Hervé Dzitko

Beam Induced Damage Studies of the IFMIF/EVEDA 125 mA CW 9 MeV D⁺ Linear Accelerator

IFMIF (International Fusion Material Irradiation Facility) will be a Li(d,xn) neutron source providing equivalent neutron spectrum of DT fusion reactions and comparable neutron flux of future commercial reactors. Such a facility is an essential step in world fusion roadmaps to qualify suitable structural materials capable of holding the unrivalled neutron irradiation inside the nuclear vessel of a fusion reactor. IFMIF, presently in its EVEDA (Engineering Validation and Engineering Design Activities) phase, is installing LIPAc (Linear IFMIF Prototype Accelerator) in Rokkasho (Japan), a 125mA CW 9 MeV deuteron beam as validating prototype of IFMIF accelerators. The MPS of LIPAc manages the interlocks for a fast beam stop during anomalous beam losses or other hazardous situations. High speed processing is essential to achieve MPS goals driven by investment protection principles. Beam losses may lead to severe damages by excessive thermal stresses, annealing or even burn/melting of materials. The assumptions to estimate the practical safe times for a fast beam shutdown during the accelerator operational life are here described. IFMIF AND LIPAC, ITS ACCELERATOR PROTOTYPE Fusion materials research has fuelled for decades the world endeavours towards high current linacs [1]. The required neutron flux >10 m·s with a broad peak at 14 MeV to simulate the irradiation conditions of the plasma facing components in a fusion reactor is obtainable through Li(d,xn) stripping reactions; however those fluxes demand deuteron currents in the 10 mA range in CW mode. The first world attempt of such conditions was framed by the Fusion Materials Irradiation Test Facility, FMIT, in the early 80s; with unexpected difficulties and lessons learnt in operating in CW mode [2]. The International Fusion Materials Irradiation Facility, IFMIF, consists of two deuteron accelerators at 125 mA in CW and 40 MeV impacting on a flowing lithium screen. It is since 2007, in its Engineering Validation and Engineering Design Activity phase, EVEDA, where the only remaining activity of its broad mandate (that has provided an engineering design [3] of the plant and, among many other technical challenges, validated the stable operation of its lithium loop [4] and its irradiation modules capable of housing above 1000 specimens and characterize structural materials simultaneously in twelve different irradiation capsules independently cooled [5]) is its Linear IFMIF Prototype Accelerator, LIPAc, presently under installation and commissioning in the International Fusion Energy Research Center (IFERC) in Rokkasho (Japan), by European and Japanese laboratories [6]. A full account of the validation activities under IFMIF/EVEDA has already been provided [7]. Collective phenomena driven by space charge forces become the main limitation on achieving high intensity beams. In low β regions, the beam outward radial Coulomb forces prevail over the inward radial Ampere ones, but they mutually cancel in the relativistic domain. Thus, space charge repulsive forces are stronger the lower the beam energy is. The successful operation of LIPAc, with its deuteron beam current of 125 mA in CW at 9 MeV as the output of the first planned cryomodule of IFMIF will validate the 40 MeV required for the Li(d,xn) source [1,8]. Figure 1: Above Comparison of IFMIF accelerators and LIPAc, their 1.125 MW beam average power prototype accelerator, matched up to the 1st SRF linac at 9 MeV. Below breakdown of the contribution for LIPAc. IONS INTERACTION WITH MATTER The physics of heavy ions with matter was first unravelled semi-classically by Bohr in 1913 based on his atomistic model (making use of the impact parameter between target nuclei and impacting particle) [9], and through relativistic quantum mechanics by Bethe in 1932 [10] (making use of the momentum transfer by the particle to the cloud of electrons) being both expressions for the stopping power − of the absorber identical for nonrelativistic ions ( ≪ 1), and with the ion only dependent variables its kinetic energy and charge. Among the different possible types of radiation, only ions show a fixed range; a mono-energetic beam of ions traversing matter loses its energy without any change in the number of particles, and eventually all are stopped reaching practically the same depth. The combination of the logarithmical dependence of the stopping power on the ions speed and their slowing Proceedings of IPAC2016, Busan, Korea WEPMR044 07 Accelerator Technology T33 Subsystems, Technology and Components, Other ISBN 978-3-95450-147-2 2373 C op yr ig ht