External Resonant Magnetic Perturbations Research Articles

Dynamic characteristics of edge plasma profiles during the opening of edge islands on the J-TEXT tokamak

Abstract On the J-TEXT tokamak, the dynamics of edge magnetic topology during the opening of the edge magnetic islands induced by the external Resonant Magnetic Perturbation (RMP) are investigated. The edge island chain is pushed outward by increasing plasma toroidal current to intersect the poloidally and toroidally localized divertor plate, forming an open magnetic island in the Scrape-Off Layer (SOL). The location of the strike points on the divertor plate predicted by the HINT code is in agreement with the experimental observations. The influence of the magnetic topology on edge plasma profiles has also been investigated using Langmuir probes. The properties of edge T e , n e , P e and E r profiles are studied as function of three magnetic structures in q a -dependence experiments and two magnetic structures in RMP configuration-dependence experiments. Some common features are observed. Inside the edge closed and SOL remnant islands, flat P e but non-flat T e and n e profiles are detected. When transitioning from the edge closed island to the partially open island with remnant island, the local flat P e profile is shifted outward accompanied by a narrower flattened P e region. The steep slopes of T e , n e and P e are measured in the SOL regions characterized by a long connection length L c , and the longer L c , the steeper slopes. E r exhibits a negative well inside the remnant island with infinite L c and in the SOL regions where L c significantly exceeds than the electron mean free path λ e , but develops a positive value in the SOL regions where L c is relatively short.

Overview of the Recent Experimental Research on the J-TEXT Tokamak

Abstract The J-TEXT capability is enhanced compared to two years ago with several upgrades of its diagnostics and the increase of electron cyclotron resonance heating (ECRH) power to 1 MW. With the application of electron cyclotron wave (ECW), the ECW assisted plasma startup is achieved; the tearing mode is suppressed; the toroidal injection of 300 kW ECW drives around 24 kA current; fast electrons are generated with toroidal injected ECW and the runaway current conversion efficiency increases with ECRH power. The mode coupling between 2/1 and 3/1 modes are extensively studied. The coupled 2/1 and 3/1 modes usually lead to major disruption. Their coupling can be either suppressed or avoided by external resonant magnetic perturbation (RMP) fields and hence avoids the major disruption. It is also found that the 2/1 threshold of external field is significantly reduced by a pre-excited 3/1 mode, which can be either a locked island or an external kink mode. The disruption control is studied by developing prediction methods capable of cross tokamak application and by new mitigation methods, such as the biased electrode or electromagnetic pellet injector. The high-density operation and related disruptions are studied from various aspects. Approaching the density limit, the collapse of the edge shear layer is observed and such collapse can be prevented by applying edge biasing, leading to an increased density limit. The density limit is also observed to increase, if the plasma is operated in the poloidal divertor configuration or the plasma purity is increased by increasing the pre-filled gas pressure or ECRH power during the start-up phase.

Modeling of heating scheme impacts on neon seeded plasmas with the resonant magnetic perturbation application in the large helical device: ECR and NBI heating

Plasma parameter profiles along the effective minor radius of flux surfaces are modeled for the conditions with neon impurity puffing and external resonant magnetic perturbations at the plasma edge in the large helical device. For the electron cyclotron heating scenario, calculations qualitatively reproduce stable steady states with plasma detached from divertor target plates and radiation losses exceeding 90% of the heating power. It is unlikely with neutral beam injection heating that the plasma collapses if the radiation approaches a critical level noticeably below 100%. The latter is explained by the dependence of neutral beam attenuation on the plasma ion effective charge.

Reduction of the external resonant magnetic perturbation threshold for 2/1 locked mode excitation by the pre-excited mode on J-TEXT

A significant reduction of the external resonant magnetic perturbation (RMP) required for exciting the locked mode (LM) has been observed experimentally due to the presence of an existing LM on J-TEXT. In a plasma with edge safety factor q a slightly smaller than 3, 3/1 and 2/1 modes are observed to be destabilized successively by a rotating RMP field. The 3/1 mode is excited first and identified as an external kink mode, whose excitation threshold decreases with the operating q a increasing from 2.67 to 2.8 with the absence of a 3/1 rational surface. Shortly after excitation of the 3/1 external kink mode, 2/1 field penetration occurs and the 2/1 mode is excited, as identified by the phase between the 2/1 magnetic response and the RMP field jumping by π. The 2/1 mode is excited with a dominant kink structure and then converts into tearing mode for a duration of several milliseconds. During the excitation of the 2/1 mode, the phase difference between the 2/1 and 3/1 modes evolves from π to around 0 and the 2/1 mode amplitude grows to a saturation value comparable with that of the 3/1 mode. The presence of the 3/1 mode leads to a remarkable reduction of the external RMP current required for the 2/1 mode excitation, which can be explained by the contribution of a 2/1 resonant field from the 3/1 mode via the toroidal coupling effect. The larger 3/1 mode amplitude can lead to a greater reduction. This work reveals that the mode can be excited more easily with a reduced amplitude of RMP or error field due to the impact of the pre-existing mode and this might be a new consideration for error field correlation predictions and active magnetohydrodynamics control actuator designs.

