Lithium-fed arc multichannel and single-channel hollow cathode: Experiment and theory

Lithium-fed arc multichannel hollow cathode (MCHC) and single-channel hollow cathode (SCHC) physical processes, including the current conduction mechanism and the conditions that determine the plasma penetration depth, cathode temperature, and cathode voltage drop, are investigated experimentally and theoretically. The ability of the MCHC to conduct high current was previously not understood. Knowledge of the SCHC and MCHC physical processes is required for informed cathode design, especially for that of the lithium Lorentz force accelerator (LiLFA).; Experiments were conducted to measure the plasma penetration depth, cathode temperature, and plasma potential at the cathode tip of four lithium-fed SCHCs (with inner diameters of 2, 4, 6, 8 mm) and an MCHC (19 - 1 mm channels in a 10 mm diameter rod) at mass flow rates of 0.2--4.0 mg/s and currents of 5--210 A. It was found that the plasma penetration depth decreases with mass flow rate, increases with channel diameter, and increases with current. The peak cathode temperature was found to depend on current and channel diameter, but not mass flow rate. From the new findings, it was determined that (1) the arc penetrates to a location of optimum plasma density that depends on current and channel diameter, (2) the arc attaches to the cathode in an annulus of width equal to three times the wall thickness, and (3) the plasma density can be determined from the cathode temperature. The theoretical models predict the cathode voltage drop, cathode temperature, and plasma penetration depth as a function of mass flow rate, cathode material type, cathode geometry, and current. The MCHC model also includes the differences of the multichannel configuration---the thermal interaction of adjacent channels and the division of current and mass flow. The models are validated by the experimental data. They lead to the physical insight that thermal radiation and thermionic cooling are the most significant power loss mechanisms, and that the MCHC operates at a lower voltage than the SCHC because of lower thermal radiation losses due to less exposed surface area, and a reduced temperature due to a larger arc attachment area. Finally, a procedure for the design of an MCHC is presented, which is applied to the cathode of an LiLFA.