Neutron emission from beryllium reactions in JET deuterium plasmas with 3He minority

Abstract

Recent fast ion studies at JET involve ion cyclotron resonance frequency (ICRF) heating tuned to minority 3He in cold deuterium plasmas, with beryllium evaporation in the vessel prior to the session. During the experiments, the high-resolution neutron spectrometer TOFOR was used to study the energy spectrum of emitted neutrons. Neutrons of energies up to 10 MeV, not consistent with the neutron energy spectrum expected from d(d,n)3He reactions, were observed. In this paper, we interpret these neutrons as a first-time observation of a 9Be(3He,n)11C neutron spectrum in a tokamak plasma, a conclusion based on a consistent analysis of experimental data and Monte Carlo simulations. 9Be(α,n)12C and 9Be(p,n)9B reactions are also simulated for p and α fusion products from d(3He,α)p reactions; these two-step processes are seen to contribute on a level of about 10% of the single-step process in 9Be(3He,n)11C. Contributions to the total neutron yield from the 9Be(3He,n)11C reaction are found to be in the range 13±3 to 57±5%. We demonstrate how TOFOR can be used to simultaneously (i) probe the deuterium distribution, providing reliable measurements of the bulk deuterium temperature, here in the range 3.2 ± 0.4 to 6.3 ± 1.0 keV and (ii) provide an estimate of the beryllium concentration (in the range 0.48±0.17 to 6.4±1.7% of ne assuming ). The observation of 9Be related neutrons is relevant in view of the upcoming installation of a beryllium-coated ITER-like wall on JET and for ITER itself. An important implication is possible neutron-induced activation of the ITER vessel during the low-activation phase with ICRF heating tuned to minority 3He in hydrogen plasmas.