Omnigeneous Toroidal Plasma Equilibria

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A simple condition is derived for omnigeneous toroidal plasma equilibria, which means that in a collisionless plasma the turning points of a trapped particle remain on the same magnetic surface. Omnigeneity is important for it assures that collisionless particle trajectories are consistent with achieving ignition in toroidal fusion systems. When the magnetic field strength depends on only one angular coordinate in Boozer coordinates, the magnetic field is quasi-symmetric, and drift trajectories are confined by a conserved canonical momentum. It is shown that a magnetic field is omnigeneous when it obeys the single-angle constraint at extrema of the field strength. Elsewhere it can be far from quasi-symmetric, but must obey a symmetry in a function  about field strength minima. When the field strength depends only on the poloidal angle near extrema, it is called quasi-poloidally symmetric. For this case, it is shown that bootstrap current need not be zero and the sign of the electric potential is more obscure than generally assumed.

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