Alpha particle velocity- and orbit-space sensitivity of gamma-ray spectroscopy diagnostics based on the ¹⁰ B(α,pγ)¹³C 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 is a key method. Previous demonstrations at JET were based on reactions between alpha particles and beryllium as the target impurity, but this is no longer applicable, neither at ITER with a tungsten-based first wall, nor in other forthcoming burning plasma experiments.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 gamma-ray spectroscopy to the alpha particle phase space, focusing on the 10 B(α,pγ) 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 gamma-ray spectroscopy 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 gamma-ray spectroscopy, depending on the observation geometry.