L.D. Horton

Second stability in the ATF torsatron—Experiment and theory

Access to the magnetohydrodynamic (MHD) second stability regime has been achieved in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)]. Operation with a field error that reduced the plasma radius and edge rotational transform resulted in peaked pressure profiles and increased Shafranov shift that lowered the theoretical transition to ideal MHD second stability to β0≊1.3%; the experimental β values (β0≤3%) are well above this transition. The measured magnetic fluctuations decrease with increasing β, and the pressure profile broadens, consistent with the theoretical expectations for self‐stabilization of resistive interchange modes. Initial results from experiments with the field error removed show that the pressure profile is now broader. These later discharges are characterized by a transition to improved (×2–3) confinement and a marked change in the edge density fluctuation spectrum, but the causal relationship of these changes is not yet clear.

Recent results from the ATF torsatron

Recent experiments in the Advanced Toroidal Facility (ATF) torsatron [Plasma Physics and Controlled Nuclear Fusion Research 1990 (IAEA, Vienna, in press)] have emphasized the role of magnetic configuration control in transport studies. Long‐pulse plasma operation up to 20 sec has been achieved with electron cyclotron heating (ECH). With neutral beam injection (NBI) power of ≥1 MW, global energy confinement times of 30 msec have been obtained with line‐average densities up to 1.3×1020 m−3. The energy confinement and the operational space in ATF are roughly the same as those in tokamaks of similar size and field. The empirical scaling observed is similar to gyro‐reduced Bohm scaling with favorable dependences on density and field offsetting an unfavorable power dependence. The toroidal current measured during ECH is identified as the bootstrap current. The observed currents agree well with predictions of neoclassical theory in magnitude and in parametric dependence. Variations of the magnetic configuration ...

Helium exhaust studies with the ALT-II pump limiter in TEXTOR

Abstract In TEXTOR helium removal experiments with the pump-limiter ALT-II have started. To simulate the presence of helium ash in the plasma, helium is injected into the discharge (e.g. at t = 0.7s) as a short pulse of Δt = 20ms. It is found that the He is removed from the discharge in an e-folding time of about half a second for neutral beam heated plasmas and in an e-folding time of about 1.5 s in an OH plasma. The exhaust efficiency of helium amounts to about 8% and is close to the one for deuterium. The fuelling efficiency for the injected helium is found to be in the range of 50–100%; the remaining part seems to be stored in the TEXTOR walls. An estimate of the surface density leads to a value of several times 1013 cm−2. This helium can easily be liberated in succeeding discharges and can be removed efficiently when ALT is pumping.

Fluctuation and modulation transport studies in the Advanced Toroidal Facility (ATF) torsatron

The Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] has completed experiments focusing on microwave scattering measurements of density fluctuations and transport studies utilizing the modulation of dimensionless parameters. Microwave scattering measurements of electron density fluctuations in the core of low‐collisionality electron cyclotron heated (ECH) plasmas show features that might be evidence of trapped electron instabilities. Starting from gyro‐Bohm scaling, the additional dependence of confinement on the dimensionless parameters ν* and β (collisionality and beta) has been investigated by modulating each of these parameters separately, revealing the additional favorable dependence, τE∝τgBν*−0.18β+0.3.

Particle exhaust studies in Tore Supra with a pump limiter

The aim of the Tore Supra pump limiter program is to study particle exhaust with a pump limiter system in long-pulse discharges with continuous pellet fueling and strong auxiliary heating. The pump limiter system consists of six vertical modules, located at the bottom of the machine, and one horizontal module at the outer midplane. The results presented here, were obtained with the horizontal module only. This module was equipped with two titanium pumps with a total pumping speed of 100000 L/s. The instrumentation of the limiter included pressure gauges, a residual gas analyzer, Langmuir probes, a spectrometer viewing the neutralizer plate for Hα and impurity measurements, and water calorimeters. All diagnostics have been commissioned and are operational. Initial results were obtained in low-density discharges, with no gas puffing during the shot. While only a modest effect on the plasma density was observed, large exhaust fluxes were measured in the pump limiter. The most likely source of this gas was outgassing of the graphite walls. Straightforward particle balance between the plasma efflux and the pump limiter exhaust, as applied in previous pump limiter experiments, did not apply. The core plasma and the edge plasma seemed to be largely decoupled and a multi-layer model is being developed to explain the experimental results.

