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...

Here we report the experimental analysis on the low-n (mostly n  =  1, sometimes n  =  2) magnetic coherent mode (MCM) at a characteristic frequency 20–, which has been frequently observed in various H-mode discharges on EAST. This mode can be easily identified in the magnetic fluctuations measured by the fast Mirnov coils mounted on the vacuum vessel wall, but is detected by the local measurements of edge electrostatic fluctuations only when the mode is sufficiently strong. The apperance of the MCM is summarized covering broad ranges of discharge parameters, in particular, the different heating schemes including pure neutral beams injected in either co- or counter-current direction as well as pure ratio-frequency waves. This may rule out the possibility of fast particle driven modes. Radial distribution and poloidal propagation of the MCM are investigated using the Doppler backscattering system and Langmuir probes inserted at the outer midplane, respectively. Temporal evolution of MCM amplitude during large ELM crashes is evaluated in detail, may suggesting the mode is closely correlated with pedestal buildup. Dedicated experiments reveal the possible correlations of MCM’s frequencies with edge line-averaged density and edge safety factor q95. We also present the observation of multi MCMs at relatively high q95, which are speculated locating at different rational surfaces in the pedestal via analyzing their mode structures and nonlinear interactions. Finally, effect of the MCM on edge particle transport is explored via surveying the correlation between the intermittent events of the mode and the particle fluxes deposited on the divertor target plates, utilizing the conditional analysis method. Corresponding results suggest that the MCM seems to primarily result in a notable poloidal redistribution of the divertor particle flux, rather than a considerable net increase of the total flux.

A Doppler reflectometer system has recently been installed in the Experimental Advanced Superconducting (EAST) Tokamak. It includes two separated systems, one for Q-band (33-50 GHz) and the other for V-band (50-75 GHz). The optical system consists of a flat mirror and a parabolic mirror which are optimized to improve the spectral resolution. A synthesizer is used as the source and a 20 MHz single band frequency modulator is used to get a differential frequency for heterodyne detection. Ray tracing simulations are used to calculate the scattering location and the perpendicular wave number. In EAST last experimental campaign, the Doppler shifted signals have been obtained and the radial profiles of the perpendicular propagation velocity during L-mode and H-mode are calculated.

Dedicated experiments focusing on the influence of lower hybrid waves (LHWs) on edge-localized modes (ELMs) were first performed during the 2012 experimental campaign of EAST, via modulating the input power of LHWs in the high-confinement-mode (H-mode) plasma mainly sustained by ion cyclotron resonant heating. Natural ELMs are effectively mitigated (ELM frequency increases, while its intensity decreases dramatically) as the LHW is applied, observed over a fairly wide range of plasma current or edge safety factor. By scanning the modulation frequency (fm) of LHW injected power in a target plasma dominated by the so-called small ELMs, we conclude that large ELMs with markedly larger amplitudes and lower frequencies are reproduced at low modulation frequencies (fm < 100 Hz). Analysis of the evolution of edge extreme ultraviolet radiation signals further indicates that plasma fluctuations at the pedestal region indistinctively respond to rapid modulation (fm ⩾ 100 Hz) of LHW injected power. This is proposed as the mechanism responsible for the observed fm dependence of the mitigation effect induced by LHWs on large ELMs. In addition, a critical threshold of LHW input power PLHW is estimated as , beyond which the impact of applied LHWs on ELM behaviours can be achieved. Finally, Langmuir probe measurements suggest that, rather than the concentration of free energy into a narrowband quasi-coherent precursor commonly observed growing until the ELM crash, the continuous development of broadband turbulence during the ELM-absent phase with the application of LHWs might contribute to the avoidance of ELM crashes. These results present new insights into existing experiments, and also provide some foundations and references for the next-step research about exploring in more depth and improving this new attractive method to effectively control the ELM-induced very large transient heat and particle flux.

Doppler backscattering system can measure the perpendicular velocity and fluctuation amplitude of the density turbulence with intermediate wavenumber. An eight-channel Doppler backscattering system has been installed in the Experimental Advanced Superconducting Tokamak (EAST), which can probe eight different radial locations simultaneously by launching eight fixed frequencies (55, 57.5, 60, 62.5, 67.5, 70, 72.5, 75 GHz) into plasma. The quasi-optical system consists of circular corrugated waveguide transmission, a fixed parabolic mirror, and a rotatable parabolic mirror which are integrated with quasi-optics front-end of the profile reflectometer inside the vacuum vessel. The incidence angle can be chosen from 5° to 12°, and the wavenumber range is 2-15/cm with the wavenumber resolution Δk/k≤0.21. Ray tracing simulations are used to calculate the scattering locations and the perpendicular wavenumber. The dynamic range of this new eight-channel Doppler backscattering system can be as large as 40 dB in the EAST. In this article, the hardware design, the ray tracing, and the preliminary experimental results in the EAST will be presented.

A strong relationship between the fishbone instability and internal transport barrier (ITB) formation has been found on the Experimental Advanced Superconducting Tokamak (EAST) in high β N ELMy H-mode discharges. ITB formation always appears after the fishbone instability, and the fishbone disappears when the ITB grows to a certain extent. Hybrid simulations with the global kinetic-magnetohydrodynamic (MHD) code M3D-K have been carried out to investigate the linear stability and non-linear dynamics of beam-driven fishbone instabilities in these shots. The simulation results show that the fishbone instability absorbs the energy of the fast ions and changes the distribution function of the fast ions, leading to the accumulation of fast ions near the ITB, which might eventually assist in the formation of the ITB. The q = 1 surface disappearance caused by the bootstrap current generated by the steep pressure gradient in the ITB region has been considered as the reason for the fishbone instability vanishing. This process has also been reproduced in simulation. However, the timescale of this change in the q profile is not sufficient under classical current diffusion times. The simulation utilizes another assumption explaining the disappearance of the fishbone instability. The density will form a barrier in the ITB region, which should broaden the distribution of the fast ions, and the broadening profile of the distribution of the fast ion mitigates the growth of the fishbone instability.

A toroidal Alfvén eigenmode (TAE) excited by barely trapped energetic electrons during the application of a static magnetic perturbations (MPs) is observed for the first time in tokamak ohmic heating plasmas. This TAE appears when the current of a static n  =  2 MPs exceeds a threshold value, at which a forced n  =  2 magnetic reconnection happens. Here, n is the toroidal mode number. This TAE is located near the plasma edge, which agrees with the calculation of the TAE gap. It propagates in the ion diamagnetic direction and has a dominant toroidal mode number n  =  2. The frequency of the TAE is consistent with the precessional frequency of the barely trapped energetic electrons with energy around 150 keV. An increase of the hard x-ray emission intensity in this energy range is observed just after the n  =  2 magnetic reconnection. The energetic electrons may be accelerated by the magnetic islands formed after reconnection. The TAE is stabilized after a reduction in the contents of energetic electrons. This observation suggests that the application of static MPs may provide us with a novel method to actively control Alfvén eigenmodes in fusion plasmas.

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