The bootstrap current and its consequences

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Abstract
The physical mechanism behind the bootstrap current is explained, and the consequences are discussed with the emphasis on the two main objectives of fusion plasma, confinement and MHD stability. For a tokamak reactor that is optimized for good confinement and stability, and that has a limited size, the total plasma current exceeds the bootstrap current by a factor of three to five and therefore almost all the plasma current must be driven through other means. Furthermore, the neoclassical tearing mode which is driven by the bootstrap current is expected to be the limiting MHD instability in these reactors. Raising the fraction of the bootstrap current is not expected to be beneficial for confinement and stability expect when broad pressure profiles (internal transport barriers) can be realized. In stellarators several optimizations are possible, either optimizing the current to zero such that it does not destroy the desired topology, and it does not generate any current-driven instabilities, or using the current to generate some of the poloidal field.

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