Triton burnup measurements by neutron activation at JT-60U

Abstract

This paper describes measurements on triton burnup in a deuterium plasma by the detection of the 2.5 MeV neutrons (from DD fusion) and the 14 MeV neutrons (from DT fusion). The 2.5 MeV neutrons have been measured by fission chambers and activation of indium foils while the 14 MeV neutrons have been detected by activation of silicon, aluminum, and copper foils. The measured yields of the 2.5 MeV neutrons utilizing In foils are ∼20–40% higher than the yields obtained from fission chambers depending on what calibration factors are used. The deviation decreases with the plasma major radius (or increasing plasma volume). When the triton burnup is measured by utilizing neutron threshold reactions ( E n>2.5 MeV) and In foils, then systematic errors in the calibration factors cancel and the maximum deviation between the measured triton burnup for different calibration factors is reduced to ∼5%. The measurements indicate that triton burnup increases with the 14 MeV neutron yield, indicating that the relative yield of 14 MeV neutrons increases depending on the time duration of the deuterium neutral beam injection (NBI). Furthermore, the triton burnup decreases with an increased plasma major radius, indicating increased triton ripple losses, and increases with plasma current, indicating reduced banana orbit losses.