Suppression of Edge Localized Modes in High-Confinement KSTAR Plasmas by Nonaxisymmetric Magnetic Perturbations

Edge localized modes (ELMs) in high-confinement mode plasmas were completely suppressed in KSTAR by applying n=1 nonaxisymmetric magnetic perturbations. Initially, the ELMs were intensified with a reduction of frequency, but completely suppressed later. The electron density had an initial 10% decrease followed by a gradual increase as ELMs were suppressed. Interesting phenomena such as a saturated evolution of edge T_{e} and broadband changes of magnetic fluctuations were observed, suggesting the change of edge transport by the applied magnetic perturbations.     

Fine structure of type-I edge-localized modes in the steep gradient region

Fast, high resolution multichannel Thomson scattering is used to quantitatively determine plasma perturbations induced by type-I edge-localized modes (ELMs) in the low-field side edge of ASDEX Upgrade H-mode plasmas. 2D snapshots of temperature and density, deduced from the laser light scattered in a vertically elongated, poloidal array of 5 x 10 scattering volumes, are obtained in the hot, steep edge gradient zone, which is difficult to access by other diagnostics. Local maxima and minima with large amplitude are identified during ELMs and even in the precursor phase, both in density and temperature. Interpreting these structures as footprints of approximately field aligned helical modes in accordance with previous experimental and theoretical work, toroidal mode numbers between 8 and 20 are obtained, roughly consistent with corresponding scrape-off layer and divertor measurements.     

Mechanisms of edge-localized-mode mitigation by external-magnetic-field perturbations

Particle and energy transport in the tokamak edge transport barrier is analyzed in the presence of magnetic field perturbations from external resonant coils. In recent experiments such coils have been verified as an effective tool for mitigation of the edge-localized modes of type I. The observed reduction of the density in plasmas of low collisionality is explained by the generation of charged particle flows along perturbed field lines. The increase of the electron and ion temperatures in the barrier is interpreted by the reduction of perpendicular neoclassical transport with decreasing density and nonlocality of parallel heat transport. The found modification of the pressure gradient implies the stabilization of ballooning-peeling MHD modes responsible for type I ELMs.     

Suppression of large edge-localized modes in high-confinement DIII-D plasmas with a stochastic magnetic boundary

A stochastic magnetic boundary, produced by an applied edge resonant magnetic perturbation, is used to suppress most large edge-localized modes (ELMs) in high confinement (H-mode) plasmas. The resulting H mode displays rapid, small oscillations with a bursty character modulated by a coherent 130 Hz envelope. The H mode transport barrier and core confinement are unaffected by the stochastic boundary, despite a threefold drop in the toroidal rotation. These results demonstrate that stochastic boundaries are compatible with H modes and may be attractive for ELM control in next-step fusion tokamaks.     

Reduced heat transport between edge-localized-mode bursts at low collisionality and small poloidal Larmor radius

Nondimensional parameter dependence of heat transport between edge localized modes (ELMs) is examined for H mode plasmas. The electron heat diffusivity between ELMs is reduced to the level of ion neoclassical transport in the plasma edge region which is affected by ELM burst. At lower edge collisionality, the heat flux assigned to the heat transport between ELMs is reduced and the ELM loss power is enhanced. During the inter-ELM phase, the energy confinement time becomes larger with decreasing the edge collisionality and poloidal Larmor radius.     

Effects of edge-localized mode-induced neoclassical toroidal viscosity torque on the toroidal intrinsic rotation in the EAST tokamak

Intrinsic rotation has been observed in lower hybrid current-driven (LHCD) H-mode plasmas with type-III edge-localized modes (ELMs) on Experimental Advanced Superconducting Tokamak (EAST), and it is found that the edge toroidal rotation accelerated before the onset of the ELM burst. Magnetic perturbation analysis shows there is a perturbation amplitude growth below 30 kHz corresponding to the edge rotation acceleration. Using the filament model, the neoclassical toroidal viscosity (NTV) code shows there is a co-current NTV torque at the edge, which may be responsible for the edge rotation acceleration. For maximum displacement ～1 cm and toroidal mode number n=15, the calculated torque density is ～0.44 N/m², comparable with the average edge toroidal angular momentum change rate ～1.24 N/m². Here, the 1 cm maximum magnetic surface displacement estimated from the experimental observation corresponds to a maximum magnetic perturbation ～ 10^?3–10^?2 T, in accordance with magnetic perturbation measurements during ELMs. By varying n from 10 to 20, the magnitude of the edge NTV torque density is mainly ～0.1–1 N/m². This significant co-current torque indicates that the NTV theory may be important in rotation problems during ELMs in H-mode plasmas. To better illuminate the problem, magnetic surface deformation obtained from other codes is desired for a more accurate calculation.     

