Development and Construction of a 2.5-MeV Neutron Time-of-Flight Spectrometer Optimized for Rate (TOFOR)

Abstract: A new neutron time-of-flight spectrometer optimized for high count rate (TOFOR) has been developed for the JET research tokamak. TOFOR will measure the energy distribution of neutrons emitted from the d+d ? 3He+n fusion reactions in deuterium plasma. It will serve as the principal neutron spectroscopy diagnostic of high fusion power plasmas produced by injection of radio frequency waves (RF) and neutral beams (NB). The objective is to study plasma effects of RF and NB injection, with regard to temperature rise of the bulk deuteron population and the characteristics of supra-thermal components manifesting themselves over an extended energy range and with large spectral intensity variations. To meet the plasma diagnostic objectives, special demands have been put on the design and characterization of TOFOR which, to a great extent, has relied on extensive neutron transport calculations. These calculations were used to optimize the design and to determine the TOFOR neutron response function. For the response function, TOF spectra were simulated for 81 quasi mono-energetic neutron energies in the range 1 to 5 MeV.This thesis presents new results on instrumental solutions on the problem to reach high count rates, leading to a factor of hundred improvement compared to earlier designs.With regard to the analysis of measured TOF spectra, the determined response function was folded with models and fitted to measurement data. The general issue of the energy dependence of the response function is raised and its importance is illustrated with analysis of high-quality TOF spectra for NB and RF heated plasmas. Potential for future developments are identified in the use of hybrid cards able to provide digital infromation on both time and pulse height.