Calibration Experiments of He3 Neutron Detectors for Analyzing Neutron Emissivity in the Hot-Ion Mode on the GAMMA 10 Tandem Mirror

Under the international fusion cooperating research, He3 neutron detectors in the GAMMA 10 tandem mirror are calibrated by the use of a Cf252 spontaneous fission neutron source (8.96×104 n/s). The calibration experiments are carried out with a “rail system” placed along the magnetic axis of the GAMMA 10 central-cell region, where hot ions in the plasma experiments with the bulk temperatures of ∼10 keV are produced. As compared to a previous neutron monitoring system with a BF3 detector in GAMMA 10, the present He3 systems are designed with about two orders-of-magnitude higher neutron-counting efficiency for analyzing a neutron emissivity from the plasmas in a single plasma discharge alone. Two He3 systems are installed near the middle and the end of the central cell so as to identify the central-cell hot-ion axial profile. The filling pressure of He3, the effective length, and the diameter of the detector are designed as 5 bar, 300 mm, and 50 mm, respectively. The detector output spectra are carefully analyzed by the use of a preamplifier, a shaping amplifier, as well as a multichannel analyzer for each He3 detector. In the present article, the neutron-counting data from the two He3 detectors due to the on-axis Cf252 scan are interpreted in terms of the d−2 intensity dependence (d being the distance between the detector and the neutron source) as well as the effects of the central-cell magnetic coils and the other machine structural components.