Department of Nuclear Engineering
THESIS PROSPECTUS
for the Degree of
Doctor of Philosopy

Christopher James Boswell


Visible Spectroscopic Imaging on the Alcator C-Mod Tokamak

Dr. James Terry, Thesis Supervisor
Prof. Ian Hutchinson, Thesis Reader

Introduction

The use of linear diode arrays for the purpose of measuring visible light emission from tokamak devices has been well established, and is common practice. Although the diodes are useful, they do have a drawback; they measure the brightness of emission along a chord and cannot determine the spatial location, on that chord, of where the light was emitted. The use of diodes is adequate if you can make assumptions about the source of the emission. If you need two-dimensional information about the emission, several diode arrays need to be placed around the region of interest, absolutely calibrated, and tomographically inverted. [1--3] A simpler solution is to continue to assume toroidal symmetry, but use a tangentially viewing CCD camera in place of several linear diode arrays. The single camera needs to be calibrated only once, as opposed to the multiple calibrations of each diode array. A tomographic inversion is still necessary to obtain the two-dimensional profile of emission.

With the ability to measure two-dimensional profiles of visible light emission (specifically D_α, D_β, and D_γ), profiles of plasma sources and sinks can be obtained. In other words, the emission profiles in conjunction with temperature and density measurements can yield the ionization and recombination rate profiles of the deuterium atoms in the plasma.[4,5]

In addition, comparisons between two-dimensional profiles of D_γ, CII and CIII emission during various operating regimes allows for the determination of spatial evolution of the radiative and recombining regions of the divertor. The D_γ emission is a signature of the recombining region while strong CII and CIII emission are signatures of a dominantly radiating region. The determination of the two-dimensional location and evolution of the radiating region has not yet been accomplished on Alcator C-Mod with sub-centimeter spatial resolution.

Anomalous continuum radiation has been observed in the Alcator C-Mod divertor region.[6] This continuum cannot be explained by bremsstrahlung or radiative recombination of deuterium atoms. One explanation for this emission has been put forth by A. Yu. Pigarov claiming the continuum is due to processes involving molecular deuterium.[7] The measurement of the two-dimensional profile of this continuum will help in determining its possible cause and diagnostic potential.

Another use of the cameras is the identification and localization of impurity sources generated by the ICRF antennas, which supply the auxiliary heating on Alcator C-Mod. The impurities generated by the antennas are identified by correlating in time the injections seen by the cameras with measurements made with core diagnostics. Visible spectroscopic views aligned with the camera views are also used to identify the species of the impurities injected.

Thesis Goal

The goal of this thesis is to implement a camera system on Alcator C-Mod including: (1) two divertor views, to be used to obtain two-dimensional profiles of deuterium, carbon, and continuum emission, and (2) three views of the ICRF antennas, to be used to identify and locate impurity sources from the antennas. A second part of the thesis is to analyze the images from these cameras to: (1) obtain volumetric recombination profiles during different plasma operating regimes, (2) measure and identify regions of radiational cooling and recombination, and (3) measure and identify regions of unexplained continuum emission.

Background

CCD cameras have been used on the Alcator C-Mod tokamak[8] and the DIII-D tokamak at General Atomics in San Diego[9] to obtain the two-dimensional profiles of visible emission in the divertor region. The camera system on the Alcator C-Mod tokamak has been used to determine the volumetric recombination rate in the divertor under various conditions.[10] The camera system on the DIII-D tokamak has been used to diagnose some of the physical processes that occur during the partially detached divertor operation.[11] This use has been extended at DIII-D to include the viewing of ultraviolet emission in the divertor with a CCD camera.[12] All of the above described uses of the tangentially viewing camera systems invert the recorded images to obtain two-dimensional profiles.

Calendar of Plan

Previously a three-camera system, primarily focussed on the divertor views, had been used on Alcator C-Mod. This system was implemented during the 1999 campaign yielding interesting results on plasma in the private flux region of the divertor[13], and observations of impurity injections from the ICRH antennas.[14] The three-camera system limited the views to either the antennas or the divertor, but never both. Therefore, the goal is to install the five camera system and record the divertor and antennas during the entire 2000 campaign.

Core/Major/Minor requirements