Proton Detector Calibration in a Gridded Inertial Electrostatic Confinement Device

Recent work [S. Krupakar Murali, J. F. Santarius, and G. L. Kulcinski, Phys. Plasmas, 15, 122702, (2008)] indicates that fusion reactions in an inertial electrostatic confinement (IEC) device primarily occur in microchannels. Since microchannels form discretely all around the cathode, the proton calibration procedure necessitated the estimation of fusion reactivity within the microchannels. Unlike neutron detectors that see a point source, the proton detectors can follow the nonuniformities in the fusion source regime. Hence, the variation in microchannel distribution around the cathode has to be taken into account for independently calibrating a proton detector. Experiments were conducted to characterize the microchannels generated within an IEC device. A new calibration factor has been derived for specific grid geometry (5 latitudes and 12 longitudes) based on the experimental results. The new factor is 16% lower than the previously used values. Moreover, the wild variation in the proton data by as much as 50% between two measurements has been determined to be caused by the variation in grid orientation. It is suggested that for consistent proton measurements, the grid orientation with respect to the detector should be kept constant such that the least number of protons are detected by the proton detector. This not only prevents detector saturation, but also ensures that nonlinear effects in proton rate measurements are eliminated.