E. Meservey

The ontogeny of a Tokamak discharge

A detailed description of the time behaviour of a hydrogen discharge in the ST-Tokamak is based on measured radial electron temperature and density profiles at 12 different times, together with measurements of the Ohmic-heating current and voltage, the temporal, spatial, and spectral distributions of hydrogen light, the ion temperatures, and impurity concentrations. Early in the discharge the electron temperature profiles show evidence of a skin effect that develops on a time-scale of several milliseconds into a peaked profile of about 2.2 keV maximum. Thereafter the peak temperature stops growing and develops into a flat plateau, the width of which appears to be determined by the Kruskal-Shafranov limit. The average particle confinement time scales with , and reaches a maximum of 13-14 ms. The power balance is dominated by electron loss and re-cycling, rather than ion loss or radiation. The recycling process at the aperture limiter appears to involve sufficiently energetic neutral atoms to provide a fairly flat radial source function for particles, and hence to influence directly the development of the radial distribution of power input and energy balance.

The effect of current profile evolution on plasma-limiter interaction and the energy confinement time

Experiments conducted on the PLT tokamak have shown that both plasma-limiter interaction and the gross energy confinement time are functions of the gas influx during the discharge. By suitably controlling the gas influx, it is possible to contract the current channel, decrease impurity radiation from the core of the discharge, and increase the gross energy confinement time, whether the aperture limiters are of tungsten, stainless steel or carbon.

Radiation losses in PLT during neutral-beam and ICRF heating experiments

Radiation and charge-exchange losses in the PLT tokamak are compared for discharges with Ohmic heating only (OH), and with additional heating by neutral beams (NB) or RF in the ion cyclotron frequency range (ICRF). Spectroscopic, bolometric and soft-X-ray diagnostics were used. The effects of discharge cleaning, vacuum wall gettering, and rate of gas inlet on radiation losses from OH plasmas and the correlation between radiation from plasma core and edge temperatures are discussed. – For discharges with neutral-beam injection the radiation dependence on type of injection (e.g. co-injection versus counter- and co- plus counter-injection) was investigated. Radial profiles of radiation loss were compared with profiles of power deposition. Although total radiation was in the range of 30–60% of total input power into relatively clean plasma, nevertheless only 10–20% of the total central input power to ions and electrons was radiated from the plasma core. The radiated power was increased mainly by increased influx of impurities, however, a fraction of this radiation was due to the change in charge-state distribution associated with charge-exchange recombination. – During ICRF heating radiation losses were higher than or comparable to those experienced during co- plus counter-injection at similar power levels. At these low power levels of ICRF heating the total radiated power was ~ 80% of auxiliary-heating power. Radiation losses changed somewhat less rapidly than linearly with ICRF power input up to the maximum available at the time of these measurements (0.65 MW).

Effects of tungsten radiation on the behaviour of PLT tokamak discharges

Bands of radiation in the 30–70 A wavelength range, ascribed to unresolved resonance lines of various tungsten ions, have been observed in a variety of discharges in the PLT tokamak. The amount of tungsten present in the discharge appears to depend sensitively on the peripheral plasma temperature, and it may be dropped significantly by adding small amounts of neon or oxygen, and also by appropriate programming of hydrogen influx. In high-tungsten discharges the observed bands account for a large fraction (~ 0.5 or more) of the power input near the centre of the plasma.

CHANGES IN TOKAMAK PLASMA PROPERTIES DURING IMPURITY INJECTION.

Certain applications of laser blow‐off impurity injection, such as ion temperature measurement and spectral line identification, may require the ratio of injected ions‐to‐plasma electron density to exceed 0.001. To achieve these concentrations we have relied on injection of micron‐sized clusters as well as monatomic species. In these cases the injected impurity can no longer be regarded as a nonperturbing trace sourced at the plasma edge. We therefore have studied the variation of plasma parameters in PLT and PDX as a function of the amount of injected impurities, the species mix, and the type of impurity. We find that for ohmically‐heated plasmas, impurity injection can increase ?e more than 4×1012 cm−3 without causing disruptions. During Ge‐injection experiments where Δne/ne ∠0.05, Te initially rises ∠5% on axis and decreases ∠20% at the edge. The plasma current drops 1–3 % and the loop voltage and radiated power both double. Varying the amount of injected impurities changes the peaking and decay times ...