Suppression of resistive interchange instability by external RMP

We experimentally investigate the effect of external resonant magnetic perturbation (RMP) on resistive interchange MHD instability which is typically observed in the Large Helical Device. We apply the m/n = 1/1 external RMP to discharges with the m/n = 1/1 interchange instability. We find that the electron density fluctuation as well as the magnetic field one is clearly reduced without a reduction of the pressure gradient by imposing the external RMP. Moreover, the achieved beta value as well as the beta gradient at the resonant surface is a little improved under certain conditions. Next, we investigate the responses of interchange instability to the external RMP under some different operational conditions. We find that the amplitude of the external RMP to completely suppress the instability with the shielding of the external RMP has a higher correlation with the volume-averaged beta value than other plasma parameters.

Behavior of multiple modes before and during minor disruption with the external resonant magnetic perturbations on J-TEXT tokamak

The behavior of multiple modes before and during minor disruption with the external resonant magnetic perturbations (RMPs) has been studied on a J-TEXT tokamak. The main component of RMPs is m/n = 2/1, where m and n are the numbers of the poloidal and toroidal modes, respectively. During the mode-locking caused by RMPs, it is found that before a minor or a major disruption (if there is no minor disruption), strong oscillations in both electron temperature and density occur if the edge safety factor q a > 3. The analysis shows that the oscillations are caused by the m/n = 3/1 mode. In addition, using the ECE, Mirnov coils and 2D electron cyclotron emission imaging diagnostic systems, it is found that a thermal collapse occurs on the inner side of the 2/1 magnetic island during the minor disruption, and before the collapse, a 3/2 island increases, after the collapse, the 3/2 island may disappear. Moreover, the study also shows that these 3/1, 2/1 and 3/2 modes play roles in the thermal collapse of disruptions.

Physical design of a new set of high poloidal mode number coils in the EAST tokamak

An external resonant magnetic perturbation (RMP) field, which is an effective method to mitigate or suppress the edge localized mode (ELM), has been planned to be applied on the ELM control issue in ITER. A new set of magnetic perturbation coils, named as high m coils, has been developed for the EAST tokamak. The magnetic perturbation field of the high m coils is localized in the midplane of the low field side, with the spectral characteristic of high m and wide n, where m and n are the poloidal and toroidal mode numbers, respectively. The high m coils generate a strong localized perturbation field. Edge magnetic topology under the application of high m coils should have either a small or no stochastic region. With the combination of the high m coils and the current RMP coils in the EAST, flexible working scenarios of the magnetic perturbation field are available, which is beneficial for ELM control exploration on EAST. Numerical simulations have been carried out to characterize the high m coil system, including the magnetic spectrum and magnetic topology, which shows a great flexibility of magnetic perturbation variation as a tool to investigate the interaction between ELM and external magnetic perturbation.

Influence of toroidal rotation on plasma response to external RMP fields in tokamak

