Gary Hallock

An experimental counter‐example to the local transport paradigm

The response of a tokamak discharge to a sharp drop in edge temperature differs significantly from that expected from typical local transport models in several important respects. Laser ablation of carbon induces large (ΔT/T≤70%), rapid (<200 μs) electron temperature drops in the outermost region of the plasma, r/a≥0.9. This cold pulse proceeds through the outer plasma (r/a≥0.75), rapidly compared with power balance or sawtooth predictions. However, the pulse shrinks markedly thereafter, disappearing near r/a∼0.5. Within r/a∼0.3, the temperature rises promptly. The results are inconsistent with conventional local transport models; a nonlocal phenomenology, in which transport coefficients increase in the edge and decrease in the core, is suggested. The turbulence levels measured with a heavy ion beam probe increase near the edge but are unchanged in the core.

Confinement in beam-heated plasmas: the effects of low-Z impurities

Confinement studies on the Impurity Study Experiment (ISX-B) in beam-heated plasmas contaminated with small quantities of low-Z impurities are reported. Experimental results on the correlation of particle and energy confinement are presented. A linear relationship of energy confinement and plasma density is observed. As density is increased further, this effect saturates and energy confinement becomes independent of electron density. The experiments have been extended to higher beam power, resulting in an expansion of the ISX-B operating space. Impurities other than neon (carbon and silicon) have been tried and do not produce an enhancement in confinement. Edge cooling by the introduction of impurities has been demonstrated. The change in confinement has been shown to be correlated with changes in the normalized poloidal field fluctuation level (θ/Bθ) but not with the density fluctuation level (ne/ne). The experimental results are compared with models of drift-wave and resistive ballooning turbulence and an explanation is offered for the difference between the results with recycling and non-recycling impurities.

Ambipolar potential formation and axial confinement in TMX

TMX experimental data on ambipolar potential control and on the accompanying electrostatic confinement are reported. In the radial core of the central cell, measurements of electrostatic potentials of 150 V which augment axial ion confinement are in agreement with predictions using the Maxwell-Boltzmann result. Central-cell ion confinement was observed to scale according to electrostatic potential theory up to average enhancement factors of eight times over mirror confinement alone.

The evidence for nonlocal transport in the Texas Experimental Tokamak

The electron temperature response of a tokamak to rapid edge cooling has characteristics difficult to reconcile with local transport analysis. The initial observations in the Texas Experimental Tokamak [K. W. Gentle, Nucl. Tech. Fusion 1, 479 (1981)] have been extended to a wider range of plasma and perturbation parameters, including auxiliary heating, and the associated turbulence changes have been measured across the plasma radius. The fast edge temperature drops and core temperature increases are quantified by more extensive analysis. A perturbation complementary to edge cooling, edge heating by a fast current ramp, evokes a completely complementary plasma response.

Modeling the Growth of Streamers During Liquid Breakdown

Earlier work by Fowler, Davaney, and Hagedorn showed that the morphology of an anode streamer could be modeled as stochastic growth of a branching fractal tree in point-plane geometry. This investigation reproduces the results of that earlier study. Because one of the concerns about the earlier work is that the electric field dependence appeared to be unphysical, the model was modified to operate under assumptions that are consistent with those that have proven useful in earlier investigations. Specifically, linear electric field dependence was assumed and there is an assumed variability in the number density of available electrons. Computations using this assumption also produce the same range of morphologies that has been measured in experiments. In addition, some assessments of sensitivity to other possible variables are made. First, the sharp cutoff in the electric field strength is replaced with a presumably more realistic exponential dependence on energy. Under this assumption, it is also possible to simulate the experimentally observed behavior of anode streamers. It is shown that three possible refinements to the model have small, and likely negligible, effects. The first is using variable streamer step lengths in the calculation rather than the fixed step length used in the earlier work. The second is to assume growth at one point in the streamer makes growth in other parts somewhat less likely. The third is the assumption that the probability of a streamer making the next step in growth is influenced by the distance of the inter-electrode gap that has already been traversed.

