J.L. Dunlap

Impurity transport and plasma rotation in the ISX-B tokamak

Recent calculations have shown that when external momentum sources and plasma rotation are included in the neoclassical theory, the standard results for impurity transport can be strongly altered. Under appropriate conditions, inward convection is reduced by co-injection and enhanced by counter-injection. In order to examine the theoretical predictions, several observations of impurity transport have been made in the ISX-B tokamak during neutral-beam injection for comparison with the transport seen with Ohmic heating alone. Both intrinsic contaminants and deliberately introduced test impurities display a behaviour that is in qualitative agreement with the predicted beam-driven effects. These correlations are particularly noticeable when the comparisons are made for deuterium where the impurity transport in the Ohmically heated discharges exhibits neoclassical-like characteristics, i.e. accumulation and long confinement times. Similar but smaller effects are observed in beam-heated hydrogen discharges; neoclassical-like behaviour is not seen in Ohmically heated hydrogen sequences. Emphasis has been placed on measuring toroidal plasma rotation, and semi-quantitative comparisons with the theories of beam-induced impurity transport have been made. It is possible that radial electric fields other than those associated with momentum transfer and increased anomalous processes during injection could also play a role.

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.

Plasma fluctuations near the shear layer in the ATF torsatron

Electrostatic turbulence has been investigated in the edge region of the Advanced Toroidal Facility (ATF). A reversal in the poloidal phase velocity of the fluctuations has been observed (velocity shear) which determines a characteristic plasma radius. The location of this shear layer depends on the magnetic configuration, the limiter radius and the plasma conditions. Using the shear position as a reference point, the density fluctuation levels in ATF (currentless stellarator) are very similar to those previously reported in TEXT (ohmically heated tokamak), suggesting that the plasma current is not an important drive for the edge turbulence. The drives for the turbulence appear to be different inside and outside the shear location (ashear), with e/Te 1) and possibly larger e/Te in the plasma edge edge (r/ashear < 1). There is a spatial decorrelation in the fluctuations at the shear location; this suggests that the poloidal shear flow has an important influence on the edge turbulence. The poloidal correlation length depends on local plasma parameters (e.g. velocity and temperature). When neutral beam injection is added, the high frequency components of n increase.

Results of hydrogen pellet injection into ISX-B

High speed pellet fueling experiments have been performed on the ISX-B device in a new regime characterized by large global density rise in both ohmic and neutral beam heated discharges. Hydrogen pellets of 1 mm in diameter were injected in the plasma midplane at velocities exceeding 1 km/s. In low temperature ohmic discharges, pellets penetrate beyond the magnetic axis, and in such cases a sharp decrease in ablation is observed as the pellet passes the plasma center. Density increases of approx. 300% have been observed without degrading plasma stability or confinement. Energy confinement time increases in agreement with the empirical scaling tau/sub E/ approx. n/sub e/ and central ion temperature increases as a result of improved ion-electron coupling. Laser-Thomson scattering and radiometer measurements indicate that the pellet interaction with the plasma is adiabatic. Penetration to r/a approx. 0.15 is optimal, in which case large amplitude sawtooth oscillations are observed and the density remains elevated. Gross plasma stability is dependent roughly on the amount of pellet penetration and can be correlated with the expected temporal evolution of the current density profile.

Confinement improvement in beam heated ISX-B discharges with low-z impurity injection

Abstract Results are reported on improved confinement in the Impurity Study Experiment (ISX-B) neutral beam heated plasmas when a small amount of neon is injected shortly after the start of beam heating. The scaling of energy confinement is modified by the introduction of a dependence on line-averaged density. Calculations show the improvement is primarily caused by a reduction in electron heat conduction.

Characteristics of edge plasma turbulence on the ATF torsatron

Measurements of electrostatic fluctuations on the edge of the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] are used to study the role of the edge turbulence in the particle transport in this current‐free magnetic configuration. Spatial profiles of the plasma electron density ne, temperature Te, and fluctuations in density (ne) and in the plasma floating potential (φf ) are measured at the edge in electron cyclotron heated plasmas using a Langmuir probe array. At the last closed flux surface (LCFS), r/a≊1, Te≊20–40 eV, and ne≊1012 cm−3 for a line‐averaged electron density ne=(3–6)×1012 cm−3. The relative fluctuation levels decrease as the probe is moved into the core plasma. For Te≳20 eV, ne/ne≊5%, and eφf /Te≊2ne/ne at r/a=0.95. The measured fluctuation spectra are broadband (40–300 kHz) with kρs≊(0.05–0.1), where k is the average wave number of the fluctuations and ρs is the ion Larmor radius at the sound speed. Near the LCFS, the density fluctuations can be approxi...

