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Plasma current is used to ohmically heat fusion plasmas and provide stabilization of grad-B drift in tokamaks as the resulting poloidal motion of the charged particles results in the particles remaining in favourable curvature region for the majority of their path in the tokamak. Equilibrium field coils are additionally used for shaping/stability, and the current running through these is dependent on the direction of plasma current. But I am simply wondering what reason(s), whether engineering- or physics-based, initially determines which direction (i.e. clockwise or counterclockwise) a plasma current should be directed in a tokamak?

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  • $\begingroup$ For a tokamak that is not perfectly up-down symmetric the direction of toroidal magnetic field is important because it determines the direction of the curvature and gradB drift, up or down, and this has significant effects in the divertor. On the other hand, the poloidal field (and plasma current) cannot be reverted without doing so for the toroidal field as well because the divertor tiles design is optimized for a specific direction of impinging fluxes of plasma particles and energy. $\endgroup$ – Maxim Umansky Aug 5 '18 at 6:44

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