Development of gamma-ray diagnostics for ITER

Gamma-ray spectrometry is a diagnostic tool for fast ions in large tokamak plasmas. The information provided allows us to identify and simultaneously distinguish the presence of fast alpha-particles and other ions (H, D, T, He-3) to obtain information on their energy distribution and relative densities, in addition to performing a tomographic radial profile reconstruction of the gamma-emission sources. The lack of vertical diagnostic ports in ITER makes the implementation of tomographic neutron and gamma-ray systems more complicated. The use of a vertical divertor port for the implementation of a vertical viewpoint is currently suggested. The strong magnetic field (similar to 2 T) found there makes it hard to use conventional multi-dynode photomultipliers as light detectors. Instead, the use of micro-channel plate photomultipliers is suggested here. Investigations of the magnetic field impact on the performance of the gamma-spectrometer with a micro-channel photomultiplier are carried out. A high-speed pulse height analysis technique, which allows us to trace gain changes in the photomultiplier tube, is developed at the Ioffe Institute. The tests demonstrate the feasibility of gamma-spectrometric measurements in the divertor port provided that micro-channel photomultiplier detectors and the developed high-speed technique are used.