Density fluctuation measurements in ATF using correlation reflectometry

A two-frequency correlation reflectometer has been operated on the Advanced Toroidal Facility (ATF) to measure plasma electron density fluctuations. This reflectometer uses quadrature phase detection to permit true phase measurement of the reflected microwave signal (probing beam). By measuring the phase fluctuations in the reflected probing beam, the amplitude of the density fluctuations can be estimated. Simultaneous two-frequency operation makes it possible to measure the coherence between fluctuations at two radially separated cut-off layers, from which the radial correlation lengths and wavenumbers can be estimated. This reflectometer has been used to study the density fluctuations in the edge gradient region of low density ATF plasmas produced by electron cyclotron heating. These studies have revealed globally coherent turbulence with a radial correlation length of up to approximately 5 cm, a radial wavenumber kr ≈ 0 cm-1 and a poloidal wavenumber kθ ≈ 1 cm-1. The rms amplitude of the fluctuations reaches a maximum of ≈ 5% at the plasma edge (ρ = 1, where ρ is the flux surface normalized radius) and decreases with decreasing radius to a level of 1%. Simultaneous measurements of the fluctuations with the reflectometer, the heavy ion beam probe and the fast reciprocating Langmuir probe provide consistent results. A comparison of the measurements with simplistic mixing length estimates, ne/ne = 1/kθLn or ne/ne = 1/krLn, shows that these estimates are too high by factors of two to more than 100, while a comparison with a more detailed estimate for the pressure gradient driven resistive interchange turbulence yields reasonable agreement with the experimentally measured fluctuation characteristics

ATF two-frequency correlation reflectometer

The Advanced Toroidal Facility (ATF) density fluctuation reflectometer system consists of two individual reflectometers operating in the 30- to 40-GHz band. Each reflectometer consists of a tunable microwave source and a quadrature phase detector connected to the same antenna system. This arrangement allows two-frequency operation along the same radial chord for radial coherence measurements. The technique used in making radial coherence measurements is discussed and the results of such experiments are given. Initial experiments have shown high coherence when the frequencies of the two reflectometers are tuned close together and a clear loss of coherence as the radial separation of the cutoff layers is increased by increasing the frequency separation of the two reflectometers. Recent results have shown that local measurements of density fluctuations in plasmas with electron cyclotron heating (ECH) are possible and that detailed structure can be seen in the fluctuation spectra. In addition, radial correlation lengths have been found to be from 0.5 to 1.0 cm in ECH plasmas, with some frequency structures having correlation lengths up to 3 cm. In plasmas with neutral beam injection (NBI), the radial correlation lengths in the edge region have been found to be approximately 0.1--0.2 cm. 4 figs.

Experimental exploration of profile control in the Princeton Beta Experiment‐Modified (PBX‐M) tokamak

The experimental program of the Princeton Beta Experiment‐Modified (PBX‐M) tokamak [Phys. Fluids B2, 1271 (1990)] is directed toward tailoring plasma profiles to achieve greater stability and confinement and to gain access to the second stability region. Modification of the current density profile has been achieved with lower‐hybrid current drive (LHCD), leading to a regime free of global magnetohydrodynamic modes, while raising the value of q(0) above unity. The diffusion of the fast electrons produced by LHCD has been examined using two‐dimensional hard x‐ray imaging. Ion Bernstein waves (IBW) have been used for ion heating: a preliminary analysis shows that ion heating was spatially localized and in agreement with theoretical calculations. Divertor biasing has modified the electric field inside the last closed surface, resulting in the formation of a transport barrier, which in turn has reduced the threshold power of neutral beam injection (NBI) for H‐mode transition by 25%.

Particle removal with pump limiters in ISX-B

Abstract The first pump limiter experiments were performed on ISX-B. Two pump limiter modules were installed in the top and bottom of one toroidal sector of the tokamak. The modules consist of inertia cooled, TiC-coated graphite heads and ZrAl getter pumps each with a pumping speed of 1000–2000 l/s. The objective of the initial experiments was the demonstration of plasma particle control with pump limiters. The first set of experiments were performed in ohmic discharges (OH) in which the effect of the pump limiters on the plasma density was clearly demonstrated. In discharges characterized by Ip = 110 kA, B T = 15 kG , n e = 1−5 × 10 13 cm −3 and t = 0.3 s, the pressure rise in the pump limiters was typically 2 mTorr with the pumps off and 0.7 mTorr after activating the pumps. When the pumps were activated, the line-average plasma density decreased by up to a factor 2 at identical gas flow rates. The second set of measurements were performed in neutral beam heated discharges (NBI) with injected powers between 0.6 MW and 1.0 MW. Due to a cooling problem on one of the ZrAl pumps, the NBI experiments were carried out with one limiter only. The maximum pressure observed in NBI-discharges was 5 mTorr without activating the pumps, i.e., approximately twice as high as in OH-discharges. The exhaust efficiency, which is defined as the removed particle flux divided by the total particle flux in the scrape-off layer, is estimated to be 5%.