ANOMALOUS PINCH FLUX IN TOKAMAKS DRIVEN BY THE LONGITUDINAL ADIABATICINVARIANT

The turbulent particle flux in tokamaks has an inward component, the p inch flux, which is independent of the density gradient. It is propose d that this flux represents a tendency of the particle distribution to approach turbulent equipartition, which means that the phase space de nsity is constant on hypersurfaces defined by those invariants that ar e not destroyed by the turbulence. For tokamaks the two first adiabati c invariants give the peaked density profile n similar to l/q of trapp ed particles. Sharp gradients are predicted near the separatrix of div ertor plasmas. The physical mechanism of the pinch is as follows. When a parcel of trapped particles is displaced inward by the turbulent fl uctuations, their parallel velocity must increase in order to keep the longitudinal invariant J constant. This is equivalent to adiabatic co mpression, and increases the density. The turbulence is assumed to be caused by thermally driven electrostatic modes. (C) 1995 American Inst itute of Physics.