Soft x-ray computer tomography system on the WT-3 tokamak

Soft x-ray (SXR) computer tomography (CT) system for the WT-3 tokamak was completed. The system was composed of SXR cameras, on-line data acquisition and software for CT reconstruction. Each SXR camera consisted of a pinhole and an integrated twenty channel PIN photodiode array on a compact board. The size of the SXR camera was so small that we could install five SXR cameras at the same toroidal section. SXR experimental data were transferred to a workstation through a CAMAC system in a minute and stored on the magnetic disk. The Fourier-Bessel expansion technique was employed to reconstruct the contour map of the SXR emissivity from SXR signals along a hundred viewing chords of five SXR cameras. It became possible to reconstruct CT images with relatively high poloidal mode number (m=4) during the interval of tokamak discharges. The internal mode structure of a plasma with steady magnetohydrodynamic oscillation was investigated. It was found that there was an m=1 mode near the plasma center in addition to an m=2 mode localized near the plasma periphery. The internal structure of the plasma during mode locking of the m=2 mode was also investigated. The CT results showed that the m=1 mode was locked as well as the m=2 mode. (author)