Neutron emission profile and neutron spectrum measurements at JET: Status and Plans

In large tokamak devices, such as JET, TFTR, JT-60U and ITER, a neutron emission profile monitor is considered as one of the principal diagnostic tools, firstly in providing information to characterize the fusion source in terms of its location, shape, intensity and secondly in developing an understanding of fast ion physics in tokamak plasmas. The JET neutron profile monitor is one of the most advanced such instruments currently operating. It enables 2-D imaging of the non-circular plasma neutron source through a tomographic deconvolution of line-integrated measurements. Historically it was usually assumed that the fusion neutron source is a function of the magnetic flux surfaces (MFS), but recent JET results have clearly demonstrated the influence of non-uniform fast particle populations on the neutron emission profile. Neutron energy spectroscopy also carries valuable information about fast ion behaviour and the fusion and nuclear reactions taking place in the plasma. Spectrum measurements of neutrons will be particularly important for the up-coming JET experiments with a Be wall. The neutron spectroscopy capabilities at JET were recently enhanced by several instruments: a new time-of-flight spectrometer optimized for high count rates (TOFOR), compact neutron spectrometers based on organic scintillator (NE213) and an upgrade of the Magnetic Proton Recoil system (MPRu). In recent JET campaigns, a prototype digital pulse shape discriminator (DPSD) has been successfully tested on one central chord of the JET neutron profile monitor and spectral information has also been obtained. Neutron spectrometry results obtained during the last JET experimental campaigns illustrate the measurement capabilities and indicate their potential use on ITER. Several projects are in progress to further develop such diagnostic capabilities and these are briefly summarized.