Visible light tomography system for a high-beta field-reversed configuration plasma

A simple and adjustable tomographic system has been developed for studying the time evolution of the internal structure of a field-reversed configuration plasma. The optical diagnostic system consists of 35 collimators. In each collimator, a plane-convex lens made of BK7 with a focal length of 100 mm collimates emissions from the plasma and focuses them onto the end of an optical-fiber tube. The collimator system gives a spatial resolution of 6 mm at the center of the 0.256 m diameter discharge chamber. The collimated light is transmitted through the optical-fiber tube to a photomultiplier tube (PMT) detector. In this system, the wavelength is selected by a bandpass interference filter with a central wavelength of 550 nm and bandwidth of 10 nm to eliminate impurities and deuterium-line spectra. The window is verified to be free of impurity lines (Matsuzawa et al., Rev. Sci. Instrum., these proceedings). Thus, only bremsstrahlung is detected by the PMTs. The 35 collimators are arranged on five holders in a fan-shaped arrangement with seven collimators per fan. A profile of the emissivity has been reconstructed by using the algebraic reconstruction technique, without any assumption regarding the symmetry of the plasma