Abstract
Edge localized mode (ELM) mitigation accompanied by density pump-out has been achieved during the application of resonant magnetic perturbation (RMP) with a toroidal mode number of n = 4 on EAST recently. The mean ELM frequency increases by a factor of 2.7 from 86 to 235 Hz with a decreased reduction in ELM loss. The evolution of pedestal electron density measured by a profile reflectometer before and after turning on the RMP current is presented. Both the pedestal density and density gradient show a decrease with application of RMP. The density fluctuation in the pedestal region has been measured by an O-mode fluctuation reflectometer. The broadband density fluctuation with a frequency range of 20–115 kHz is enhanced at the later period of the inter-ELM phase during ELM mitigation. This phenomenon is also observed for magnetic fluctuation measured by magnetic probes mounted in the vacuum vessel. A further study shows that the enhanced broadband fluctuations lead to a decrease in the growth rate of the pedestal density and an increase in divert or particle flux. This result implies that these enhanced broadband fluctuations could lead to an enhancement of outward particle transport. The possible roles of the enhanced fluctuations observed in ELM mitigation are also discussed.
Highlights
We further present that the turbulence in the pedestal gradient region is enhanced during the Edge localized mode (ELM) mitigation phase by an n = 4 resonant magnetic perturbation (RMP) and the enhanced turbulence has an effect to increase the outward particle transport
The enhanced fluctuation has an effect to preclude the increase in the density and pressure of the pedestal. This would make the pedestal evolution path change from B to A, resulting in that the ELM behaviors may scitation.org/journal/adv change from peeling-ballooning to more peeling. This could partly explain why density pump-out and ELM mitigation are observed by RMP
ELM mitigation has been achieved with n = 4, IRMP = 14 kAt, and δφUL = 180○ RMP on EAST
Summary
High-confinement (H-mode) is usually accompanied by quasi-periodic bursts of magnetohydrodynamic (MHD) instability called edge localized mode (ELM), which can eject large amounts of energy and particles from the confined plasma to divertor targets. For a future tokamak fusion reactor, such as ITER, the robust control method of type I ELMs is an essential requirement, in order to ensure an acceptable lifetime of plasma facing components (PFCs). Three-dimensional (3D) resonant magnetic perturbation (RMP), which is considered for ITER, is the most effective technique to mitigate or suppress completely the ELMs. RMP has been demonstrated in multiple fusion devices, such as DIII-D,6,7 JET, ASDEX Upgrade, MAST, KSTAR, and EAST. RMP can induce ELM suppression or ELM mitigation, the two are very different phenomena from the view of MHD stability since the former corresponds to a MHD stable pedestal, while the latter corresponds to a MHD more unstable pedestal. The pedestal recovers more frequently at a lower pressure gradient to trigger ELMs and ELM mitigation occurs.24–27 It was observed in the nonlinear simulation that the most unstable mode for ELMs could strongly couple to RMP and the magnetic energy redistributed to lower toroidal mode number modes. The relaxations of the lower n modes, manifested as more frequent small ELMs or MHD turbulence similar to type II ELMs, can provide sufficient transport and prevent large ELM crash due to the most unstable mode. For both ELM suppression and ELM mitigation by RMP, density pump-out, i.e., a reduction of plasma density, is usually observed.
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