During the slowing-down process, fast ions injected into a tokamak plasma may pitch-angle-scatter into orbits whose deviations from flux surfaces are of order a, the limiter radius. Ions on such orbits may hit the wall or limiter or be charge-exchanged out of the system and are thus in the “loss region”. The boundary of this region is calculated and its effect on plasma heating by neutral-beam injection is evaluated by using the Fokker-Planck equation. In present injection experiments, the loss region may have a significant effect if the impurity level of the plasma is high. Counter-injected ions are more strongly affected by the loss region than are co-injected ions since they have to scatter through a smaller pitch angle to reach it.
Physics issues are discussed for compact stellarator configurations which achieve good confinement by the fact that the magnetic field modulus |B| in magnetic co-ordinates is dominated by poloidally symmetric components. Two distinct configuration types are considered: (1) those which achieve their drift optimization and rotational transform at low β and low bootstrap current by appropriate plasma shaping; and (2) those which have a greater reliance on plasma β and bootstrap currents for supplying the transform and obtaining quasi-poloidal symmetry. Stability analysis of the latter group of devices against ballooning, kink and vertical displacement modes has indicated that stable β values on the order of 15% are possible. The first class of devices is being considered for a low β near term experiment that could explore some of the confinement features of the high β configurations.
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