PROPOSED MAGNETIC PROTON RECOIL NEUTRON SPECTROMETER UPGRADE (MPRU)

摘要

The Magnetic Proton Recoil (MPR) neutron spectrometer was installed at the Joint European Torus (JET) in 1996. The present prototype instrument is designed for 14-MeV neutrons. Its capability as a neutron emission spectroscopy (NES) diagnostic was demonstrated during the Deuterium-Tritium Experimental (DTE1) campaign in 1997 and it has since been continuously used during subsequent D-plasma operation. In this paper we describe an upgrade of the spectrometer's focal plane, recoil-proton detector and radiation shield. The MPRu is intended to make it possible fully to exploit the MPR capabilities in the forward JET experimental programme and to demonstrate the potential role of NES diagnostics in the next step fusion experiments such as planned with ITER. The MPR instrument, its data acquisition, monitoring and control systems, as well as the calibration and data reduction procedures, have been described elsewhere. The use of fast plastic scintillators as detectors, in the form of a 37-element hodoscope, and simple signal-handling electronics, has ensured that the high count-rate capability of the instrument is well matched to the high neutron flux offered by the position close to the plasma. This made it possible to reach count rates up to 611 kHz during the world record pulse with P_(fus)=16.1 MW (with a neutron rate of Y_n = 6·10~(18) n/s) in 1997. High count rate can be used for time resolved measurements and is a prerequisite to reach the high sensitivity needed to observe weak components in the neutron spectrum. Thus, high sensitivity is attained by a combination of high count rate, providing sufficient counting statistics, and efficient background suppression, i.e., a high signal to background (S:B) ratio. With the present MPR, it has been possible to perform detailed studies of auxiliary (ICRH or NB) heated plasmas and to make the first observation of the so-called Alpha Knock-on Neutron (AKN) emission, the latter being the imprint of the energetic alpha particles in the neutron spectrum. Additional information contained in the neutron emission can be accessed if the instrumental sensitivity is further increased.