Free energy minimization approach to penetration of resonant magnetic perturbations in tokamaks

Abstract

By applying the principle of minimum free energy an analytical model for the plasma response to externally applied resonant magnetic perturbations ͑RMPs͒ is proposed. The results are compared with ATTEMPT code calculations ͓D. Reiser et al., Phys. Plasmas 16, 0042317 ͑2009͔͒ and reproduce qualitatively and quantitatively the numerical results on the collisionality dependence of RMP penetration characteristics. Strong increase in the radial electric field with reduced screening at RMPs above a certain threshold is also reproduced by the model.

Local measurements of screening currents driven by applied RMPs on TEXTOR

“…As I DED is increased further, there is an increase in the amplitude of the plasma response deeper inside the plasma followed by complete penetration of the RMP field. This initially gradual change followed by a very sudden field penetration is consistent with predictions from modelling [19]. After RMP field penetration, the fluctuating part of the magnetic field has a significantly greater amplitude and a ∼180 • phase difference compared to the vacuum case.…”

“…This very sudden field penetration may seem like a bifurcation. In fact, a bifurcation in RMP screening is predicted in [19] but only for low plasma resistivity, whereas the edge of a TEXTOR plasma is highly resistive. However, the prediction for high plasma resistivity is for an initial gradual reduction in screening with increasing RMP strength followed by a sudden drop, which is also consistent with the results presented here.…”

“…This modelling was extended in [19] using an analytical free energy minimization approach, and the effect of the strength of the RMP field on the screening was investigated. In section 4, at least for the −1 kHz case, the RMP field penetration is shown to occur very suddenly with a small increase in DED current.…”

Magnetic field topology modeling under resonant magnetic perturbations on EAST

“…While a possible plasma response field and the related change in the magnetic field topology also should be evaluated with respect to the weak mitigation case when RMP is shielded. The screening of RMP results from a complex nonlinear interrelation of plasma current, RMP field strength and, in particular, plasma rotation [17].…”

“…From the viewpoints of theoretical works, the interaction between the RMP fields and plasma rotation is a key point of plasma response [16]. Plasma response resulted from plasma rotation will induce the current on rational surfaces which may shield out the RMP fields [17]. To date, some models have been developed for understanding of the three-dimensional (3D) magnetic topology more clearly with plasma response under RMP.…”