D. Gwinn

Disruptions and halo currents in Alcator C-Mod

Disruptions in Alcator C-Mod can generate large eddy currents in the highly conducting vacuum vessel and internal structures, including a significant poloidal component due to halo currents. In order to understand better the stresses arising from the resulting J*B forces, Alcator C-Mod has been fitted with a comprehensive set of sensors to measure the spatial distribution and temporal behaviour of the halo currents. It is found that they are toroidally asymmetric, with a typical peaking factor of 2. The asymmetric pattern usually rotates toroidally at a few kilohertz, thus ruling out first wall non-uniformities as the cause of the asymmetry. Analysis of the information compiled in the C-Mod disruption database indicates that the maximum halo current during a disruption scales roughly as either Ip2/Bphi or Ip/q95, but that there is a large amount of variation that is not yet understood

Effect of pellet fuelling on energy transport in ohmically heated alcator C plasmas

Time-dependent transport analysis calculations have been carried out, using experimentally determined plasma parameters to obtain the variation of electron and ion thermal diffusivities following pellet injection into moderate-density Alcator C discharges. The ion thermal diffusivity, which is typically higher than neoclassical predictions by a factor of three to five in the gas-fuelled target plasma, is found to decrease after pellet injection to approximately the neoclassical value. The electron thermal conductivity is not reduced after pellet injection. The improvement in ion transport correlates with the peaking of the density profile and may be related to the reduction in the quantity ηi ≡ d ln Ti/d ln n, which is inferred to lie close to the critical value for stability of drift modes driven by the ion temperature gradient. Extrapolation of these results to higher-density plasmas, for which the electron and ion losses cannot be unambiguously measured, is consistent with previously reported increases in global energy confinement time accompanying pellet injection.

Lower hybrid heating in the Alcator A tokamak

The results and interpretation of the modest-power (~90 kW) lower-hybrid-heating experiment on Alcator A are presented. The expected results from linear waveguide-plasma coupling theory are outlined, and the possible effects of parametric instabilities, scattering from density fluctuations, and imperfect energetic ion confinement are addressed. It is found experimentally that good coupling and the absence of RF breakdown are achieved with a double waveguide array at available RF power densities PRF ≤ 4.5 kW.cm·−2, the waveguide vacuum windows being outside the toroidal field magnets; a waveguide array having vacuum windows near the waveguide mouth so that the ω = ωce layer can be pressurized shows no breakdown at PRP > 8 kW/cm2 when a single waveguide is energized. Energetic ion production and a factor-of-50 increase in the fusion neutron rate are observed to take place at well defined values of central plasma density; below these densities electron heating occurs. The ion tail production is found to be independent of the relative phase of the double waveguide array employed. This ion heating occurs at a lower density than theoretically expected; together with the electron heating this indicates waves having n|| ~5 being absorbed near the plasma centre. Probes at the plasma edge observe a frequency-down-shifted and broadened RF pump signal that is strongly attenuated as the plasma density increases through the neutron production band. These anomalous heating results and probe signals can be explained by a parametric decay of the pump wave into higher n|| lower hybrid waves near the plasma edge. An alternate qualitative explanation would be the poloidal scattering of the lower hybrid waves at the plasma periphery due to density fluctuations; the n|| of the scattered lower hybrid waves would then increase as they propagated inward because of magnetic shear. The neutron rate decay times imply that the RF creates ion tails having a substantial fraction of their energy above 50 keV. The neutron decay times and rates strongly depend on plasma current and indicate the expected influence of ion confinement on RF heating efficiencies. Finally, the RF heating efficiencies are assessed.

Impurity generation during intense lower hybrid heating experiments on the Alcator C tokamak

Abstract Experiments are underway on the Alcator C tokamak with over 1 MW of RF power injected into the plasma at a frequency of 4.6 GHz to study both heating and current drive effects. During these studies, impurity generation from limiter structures has been observed. The RF induced impurity influx is a strongly nonlinear function of net injected power. For PRF P RF = 1.0 MW , n e = 1.3 × 10 14 cm −3 , and the SiC coated limiters, Zeff is seen to increase from 1.5 before the RF pulse to about 4 during the heating. At the same time, central Te increases from 2000 to 3000 eV and central Ti from 1200 to 1800 eV. Similar effects are seen in both H2 and D2 working gas discharges. The contribution to impurity generation of nonthermal electrons, which are produced by the RF, is under investigation. Changes in edge plasma temperature and density, as well as the possibility that the particle transport is affected by the RF, are also being examined. Results of the experiments with the three different limiter materials are compared, and contributions of impurity radiation to the overall power balance are estimated.

