Among various approaches proposed in the field of temperature diagnostics of hot plasmas, diagnostics based on detection of reaction products are of particular importance. Most appropriate diagnostic reactions seem to be those generating neutron or γ-ray. Such neutrons have already been used for plasma diagnostics. Alternatively, detection of γ-rays may provide another possibility of ion temperature measurements.
Previously, we proposed to use ⁶Li isotopes in D-T fusion plasmas to induce diagnostic γ-ray modes of nuclear reactions[1]. We considered the following nuclear reactions between ⁶Li and fuel ions:
⁶Li + d → ⁷Be^*[0.429MeV] + n + 2.95MeV ...(1)
⁶Li + d → ⁷Li^*[0.478MeV] + p + 4.55MeV ...(2)
⁶Li + t → ⁷Li^*[0.478MeV] + d + 0.51MeV ...(3)
⁶Li + t → ⁸Li^*[0.981MeV] + p - 0.18MeV ...(4)
Excited nuclei ^7,8Li^* and ⁷Be^* generate diagnostic γ-rays which have energies in above brackets in their decay to ground states. The yield ratios Y(0.478)/Y(0.429), Y(0.981)/Y(0.471) and Y(0.981)/Y(0.429) show strong ion temperature dependence. It was hence concluded that comparative measurements of the respective γ-ray yields may provide a possibility of ion temperature diagnostics. However, there has remained a drawback that the yield ratios depend on an unknown parameter, the fuel ion density ratio n_T/n_D.
The present work is a further improvement of the diagnostic method[1]. We have attempted to resolve the drawback by deriving a specific density-independent combination C of the yield ratios. It is found that a simultaneous detection of the γ-lines may provide a possibility of absolute temperature measurements which are not affected by other plasma parameters. An important point is sensitivity of the diagnostics, which is tightly interrelated with temperature dependence of the γ-ray combination C. Thus, the behavior of C(T_i) should be analyzed in a proper kinetic model taking into account the influence of distortion of ion velocity distribution functions. Such distortion is mainly caused by large angle scattering of D-T fusion α-particles by the fuel ions. We have calculated realistic distribution functions of D and T as numerical solution of the kinetic model equation. Then, the value of C has been derived from the calculated distribution functions.

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