William H. Miner

Electron thermal confinement studies with applied resonant fields on TEXT

Externally applied magnetic fields are used on the Texas Experimental Tokamak (TEXT) to study the possibility of controlling the particle, impurity and heat fluxes at the plasma edge. Fields with toroidal mode number n = 2 or 3 and multiple poloidal mode numbers m (dominantly m = 7) are used, with a poloidally and toroidally averaged ratio of radial to toroidal field components 〈|br/Bo〉 ≅0. 1%. Calculations show that it is possible to produce mixed islands and stochastic regions at the plasma edge (r/a ≥ 0.8) without affecting the interior. The expected magnetic field structure is described and experimental evidence of the existence of this structure is presented. The edge electron temperature decreases with increasing 〈|br/Bo〉, while interior values are not significantly affected. The implied increase in edge electron thermal diffusivity is compared with theoretical expectations and is shown to agree with applicable theories to within a factor of three.

Stability of space‐charge limited electron flow

The stability of space‐charge limited flow in planar diodes is investigated. It is found that the flow is unstable whenever the electron transit time is approximately n+ (1)/(4) , cycles where n is an integer, of the high‐frequency modes supported by the diode structure. Analytical and numerical solutions are found for the resulting dispersion relation in the nonrelativistic limit. The analytical results lead to a simple physical explanation of the instability and provide accurate solutions over a wide range of parameters. Numerical solutions are also obtained for the relativistic case.

Current diffusion in TEXT

A study of current diffusion in the TEXT device has been performed. The plasma current was ramped up from steady state conditions (150 kA) on a time-scale (20 ms) much faster than the classical skin time (100 ms), with ∂lp/∂t 8 MAs−1. A stable discharge was obtained and held at the resulting higher current (300 kA). Density (4 × 1019 m−3) and position were held constant during current ramping. The profiles derived from Thomson scattering and the surface barrier diode soft-X-ray array were used in the solution of the poloidal magnetic field diffusion equation. The experimental data were also compared with the prediction of a three-zone model of radial particle and energy transport. No enhanced or anomalous resistivity is required to adequately match the data for the case where the current was ramped from a steady state condition.

Transport with pellet fuelling in the Texas Experimental Tokamak

In the Texas Experimental Tokamak, a discharge regime characterized by persistent peaked profiles was observed to be induced by pellet fuelling, and its transport properties were studied. The hallmark of the regime is the suppression of sawteeth, and the regime was attained by injecting hydrogen pellets to promptly double the plasma density. In each of several pairs of experiments, a pellet fuelled discharge was compared with an edge fuelled discharge with similar averaged electron density, plasma current and toroidal magnetic field in order to characterize the transport change and to look for causal changes in the plasma turbulence. The impurity, radiation and working gas particle profiles were more peaked for the pellet fuelled case. The values of rjt derived from measured ion temperature and density profiles for high density edge fuelled and pellet fuelled discharges indicate that ion pressure gradient driven turbulence should be reduced in the pellet fuelled case. The macroscopic effects were accompanied by microscopic changes. Measurements of turbulent density fluctuations in high density edge fuelled discharges give strong evidence that a component of the turbulence propagates in the ion diamagnetic direction and that this particular mode is reduced in pellet fuelled discharges. The effects of the reduction of an ion mode turbulence were sought in the energy confinement of the discharges, but it was found that for these experiments (tailored for the turbulence diagnostics) the energy flowing in the ion channel was not large enough to affect the energy confinement. Simulations were used to interpret some of the results. Discharge simulations which include the pellet injection can reproduce the sawtooth suppression. This result and known properties of discharges in which sawteeth are suppressed suggest that some of the profile effects (including peaking of the working gas particles) induced by pellet injection are due to sawtooth suppression. The particle peaking may cause the observed reduction in the turbulence which follows pellet injection.

Drift wave eigenmodes in a torus are not localized by shear

The radial equations for the poloidal Fourier components of a drift wave in a torus are solved by a numerical finite element method. It is shown that in the trapped electron regime the solutions are not localized by shear.

Use of a genetic algorithm for compact stellarator coil design

A new global optimization technique for designing stellarator coils has been developed and applied to the design of coils for the National Compact Stellarator Experiment. Using this technique coil sets were found with fewer coils and lower current densities than those obtained with traditional methods. A new coil design procedure which uses a genetic algorithm as the core optimization method is described and the resulting advanced coil designs presented.

Design studies of low aspect ratio quasi-omnigenous stellarators

Significant progress has been made in the development of new modest-size compact stellarator devices that could test optimization principles for the design of a more attractive reactor. These are 3 and 4 field period low-aspect-ratio quasi-omnigenous (QO) stellarators based on an optimization method that targets improved confinement, stability, ease of coil design, low-aspect-ratio, and low bootstrap current.

The TEXT data base

The TEXTDB data base organizes the analyzed experimental data from the TEXT experiment so as to aid in understanding the physics of a complete discharge. The data base was constructed using a commercial relational data base. The relational model provided a flexible structure for organizing data from many different diagnostics into a unified description of the plasma discharge.