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Description
NUMERICAL ANALYSIS OF NEUTRON IRRADIATION FIELDS
FOR A HIGH-EFFICIENCY NEUTRON SOURCE BASED ON DEUTERON CARA
K. ARAI, M. Cuvelier, T. Itagaki, J. Hasegawa (Institute of Science Tokyo, Japan), Y. Miyake, H. Okuno (Riken Nishina Center for Accelerator-Based Science, Japan)
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INTRODUCTION
In the development of fusion reactor materials, establishing high-intensity and high-energy neutron sources capable of simulating actual reactor environments is an urgent priority. As a promis-ing candidate, an accelerator-based neutron source utilizing the Cyclotron Auto-Resonance Accelera-tion (CARA) [1] principle has been proposed and is currently under development. CARA enables continuous acceleration within a remarkably compact cavity of approximately 1.8 m through syn-chronization between particle cyclotron motion and a rotating electric field. Historically, matching the cyclotron frequency with standard radio-frequency (RF) bands required magnetic fields exceeding 1 T, limiting its application to electron acceleration. However, with the technological advancements in high magnetic field generation following extensive research on MRI scanners, the application of CARA to light ions such as deuterons has become feasible. This deuteron CARA (dCARA) neutron source aims to generate a neutron flux with a peak at 10–20 MeV by accelerating a DC deuteron beam to 40 MeV and directing it onto a liquid lithium target to induce reactions such as the 7Li(d,n)8Be stripping reaction. The purpose of this study is to evaluate the performance and charac-teristics of the neutron irradiation field through numerical simulations toward the practical imple-mentation of this system. -
SIMULATION METHOD
In this study, a comprehensive computational model including the dCARA, beam transport system, liquid lithium target, and shielding is constructed in PHITS (Particle and Heavy Ion Transport code System) [2] to evaluate the irradiation field. A distinctive feature of the dCARA sys-tem is that the extracted deuteron beam possesses a gyration motion with a diameter of approximate-ly 10 cm. This motion is a characteristic resulting from the interaction between the static magnetic field and the radio-frequency electromagnetic field within the acceleration cavity, resulting in a spa-tial spread distinct from conventional linear beams. Since the angular and energy distributions of the generated neutrons are highly sensitive to the beam's incident conditions on the target, precise design of the transport is essential. Specifically, it is necessary to achieve both uniformity of the neutron flux and dispersion of the thermal load on the target by controlling the pitch angle and incident posi-tion distribution of the beam just before the target. The computational procedure involves first devis-ing the design of the deuteron beam transport system using Particle-in-Cell (PIC) simulations. In this step, the evolution of the beam's phase-space distribution is analyzed in detail, taking into account space-charge effects within the high magnetic field. Next, the obtained three-dimensional phase-space distribution is implemented as the source term in PHITS to calculate the spatial distribution and energy spectra of the neutron flux. These results are fed back into the design of the transport and shielding systems to investigate a more efficient neutron source configuration. -
CONCLUSION
This report describes a coupled simulation framework integrating PIC and PHITS to analyze the irradiation field characteristics of the dCARA neutron source. While the interaction be-tween a 40 MeV deuteron beam and a lithium target is effective for simulating D-T fusion environ-ments, the transport control of the gyrating beam remains a primary technical challenge. Currently, the impact of these complex beam dynamics on the incident angle and the neutron flux distribution is being evaluated using this simulation. The findings discussed in this report are expected to serve as crucial design guidelines for determining the optimal configuration of the dCARA system to meet the stringent requirements of fusion material testing, such as achieving high displacement-per-atom (dpa) rates in specimen volumes.
[1] C. Wang et al., Phys. Rev. E. 51(3), 2456 (1995).
[2] T. Sato et al., Recent improvements of the Particle and Heavy Ion Transport code System - PHITS version 3.33, J. Nucl. Sci. Technol, 61(1), 127-135 (2024).