Visible spectroscopic imaging on the Alcator C-Mod tokamak

Abstract

This dissertation reports on the development of a diagnostic visible imaging system on the Alcator C-Mod tokamak and the results from that system. The dissertation asserts the value of this system as a qualitative and quantitative diagnostic for magnetically confined plasmas. The visible imaging system consists of six CCD cameras, absolutely calibrated and filtered for specific spectral ranges. Two of these cameras view the divertor region tangentially, two view RF antenna structures and two are used for a wide-angle survey of the vacuum vessel. The divertor viewing cameras are used to generate two-dimensional emissivity profiles using tomography. Three physics issues have been addressed using the visible imaging system: 1) Using two-dimensional emissivity profiles of Da, volumetric recombination rate profiles have been measured and found to have a structure that depends on a poloidal temperature gradient in the outer scrape-off-layer. 2) A camera viewing the inner wall tangentially was used to measure Da emission profiles. A sharp break in slope of the radial density profile was found at the location of the secondary separatrix near the inner wall by using these profiles and a kinetic model of the neutrals. 3) Two-dimensional emissivity profiles of visible continuum (420-430nm) have been measured and found to be an order of magnitude too large when compared to expected levels from electron-ion bremsstrahlung and radiative recombination. Several atomic and molecular processes have been considered to explain the enhanced continuum. However, none of the considered processes could explain the continuum level without particle densities inconsistent with current modeling efforts.

(cont.) The visible imaging system was also used in identifying the causes of impurity injections during discharges, in identifying the failure of invessel components, and as a monitor of vessel and plasma conditions. Both the physics results and the operational benefits of the visible imaging system show that the system is a valuable quantitative and qualitative diagnostic.