G. Cima

An experimental counter‐example to the local transport paradigm

The response of a tokamak discharge to a sharp drop in edge temperature differs significantly from that expected from typical local transport models in several important respects. Laser ablation of carbon induces large (ΔT/T≤70%), rapid (<200 μs) electron temperature drops in the outermost region of the plasma, r/a≥0.9. This cold pulse proceeds through the outer plasma (r/a≥0.75), rapidly compared with power balance or sawtooth predictions. However, the pulse shrinks markedly thereafter, disappearing near r/a∼0.5. Within r/a∼0.3, the temperature rises promptly. The results are inconsistent with conventional local transport models; a nonlocal phenomenology, in which transport coefficients increase in the edge and decrease in the core, is suggested. The turbulence levels measured with a heavy ion beam probe increase near the edge but are unchanged in the core.

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...

Characteristics of equilibrium and perturbed transport coefficients in tokamaks

Although the evolution of a perturbation to a tokamak equilibrium can generally be described by local transport coefficients modestly enhanced above the equilibrium values, there are some significant cases for which this is inadequate. The density profile evolution in ASDEX-U occurs far more rapidly than is consistent with reasonable particle confinement times, and the evolution of cold pulses in TEXT requires nonlocal behavior in the core and some kind of anomaly near the periphery. The experiments are suggesting effects beyond standard local turbulent transport models.

Electron cyclotron emission correlation radiometry in toroidal plasma confinement devices

Abstract Very low level plasma density and temperature fluctuations can be responsible for anomalous transport in thermonuclear magnetic traps. Electron cyclotron emission (ECE) detection has proven to be a very effective diagnostic for monitoring plasma temperature fluctuations at the frequencies of interest. Diffraction unfortunately limits the space resolution of this diagnostic with potentially important consequences on estimates of turbulent transport. This issue will be investigated both theoretically and with regard to technical implications. Experimental results obtained on the TEXT-U tokamak will be used to illustrate the previous point. Recent results obtained on TEXT-U with ECE, and corroborated by other fluctuation diagnostics, on the anisotropy of the high frequency fluctuations will be presented.