T. Uckan

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...

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 ...

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.

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.

Modeling of spectroscopic measurements of edge recycling

The scaling of particle confinement time with density is studied in detail First, we fit the poloidal Hα brightness distributions measured in a density scan series in the TEXT tokamak. Results of the fit suggest that there is not a monotonic decrease of the mean ionization radius with increase in density. While this detailed fitting procedure is an improvement over fitting global Hα data, it still requires the use of free parameters. We show that three-dimensional (3D) Monte Carlo calculations may be used to compute the expected Hα lineshape. As several experiments have recently reported measurement of this quantity, comparison of theoretical and measured values should allow improvement in confinement analysis. We have carried out experiments in the ORNL Plasma Materials Interaction Test Facility to measure the Hα lineshape in discharges with CH4 injection into pure D2 plasmas. We have observed the production of H0 by CH4 breakup in PMITF, and have measured the lineshape contribution due to dissociation products in a plasma environment. Finally, since many confinement studies are conducted in “carbonized” devices, we have studied the problem of determining the fueling profile in carbon-rich plasmas by calculating the H0 deposition due to CH4 breakup. We find that the penetration of H0 through the scrape-off layer by CH4 breakup should introduce a significant density scaling in the particle fueling source term.

Modeling of core-wall particle dynamics in Tore Supra pumping experiments

Particle control experiments in Tore Supra have shown that the change in core plasma density with pumping at the outboard pump limiter amounts only to a small fraction of the total number of particles extracted by the outboard pump limiter system. To develop a model for the core-wall particle exchange at the level of individual basic physical processes, the particle exchange between the core plasma and the wall has been modeled in detail for a series of Tore Supra discharges. Core and scrape-off layer particle balance and wall diffusion calculations were performed using a radial transport code and a 1D wall diffusion code. Core-wall particle exchange for the case of the outboard pump limiter is found to be mediated by the localized charge exchange flux in the near limiter region. The wall particle efflux required for detailed balance does not match that calculated to be available from the near limiter wall region unless some local heating of the wall is assumed.