Effects of magnetic geometry, fluctuations, and electric fields on confinement in the Advanced Toroidal Facility

Recent experiments in the Advanced Toroidal Facility (ATF) [Fusion Technol. 10, 179 (1986)] have been directed toward investigations of the basic physics mechanisms that control confinement in this device. Measurements of the density fluctuations throughout the plasma volume have provided indications for the existence of theoretically predicted dissipative trapped electron and resistive interchange instabilities. These identifications are supported by results of dynamic configuration scans of the magnetic fields during which the magnetic well volume, shear, and fraction of confined trapped particles are changed continuously. The influence of magnetic islands on the global confinement has been studied by deliberately applying error fields which strongly perturb the nested flux‐surface geometry, and the effects of electric fields have been investigated by means of biased limiter experiments.

Overview of results from the ATF torsatron

Experiments involving plasma improvement, confinement scaling, bootstrap currents, and edge fluctuations have been carried out in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)]. Average densities ne≤9×1019 m−3 have been obtained, with global energy confinement times τ*E≤20 msec. Confinement times generally follow the stellarator/torsatron empirical scaling law, τSL =0.17×P−0.58n0.69eB0.84a2R0.75 (with τSL in seconds, power P in megawatts, density ne in 1020 m−3, and plasma radius a and major radius R in meters). Gas injection during neutral beam injection (NBI) causes increases in ne, so that τ*E does not decrease during NBI. Edge plasma fluctuations are found to exhibit a mode change near the peak of the energy confinement time. Plasma currents observed during electron cyclotron heating have been identified as bootstrap currents.

Analysis of low-Z impurity behaviour in jet by charge exchange spectroscopy measurements

The absolute densities of the dominant light impurities in the plasma centre have been measured in JET during neutral beam heating using charge exchange spectroscopy. The behaviour of carbon, oxygen and helium (in helium discharges) is investigated for limiter and X-point discharges. When the plasma is in the X-point configuration, the central carbon concentration is significantly lower than in the material limiter configuration (by a factor of two to three). After the transition from the L-mode to the H-mode, a strong impurity buildup is observed in the plasma centre which significantly reduces the central dilution factor nD/ne from initial values near 0.8 to about 0.6 at the collapse of the H-mode. Nonetheless, during the H-mode, the product of deuteron density and ion temperature nDTi (both derived by the charge exchange spectroscopy diagnostic) shows a clear enhancement with respect to the L-mode at the same input power. Helium discharges as compared to deuterium discharges have a higher carbon concentration (consistent with physical sputtering) and a much reduced oxygen concentration (no/ne < 0.005, consistent with chemical release of oxygen). The transient behaviour of oxygen and carbon following deuterium pellet injection is discussed using a comparable non-pellet reference discharge. An increase of the oxygen content in the plasma centre is observed some 600 ms after injection of a deuterium pellet, while the central carbon density appears only transiently diluted by the sudden addition of pure deuterium. No evidence of a cleaning action through sawtooth relaxations is detected in the case of low-Z impurities.

Radiative losses and improvement of plasma parameters after gettering in the advanced toroidal facility

The characteristics of plasmas in the Advanced Toroidal Facility (ATF) have proven to be strongly dependent on the type of wall conditioning employed. A succession of techniques, beginning with glow discharge cleaning and baking, and evolving to gettering with chromium and titanium, have led to progressive improvement of the plasma parameters. Gettering with titanium has reduced the low-Z impurity content by a factor of 3, lowered the radiated power by a factor of 2.5–3.5, and improved the control over the electron density. The maximum values achieved for stored energy, line averaged density and confinement times are 28 kJ, 1.2 × 1014cm−3 and 25 ms, respectively. These parameters are comparable to the best results achieved in the ISX-B tokamak which had the same average minor radius and one half the major radius of ATF. Quasi-steady operation for 200 ms of neutral beam injection (NBI) has been obtained in high density, titanium gettered plasmas without the collapses that were typical earlier periods of operation. Neon injection experiments have helped to delineate the limits on the global levels of radiation that can be maintained and have supported the conclusion that mechanisms other than radiative losses are important for initiating the collapses still observed in low density NBI plasmas.

Effect of impurities on particle collection studies with a modular pump limiter on Tore Supra

Particle collection experiments with a modular pump limiter in Tore Supra, for ohmic and additional power heating, are reported. The essential feature of these experiments is that the pressure buildup in the pump limiter plenum is only driven by the hydrogen influx whereas the total ion influx in the throat is shown to be partly due to impurities. This is consistent with experimental results which show similar evolution of Z eff in the plasma and of the ratio Γ T / p of the total ion influx Γ T in the throat to the pressure p in the plenum. Particle collection efficiency, in terms of the hydrogen influx Γ D to the pressure ratio Γ D / p , is shown to be roughly constant over the density (ohmic) and additional power ranges studied.