Alpha particle velocity- and orbit-space sensitivity of gamma-ray spectroscopy diagnostics based on the 10 B ( α , p γ ) 13 C reaction

A key challenge on the path towards burning plasmas is the measurement of the alpha particle phase space. Among the few experimental options available, gamma-ray spectroscopy (GRS) is a key method. Previous demonstrations at Joint European Torus were based on reactions between alpha particles and beryllium as the target impurity, but this is no longer applicable, neither at ITER with a tungsten-based first wall, nor in other forthcoming burning plasma experiments. On the other hand, owing to the use of boron injection for plasma operations in tungsten devices, gamma-ray reactions between the alpha particles and boron impurities have been proposed as an alternative. This work presents the first numerical evaluation of the velocity-space and orbit-space sensitivity of boron-based GRS to the alpha particle phase space, focusing on the 10 B ( α , p γ ) 13 C reaction. Depending on the emission peak, we find that measurements can be separately sensitive to selected super- and sub-Alfvenic velocities, with an enhanced sensitivity towards particles on trapped and potato orbits, for the case of radial lines of sight. Information on co- and counter-going alphas may be accessed only by more refined spectral shape analysis or if a tangential line of sight is deployed. The results are relevant to establish a bridge between the indirect GRS measurements and the alpha particle phase space. They furthermore provide a quantitative input to determine which wave-alpha particle resonant interactions may (or may not) be measurable with boron based GRS, depending on the observation geometry.