High resolution neutron sp ectrometer system for ITER

Content The High Resolution Neutron Spectrometer (HRNS) system for ITER is an array of neutron spectrometers with the primary function to provide measurements of the fuel ion ratio, nT/nD, in the plasma core. Supplementary functions are to assist or provide information on fuel ion temperature and energy distributions of fuel ions and confined alpha-particles. The ITER requirement for the HRNS primary function is to obtain nT/nD with 20% uncertainty and a time resolution of 100 ms. In this contribution, a conceptual HRNS system design and its measurement performance for nT/nD will be presented. The HRNS system studied, is based on established instrumental techniques and its performance is assessed using realistic response functions for the individual spectrometers in the system. The main interfacing requirements for the HRNS is a 10 cm diameter aperture in the ITER first wall, tapered collimation resulting in a neutron flux in the order of 1099 n/cm22/s on an area of 12 cm22 at a distance of 16 m from the first wall. For optimum use of the available neutron flux, the system is divided into two sections; "low efficiency" neutron spectrometers in the front and "high efficiency" in the rear. Also investigated is to use an adjustable collimator between the front and rear spectrometers in order to enhance the dynamic range and overlap of the two sections. For the combination of neutron spectrometers presented here, it is shown that the system fulfills the ITER requirement, on nT/nD, over an order of magnitude in fusion power, 50 < Pfus < 500 MW. In the performance study, contributions due to neutron scattering in the vessel walls, collimator and beam dump are included together with a neutron induced gamma background.