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Abstract

One of the important problems in designing the future fusion reactors is the assessment of the effect of fusion neutrons on the reactor components. The intense flux of 14 MeV D+T fusion neutrons may result in a displacement of atoms in atomic lattices, causing material degradation. It may also initiate nuclear reactions which result in substantial material activation of trace admixtures, or material transmutation that may alter the construction material properties. Fusion neutron effects are also central to any tritium breeding schemes. This problem is common - with some small differences - both to the MCF and ICF approaches to fusion energy. In order to study the fusion neutron-induced phenomena dedicated facilities are presently being built, based on conventional accelerators.

However, in recent years there was a great progress in the development of laser-driven sources of fast (multi-MeV) neutrons, so one may ask whether some of the neutron irradiation studies could be done at laser facilities, perhaps more conveniently and at a lower cost. This was the motivation of our study. The simplest scheme for the laser production of neutrons involves acceleration of protons, which are then used to irradiate a converter, in which protons undergo nuclear reactions that generate the neutron output. Since we are interested in near-14 MeV neutrons, the protons do not have to be very energetic, so in principle we are not interested in maximizing the laser intensity on target. However, a high repetition rate is of crucial importance for the high neutron flux. For these reasons in a first step we had chosen to study the neutron production capabilities of a prospective high average power laser at ELI-Beamlines that would deliver 3 J/25 fs high-contrast pulses with the repetition rate of at least 50 Hz. We performed particle-in-cell simulations to estimate the proton parameters and then used FLUKA Monte Carlo code to estimate neutron generation off a Be converter. To ensure high proton cutoff energy and high proton count we used composite targets consisting of few micrometer thick carbon foam layer attached to a metal foil. Our simulations indicate the possibility of ~109 neutron multiplicity in a single shot, with the neutron spectrum extending well beyond 14 MeV. This implies the potential for 5×1010 n/s source intensity if the laser is operated with the maximum repetition rate. Such a laser neutron source could have many other interesting applications besides fusion material studies.