Since the installation of an ITER-like wall, the JET programme has focused on the consolidation of ITER design choices and the preparation for ITER operation, with a specific emphasis given to the bulk tungsten melt experiment, which has been crucial for the final decision on the material choice for the day-one tungsten divertor in ITER. Integrated scenarios have been progressed with the re-establishment of long-pulse, high-confinement H-modes by optimizing the magnetic configuration and the use of ICRH to avoid tungsten impurity accumulation. Stationary discharges with detached divertor conditions and small edge localized modes have been demonstrated by nitrogen seeding. The differences in confinement and pedestal behaviour before and after the ITER-like wall installation have been better characterized towards the development of high fusion yield scenarios in DT. Post-mortem analyses of the plasma-facing components have confirmed the previously reported low fuel retention obtained by gas balance and shown that the pattern of deposition within the divertor has changed significantly with respect to the JET carbon wall campaigns due to the absence of thermally activated chemical erosion of beryllium in contrast to carbon. Transport to remote areas is almost absent and two orders of magnitude less material is found in the divertor.

A high resolution neutron spectrometer (HRNS) system has been designed as a neutron diagnostic tool for ITER. The HRNS is dedicated to measurements of time resolved neutron energy spectra for both deuterium and deuterium-tritium (DT) plasmas. The main function of the HRNS is to determine the fuel ion ratio n t /n d in the plasma core with 20% uncertainty and a time resolution of 100 ms for a range of ITER operating scenarios from 0.5 MW to 500 MW in fusion power. Moreover, neutron spectroscopy measurements should also be possible in the initial deuterium phase of ITER experiments. A supplementary function of the HRNS is to provide information on the fuel ion temperature. Furthermore, the HRNS can be used as an additional line-of-sight (LOS) for the radial neutron camera. To meet these requirements, a set of four spectrometers positioned after each other along a single LOS has been designed. The detector techniques employed include a thin foil proton recoil spectrometer (TPR), a neutron diamond detector (NDD), a back-scattering time-of-flight system (bToF) and a forward timeof-flight system (fToF). The TPR system, positioned closest to the plasma, provides data at high fusion powers. For plasma conditions producing intermediate fusion power two neutron spectrometers are installed: NDD and bToF. The NDD is installed as the second instrument along the HRNS LOS after the TPR. The fToF spectrometer is dedicated for low tritium densities and pure deuterium operation.The paper summarizes the current state of the art of neutron spectroscopy useful in plasma diagnostics and the possibility of installing a dedicated HRNS for ITER in the designated diagnostic port. We conclude that the proposed HRNS system can fulfil the ITER

The paper presents a method of solving the diffusion equation for the thermal neutron flux in a heterogeneous medium. Perturbation calculation is successfully applied for a cylindrical concentric system after testing this method for a spherical concentric geometry, analytically solved by Czubek (1981). The method makes possible the calculation of the thermal neutron decay constant and the space distribution of the thermal neutron flux in a two-region system. The condition of a constant value of the neutron flux in the inner part of the system must be met in this method. This method has an application in the measurement of the thermal neutron absorption cross-section (Czubek 1981).

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