A.J. Wootton

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.

The beryllium limiter experiment in ISX-B

An experiment to test beryllium as a limiter material has been performed in the ISX-B tokamak. The effect of the plasma on the limiter and the effect of the limiter on the plasma were studied in detail. Heat and particle fluxes to the limiter were measured, and limiter damage by melting was documented as a function of power flux. Strong melting and evaporation of the limiter caused beryllium gettering of the vacuum vessel. Postmortem analysis of the limiter was performed to document the amount of retained hydrogen and the erosion and impurity deposition on the limiter. The effect of the limiter on the plasma performance was studied in terms of parameter space, impurity content, and confinement for the ungettered and gettered cases. Operational experience with beryllium in a fusion experiment is discussed.

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.

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.

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%.

Measurements of periodic ripple transport in the ISX-B tokamak

The effect of periodic toroidal field (TF) ripple on ion confinement has been studied in the ISX-B tokamak by comparing neutral-beam-heated plasma performance with 9 and 18 TF coils. Three ripple physics issues were treated by these experiments: (1) enhanced ion thermal conductivity, (2) enhanced loss of energetic ions, and (3) ripple damping of beam-induced toroidal plasma rotation, which may affect the plasma losses. Under a wide variety of plasma conditions, ripple reduced the central-ion temperature by a factor of approximately two (600 eV → 300 eV). Ion temperature was found to be nearly independent of applied neutral-beam power in the large ripple configuration (9 TF coils). These results are shown to be in reasonable agreement with theoretical models of ripple transport. Charge-exchange measurements of the fast-neutral flux indicated no loss of fast passing ions due to ripple, but a large depletion of the fast ions trapped in local ripple wells, as expected theoretically. The central toroidal rotation velocity was reduced by a factor of six by ripple, yielding a momentum confinement time substantially less (factor of about seven) than that expected from standard theoretical expressions for ripple-enhanced ion viscosity.

Rotation scalings and momentum confinement in neutral-beam-injected ISX-B plasmas

Scalings of the central rotation in non-gettered, co-injected ISX-B discharges have been measured as a function of beam power, electron density and plasma current. Extensive studies are made possible by exploiting charge-exchange excitation (CXE) of 0 VIII lines to measure Doppler shifts. The rotation velocity, v(0), tends to saturate at (1.0 − 1.2) × l07 cms−1 when Pb0.5 MW, showing little further increase up to the maximum input of 2 MW; v(0) is independent of ne and Ip. Momentum confinement times in quasi-steady plasmas are 10–16 ms for e = 4.5 × 1013 cm−3. Counter-injection discharges always disrupt, but before this event v(0) is the same as for co-injection plasmas. The addition of a third beam line, permitting injection of up to 2 MW of balanced neutral-beam power, has allowed comparisons of the energy and particle confinement in rotating and non-rotating plasmas with the same total neutral-beam input. In those cases where impurity buildup can be avoided, it is found that the ISX-B empirical scaling of energy confinement time is reproduced with balanced injection. Thus, the unfavourable dependence of is not the result of rotation. Studies of impurity behaviour under differing injection conditions have been extended to include fully stripped low-Z ions. The results are consistent with previous investigations of metallic elements which revealed strong dependences on the sense (co versus counter) of injection. The potentials calculated from momentum balance, using measured rotation profiles and typical plasma density and temperature profiles, are in qualitative agreement with the potentials measured directly for various combinations of co- and counter-injection.

Beta and confinement scaling studies with neutral-beam heating in the ISX-B tokamak

Experiments to investigate the scaling of volume-averaged beta and a global energy confinement time for neutral-beam-heated (Pb ≤ 2.5 MW) discharges in the ISX-B tokamak are described. The results are condensed into a set of empirical scaling formulas which can be used as a guide for other theoretical and experimental studies of confinement in high-beta, neutral-beam-heated plasmas. The dependence on toroidal field BT, plasma current Ip, and line-averaged electron density e was determined by varying each of these while keeping other external variables fixed. Magnetic diagnostics were used to obtain global properties, and Thomson-scattering-based profile analysis was carried out to permit more detailed investigation of selected cases. The poloidal beta, βp, is found to be independent of BT and e at fixed beam power Pb; confinement is found to deteriorate with increasing Pb but to improve with IP, consistent with previous results. The mechanisms which govern this confinement scaling have not been discerned, but it apparently does not depend on β, BT, or the (m = 1; n = 1) MHD activity, which typically dominates the MHD diagnostic signals. Losses are primarily through the electron channel, and the scaling of electron energy confinement time is similar to that of .

Plasma edge turbulence probing and feedback control and stabilization experiments

Experiments in modifying plasma edge turbulence were carried out on the Texas Experimental Tokamak (TEXT) by launching waves using two electrostatic probes in the shadow of the limiter. Measurements indicate that the wave, launched with a typical frequency range of 15 to 50 kHz from the edge of the machine top, is received by sensing probes located halfway around the torus. The detected signal strength depends weakly on the frequency of the wave and on the phasing of the applied AC signal between the launching probes. Modifications to the spectrum of the edge potential fluctuations are observed at the launching frequency. These experiments have been extended to the control of the edge plasma fluctuation level using feedback. The launcher is driven by the floating potential of the fluctuating plasma at the location of the launching probes. Edge fluctuations are reduced or excited, depending on the phasing between the launching probes, both locally and at the downstream sensing probes. The fluctuation induced particle flux as well as the local plasma parameters are affected by feedback phasing

Gettering in ISX-B☆

Abstract Gettering is used in the ISX-B tokamak to reduce the impurity concentration. This paper documents the gettering process used, and compares the expected changes in recycling and radiation with those observed experimentally. The enlargement of the operating regime ( 1/q, n e R/B φ space) is discussed. Finally, the effect on one of the objectives of the exerimental program, that of obtaining high values of beta, is described.