Studies of Achieving Buildup Against Charge-Exchange Losses in a Mirror-Confined Plasma

Mirror plasma buildup via neutral beam injection into a small low-density target is strongly affected by plasma losses resulting from charge-exchange (CX) with cold neutrals entering via chamber structures. The influence of CX events extends beyond the collision site due to the large ion orbits typical of small mirror plasmas. This study examines effects of key parameters that influence plasma buildup, using a 2½-dimensional, energy-dependent finite gyroradius model. Results presented for a 2X-like plasma show that buildup occurs if a “critical density” is achieved before CX losses erode the central plasma region. An efficient way to attain this density is to position the injected beams so that trapped ions have orbits circling inward, toward the plasma center.