Alcator C-Mod

Liang Lin

Thesis Title: Experimental Study of Turbulence and Waves in the Alcator C-Mod Tokamak with Phase Contrast Imaging Diagnostic

Thesis Advisor: Miklos Porkolab

Updated on July 7, 2006

Motivation and Objective

The origin and nature of the anomalous electron transport is perhaps the most important unresolved issue in the plasma transport area . In reactor scale fusion devices like ITER, ions and electrons will be closely coupled thus the thermal transport through the electron channel may dominate . Gyro-kinetic simulations (GS2 and GYRO) predict that in tokamak devices the short wavelength Electron Temperature Gradient (ETG) modes are likely responsible for the observed electron thermal transport properties. An experimental test of the prediction is necessary for the validation of the codes.

The primary goal of the proposed experiment is to search for and characterize the turbulence with the upgraded Phase Contrast Imaging (PCI) diagnostic and make direction comparison with the gyro-kinetic codes, as well as with the global and local confinement properties.

Phase Contrast Imaging Diagnostic in C-Mod

Phase contrast imaging (or PCI) is an internal reference interferometer technique. The entire expanded CO 2 laser beam is sent through the plasma and modulated by longer wavelength electron density fluctuations. The PCI technique relies on spatial filtering at the phase plate to additionally phase shift the reference beam (part of the CO 2 beam) by p /2, and this will transform phase variations in the plasma into the intensity variations at the detector array. The scattered signal and reference beam are finally recombined across linear detectors, which provide sampling of the density fluctuations along the tokamak major radius with an effective spacing that is set by the magnification of the imaging optics system.

In the past, the C-Mod PCI diagnostic has generally been used to study long wavelength waves and fluctuations with coherent frequency. Recently, the detector channel number has been increased from 12 to 32 and the digitizer sampling rate from 1 MHz to 10 MHz. Presumably, it is necessary to have higher power to search for ETG, since these are low amplitude modes. A new CO 2 laser has been installed, increasing laser power from 25 W to 60 W, to increase the signal-noise-ratio. Together with the new implemented imaging optics, this enabled the C-Mod PCI to measure turbulent density fluctuations in the wavenumber range [0.5-60 cm -1 ] and in the frequency range of 2kHz to 5 MHz. Vertical localization through the variation of the magnetic field pitch angle has been attempted, and Quasi-Coherent (QC) modes localized to the top (bottom) have been differentiated, and an ITG like turbulence has also been measured. Short wavelength turbulence measurement in the ETG regime has been started with the enhanced PCI system.