Shielding of External Magnetic Perturbations By Torque In Rotating Tokamak Plasmas

Abstract

The imposition of a nonaxisymmetric magnetic perturbation on a rotating tokamak plasma requires energy and toroidal torque. Fundamental electrodynamics implies that the torque is essentially limited and must be consistent with the external response of a plasma equilibrium f = j × B.Here magnetic measurements on National Spherical Torus eXperiment (NSTX) device are used to derive the energy and the torque, and these empirical evaluations are compared with theoretical calculations based on perturbed scalar pressure equilibria f = ∇p coupled with the theory of nonambipolar transport. The measurement and the theory are consistent within acceptable uncertainties, but can be largely inconsistent when the torque is comparable to the energy. This is expected since the currents associated with the torque are ignored in scalar pressure equilibria, but these currents tend to shield the perturbation.

Importance of plasma response to nonaxisymmetric perturbations in tokamaks

“…The large torque up to |α| ∼ |s| implies the phase shift of the plasma response [46], and then the perturbation can no longer tap the energy from the plasma. When |α| ∼ |s|, the shielding by currents associated with the torque has been theoretically expected [46][47][48], and also from recent NSTX observations based on the single (s, α) model [49]. This is important for n = 1 feedback control of Resistive Wall Mode in high β N plasmas.…”

Calculation of Neoclassical Toroidal Viscosity with a Particle Simulation in the Tokamak Magnetic Breaking Experiments

“…Note that the combined NTV calculation here is updated with the higher resolutions and thus improved integrations on both spatial and velocity space, and also with the flow offset for the damping rate, compared to Ref. [28]. The conclusion in Ref.…”

“…The poloidal rotation is neglected in the simulations due to the fast toroidal rotation in the selected discharges. For the NSTX discharges, NTV torque profiles were measured from toroidal rotation changes by the magnetic braking, as described in Ref [28]. In the discharge 124439, where κ = 2.3, I p = 0.8 MA, and B T = 0.45 T in the lower single null configuration, the toroidal rotation was observed to damp and relax to a different rotational equilibrium after n = 3 magnetic braking was fully applied by EF/RWM coils.…”

“…In the discharge 124439, where κ = 2.3, I p = 0.8 MA, and B T = 0.45 T in the lower single null configuration, the toroidal rotation was observed to damp and relax to a different rotational equilibrium after n = 3 magnetic braking was fully applied by EF/RWM coils. Particle transport was not changed even though the non-axisymmetric fields produce a strong momentum transport, and it was clearly indicated that the damping is purely driven by the n = 3 magnetic braking [28].…”

“…The rotational damping rate can be measured by calculating the rotation changes of the target discharge compared to the reference shot. A short time period after turning on EF/RWM coils is considered so that an exponential decay of the rotation can be linearized [28]. Then, the NTV is calculated from the damping rate using…”