Nuclear fusion: bringing a star down to Earth

Nuclear fusion offers the potential for being a near limitless energy source by fusing together deuterium and tritium nuclei to form helium inside a plasma burning at 100 million K. However, scientific and engineering challenges remain. This paper describes how such a plasma can be confined on Earth, and discusses the similarities and differences with fusion in stars. It focuses on the magnetic confinement technique and, in particular, the method used in a tokamak. The confinement achieved in the equilibrium state is reviewed and it is shown how the confinement can be too good, leading to explosive instabilities at the plasma edge called edge localised modes (ELMs). It is shown how the impact of ELMs can be minimised by the application of magnetic perturbations and discusses the physics behind the penetration of these perturbations into what is ideally a perfect conducting plasma.     

Nonaxisymmetric energy deposition pattern on ASDEX upgrade divertor target plates during type-I edge-localized modes

In the ASDEX Upgrade tokamak, complex power deposition structures on the divertor target plates during type-I edge-localized modes (ELMs) have been discovered by fast (few microseconds), two-dimensional (40 x 40 cm(2)) infrared thermography. In addition to the usual axisymmetric power deposition line near the separatrix, there appear, statistically distributed, several laterally displaced and inclined stripes, mostly well separated from each other and from the main strike zone. These structures are interpreted as footprints of approximately field aligned, helical perturbations at the low field side of the main plasma edge related to the nonlinear ELM evolution. Based on this picture, the ELM related mode structure can be derived from the target load pattern, yielding on average toroidal mode numbers in a range of 8-24.     

Evolution of filament structures during edge-localized modes in the MAST Tokamak

Edge-localized modes (ELMs) are repetitive instabilities that occur in the outer region of tokamak plasmas. This Letter provides new information on and the implications of the evolution of the filament structures observed during ELMs in the MAST tokamak. The filaments exist for the time over which particles are being released into the scrape off layer. They start off at the plasma edge rotating at the velocity of the pedestal, and then decelerate toroidally and accelerate radially outwards. As the filaments propagate radially they remain aligned with the local magnetic field line.     

Direct observation of current in type-I edge-localized-mode filaments on the ASDEX Upgrade tokamak

Magnetically confined plasmas in the high confinement regime are regularly subjected to relaxation oscillations, termed edge localized modes (ELMs), leading to large transport events. Present ELM theories rely on a combined effect of edge current and the edge pressure gradients which result in intermediate mode number (n?10-15) structures (filaments) localized in the perpendicular plane and extended along the field lines. By detailed localized measurements of the magnetic field perturbation associated to type-I ELM filaments, it is shown that these filaments carry a substantial current.     

Tearing mode suppression by using resonant magnetic perturbation coils on J-TEXT tokamak

A series of experiments on the interactions between external resonant magnetic perturbations (RMP) and plasmas has recently been conducted, using static resonant magnetic perturbation (SRMP) coils on the Joint Texas Experimental Tokamak (J-TEXT). The tearing mode can be completely suppressed by applying SRMP. However, the locked mode is likely to be stimulated under a larger SRMP field even though the tearing mode has been first suppressed. A numerical simulation shows three typical regimes of RMP?s effects on the tearing mode, which are consistent with experimental results.     

Plasma Response to Resonant Perturbations at Tokamak Edge

In certain circumstances, plasma response suppresses magnetic islands expected at perturbed resonant magnetic surfaces. We investigate the plasma response to the resonant magnetic perturbations in a large aspect ratio tokamak perturbed by external resonant helical windings, considering polar toroidal coordinates for which analytical toroidal equilibrium solutions and perturbing fields are available. We apply an empirical approach to mimic the plasma screening effects by introducing presumed plasma current sheets on the resonance surfaces to cancel the RMP effects. Numerical examples show the effect of plasma response reducing magnetic islands at the plasma edge and also regularizing field lines around the resonant surface. The distribution of connection lengths along the plasma cross section indicates that the plasma response increases the connection lengths since more toroidal turns are performed until a field line reaches the tokamak wall.     

Monitoring Alfvén cascades with interferometry on the JET Tokamak

A microwave interferometry technique is applied for the first time for detecting a discrete spectrum of Alfvén cascade (AC) eigenmodes excited with fast ions in reversed magnetic shear plasmas of the Joint European Torus. The interferometry measurements of plasma density perturbations associated with ACs show an unprecedented frequency and time resolution superior to that obtained with external magnetic coils. The measurements of ACs are used for monitoring the evolution of the safety factor and density of rational magnetic surfaces in the region of maximum plasma current.     

Plasma confinement in tokamaks with robust torus

We present a non-linear symplectic map that describes the alterations of the magnetic field lines inside the tokamak plasma due to the presence of a robust torus (RT) at the plasma edge. This RT prevents the magnetic field lines from reaching the tokamak wall and reduces, in its vicinity, the islands and invariant curve destruction due to resonant perturbations. The map describes the equilibrium magnetic field lines perturbed by resonances created by ergodic magnetic limiters (EMLs). We present the results obtained for twist and non-twist mappings derived for monotonic and non-monotonic plasma current density radial profiles, respectively. Our results indicate that the RT implementation would decrease the field line transport at the tokamak plasma edge.     