Radial transport in the central cell of the tandem mirror experiment

An experimental study of radial transport in the Tandem Mirror Experiment is reported here. Plasma parameters were measured in a series of well‐diagnosed plasma discharges. A negative electric current (80±40 A within a 30‐cm radius) flowed to the end wall, implying an equal radial loss of plasma ions. The axial losses of plasma ions were 100 A from the same volume. The nonambipolar radial ion flux was of the same order as the flux resulting from resonant‐neoclassical and ion‐neutral transport, but the uncertainties are large. The ambipolar radial transport (of both ions and electrons) was investigated by comparing the observed end losses with calculations of the plasma fueling by gas penetration and neutral beams. The ambipolar radial losses are probably smaller than the loses through other processes and may be as small as the classical losses resulting from Coulomb collisions.

Effects of different plasma species (atomic N, metastable N2*, and ions) on the optical properties of dilute nitride materials grown by plasma-assisted molecular-beam epitaxy

This letter studies the effects of atomic N, metastable N2*, and ionic species on the optical properties of dilute nitride materials. Ga0.8In0.2N0.01As0.99 was grown using a 1% N2 in Ar gas mix from an Applied-Epi Unibulb™ rf plasma source. Isonitrogen samples with and without ions were studied using various plasma operating conditions. Optical emission spectrometry was used to characterize relative proportions of different active nitrogen plasma species (atomic N and metastable N2*). Samples grown without ions and with a higher proportion of atomic N resulted in the best overall material quality, although this improvement was observed at high annealing temperatures. At lower annealing temperatures, increased blueshifts were observed for samples grown with a higher proportion of atomic N; however, there was no noticeable influence of ions on blueshift regardless of whether atomic N or metastable N2* was the dominant species present in the plasma. The key implication of this work is that it helps to elucid...

Ion damage effects from negative deflector plate voltages during the plasma-assisted molecular-beam epitaxy growth of dilute nitrides

We studied the effects of ion damage on the optical properties of dilute nitrides grown by plasma-assisted molecular-beam epitaxy. A dual-grid retarding field ion energy analyzer was used to measure the ion flux and ion energy distribution at the substrate position from an Applied-EPI UniBuilb™ rf plasma cell. These changes were measured as the negative deflector plate voltage varied from 0 to −800V. The largest ion flux resulted with a −100V setting, while the greatest ion energies occurred with −200V. Deflector plate voltages more negative than −300V resulted in a significant reduction in both the ion flux and ion energy. The damage caused by these ions was determined by measuring the pre- and postanneal photoluminescence properties of Ga0.8In0.2N0.01As0.99 quantum wells. Comparable optical properties were possible with various combinations of ion fluxes and ion energies, which demonstrate how the ion flux and ion energy each impart an individual effect on the sample’s optical properties. An awareness o...

The TMX heavy ion beam probe

A heavy ion beam probe has been used to measure the radial space potential distribution in the central cell of TMX. This was the first beam probe system to utilize computer control, CAMAC instrumentation, and fast time response for broadband fluctuation capabilities. The fast time response was obtained using off-line processing of the energy analyzer detector signals and wideband transimpedance amplifiers. The on-axis space potential was found to be 300-400 V, with /spl phi//sub e//T/sub ec//spl sim/8. The radial potential profile is parabolic when gas box fueling is used. The frequency of observed fluctuations was found to agree with the E/spl times/B plasma rotation frequency during the discharge. The measured Tl/sup ++/ secondary ion current level is consistent with calculations, given reasonable assumptions for beam attenuation. >

Measurements of plasma fluctuations with a heavy‐ion beam probe

A heavy‐ion beam probe can provide simultaneous, spatially resolved measurements of φ and (nσeff) in magnetically confined plasmas. Capabilities have been demonstrated on TMX, RENTOR, and ISX‐B using single detector systems, which permit the evaluation of the frequency spectra out to several hundred kilohertz. The cross correlation and phase difference between n and φ are also obtained by assuming Te fluctuations to be negligible. Present development effort is focused on the use of multiple detectors and multiple ion species. These should allow the determination of both the frequency and wave number spectra for n, φ, and Te. The resolution and limitation of these techniques and their application to studying fluctuation‐induced transport will be discussed.