Modelling γ-spectrum to underpin plasma temperature monitoring in fusion facilities
Measuring temperatures between 100 and 150 million degrees, expected to be achieved in future fusion reactors, is a great challenge. Existing methods deliver ion temperature results with a delay from 5-10 minutes to several days after taking the measurements. A promising novel approach to real-time monitoring of deuterium-tritium plasma temperature could be based on detecting γ-rays coming from the d + t → α + n + γ reaction which occurs approximately one in ten to hundred thousand d + t → α + n fusion events. The new approach requires detailed knowledge of the γ-emission spectrum, especially in the region of 17 MeV where a relatively narrow peak from the lowenergy α-n resonance should be most prominent. We report the first model calculations of the dtγ-spectrum assuming that the main mechanism of its formation is bremsstrahlung from an intermediate α − n state. We show that, as a consequence of the bremsstrahlung, the γ-spectrum contains nonzero contributions at all energies thus making inclusive-measurement-based temperature monitoring sensitive to the low-energy cutoff of the detected γ-events. This sensitivity has not been discussed before. Comparison of our predictions to existing d + t → α + n + γ measurements in accelerators, employing cutoffs of 13 and 14 MeV, and inertial confinement fusion facilities, with the low-limit cutoff of 0.4 to 6.3 MeV, reveals previously unnoticed contradiction between results from these two types of experiments. Our predictions favour accelerator measurements and corroborate cutoff dependence observed in inertial confinement experiments. These predictions are sensitive to the wave function details inside the short-range area of the α − n interaction, with uncertainty comparable to that of available experimental data, but become model-independent below 4-5 MeV. This part of the γ-spectrum features previously unexpected rise, which further down below 0.5 MeV surpasses the main 17 MeV γ-peak in strength. The reactivity of the d + t → α + n + γ reaction was found to be proportional to its cross section. It strongly depends on reactor temperature, which makes it suitable for deuterium-tritium plasma temperature diagnostics.