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Halo Currents and Their Rotation

Author: Allen H Boozer
Requested Type: Consider for Invited
Submitted: 2015-01-18 19:18:23

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Columbia University
500 W. 120th St
New York, New York   10027
USA

Abstract Text:
Halo currents flow for part of their path along the open magnetic field lines near the plasma edge and for part through the surrounding conducting structures. Halo currents act much as a resistive wall to slow the evolution of the plasma shape to the resistive time scale of the halo current τh and occur whenever the plasma would otherwise evolve more rapidly. Strong halo currents arise in tokamaks when axisymmetric equilibrium is lost. As the plasma pushes into the wall, which has a resistive time scale τw, the edge q drops when the resistive decay time of the plasma current τp satisfies τp>τw. When q~2, a strongly unstable n=1 kink occurs, which in the absence of a halo current would have an Alfvénic growth. A halo current arises in proportion to the amplitude Δk of the kink and slows its growth to τh.
The evolution of the halo current is dominated by two properties when flowing through the edge plasma. (1) The edge plasma cannot balance a significant jxB force, so j is aligned with B with j/B constant along each field line. Each tube of magnetic flux δψ acts as a wire carrying a current (j/B)δψ. (2) The halo current channel has a minimum radial width Δh, which must be comparable to the amplitude Δk of the n=1 kink. Geometry implies the halo current can only flow between the plasma and the wall in a region of toroidal angular extent δφ ~ sqrt(2Δh/Δk). When δφ<<1, the toroidal coupling is so strong that the energy released by the unstable n=1 kink is not sufficient to drive the coupled n≠1 kinks, which are stable.
Toroidally rotating halo currents can be particularly destructive. Rotation arises when the energy released by the n=1 kink depends on the toroidal frequency ω of the kink. The torque required to change the kink rotation from the frequency that maximizes the energy release is related to the ω dependence of this release by the Kramers-Kronig relations, which are implied by causality.
Supported by DoE-OFES award De-FG02-03ER54696.

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