A gamma-ray spectrometer system for fusion applications

A NaI scintillator spectrometer system for the measurement of gamma-ray spectra in tokamak discharges has been developed and installed on the Frascati Tokamak Upgrade. Two NaI scintillators are viewing the plasma at two different angles with respect to the equatorial plane. The main features of the spectrometer system (energy range: 0.3–23MeV) and of the unfolding technique used to restore physical spectra from the pulse-height distributions are described: a method of solution with regularisation for matrix equations of large size, allowing to process count distributions with significant statistical noise, has been developed. A dedicated software, portable to any platform, has been written both for the acquisition and the analysis of the spectra. The typical gamma-ray spectra recorded in hydrogen and deuterium discharges, also with additional heating, are presented and discussed; two components have been observed: (a) thick-target bremsstrahlung gamma-rays produced by runaway electrons hitting the inconel poloidal limiter and/or the vessel; and (b) neutron capture gamma-rays generated in the detector shielding and tokamak structures. The maximum energy resulting from the bremsstrahlung spectra is in agreement with the runaway energy predicted by a test particle model of runaway electron dynamics.