Experimental assessment of bulk plasma distribution in a multi-cusp device

The physics of plasma transport from the bulk plasma through the magnetic cusp to the anode remains poorly understood. A proper accounting of plasma losses to the anode is critical to accurate modeling of multicusp device performance. The objective of this work is to analyze plasma transport to the anode in multicusp discharge chambers and evaluate its impact on discharge performance. In this work, plasma transport in two magnetic circuit configurations of a 16-cm multicusp discharge chamber was studied. Each ring is covered with an electrically isolated electrode, which enables the direct measurement of current to each individual ring, as well as the discharge chamber wall. Ring electrode measurements coupled with spatially resolved plasma parameter measurements throughout the discharge chamber allow for an assessment of plasma losses to each ring in terms of an “effective loss area” which, multiplied by electron current density incident on the bulk/cusp boundary, gives the correct collected current to each ring. The effective loss area is compared with parameters such as cusp mirror ratio, collision frequency, and sheath potential. Spatially resolved maps of plasma density and temperature in the bulk region are be obtained and used to compare the location of the plasma centroid with current distribution at the cusps.