (also called 'helicons' or ‘whistlers’) have application to off-axis current drive in tokamaks with high electron beta. The high frequency causes the whistler-like behaviour of the wave power nearly following field lines, but with a small radial component, so the waves spiral slowly towards the plasma centre. The high frequency also contributes to strong damping. Modelling predicts robust off-axis current drive with good efficiency compared to alternatives in high performance discharges in DIII-D and Fusion Nuclear Science Facility (FNSF) when the electron beta is above about 1.8%. Detailed analysis of ray behaviour shows that ray trajectories and damping are deterministic (that is, not strongly affected by plasma profiles or initial ray conditions), unlike the chaotic ray behaviour in lower frequency fast wave experiments. Current drive was found to not be sensitive to the launched value of the parallel index of refraction n‖, so wave accessibility issues can be reduced. Use of a travelling wave antenna provides a very narrow n‖spectrum, which also helps avoid accessibility problems.

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Second harmonic 60 GHz electron cyclotron (EC) pre-ionization using low field side (LFS) X-mode launchers has been effective in producing target plasmas for startup of the DIII-D tokamak with electron densities comparable to fundamental LFS pre-ionization plasmas using the same EC system. A visible Bremsstrahlung array showed that breakdown occurred at the 2 nd harmonic resonance location and after a few milliseconds the EC driven plasma filled the entire vessel, independent of the resonant location, which was varied from near the inner wall to the center of the torus. The power threshold for ionization of ~0.4 MW was observed in DIII-D for these 2 nd harmonic preionization experiments. An orbit following code calculated that cold (0.03 eV) electrons could be heated to energies above 20 eV where ionization of the neutral deuterium gas can occur. Scaling from DIII-D to ITER indicates that ECH 2 nd harmonic pre-ionization and initial plasma formation is possible if ITER operation at reduced toroidal field is desired, but additional experiments are required to extrapolate the entire ECH start-up scenario from DIII-D to ITER.

The first observation of //-mode confinement with electron cyclotron heating as the sole auxiliary heating method has been made in divertor discharges in the DIII-D tokamak. These discharges exhibit the usual characteristics of the H mode, including improved confinement of particles and energy, when electron cyclotron heating is added at a power level above 0.7 MW. The //-mode transition is accompanied by the development of an electron temperature pedestal of 0.25 keV and a dramatic steepening of the density gradient near the separatrix.PACS numbers: 52.55.Fa, 52.50.GjThe plasma energy-confinement time in tokamaks has generally been found to decrease when auxiliary power is added to the Ohmic heating. This phenomenon, L-mode scaling,' makes it difficult to heat a tokamak plasma to reactor temperatures. However, a regime of improved confinement, ^-mode scaling, has been observed in several tokamaks with neutral-beam injection (NBI) heating. ^"^ In the H mode, the energy confinement time may remain at the Ohmic level without degradation when auxiliary power is added, or the confinement time may degrade with power but remain a factor of about 2 larger than the confinement time under the L-mode scaling, as in the JET tokamak.^ Recently, the H mode has also been obtained with ion cyclotron heating,^ and, in the JFT-2M tokamak, with ion cyclotron heating,^ with a combination of NBI and electron cyclotron heating (ECH),^^ and with NBI in limiter discharges. ^^ The present work reports the first observation of the H mode with electron cyclotron heating as the sole source of auxiliary heating.These experiments have been carried out in the DIII-D tokamak operated in the expanded boundary divertor configuration under conditions which have facilitated the //-mode transition in discharges heated by NBI.^'^^ Typical plasmas had Ro = \.6S m, a-0.62 m, elongation = 1.8, with a single null divertor located in the direction of the ion \B drift, as preferred for the H mode. ^^ The gap between the separatrix flux surface and the nearest material wall was typically 3.5 cm. The plasma was developed from deuterium gas, and the plasma current was 0.48 MA.The ECH power was applied at a frequency of 60 GHz, for which the second-harmonic resonance occurs at a local magnetic field of 1.07 T. The antennas were located near the outside midplane of the plasma, and launched 85% of the incident power in the extraordinary mode and 15% in the ordinary mode. The total incident power, after allowance for waveguide losses, was up to 0.9 MW. The antennas launch power in the HEn mode at an angle ±17° to the radial with a Gaussian dispersion 11 ° in half width. When the resonance is at the plasma center, for the 17° launch angle, the local cutoff density for the extraordinary mode is 1.86x10^^ m"^, and for densities well below cutoff", the power is deposited in a volume with approximate minor radius of 20 cm in the midplane. The ECH power was typically a factor of 8 larger than the Ohmic power during the heating, and so the plasma power balance was domi...