Reduction of the neutron induced noise in the compact neutral particle analyzer for LHD deuterium plasma experiments

The compact neutral particle analyzer (CNPA) combined with the impurity pellet measurement is one of the few instruments that can directly measure the radial high energy particle distribution in the Large Helical Device (LHD). For this purpose, it is suitable to set CNPA near LHD. On the other hand, sufficient and heavy shielding against DD-neutrons, generated in deuterium experiment, is required, especially when the deuterium neutral particle beam injection heating is applied. The shield is insufficient due to the weight limitation on the stage. However, if the neutron energy is thermalized on the detector, the neutron noise can be estimated only from the total neutron yield, which is monitored by the ²³⁵U fission chamber, etc. In the experiments, the pure neutron noise on the CNPA has been measured by closing the gate-valve to avoid the charge exchange neutral particle signals. The neutron noise on the CNPA has been proportional to the total neutron yield over three-order magnitudes. This means that the neutron noise can be estimated from the total neutron yield. Therefore, the calibrated charge exchange neutral signal can be obtained simply by subtracting the estimated neutron noise from the measured signal in ordinary experiments. (author)