The type-I edge localized mode (ELM) is a critical event associated with magneto-hydrodynamic(MHD) instabilities occurring in tokamak high-confinement (H-mode) discharges, that leads to huge heat loads on the plasma phasing components (PFC) and may result in material damages. It is important to effectively control large ELMs, in order to ensure safe operation of the future reactor-scale devices such as ITER and DEMO. Resonant magnetic perturbation (RMP) has been experimentally demonstrated to be a mature and robust technique for controlling ELMs. A set of parameters, such as the edge safety factor, the plasma flow, the RMP coil geometry and the spectrum of the applied external field, have been found to play important roles in controlling ELMs by RMP. Furthermore, the plasma pressure is known to affect the plasma response to the RMP field, in particular near the no-wall beta limit. This is because high plasma pressure drives the resonant field amplification of the external field by the plasma response. The ITER 10 MA steady state scenario will be operated near the no-wall stability limit. The new tokamak device HL-2M will also operate in the relatively high-beta regimes. On the other hand, more investigations are still needed to understand the influence of toroidal flow on the high-beta plasma response. This work employs a single fluid toroidal model to compute the plasma RMP response in HL-2M, emphasizing on the roles of two key physical quantities: the plasma resistivity and the toroidal rotation. The former allows penetration of the external RMP field into the plasma, while the latter mainly provides screening effect on the resonant field component. More specifically, the MARS-F code is utilized to study the plasma response to the externally applied <i>n</i> =1 ( n is the toroidal mode number) RMP field for high-beta HL-2M discharges, while varying the plasma toroidal rotation profile. The plasma response is found to (i) substantially modify the poloidal spectrum of the applied vacuum RMP field, (ii) change the amplitude of the resonant radial field amplitude near the plasma edge, and (iii) affect optimal current phasing between the two rows of RMP coils on HL-2M. A sufficiently slow toroidal flow near the plasma edge amplifies the radial field at rational surfaces associated with the perturbation. Since the latter serves as a reliable indicator for controlling the type-I edge localized mode (Type-I ELM) by RMP, varying rotation profile near the plasma edge offers a promising approach to optimize ELM control.

External RMP effect on locked-mode-like instability in helical plasmas

The slowing-down mechanism of the locked-mode-like instabilities with and without an island structure is investigated through the effects of an external RMP (resonant magnetic perturbation) on the instabilities. For both instabilities, the slowing-down duration decreases with the increase in the external RMP, and the RMP dependence is consistent with the braking model of the j × B force due to the interaction between the instabilities and the external RMP. Moreover, the relationship between the amplitude and the frequency of both locked-mode-like instabilities during the slowing down is consistent with the force balance model between the j × B force due to the external RMP and a viscous force. These results suggest that the slowing down of both locked-mode-like instabilities with finite external RMP occurs due to the j × B force driven by the external RMP.

An improved theory of the response of DIII-D H-mode discharges to static resonant magnetic perturbations and its implications for the suppression of edge localized modes

The plasma response to an externally generated, static, n = 2, resonant magnetic perturbation (RMP) in the pedestal region of DIII-D H-mode discharge #158115 is investigated using a toroidal generalization of the asymptotic matching model presented by Fitzpatrick [Phys. Plasmas 27, 042506 (2020)]. Just as in a recent paper [Q. M. Hu et al., Phys. Plasmas 26, 120702 (2019)], it is hypothesized that the density pump-out phenomenon is due to locked magnetic island chains induced at the bottom of the pedestal, whereas the suppression of edge localized modes is associated with a locked magnetic island chain induced at the top of the pedestal. Neutral penetration inside the last closed magnetic flux-surface is found to have a significant influence on locked magnetic island chain formation at the bottom of the pedestal. It is found that locked island formation at the top of the pedestal is only possible when q95 lies in certain narrow windows. Finally, it is inferred that, in order to successfully induce a locked island chain at the top of the pedestal, an external RMP field-coil system must generate a magnetic field that is simultaneously strongly amplified by the plasma (via the ideal kink response) and has a large resonant component at a rational surface that lies close to the top of the pedestal.

Compatibility of pellet fuelling with ELM suppression by RMPs in the ASDEX Upgrade tokamak

It is demonstrated that tokamak plasma can be fuelled by pellets while simultaneously maintaining ELM suppression by external resonant magnetic perturbations (RMPs). Pellets are injected vertically from high field site and deposited at outer part of plasma cross section. Each pellet triggers a benign MHD event followed by a short lived ELM-free phase. The ELM suppression phase with pellet fuelling lasts 11 pellet cycles and is terminated by intentionally increasing the pellet rate to cause a transition to the ELMy phase.

Transport at high and development of candidate steady state scenarios for ITER

On DIII-D, the high scenario has an internal transport barrier (ITB), , , and very high normalized confinement . Recently, plasmas starting with these conditions have been dynamically driven to and , where we find the ITB and high performance persist for five energy confinement times. These conditions are projected to meet the ITER steady-state goal of Q = 5. The ITB is maintained at lower with a strong reverse shear, consistent with predictions that negative central shear can lower the threshold for the ITB. There are two observed confinement states in the high scenario: H-mode confinement state with a high edge pedestal, and an enhanced confinement state with a low pedestal and an ITB. It has been observed in a scan of external resonant magnetic perturbation amplitude that when there are no large type-I ELMs, there is no transition to enhanced confinement. This is consistent with the proposed mechanism for ITB formation being a type-I ELM. Quasilinear gyro-Landau fluid predictive modeling of ITER suggests that only a modest reverse shear is required to achieve the ITB formation necessary for Q = 5 when electromagnetic physics including the kinetic ballooning mode (KBM) is incorporated.

