The effect of finite β on stellarator transport

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Abstract
A theory of the modification of stellarator transport caused by the presence of finite plasma pressure is developed and applied to a range of stellarator configurations. For typical planar‐axis stellarators, transport can change by a factor of about 4–5 in the progression from zero pressure to the equilibrium β limit of the device. For transport‐optimized configurations, the factor can be over an order of magnitude. Thus, a stellarator with transport‐optimized vacuum fields can have poor confinement at the desired operating β. Without an external compensating field, increasing β tends to degrade confinement, unless the initial field structure is very carefully chosen. The theory permits correct determination of this vacuum structure, in terms of the desired structure of the field at a prescribed operating β. With a compensating external field, the deleterious effect of finite β on transport can be partially eliminated.