Conductivity and transport in neon deuterium discharges in the PLT tokamak

Introduction of large amounts of neon into Ohmically heated deuterium discharges in the PLT tokamakl results in higher central electron temperature (Te(0) 3 keV) and values of electron energy containment time that are larger than in regular discharges at the same electron density (τEe = 44 ms at e = 2 × 1019 m−3). For steady-state discharges with high effective Z (~ 5–8) the conductance is larger than that predicted by neoclassical theory by as much as a factor of two. Transport rates of hydrogen-like and helium-like ions can be fairly well approximated by assuming a diffusion constant of between 0.4 and 1 m2s−1. Within experimental uncertainties the diffusion of hydrogen-like neon is the same for co- and counter-directed high-power neutral beams.

Low-Z impurities in the PLT Tokamak

Concentrations and influx rates of oxygen and carbon ions have been determined in a variety of discharges in the PLT Tokamak. Oxygen, and to a lesser extent carbon are found to affect strongly the plasma behavior, mostly by changes in resistivity. The principal effect appears to be peripheral cooling by oxygen radiation, which changes the radial distribution of current density and temperature, and simultaneously inhibits the influx of high-Z limiter and wall material.

Perpendicular bremsstrahlung emission of suprathermal electrons

An interpretation of the x‐ray bremsstrahlung emission by suprathermal electrons perpendicular to a magnetic field is given in terms of the parallel and perpendicular temperature of a three‐temperature distribution function. The slope (i.e., the temperature) of the distribution can be determined relatively well. Factor‐of‐two uncertainties remain for the number of electrons.

Radiated energy and impurity density changes during intensive hydrogen influx in the PLT tokamak

During a discharge a puff of hydrogen is admitted, sufficient to more than triple the plasma density, and the resulting changes in various plasma parameters are determined. The absolute densities of various wall and limiter (carbon) materials are found to decrease by a substantial fraction, probably as a result of lowered peripheral temperature. The radiation pattern deduced from spectroscopically determined plasma composition is in good quantitative agreement with direct bolometric measurements. In the interior of the discharge, radiation constitutes only a small part of the power input. Neither the radiated power nor the power input changes very markedly as a result of the density rise, since the effects of temperature and plasma composition changes tend to compensate each other.

Fast-wave ion-cyclotron heating in the Princeton Large Torus

Recent experimental results for ICRF heating in PLT are presented. For the two-ion regime in D-H or D-/sup 3/He plasmas minority H and /sup 3/He ions are found to absorb the rf power and transfer it to the deuterons and electrons in accordance with Fokker-Planck theory. The deuteron heating rate is approx. 3 eV x 10/sup 13/ cm/sup -3//kW for H and approx. 6 eV x 10/sup 13/ cm/sup -3//kW for /sup 3/He minorities. Neutron fluxes of approx. 3 x 10/sup 11/ sec/sup -1/ corresponding to a T/sub d/ approx. 2 keV (..delta..T/sub d/ approx. 1.2 keV) have been produced with P/sub rf/ approx. = 620 kW at anti n/sub e/ approx. = 2.9 x 10/sup 13/ cm/sup -3/. Neutron energy spectra and mass sensitive charge exchange spectra indicate Maxwellian deuteron distributions. In addition, D-/sup 3/He fusion reaction rates greater than or equal to 10/sup 12/ sec/sup -1/ have been produced by the energetic /sup 3/He ions. For the second harmonic regime, initial heating results for an H plasma at P/sub rf/ approx. = 140 kW are consistent with the Fokker-Planck theory and the bulk heating rate is comparable to that of D heating in the D-H minority regime.