Rex F. Gandy

Core temperature fluctuations and related heat transport in the Texas Experimental Tokamak‐Upgrade

The mechanism(s) responsible for anomalous heat transport in the tokamak plasma core has remained elusive to experimental verification. In this paper the hypothesis that high‐frequency electrostatic turbulence can account for the measured electron heat transport in Ohmically heated tokamak discharges of the Texas Experimental Tokamak‐Upgrade (TEXT‐U) [Proceedings of the 15th Symposium on Fusion Technology, Utrecht (Elsevier, Amsterdam, 1989), Vol. 1, p. 342] is tested. To accomplish this, core temperature fluctuations have been determined from the measured correlation between two electron cyclotron radiation signals detected by a multichannel high‐frequency‐resolution heterodyne radiometer. It is found that long wavelength modes (poloidal wave number ≲1 cm−1) are present, with an electron temperature fluctuation amplitude comparable to the density fluctuation amplitude. However, these modes cannot account for observed transport. An extrapolation of the observed turbulent temperature spectrum to the shorte...

Magnetic surface mapping with highly transparent screens on the Auburn torsatron

In stellarator‐type magnetic confinement devices (of which the torsatron is one), the magnetic field is produced entirely by external, current‐carrying coils. Two methods for mapping magnetic surfaces in the Auburn torsatron were tested and compared, both of which involve the use of highly transparent screens. The first method consists of coating the screen with a phosphor that emits light when struck by electrons emitted by an electron gun. A pattern representative of a magnetic surface is formed on the screen, and this pattern is recorded photographically. The second method uses an uncoated screen to collect electrons emitted from an emissive probe, which is scanned over a poloidal cross section of the torus. Under certain conditions, the collected current is a constant over a particular magnetic surface so that a contour plot of the current versus position is equivalent to a plot of the magnetic surfaces. Parametric studies of the two methods are presented, and the effectiveness of each technique is di...

Correlation radiometry of electron cyclotron radiation in TEXT‐U (invited)

Very low level plasma density and temperature fluctuations can be responsible for anomalous transport in thermonuclear magnetic traps. Measuring these fluctuations by detecting plasma electron cyclotron emission (ECE) might indicate where, in ω−k space, the most dangerous instabilities are located. The most attractive feature of ECE is the good localization of the source, unique to this range of frequencies among plasma radiation. However, to maintain spatial resolution, only a small number of radiation modes should be detected. The consequent poor statistics of the ECE measurement allows one to measure only the average value for the temperature fluctuation amplitude via correlation analysis of two ECE signals with suitable correlation properties. Initial measurements of this kind have been performed successfully. But, results have only been partially conclusive as to the relevance of electrostatic turbulence to heat transport, due to still too limited poloidal spatial resolution and the inability to corr...

Design of the Compact Auburn Torsatron

This paper describes the design and optimization procedure for the Compact Auburn Torsatron. Included in this is a description of the Cary-Hanson Optimization technique. In this paper the properties of the magnetic fields are presented and a description of the machine and the construction procedure. The experimental setup for the surface mapping is described and conclusions are presented. 10 refs., 9 figs., 1 tab.

Poloidal asymmetry and gradient drive in core electron density and temperature fluctuations on the Texas Experimental Tokamak-Upgrade

Electron temperature and density fluctuations are measured in the core of the Texas Experimental Tokamak‐Upgrade (TEXT‐U) [P. H. Edmonds, E. R. Solano, and A. J. Wootton, in Proceedings of the 15th Symposium on Fusion Technology, Utrecht (Elsevier Science, Amsterdam, 1989), Vol. 1, p. 342] plasma across the poloidal cross section. The high spatial resolution of the heavy‐ion beam probe (HIBP) and correlation radiometry of electron cyclotron emission (CRECE) reveal that both the density and temperature fluctuations are strongly poloidally asymmetric. Temperature fluctuation measurements indicate a broadband drift wave feature localized near the plasma equatorial plane on both the high‐ and low‐field sides, which is consistent with density fluctuation measurements by far infrared (FIR) scattering. In contrast, the HIBP observes this feature localized only to the low‐field side. Excellent spatial resolution allows us to investigate whether changes in the gradient affect the fluctuation amplitudes. We find th...

