The orbital dynamics and collisional transport of trace massive impurity ions in rotating tokamaks

Abstract

Massive impurity ions in rotating tokamak plasmas have mean toroidal velocities that can greatly exceed the thermal speed of those ions. The effects of this hypersonic rotation on particle orbits and collisional transport are explored in the trace limit by considering the role of centrifugal and Coriolis forces in a co-rotating frame. Impurity ions of sufficiently high mass are deeply trapped by a centrifugal potential well in the outer plasma midplane, with a bounce period that is shorter than both the bounce period of magnetically trapped ions and the collision time. The collisional diffusivity in this regime is shown to be higher than that of a non-rotating plasma. Irrespective of the collisionality regime, it is demonstrated that the interaction of massive impurity ions with bulk ions leads to an outward advection that is proportional to the impurity ion mass and can exceed the pinch velocity associated with the loop voltage. Due to modifications to the effective magnetic field arising from the Coriolis force, the increase in transport is greatest for relatively low charge states of massive impurity ions in plasmas rotating in the same direction as the plasma current. These effects are quantified analytically and using test-particle simulations of tungsten (W) and molybdenum (Mo) transport in transonically rotating spherical tokamak plasmas. It is shown that the collisional confinement time of W ions in such plasmas can be two orders of magnitude shorter than the confinement time in the absence of rotation.