Fusion gamma-ray measurements for D-³He experiments at JT-60U

Fusion gamma rays were measured in D-³He experiments using negative ion-based neutral beam injection (N-NBI) in reverse shear plasmas of the JT-60 tokamak. ³He gas was puffed at plasma initiation and just before N-NB injection. The D-³He reaction produces 3.6 MeV alphas and 14.7 MeV protons, but there is also a small branch which provides ⁵Li and 16.7 MeV gamma rays. The total D-³He reaction rate can be evaluated from measurement of gamma rays of the ³He (d, γ) ⁵Li reactions using a 3 in. diam by 3 in. long Bi₄Ge₃O₁₂ scintillator. The gamma-ray detector was located 17 m below the plasma center and measured the gamma-rays in a vertical line of sight. The detector was mounted inside a heavy collimator with polyethylene and lead shielding. The floor penetration, a 4 × 8 cm² hole, was used as a precollimator. Energy calibration of the detector was done with photopeaks for neutron capture gamma rays from the structural materials in D-D discharges. The detection efficiency was calculated with Monte Carlo code MCNP-4B for 16.7 MeV gammas. The pulse height analysis of the gamma rays resulted in the D-³He fusion power of 110±30kW in this experiment.