S.C. McCool

Electron thermal confinement studies with applied resonant fields on TEXT

Externally applied magnetic fields are used on the Texas Experimental Tokamak (TEXT) to study the possibility of controlling the particle, impurity and heat fluxes at the plasma edge. Fields with toroidal mode number n = 2 or 3 and multiple poloidal mode numbers m (dominantly m = 7) are used, with a poloidally and toroidally averaged ratio of radial to toroidal field components 〈|br/Bo〉 ≅0. 1%. Calculations show that it is possible to produce mixed islands and stochastic regions at the plasma edge (r/a ≥ 0.8) without affecting the interior. The expected magnetic field structure is described and experimental evidence of the existence of this structure is presented. The edge electron temperature decreases with increasing 〈|br/Bo〉, while interior values are not significantly affected. The implied increase in edge electron thermal diffusivity is compared with theoretical expectations and is shown to agree with applicable theories to within a factor of three.

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.

The space potential in the tokamak text

A heavy ion beam probe has been used to measure the plasma space potential profiles in the tokamak TEXT [Nucl. Fusion Technol. 1, 479 (1981)]. The Ohmic discharges studied were perturbed by externally produced resonant magnetic fields (an ergodic magnetic limiter or EML). Without these perturbations the plasma central potential is generally consistent with the value calculated from radial ion momentum balance, using experimental values of density and ion temperature and assuming a neoclassical poloidal rotation velocity. Exceptions to the agreement are found when operating with reduced plasma parameters. Possible reasons for this discrepancy are explored, in particular, the effects of intrinsic magnetic field fluctuations, and modifications to the self‐consistent radial electric sheath. With the application of the EML fields the edge electric field and potential increase during periods of magnetic island overlap. A test particle calculation of electron transport shows increases in diffusivity also occur d...

Particle transport studies with applied resonant fields on TEXT

Externally applied resonant magnetic fields have been used on TEXT to modify the particle flux and the radial electric field near the plasma edge. Magnetic fields with primary mode numbers m/n = 7/3 and 7/2, and an average radial field amplitude |br|/B ? 0.1% have been employed. This perturbation produces mixed islands and stochastic regions at the plasma edge (r/a ? 0.8) without affecting the interior. Working particle transport is shown to be increased by typically 30% only in the presence of (computed) magnetic islands. The effect is diminished at high perturbing field strength when the islands become stochastic. A novel transport mechanism due to ? convection is proposed to explain this. Outward impurity transport is increased as well.

High resolution visible continuum imaging diagnostic on the Alcator C-Mod tokamak

A high spatial resolution CCD based one-dimensional imaging system to measure visible continuum emissivity profiles from Alcator C-Mod tokamak plasmas is described. The instrument has chordal resolution that is better than 1 mm for the edge region of the plasma, where very sharp (1 to 10 mm) gradient lengths in plasma parameters are observed after the formation of the H-mode transport barrier. Each image has up to 2048 pixels, and total spatial coverage goes from 2 cm inside of the magnetic axis to ∼4 cm outside of the last closed flux surface in the ∼22 cm horizontal minor radius plasmas. Time resolution can be varied from 0.21 ms to 4 ms; good signal to noise is achieved with 1 ms integration under typical plasma conditions. The emission over most of the plasma volume is dominated by free–free bremsstrahlung, and can be used to infer local values of the average ion charge (Zeff). Toroidally localized puffing of deuterium, nitrogen, and helium reveals that a significant contribution to the signal in the ...

Experimental evidence for ion pressure gradient driven turbulence in TEXT

For high density Ohmic discharges in the TEXT tokamak, a distinct ion mode (i.e. density fluctuations propagating in the ion diamagnetic drift direction) is observed in the microturbulence spectra. The magnitude and spectral characteristics of the mode are identified. A microinstability based transport model is used for the purpose of interpreting anomalous confinement properties. Onset of the ion feature occurs at plasma densities where a clear saturation is evident in the global energy confinement time τE. When the ion feature in the fluctuation spectra is strongest, agreement between predictions from the transport model and experimentally measured values of the global energy confinement time is realized if anomalous ion effects due to the ion pressure gradient driven (ηi) instability are included. By injecting pellets, a high density plasma is created in which the density profile is sharply peaked. Under these conditions the ion feature in the fluctuation spectra is suppressed. A possible connection between this experimentally observed ion mode and the theoretically predicted properties of the ηi instability is explored.

