Effect of Self-Magnetic Forces on the Anode Mechanism of a High Current Discharge

When a dc discharge is operated between a central cathode and a ring-shaped anode at currents up to 6 × 103 A and pressures on the order of a few torrs, the stable discharge regime is found to be limited. The anode fail voltage is determined experimentally from calorimetric and potential measurements. In a wide range of operating conditions its variation correlates well with the parameter J2/¿ where J = current, ¿ = mass flow rate through the discharge. With increasing values of J2/¿, the anode fall increases continuously from small negative values, through zero, and up to moderate positive values. Then, within a small increment of J2/¿, it jumps to values on the order of the ionization potential of the gas. A theoretical two-dimensional description of the flow field indicates that the self-magnetic forces, which are proportional to J2, pinch the gas towards the discharge axis, thereby leading to particle starvation in the vicinity of the anode. Based upon the results of this study it is shown that, as a consequence of the altering boundary conditions, which are characterized by the parameter J2/¿ the anode responds by adopting different operating modes. Existing theories yield values for the anode fall which are in good agreement with experimental data.