Role of the anode area in the behavior of magnetic multipole discharges

A theoretical model and experimental measurements are presented for a hydrogen discharge in a magnetic multipole (bucket) plasma source where the plasma potential is negative with respect to the anode. This plasma potential depends critically on the anode cusp field, and the measurements show clearly that electrostatic ion confinement is obtained when the anode field is strong (1400 G), which leads to a high electrical efficiency for useful ion production. However, when the anode field is reduced, the plasma potential reverts to its normal positive value with respect to the anode with a consequent reduction in electrical efficiency.