R. Motley

Angular distribution of the bremsstrahlung emission during lower hybrid current drive on PLT

The bremsstrahlung emission from the PLT tokamak during lower-hybrid current drive has been measured as a function of angle between the magnetic field and the emission direction. The emission is peaked strongly in the forward direction, indicating a strong anisotropy of the electron velocity distribution. The data demonstrate the existence of a nearly flat tail of the velocity distribution, which extends out to approximately 500 keV and which is interpreted as the plateau created by Landau damping of the lower-hybrid waves.

Modelling of the electron distribution based on bremsstrahlung emission during lower-hybrid current drive on PLT

Lower-hybrid current drive requires the generation of a high-energy electron tail anisotropic in velocity. Measurements of bremsstrahlung emission produced by this tail are compared with the calculated emission from reasonable model distributions. The physical basis and the sensitivity of this modelling process are described, and the plasma properties of current-driven discharges which can be derived from the model are discussed.

Lower hybrid experiments on PLT using grills with various n∥ spectral widths

Coupling structures for lower hybrid current drive experiments have, until now, been smaller than a free space wavelength and have had a correspondingly broad wavenumber spectrum. The paper reports the results of experiments on the PLT tokamak using a 16-waveguide grill (2.2 wavelengths) which produces a very narrow n∥ = k∥c/ω spectrum. Experimental results from the 16-waveguide grill are compared with results from three other PLT grills with less sharply defined n1 spectra. The current drive figure of merit, , is approximately 40% higher for the experiments with the 16-waveguide coupler than for previously reported experiments on PLT, in spite of the larger spectral gap. The experimental results are consistent with the first-pass damping of a large fraction of the launched spectrum.

Suppression of internal disruptions in inductively driven tokamak discharges by lower hybrid wave current drive

Internal disruptions occurring in an inductively driven discharge are suppressed by lower hybrid waves that drive the plasma current in the same direction as the inductively driven current. Upon suppression, in the central region a finite-amplitude, non-growing m = 1 oscillation is observed and strong electron heating is measured.

Triton Burnup Measurements and Calculations on TFTR.

Measurements of the burnup of fusion product tritons in TFTR are presented. Interpretation of triton burnup experiments requires three accurate components: the measurement of the 2.5 MeV neutron emission, the measurement of the 14 MeV neutron emission and a calculation of the expected burnup ratio from the measured plasma parameters. The absolute calibration for the 14 MeV neutron measurements is provided by an NE213 proton recoil spectrometer. Time dependent burnup measurements for three plasma conditions selected for optimum detector operation are shown. Measurements of the time integrated triton burnup from copper activation foils (cross-calibrated to the NE213 measurements) are presented. Descriptions are provided of the neutron detectors and the plasma diagnostics whose data are used as input to the calculation of the expected burnup. All these measurements find that the triton burnup on TFTR is 1/2 ± 1/4 the classical expectations for a wide variety of discharges. The burnup decreases for relatively longer triton slowing down times, implying possible fast ion diffusion coefficients of ~0.1 m2/s. Alternatively, burnup appears to decrease with increasing major radius of the triton source and edge safety factor qcyl, implying that ripple losses may be playing a role. Triton burnup is a very sensitive measure of anomalous fast ion transport; similar levels of diffusive transport in an ignited reactor would have minimal impact on the alpha particles.

Coupling to the fast wave via a phased waveguide array

A dielectric-loaded waveguide array has been used to launch fast waves into a plasma in which ..omega../sup pi/ < ..omega.. << ..omega../sub pe/ approx. ..omega../sub ce/. The wave propagates when accessibility and cutoff requirements are satisfied. Reflection coefficients as low as 1% have been measured. Use of the fast wave for steady-state current drive is suggested.

Reduced thermal diffusion using lower hybrid waves in a tokamak plasma

Lower hybrid waves were launched into PLT tokamak plasma. The dependence of electron heating on the direction of the launched LH wave is examined. (AIP)

Current drive verus the launched N∥ spectrum

Lower hybrid experiments on PLT with a narrow wavenumber spectrum have shown a 40% improvement in the current drive figure of merit ηCD over previous PLT results. The simple assumption that the electron tail energy is determined by the launched n∥ spectrum gives a good qualitative description of all of the PLT current drive results.

Study of m=2 activity during lower‐hybrid‐driven, low q discharges in PLT

Lower hybrid driven discharges in PLT with q<2.5 are unstable to m=2 instabilities, as observed on Mirnov loops. The mode structure grows within a few milliseconds to encompass most of the plasma column. After the mode slows down from a few kilohertz and locks in, a mini disruption occurs, giving rise to a drop in the population of the high energy electron tail, a rise in the impurity level, and a loss of 30–50% of the plasma energy.

Current profile broadening during sawtooth suppression by lower hybrid current drive

Current‐carrying channel during lower hybrid current drive is broadened from that in inductively driven discharges. For the PLT sawtooth suppression experiment (ne=1×1013 cm−3, I=500 kA, B=30 kG), the rate of decrease of internal inductance (or the rate of increase of the width of current channel) with wave power is ∼0.1/MW. At Prf≂450 kW, the power level at which sawtooth oscillations are suppressed and the m=1 magnetic island is eliminated, the radius of the current‐carrying channel is estimated to increase by 1.5 cm.