Development of a Permanent-magnet Microwave Ion Source for a Sealed-tube Neutron Generator

A microwave ion source has been designed and constructed for use with a sealed-tube, high-yield neutron generator. When operated with a tritium-deuterium gas mixture the generator will be capable of producing 5 · 10 n/s in non-proliferation applications. Microwave ion sources are well suited for such a device because they can produce high extracted beam currents with a high atomic fraction at low gas pressures of 0.2 − 0.3Pa required for sealed tube operation. The magnetic field strength for achieving electron cyclotron resonance (ECR) condition, 87.5mT at 2.45GHz microwave frequency, was generated and shaped with permanent magnets surrounding the plasma chamber and a ferromagnetic plasma electrode. This approach resulted in a compact ion source that matches the neutron generator requirements. The needed proton-equivalent extracted beam current density of 40mA/cm was obtained at moderate microwave power levels of ∼ 400W. Results on magnetic field design, pressure dependency and atomic fraction measured for different wall materials are presented.