Low Pressure Operation of Radially Convergent Beam Fusion Using Differentially-Pumped Ion Sources

Radially convergent beam fusion (RCBF) has been studied for practical use as a portable neutron/proton source for various applications such as landmine detection and positron emission tomography. In a conventional RCBF device using a glow discharge, the neutron/proton production rate is proportional to the cathode current because beam-background reactions are dominant in contrast with the original RCBF concept. However, since the neutron/proton production rate of beam-beam reactions is proportional to the cathode current squared, beam-beam reactions have a potential to increase the neutron/proton production rate in a high cathode current region. In this study, a new RCBF system using differentially-pumped ion sources was designed for the low pressure operation without the glow discharge. In the RCBF chamber, a cylindrical grid cathode is concentrically placed on the axis of a cylindrical mesh anode, and two ion sources are oppositely mounted around the mesh anode. The ion sources allow the RCBF device to be operated at a pressure of 10^-4 Torr in the RCBF chamber, which is much lower than that of 10^-1 Torr in the ion sources. Generated ions in the ion sources are extracted through each orifice by the pressure gradient and the extraction electric field, and then accelerated to the RCBF cathode. At first, a performance as differential pumping system and discharge characteristics of ion sources were investigated. Then, the neutron production rate at a lower pressure compared with that of a conventional RCBF device was measured. Neutron production rate at a pressure of 0.30 mTorr was proportional to the ion current to the power of 1.19-1.23. This implies that the fraction of beam-beam reactions was increased by the reduction of background pressure in the RCBF chamber.