Impact of a resonant magnetic perturbation field on impurity radiation, divertor footprint, and core plasma transport in attached and detached plasmas in the Large Helical Device

The effects of resonant magnetic perturbation (RMP) field on impurity radiation, divertor footprint distribution, and core plasma transport are investigated in the detachment discharges of the Large Helical Device (LHD). The RMP with m/n = 1/1 mode creates an edge magnetic island in the stochastic layer, which enhances the impurity emission from low charge states, C2+ and C3+, and then triggers a detachment transition. Emission from the higher charge states, C4+ and C5+, implies enhanced penetration of impurities during the detachment phase with RMP. The toroidal divertor particle flux distribution exhibits n = 1 mode structure in both the attached and detached phases, but with a large toroidal phase shift between the two phases. The distribution in the attached phase is well correlated with the magnetic footprint of field line connection length calculated by the vacuum approximation. During the detached phase, however, the phase shift is not well explained by the vacuum approximation, where a significant plasma response to the external RMP is observed. The energy confinement time becomes systematically shorter with RMP application due to the shrinkage of plasma volume caused by the edge magnetic island. On the other hand, the pressure profile during detachment with RMP is found to be more peaked than without RMP. The analysis using the core transport code TASK3D, considering the heating profiles of neutral beam injection, shows no significant transport degradation during detachment with RMP application, even with the enhanced radiation, reduced divertor flux, and possible impurity penetration.

Phase-detection-based feedback control for the power supply in tearing mode control system on J-TEXT

Tearing mode creates magnetic islands and easily causes a disruption in the tokamak. A new method to control the tearing mode has been proposed. The external resonant magnetic perturbation (RMP) coils are used to create modulated magnetic perturbation in certain magnetic island phase region. In the region of -0.5π to -π, RMP has an accelerating and stabilizing effects on tearing mode. To verify the mechanism, two variable frequency current-pulse power supplies have been developed, which have the maximum frequency of 8 kHz and the current of 3 kA. In this paper, the phase-detection-based and frequency feedback control are introduced. A fast island phase identification theory is used in the feedback control system to provide the magnetic island phase information in real time. It uses a poloidal array of Mirnov probes to identify the O-point of the magnetic island. There is a current amplitude feedback control to adjust the strength of RMP field. A bypass is used in the power supply to establish the output current quickly in initial period and protect the inverter. Finally, the feedback control system and the new power supplies have been used on the J-TEXT tokamak and two control methods are compared.

Investigation of RMP induced density pump-out on EAST

Density pump-out has been observed on the experimental advanced superconducting tokamak under the conditions of mitigated or totally suppressed edge localized modes with n = 2 external resonant magnetic perturbation (RMP), associated with decreased core density and degradation of particle confinement. In order to investigate the particle transport during RMP induced density pump-out, density modulation using supersonic molecular beam injection is performed to obtain the particle transport coefficients. The results show that the diffusive coefficient becomes larger and the inward convection velocity decreases towards the plasma edge after RMP. The measured radial electric field (Er) became less negative in the plasma edge near rho ~ 0.97 and consequently, the E × B shearing rate was reduced significantly, which is consistent with increased density fluctuations.

Response of plasma rotation to resonant magnetic perturbations in J-TEXT tokamak

The response of plasma toroidal rotation to the external resonant magnetic perturbations (RMP) has been investigated in Joint Texas Experimental Tokamak (J-TEXT) ohmic heating plasmas. For the J-TEXT’s plasmas without the application of RMP, the core toroidal rotation is in the counter-current direction while the edge rotation is near zero or slightly in the co-current direction. Both static RMP experiments and rotating RMP experiments have been applied to investigate the plasma toroidal rotation. The core toroidal rotation decreases to lower level with static RMP. At the same time, the edge rotation can spin to more than 20 km s−1 in co-current direction. On the other hand, the core plasma rotation can be slowed down or be accelerated with the rotating RMP. When the rotating RMP frequency is higher than mode frequency, the plasma rotation can be accelerated to the rotating RMP frequency. The plasma confinement is improved with high frequency rotating RMP. The plasma rotation is decelerated to the rotating RMP frequency when the rotating RMP frequency is lower than the mode frequency. The plasma confinement also degrades with low frequency rotating RMP.