Fast alpha diagnostics using carbon pellet injection

Charge exchange interactions of alpha particles with the ablation cloud surrounding an injected carbon pellet can be used to measure the energy spectrum of the incident fast confined alpha particles in a fusion plasma. Measurement of helium neutrals from He2++C4+→He0+C6+ interactions appears to be the most attractive option. This paper describes progress on developing this diagnostic including measurements during pellet injection into the TEXT tokamak.

Current diffusion in TEXT

A study of current diffusion in the TEXT device has been performed. The plasma current was ramped up from steady state conditions (150 kA) on a time-scale (20 ms) much faster than the classical skin time (100 ms), with ∂lp/∂t 8 MAs−1. A stable discharge was obtained and held at the resulting higher current (300 kA). Density (4 × 1019 m−3) and position were held constant during current ramping. The profiles derived from Thomson scattering and the surface barrier diode soft-X-ray array were used in the solution of the poloidal magnetic field diffusion equation. The experimental data were also compared with the prediction of a three-zone model of radial particle and energy transport. No enhanced or anomalous resistivity is required to adequately match the data for the case where the current was ramped from a steady state condition.

Transport with pellet fuelling in the Texas Experimental Tokamak

In the Texas Experimental Tokamak, a discharge regime characterized by persistent peaked profiles was observed to be induced by pellet fuelling, and its transport properties were studied. The hallmark of the regime is the suppression of sawteeth, and the regime was attained by injecting hydrogen pellets to promptly double the plasma density. In each of several pairs of experiments, a pellet fuelled discharge was compared with an edge fuelled discharge with similar averaged electron density, plasma current and toroidal magnetic field in order to characterize the transport change and to look for causal changes in the plasma turbulence. The impurity, radiation and working gas particle profiles were more peaked for the pellet fuelled case. The values of rjt derived from measured ion temperature and density profiles for high density edge fuelled and pellet fuelled discharges indicate that ion pressure gradient driven turbulence should be reduced in the pellet fuelled case. The macroscopic effects were accompanied by microscopic changes. Measurements of turbulent density fluctuations in high density edge fuelled discharges give strong evidence that a component of the turbulence propagates in the ion diamagnetic direction and that this particular mode is reduced in pellet fuelled discharges. The effects of the reduction of an ion mode turbulence were sought in the energy confinement of the discharges, but it was found that for these experiments (tailored for the turbulence diagnostics) the energy flowing in the ion channel was not large enough to affect the energy confinement. Simulations were used to interpret some of the results. Discharge simulations which include the pellet injection can reproduce the sawtooth suppression. This result and known properties of discharges in which sawteeth are suppressed suggest that some of the profile effects (including peaking of the working gas particles) induced by pellet injection are due to sawtooth suppression. The particle peaking may cause the observed reduction in the turbulence which follows pellet injection.

EFfects of an ergodic magnetic limiter on the edge plasma properties of the text tokamak

Spatially resolved VUV emissions from low-Z intrinsic impurities have been used to measure the effects of an ergodic magnetic limiter (EML) on the edge region of the TEXT plasma. Using previously established techniques for determining the electron density ne and the electron temperature Te from line pair ratios within the beryllium-like O V ion, poloidal modulations in ne, and Te are found corresponding to EML induced magnetic islands. In general, increases of 50% in both ne and Te are found near the poloidal locations of X-points and decreases of the same order are found near O-points. Modulations and radial profile broadening of intrinsic carbon and oxygen ions are also found, indicating enhancement of edge impurity transport during EML perturbation.