ELM control strategies and tools: status and potential for ITER

Abstract

Operating ITER in the reference inductive scenario at the design values of I P = 15 MA and Q DT = 10 requires the achievement of good H-mode confinement that relies on the presence of an edge transport barrier whose pedestal pressure height is key to plasma performance. Strong gradients occur at the edge in such conditions that can drive MHD instabilities resulting in Edge Localized Modes (ELMs), which produce a rapid energy loss from the pedestal region to the plasma facing components. Without appropriate control, the heat loads on plasma facing components during ELMs in ITER are expected to become significant for operation in H-mode at I P = 6 -9 MA; operation at higher plasma currents would result in a very reduced life time of the plasma facing components. Currently, several options are being considered for the achievement of the required level of ELM control in ITER; this includes operation in plasma regimes which naturally have no or very small ELMs, decreasing the ELM energy loss by increasing their frequency by a factor of up to 30 and avoidance of ELMs by actively controlling the edge with magnetic perturbations. Small/no ELM regimes obtained by influencing the edge stability (by plasma shaping, rotational shear control, etc.) have shown in present experiments a significant reduction of the ELM heat fluxes compared to type-I ELMs. However, so far they have only been observed under a limited range of pedestal conditions depending on each specific device and their extrapolation to ITER remains uncertain. ELM control by increasing their frequency relies on the controlled triggering of the edge instability leading to the ELM. This has been 2 presently demonstrated with the injection of pellets and with plasma vertical movements; pellets having provided the results more promising for application in ITER conditions. ELM avoidance/suppression takes advantage of the fact that relatively small changes in the pedestal plasma and magnetic field parameters seem to have a large stabilizing effect on large ELMs. Application of edge magnetic field perturbation with non-axisymmetric fields is found to affect transport at the plasma edge and thus prevent the uncontrolled rise of the plasma pressure gradients and the occurrence of type-I ELMs. This paper compiles a brief overview of various ELM control approaches, summarizes their present achievements and briefly discusses the open issues regarding their application in ITER. IntroductionThe main goal of current research in the field of magnetically confined plasmas aiming for power generation by nuclear fusion is to optimize high confinement plasma regimes (Hmode) in order to achieve the maximum plasma energy for a given input heating power P in . Thus, in future fusion devices such as the ITER tokamak, which is expected to produce considerable amounts of fusion power P fus , the gain or amplification factor Q = P fus /P in requires to be maximized as well. High confinement H-mode plasmas are characterized by the existence of an Edge Transport Barrier (ETB) in a ...

ELM pacing and high-density operation using pellet injection in the ASDEX Upgrade all-metal-wall tokamak

“…Indications of mitigation becoming more intricate also merge from the latest investigations performed at JET-ILW. There, pellet ELM pacing resulted in enhancement of f 0 ELM by a factor of about two, but only minor ELM mitigation with respect to ∆W ELM /W 0 and virtually no reduction of even more crucial peak heat flux could be achieved [25]. For our pacing experiment, we made essentially the same observation.…”

Helical modulation of the electrostatic plasma potential due to edge magnetic islands induced by resonant magnetic perturbation fields at TEXTOR

“…In principle, the use of ECRH can be also a way of overcoming the density limit, which critically depends on the E r pattern, at least in the RFP [48,49]. Finally, it should be worth doing experiments of ECRH in conjunction with RMP, to assess the role of E r on plasma stability with respect to the so-called edge localized modes [25].…”

Correlation analysis for energy losses, waiting times and durations of type I edge-localized modes in the Joint European Torus

“…Given the importance of ELMs for the successful operation of next-step fusion devices, a large array of ELM control and mitigation techniques have emerged [3] [5]. Typically, ELM losses are influenced either by a complete suppression of the ELMs in regimes where an alternate mechanism replaces the energy and particle transport, or by increasing the ELM frequency (f ELM ) over its natural value (ELM pacing), so that the ELM losses become smaller.…”