Plasma Response to Resonant Magnetic Perturbations in Large Aspect Ratio Tokamaks

In this paper, a plasma response model is investigated for large aspect ratio tokamaks, on the presence of external nonaxisymmetric resonant magnetic perturbations. To control plasma confinement, the plasma is perturbed by external resonant helical windings, similar to those introduced by an ergodic magnetic limiter. The plasma response to the perturbation is modeled as an additional magnetic perturbation, created by a current sheet on the main perturbed resonant surface, vanishing the radial component of the total magnetic field in the considered resonant surface. In order to show the influence of the plasma response on the field line topology, the field line differential equations are integrated numerically, showing the reduction of magnetic island sizes and the regularization of chaotic regions around the resonant surface where the response was introduced, in agreement with results observed in sophisticated simulation codes. Finally, the connection lengths indicate that, regarding their distribution, the plasma response introduces a homogenization effect which may affect the deposition patterns at the tokamak wall.

Enhanced neoclassical particle transport by resonant magnetic perturbations in a tokamak

Magnetohydrodynamics mode can be triggered by external resonant magnetic perturbations. Whether it is ideal mode or resistive mode, it breaks the toroidal symmetry in tokamaks. When the toroidal symmetry is broken, the neoclassical transport is enhanced because collisionless particles are not confined. In this study, we calculate the neoclassical particle transport when kink-like modes are present near the plasma edge or boundary.

Neoclassical toroidal viscosity torque in tokamak edge pedestal induced by external resonant magnetic perturbation

The characteristic profile and magnitude are predicted in theory for the neoclassical toroidal viscosity (NTV) torque induced by the plasma response to the resonant magnetic perturbation (RMP) in a tokamak with an edge pedestal. For a low-β equilibrium, the NTV torque is dominated by the toroidal component with the same dominant toroidal mode number of RMPs. The NTV torque profile is found to be localized, whose peak location is determined by profiles of both the equilibrium temperature (pressure) and the plasma response. In general, the peak of the NTV torque profile is found to track the pedestal position. The magnitude of NTV torque strongly depends on the β value at the top of pedestal, which suggests a more significant role of NTV torque in higher plasma β regimes. For a fixed plasma β, decreasing density hence increasing temperature can also enhance the amplitude of NTV torque due to the reduced collisionality in the 1/ν regime. Based on those findings, we identify the tokamak operation regimes where the significance of NTV torque in the edge pedestal induced by RMP can approach those from other momentum sources such as the neutral beam injections.

Joint Varenna-Lausanne International Workshop on the Theory of Fusion Plasmas 2016

The joint Varenna-Lausanne international workshop on the theory of fusion plasmas took place in Varenna from August 29 to September 2 2016. Several issues of interest for fusion plasmas were addressed, namely MHD stability, RF heating, collisional and turbulent transport, plasma wall interaction, and physics of burning plasmas. The articles published in this special issue illustrate nicely the well balanced combination of physics, applied mathematics, and computer sciences that characterizes this workshop.Let us mention several attractive topics, which are addressed in this issue. The question of 3D MHD equilibrium in tokamaks has received a great deal of attention, in connection with external resonant magnetic perturbations in tokamaks, and also stochastic edge in stellarators. The reader will also find some recent developments related to the effect of current drive and heating on the stability of tearing modes. As usual, turbulent transport is addressed in much detail. Several papers address specific numerical aspects of fluid and gyrokinetic codes, including code optimisation. Physics issues are abundantly dealt with, such as the impact of fast particles on turbulence, and particle transport. New numerical techniques to model wave propagation are presented, which provide significant advances in the field. Refinements such as the effect of density fluctuation on wave propagation, or the interaction between particles and the electromagnetic field near antennas, have also been studied in depth. Finally, specific issues such as nonlocal transport, decay of zonal flows, and the effect of neutrals on rotation have been investigated.A striking feature of the 2016 edition was the large number of young faces among the participants. This is a great satisfaction for the organizers since a new generation of scientists is certainly needed whilst several devices come to operation, or will do so in a foreseeable future.The diversity and quality of the papers published in the present cluster prove once more that the Varenna-Lausanne meeting has a special place among the conferences on magnetic fusion energy. It is certainly an excellent forum for exchanging ideas on theory and modelling, and for advertising the best results obtained in that field.The present volume of Journal of Physics Conference Series contains 16 outstanding papers - all peer reviewed. Let us mention another set of about 20 papers to appear in Plasma Physics and Controlled Fusion. We hope the reader will appreciate this special issue and that it will be a source of inspiration for him.Xavier Garbet, Olivier Sauter November, 8 2016http://varenna-lausanne.epfl.ch