Alcator C-Mod: A high-field divertor tokamak

Abstract The Alcator C-Mod tokamak is a new device presently under construction at Massachusetts Institute of Technology (M.I.T.) which is scheduled to begin operation in mid-1990. The projected operating parameters are as follows: Toroidal field of 9 T; I p ≤ 3 MA, R = 66.5 cm, a = 21 cm, κ ≤ 2.0, δ ≤ 0.5, n e ≤ 10 21 m −3 , P ICRF ≤ 6 MW. The divertor configuration includes mechanical baffling as opposed to an ‘open’ geometry. Under strictly ohmic heating conditions, central T i and T e are predicted to be in the range 2.5–3.5 keV over the density range (4–8) × 10 20 m −3 . With the addition of 6 MW of ICRF heating, T i should vary from 4–8 keV over the same density range (assuming either Kaye-Goldston or Neo-Alcator scalings for electron confinement). Based on edge plasma characterizations from Alcator-C and divertor tokamaks, the scrape-off layer (SOL) properties are predicted to be: λ n ≈ 10mm, density at the divertor plate 21 m −3 , H 0 ionization mean free path between 1 and 10 mm. Maximum heat loads on various internal components are predicted to be in the range 5–10 MW/m 2 . The flexibility of the poloidal field system in forming a number of flux surface geometries will provide further comparisons of the relative impurity control capabilities of double-null, single-null and limiter plasmas.

Alcator C-MOD ICRF antenna design and analysis

Design features of the ICRF (ion cyclotron range of frequencies) fast-wave antenna to be used on the Alcator C-MOD tokamak are described. Initially, a movable, single-current-strap antenna will be used. A detailed study of the antenna performance was carried out using an antenna-plasma coupling code. Mechanical strength against disruption forces was a major consideration in the antenna design. Analyses show that the present design is mechanically and thermally satisfactory. An optimized configuration being planned for future experiments and having two double-current-strap antennas is also described.<<ETX>>

LOWER HYBRID HEATING EXPERIMENTS ON THE ALCATOR-C AND THE VERSATOR-II TOKAMAKS

ABSTRACT We report on initial results from lower hybrid wave heating experiments carried out on the MIT Alcator C and Versator II tokamaks. In the Alcator C experiments a 4 waveguide array, with internally brazed ceramic windows has been used to inject 160 kW of microwave power at 4.6 GHz into the plasma with n O ≤ 1 × 10 15 cm −3 , and B O ≤ 12 T. An RF power density of 8 kW/cm 2 has been transmitted into the plasma without RF breakdown. RF coupling studies show optimal coupling (R ≤ 10%) when the local density at the waveguide mouth is 25–50 times overdense. Initial heating experiments show an ion tail formation in hydrogen discharge peaking at a density of at B = 8.9 T, and bulk ion heating at a density of at B ≃ 11 T. Evidence of RF current enhancement has been observed at a density of n ≃ 3 × 10 13 cm −3 .

Halo current measurements on Alcator C-Mod

An aggressive experimental campaign to measure halo currents in Alcator C-Mod is described. Arrays of magnetic probes in combination with resistive shunts in outer divertor modules measure total halo currents flowing during disruptions as well as their vertical and toroidal distributions. Toroidal rotation and toroidal asymmetries dominated by n=1 structure characterize the measurements. A vertical array of magnetic probes places an upper bound on the width of the halo region, indicating that the halo currents flow in a layer close to the plasma. Dynamic calculations of the stresses on the Alcator C-Mod vacuum vessel inner cylinder indicate enhanced stresses attributable to the asymmetries observed. Halo current measurements are presented and tentative scalings of halo current with plasma parameters are proposed.