Roger D. Bengtson

Turbulent structure in the edge plasma of the TEXT tokamak

A reversal has been observed in the mean phase velocity of the turbulent fluctuations in the edge plasma of the TEXT tokamak. The observations can be described by a model in which the wave velocity in the lab frame is dominated by a nonuniform Er×B velocity and a gradient driven drift. The measurements also exhibit a localized instability which occurs in the region of maximum velocity shear.

Space/time statistics of the turbulence in a tokamak edge plasma

The results of a study of the statistical space/time properties of the turbulence observed in the edge region of the Pretext tokamak are reported. A local estimate of the wavenumber frequency spectra S(?,?) for poloidal (ky)an d toroidal (kz) wavenumbers is determined for the range 0.5 cm?1 < k < 7.5 cm?1, for both potential and density fluctuations obtained from spatially fixed Langmuir probe pairs. The S(ky, ?) spectra aredominated by low frequencies ( < 200 kHz) and small wavenumbers (< 3 cm?1) and appear broadened about an approximately linear statistical dispersion relation, (?). The broadening is characterized by a spectral width ?k (?) (root-mean-square deviation about (?)) which is of the order of (?). The turbulence appears to propagate poloidally with an apparent mean phase velocity of (1?2) ? 105 cm?s?1 in the ion diamagnetic drift direction. Measurements of the fluctuation-induced particle transport reveal the particle flux to be outward and resulting primarily from the low-frequency, long-wavelength components of the turbulence. A particle diffusion coefficient Dx, estimated from the measured particle flux and density profiles, is of the order of the coefficient estimated for Bohm diffusion, but appears to increase with density.

Experimental measurement of three-wave coupling and energy cascading

The nonlinear coupling coefficient and the energy transfer associated with three‐wave interactions are computed from measured data of the turbulent edge plasma of the Texas Experimental Tokamak(TEXT) [Nucl. Technol. Fusion 1, 479 (1981)]. The results show the presence of three‐wave interactions. The interactions cause energy to cascade away from the dominant waves of the spectrum primarily toward lower, but also toward higher frequencies. The results are obtained with a new digital spectral analysis technique based on the estimation of higher‐order cumulants. The method is discussed and tested on a simulation experiment. The same technique is useful for neutral fluids as well.

Advanced plasma fluctuation analysis techniques and their impact on fusion research (invited)

This article reviews digital spectral analysis techniques that yield experimental insight into plasma turbulence. Methods to quantify the statistical properties of the fluctuations and to measure the particle and heat flux caused by electrostatic fluctuations are presented. Furthermore, analysis techniques to study the nonlinear coupling process of turbulence and the redistribution of energy among the different modes are discussed. The impact of the analysis techniques on fusion research is demonstrated with experimental results collected with Langmuir probes, heavy‐ion beam probes, and laser scattering in the tokamak TEXT. Special emphasis is given to the characterization of the wavenumber distribution and the correlation lengths in all toroidal directions, including a first measurement of k∥ in a tokamak.This article reviews digital spectral analysis techniques that yield experimental insight into plasma turbulence. Methods to quantify the statistical properties of the fluctuations and to measure the particle and heat flux caused by electrostatic fluctuations are presented. Furthermore, analysis techniques to study the nonlinear coupling process of turbulence and the redistribution of energy among the different modes are discussed. The impact of the analysis techniques on fusion research is demonstrated with experimental results collected with Langmuir probes, heavy‐ion beam probes, and laser scattering in the tokamak TEXT. Special emphasis is given to the characterization of the wavenumber distribution and the correlation lengths in all toroidal directions, including a first measurement of k∥ in a tokamak.

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.

Characterization of tokamak edge turbulence by far-infrared laser scattering and Langmuir probes

The spectra, magnitude and spatial distribution of low-frequency (ω ωci) density fluctuation have been measured by two independent experimental methods in the edge plasma of the TEXT tokamak. Good agreement between far-infrared laser scattering and Langmuir probe measurements has been achieved and the strengths of each technique are evaluated. Langmuir probes are used to directly determine the particle flux induced by edge fluctuations (Γ ∝ n×B) and collective Thomson scattering permits an extension of these observations to the plasma interior. Results are presented for typical discharge conditions in a tokamak.

Global particle confinement in the Texas Experimental Tokamak

Particle transport in an ohmically heated tokamak plasma was investigated in the Texas Experimental Tokamak (TEXT). Spectroscopic measurements of the electron source were used with electron density measurements to derive particle confinement times from the continuity equation. Scalings were developed for particle confinement time with electron density, plasma current, toroidal field, and plasma positioning. Simultaneous measurement of electrostatic fluctuations with Langmuir probes may suggest a correlation between edge particle transport in TEXT and electrostatic turbulence. In addition, two major features of transport were isolated. First, transport is poloidally asymmetric at least in the plasma edge. Secondly, in some cases, the particle confinement scalings are closely associated with the scalings for recycling at particular surfaces. Similarities of the TEXT global particle confinement time scalings to those observed in other tokamaks may allow the conclusions of this work to be extended to other devices.

A new scheme for Langmuir probe measurement of transport and electron temperature fluctuations

A new scheme to extend the triple Langmuir probe technique for the measurement of electron temperature fluctuations and the fluctuation‐driven transport has been developed. The extension is aimed at reducing the phase delay error introduced by finite probe tip separations in standard triple‐probe method. The modified triple‐probe scheme provides a more reliable measurement of the temperature fluctuations for a proper interpretation of the density and potential fluctuations and the transport measurement from Langmuir probe data. New results on fluctuations have been obtained from Phaedrus‐T and TEXT‐U tokamaks.

Temperature fluctuations and transport in a tokamak edge plasma

Using the technique first described by Robinson and Rusbridge [Plasma Phys. 11, 73 (1969)] electron temperature fluctuations have been measured in the scrape‐off layer of the TEXT tokamak [Nucl. Technol. Fusion 1, 479 (1981)] to be Te/Te ≂0.4ne/ne. Particle and energy transport were estimated from 〈ne Eθ〉 and 〈Te Eθ〉 correlations. The fluctuation‐driven particle transport was consistent with earlier measurements and with measurements of global particle confinement. A high correlation of temperature and electric field fluctuations and a low but positive correlation between density and temperature fluctuations were observed.

Biased limiter experiments on text

Experiments using an electrically biased limiter have been performed on the Texas Experimental Tokamak (TEXT). A small movable limiter is inserted past the main poloidal ring limiter (which is electrically connected to the vacuum vessel) and biased at VLim with respect to it. The floating potential, plasma potential and shear layer position can be controlled. With ¦VLim¦⩾ 50 V the plasma density increases. For VLim 0 the results obtained are inconclusive. Variation of VLim changes the electrostatic turbulence which may explain the observed total flux changes.