T.K. Chu

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.

Measurement of the local carbon diffusion coefficient in the S-l spheromak

The local carbon diffusion coefficient has been measured in the S-l spheromak by detecting the radial spread of injected carbon impurity. The radial impurity density profile is determined by the balance of ionization and diffusion. Using the measured local electron temperature and density, the ionization rate is determined from which the particle diffusion coefficient is inferred. The results found in this work are consistent with Bohm diffusion, and the absolute magnitude of the local particle transport coefficient D⊥ is found to be (4-6) × DBohm. The measured particle transport rates are correlated with the plasma pressure gradients, suggesting a pressure driven transport mechanism.

Effect of reconnection of magnetic field lines and electron thermal conduction on the plasma pressure gradient in a sawtooth oscillation

In a sawtooth oscillation, the resistive reconnection and the internal disruption are two distinct and sequential events. At the beginning of the reconnection of magnetic field lines around an m = 1 magnetic island, the mixing of the electron temperatures associated with the reconnection results in an increasing island width and a steep pressure gradient in the boundary layer adjacent to the reconnecting surface, leading to the destabilization of ideal MHD modes. The thickness of the boundary layer is determined by the ratio of electron cross-field thermal diffusivity and the speed of reconnection. This narrow boundary layer and the short onset time of a sawtooth crash can be supported by finegrained transport processes which drive the electron energy loss in a tokamak plasma.

Control of the current density profile with lower‐hybrid current drive on PBX‐M

Lower hybrid current drive (LHCD) is being explored as a means to control the current density profile on PBX‐M with the goal of raising the central safety factor q(0) to values of 1.5–2 to facilitate access to a full‐volume second stable regime. Initial experiments have been conducted with up to 400 kW of 4.6 GHz LH power in circular and indented plasmas with modest parameters. A tangential‐viewing two‐dimensional hard x‐ray imaging diagnostic has been used to observe the bremsstrahlung emission from the suprathermal electrons generated during LHCD. Hollow hard x‐ray images have indicated off‐axis localization of the driven current. A serious obstacle to the control of the current density profile with LHCD is the concomitant generation of MHD activity, which can seriously degrade the confinement of suprathermal electrons. By combining neutral beam injection with LHCD, an MHD‐free condition has been obtained where q(0) is raised above 1.

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.