Nonlinear Transition from Mitigation to Suppression of the Edge Localized Mode with Resonant Magnetic Perturbations in the EAST Tokamak

Abstract

Evidence of a nonlinear transition from mitigation to suppression of the edge localized mode (ELM) by using resonant magnetic perturbations (RMPs) in the EAST tokamak is presented. This is the first demonstration of ELM suppression with RMPs in slowly rotating plasmas with dominant radio-frequency wave heating. Changes of edge magnetic topology after the transition are indicated by a gradual phase shift in the plasma response field from a linear magneto hydro dynamics modeling result to a vacuum one and a sudden increase of three-dimensional particle flux to the divertor. The transition threshold depends on the spectrum of RMPs and plasma rotation as well as perturbation amplitude. This means that edge topological changes resulting from nonlinear plasma response plays a key role in the suppression of ELM with RMPs. DOI: 10.1103/PhysRevLett.117.115001 Magnetic reconnection and the resultant topological change play an important role in plasma dynamics in both laboratory and space plasma physics research. The formation of an edge stochastic magnetic field caused by resonant magnetic perturbations (RMPs) is believed to be the reason for the suppression of periodic crash events near the plasma edge known as the edge localized mode (ELM) observed in the DIII-D tokamak [1]. The ELM causes transient heat loads to the plasma facing components and may degrade them on the next generation fusion device like ITER [2]. The reduction of free energy in the edge pressure gradient and current because of field stochasticity moves the plasma into a stable regime against the ELM [3]. This successful experiment motivated ELM control using RMPs in many other tokamaks [4][5][6][7]. However, the plasma response effect usually shields the external applied RMPs and may significantly reduce the magnetic field stochasticity [8][9][10][11], which makes this mechanism questionable. Different from topological change, the linear peelinglike magneto hydro dynamics (MHD) response has been found to play an important role in ELM control [12][13][14]. Nonlinear plasma response has been observed in the JET totamak [15]. The possible formation of a magnetic island near the plasma edge [16] with a toroidal Fourier mode number n ¼ 1 during ELM suppression by using n ¼ 2 RMP has been recently observed on DIII-D [17]. However, the key difference between ELM suppression and mitigation and the different roles of linear and nonlinear plasma response on ELM suppression are still not clear.In this Letter, we report the first observation of full ELM suppression using low n RMPs in slowly rotating plasmas with dominant radio-frequency (rf) wave heating, which is potentially important for the application of this method for a future fusion device. This is the first observation of full ELM suppression using RMPs in the medium plasma collisionality regime in EAST, and it expands beyond the previous observations of ELM suppression on DIII-D [1,3] and KSTAR [7]. It is found that not only the formation of a magnetic island near the edge [17] but also a critical leve...

ELM control with RMP: plasma response models and the role of edge peeling response

“…7, where an n=2 RMP field, with time-varying coil phasing, is applied to a high q 95 plasma (q 95 =5. Recent modelling results [13] for the ELM control experiments in EAST [7] also confirm the correlation between the edge peeling response and the observed ELM mitigation/suppression. We show one example in Fig.…”

“…Among several ELM control techniques, the resonant magnetic perturbation (RMP), produced by magnetic coils outside the plasma, appears to be a robust one. Several tokamak devices have experimented with this technique to control type-I ELMs, including DIII-D [2], JET [3], MAST [4], ASDEX Upgrade [5], KSTAR [6], and recently EAST [7]. Either ELM mitigation (increasing the ELM frequency accompanied by reduction of the individual ELM amplitude) or ELM suppression (complete removal of type-I ELMs) has been achieved on these devices, using the RMP fields.…”

Multi-diagnostic analysis of plasma filaments in the island divertor

“…1 This method was first demonstrated in DIII-D 2 and thereafter observed on several tokamaks worldwide. [3][4][5][6][7][8][9] Various models have been proposed to explain the causes of ELM suppression, [10][11][12][13] however important characteristics of ELM suppression remain unexplained. One characteristic of ELM suppression in DIII-D low collisionality (ν * e <0.5) ITER-similar-shape (ISS) plasmas is the low pedestal density n e,ped ≈ 2-3×10 19 m -3 required for access to ELM suppression.…”

Filamentary plasma eruptions and their control on the route to fusion energy

“…ELM suppression was first achieved on the DIII-D tokamak 73 and then on other tokamaks, including ASDEX Upgrade 74 ( fig. 5), KSTAR 75 and the Experimental Advanced Super conducting Tokamak (EAST) 76 . It is believed that resonant magnetic perturbations (RMPs) are able to suppress type I ELMs at ITER-relevant 'low collisionality' (which corresponds to a low ratio of density to the a b square of temperature) because the pressure gradient in the edge is held below the peeling-ballooning stability limit.…”