Modeling of Large ELM Suppressions for High Confinement Plasma in D Ⅲ-D

The H-mode discharges with high edge pressure gradients are expected for the economic feasibility of future fusion reactors. However, the high edge pressure gradients easily produce ELM instability , which generally can expel large, heat and particle loading to the divertor targets. These ELMs limit the core plasma performance and reduce the lifetime of divertor target plates. The transports of heat and particles outward across plasma boundary are useful to control density and impurity profiles for achieving steady state, high performance plasmas. Consequently, any technique to eliminate or mitigate large fast ELM impulses must replace the transient heat and particle transports with another slow process. Such a technique is high priority for a burning plasma device such as ITER.     

ELM triggering conditions for the integrated modeling of H-mode plasmas

Recent advances in the integrated modeling of ELMy H-mode plasmas are presented. A new model for the H-mode pedestal and for the triggering of ELMs predicts the height, width, and shape of the H-mode pedestal and the frequency and width of ELMs. The model for the pedestal and ELMs is used in the ASTRA integrated transport code to follow the time evolution of tokamak discharges from L-mode through the transition from L-mode to H-mode, with the formation of the H-mode pedestal, and, subsequently, to the triggering of ELMs. Turbulence driven by the ion temperature gradient mode, resistive ballooning mode, trapped electron mode, and electron temperature gradient mode contributes to the anomalous thermal transport at the plasma edge in this model. Formation of the pedestal and the L-H transition is the direct result of flow shear suppression of anomalous transport. The periodic ELM crashes are triggered by MHD instabilities. Two mechanisms for triggering ELMs are considered: ELMs are triggered by ballooning modes if the pressure gradient exceeds the ballooning threshold or by peeling modes if the edge current density exceeds the peeling mode threshold. The BALOO, DCON, and ELITE ideal MHD stability codes are used to derive a new parametric expression for the peeling-ballooning threshold. The new dependence for the peeling-ballooning threshold is implemented in the ASTRA transport code. Results of integrated modeling of DIII-D like discharges are presented and compared with experimental observations. The results from the ideal MHD stability codes are compared with results from the resistive MHD stability code NIMROD.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.     

Electronic detection of collective modes of an ultracold plasma

Using a new technique to directly detect current induced on a nearby electrode, we measure plasma oscillations in ultracold plasmas, which are influenced by the inhomogeneous and time-varying density and changing neutrality. Electronic detection avoids heating and evaporation dynamics associated with previous measurements and allows us to test the importance of the plasma neutrality. We apply dc and pulsed electric fields to control the electron loss rate and find that the charge imbalance of the plasma has a significant effect on the resonant frequency, in excellent agreement with recent predictions suggesting coupling to an edge mode.     

Model for current-driven edge-localized modes

Edge-localized modes (ELMs) are cyclic disturbances in the outer region of tokamak plasmas that are influential in determining present and future tokamak performance. In this Letter, we outline an approach to modeling ELMs in which we envisage toroidal peeling modes initiating a Taylor relaxation [Phys. Rev. Lett. 33, 1139 (1974)10.1103/PhysRevLett.33.1139] of a tokamak outer region plasma. Relaxation produces a peeling destabilizing flattened edge current profile and a stabilizing plasma-vacuum current sheet; the balance between the two determines the radial extent of the relaxed region. The model can be used to predict the energy losses due to an ELM and reproduces experimentally observed variations with edge safety factor and plasma collisionality. There is an intrinsic "deterministic scatter" in the model that also accords with observation.     

Sandpile model with tokamaklike enhanced confinement phenomenology

Confinement phenomenology characteristic of magnetically confined plasmas emerges naturally from a simple sandpile algorithm when the parameter controlling redistribution scale length is varied. Close analogs are found for enhanced confinement, edge pedestals, and edge localized modes (ELMs), and for the qualitative correlations between them. These results suggest that tokamak observations of avalanching transport are deeply linked to the existence of enhanced confinement and ELMs.     

Effect of resonant magnetic perturbations on edge plasma parameters in the STOR-M tokamak

Edge plasma properties have been modified in the Saskatchewan Torus-Modified tokamak by means of resonant magnetic perturbations (RMP). It has been found that the radial profiles of ion saturation current and floating potential in the edge region can be modified by an externally applied static (m?=?2, n?=?1) RMP field. An increase in the pedestal plasma density (n) and more negative electric field (E_r) have been observed in the plasma edge region. It is believed that the RMP field altered the plasma transport in the edge and scrape-off layer regions, leading to a higher density pedestal and a potential drop in some cases. During the enhanced confinement phase, it is possible to identify a region where intermittent transport events, the so-called blobs, are created and the holes of lower density left behind.