Density fluctuation effects on electron cyclotron emission correlation measurements in optically gray plasmas

Measurements of electron temperature using electron cyclotron emission (ECE) may be contaminated by density effects in plasmas with low optical depth. ECE intensity fluctuations in these plasmas will include a component dependent on density fluctuations as well. Correlation measurements, which will extract the temperature fluctuations from the dominant radiometer thermal noise, will include correlated density fluctuations in the result. In tokamak core plasmas, where normalized density fluctuations have been measured to be about 1%, this effect can be considerable (∼100%) at low levels of temperature fluctuations. However, at ‘‘relevant’’ levels, defined by the amount of anomalous heat transport, these effects are reduced, at least in TEXT‐U. While exact interpretation of temperature fluctuation amplitudes is compromised, statements regarding the relevance of the inferred temperature fluctuation amplitudes can be made relatively unambiguously.

Autocorrelation and crossed-sightline correlation of ECE for measurement of electron temperature and density fluctuations on ATF and TEXT

The great stumbling block in the quest for fusion power using magnetic confinement devices is anomalous transport. It is conjectured that turbulent plasma fluctuations may be responsible for the degraded energy confinement observed in experiments. There exists a clear need for more detailed experimental studies of plasma microturbulence. A conceptual design is presented for a diagnostic to measure electron temperature and density fluctuations using electron cylotron emission (ECE). The proposed ECE systems will employ autocorrelation and cross‐correlation techniques to measure radiation from the Advanced Toroidal Facility (ATF) at Oak Ridge National Laboratory (ORNL) and also from the Texas Experimental Tokamak (TEXT) at the University of Texas. This set of experiments on a stellarator and a tokamak will allow a unique comparative study of the fluctuation physics in the two different magnetic configurations. The paper presents the theoretical basis and conceptual design of the diagnostic.

Measurement of temperature fluctuations from electron cyclotron emission

Radial scans of high‐frequency electron temperature fluctuations Te,rms have been performed in TEXT‐U ohmic discharges. The measurements were accomplished by using a technique correlating the electron cyclotron emission at two disjoint frequency bands emitted from essentially the same plasma volume. Focusing and defocusing the optics allows one to estimate the poloidal k spectrum of these fluctuations. Applying the analysis stroboscopically synchronized with the sawtooth oscillations it is shown that, in the central region of the plasma, sawteeth are the dominant contribution to low‐frequency modes. Additionally, by adjusting the frequency of one of the bands, one can separate the two volumes and thus estimate the radial correlation length of Te,rms.

Third harmonic electron cyclotron emission measurements on the Advanced Toroidal Facility

Measurements of third harmonic electron cyclotron emission have been made on the Advanced Toroidal Facility (ATF). Electron temperatures have been extracted from third harmonic, optically thin, electron cyclotron emission (ECE) measurements. The radial and temporal evolutions of the electron temperature have been characterized for electron cyclotron resonance heated discharges and for the early stages of neutral beam heated plasmas. A slab based emission model incorporating the equation of radiation transfer and ray tracing has been used to calculate the expected ECE. The results have been compared with experimental measurements. Measurements of the polarization of the third harmonic ECE during 1 T discharges have been compared with theoretical calculations of wave polarization rotation due to magnetic shear. Significant polarization of the emission was observed, and correction factors for a given wave frequency and peak density were obtained

High resolution ECE radiometer for electron temperature profile and fluctuation measurements on Alcator C-Mod

Abstract A broadband heterodyne radiometer has been installed on Alcator C-Mod to measure second harmonic electron cyclotron emission at 234–306 GHz. The high-resolution diagnostic is now operational with 32 channels separated by