MHD Instabilities Research Articles

Kinetic modeling of ELM-induced tungsten transport in a tokamak plasma

Impurity accumulation in the core plasma leads to fuel dilution and higher radiative losses that can lead to loss of the H-mode, to thermal collapse of the plasma, and eventually even to a disruption in tokamaks. In present experiments, it has been shown that Edge Localized Modes (ELMs) at sufficiently high frequency are required to prevent W accumulation in the core, by expelling impurities from the edge plasma region, thus preventing their penetration into the plasma core. We present a full-orbit particle extension of the MHD code JOREK suitable for simulating impurity transport during ELMs. This model has been applied to the simulation of an ELM crash in ASDEX Upgrade, where we have quantified the displacement of W particles across flux surfaces. The transport mechanism is shown to be the particle E × B-drifts due to the electric field created by the MHD instability underlying the ELM. In- and outward transport is observed as a series of interchange motions, leading to a superdiffusive behavior. This causes not only the particles near the plasma pedestal to move outwards but also the particles outside of the pedestal to move inwards. This has important consequences for operation with W in ITER, where it is expected to be screened in the pedestal, and ELMs are shown here to increase the core W density. A comparison with existing diffusive modeling is made, showing a qualitative agreement and the limitations of this simplified modeling approach.

Numerical study of tearing mode seeding in tokamak X-point plasma

A detailed understanding of island seeding is crucial to avoid neoclassical tearing modes and their negative consequences like confinement degradation and disruptions. In the present work, we investigate the growth of 2/1 islands in response to magnetic perturbations. Although we use externally applied perturbations produced by resonant magnetic perturbation (RMP) coils for this study, the results are directly transferable to island seeding by other MHD instabilities creating a resonant magnetic field component at the rational surface. Experimental results for 2/1 island penetration from ASDEX Upgrade are presented extending previous studies. Simulations are based on an ASDEX Upgrade L-mode discharge with low collisionality and active RMP coils. Our numerical studies are performed with the 3D, two-fluid, nonlinear MHD code JOREK. All three phases of mode seeding observed in the experiment are also seen in the simulations: first, a weak response phase characterized by large perpendicular electron flow velocities followed by a fast growth of the magnetic island size accompanied by a reduction of the perpendicular electron velocity and finally the saturation to a fully formed island state with perpendicular electron velocity close to zero. Thresholds for mode penetration are observed in the plasma rotation as well as in the RMP coil current. A hysteresis of the island size and electron perpendicular velocity is observed between the ramping up and down of the RMP amplitude consistent with an analytically predicted bifurcation. The transition from dominant kink/bending to tearing parity during the penetration is investigated.

Development of Microwave Imaging Diagnostics for WEST Tokamak

WEST is an upgrade of the Tore Supra tokamak to test ITER like divertor elements over very long pulses. For the studies of MHD instabilities affected by tungsten impurities, two microwave imaging diagnostics have been developed to obtain density and temperature fluctuation maps. The core reflectometer was refurbished and now measures the core density profiles in few microseconds. Measurements of several thousand profiles in sequence with microsecond dead time (burst mode) will be available in 2019 enabling the reconstruction of density maps with unprecedented time resolution. The major issue for the implementation of an electron cyclotron emission imaging diagnostic was the heat radiation during long pulses on the large vacuum window and the first optics. Two metallic in-vessel mirrors inside the personnel access tube solved this issue. The first mirror can withstand the heat radiation without water cooling, the second mirror redirects the beam toward the personnel access port. The oblique and small view angle of this access eases also the thermomechanical constraints on the vacuum window. As the space between the port flange and the tokamak access lobby was tight, a compact optical enclosure with vertical aligned optics was designed. To ensure a good reproducibility of the mirrors and the optical enclosure positions after a maintenance period and thus avoid re-alignment, different mechanical systems are used to manipulate the mirrors and the optical enclosure and to fine tune their position and their orientation. The first channel covers the low field side and will provide 192 pixel images of temperature fluctuations with 1–2 microsecond time resolution. The installation of the in-vessel mirrors is scheduled during March 2019. Diagnostic qualification tests will be done before the restart of the experiments late May 2019.

Summary of the 8th Asia-Pacific Transport Working Group (APTWG) Meeting

This conference report gives a summary on the contributed papers and discussions presented at the 8th Asia-Pacific Transport Working Group Meeting held at the Engineering & Technical College of Chengdu University of Technology, Leshan, China, from 12 to 15 June 2018. The objective of the meeting is to develop a predictive understanding of the physical mechanisms responsible for particle, energy and momentum transport in magnetically confined plasmas. In order to achieve this objective, five technical working groups were organized under the headings: (A) edge/SOL turbulence and L-H/H-L transition, (B) core-edge turbulence coupling, and transport in high beta plasmas, (C) transport and turbulence in 3D magnetic topology, (D) interaction between energetic particles, MHD instabilities and turbulence, and (E) model reduction and experiments for validation. A summary of the selected significant progresses from the presentations in the five technical working groups is given. The Young Researchers Forum which was held in this meeting is described.

A Two-step Magnetic Reconnection in a Confined X-class Flare in Solar Active Region 12673

Abstract Solar flares are often associated with coronal eruptions, but there are confined ones without eruptions, even for some X-class flares. How such large flares occurred and why they are confined are still not well understood. Here we studied a confined X2.2 flare in NOAA Active Region 12673 on 2017 September 6. It exhibits two episodes of flare brightening with rather complex, atypical ribbons. Based on topology analysis of the extrapolated coronal magnetic field, we revealed that there is a two-step magnetic reconnection process during the flare. Prior to the flare, there is a magnetic flux rope (MFR) with one leg rooted in a rotating sunspot. Neighboring the leg is a magnetic null-point structure. The sunspot drives the MFR to expand, pushing magnetic flux to the null point, and reconnection is first triggered there. The disturbance from the null-point reconnection triggers the second reconnection, i.e., a tether-cutting reconnection below the rope. However, these two reconnections failed to produce an eruption, because the rope is firmly held by its strapping flux. Furthermore, we compared this flare with an eruptive X9.3 flare in the same region 2 hr later, which has a similar MFR configuration. The key difference between them is that, for the confined flare, the MFR is fully below the threshold of torus instability, whereas for the eruptive one, the MFR reaches entirely above the threshold. This study provides good evidence supporting that reconnection alone may not be able to trigger eruption; rather, MHD instability plays a more important role.

Development and preliminary results of phase contrast imaging diagnostic on HL-2A

Phase Contrast Imaging (PCI) diagnostic has been developed on HL-2 A tokamak. The chord integral density fluctuations of plasma can be obtained by measuring the phase shift of CO2 laser which has a wavelength of 10.6 μm after passing through plasma. The 32-channel detector array enables PCI to diagnose density fluctuations with wavenumbers ranging from 2 cm−1 to 15 cm−1. The temporal resolution of PCI is better than 2 μs which makes it suitable for broadband turbulence research. Calibration by the sound source confirms that PCI is able to discriminate a wide range of wavenumbers. The plasma density fluctuations measured by PCI on HL-2 A is presented for the first time. Comparison of MHD instability investigations with magnetic probe data shows the reliability of this PCI diagnostic.

Long-Lived Banana Orbit Formation of Suprathermal Electrons During MHD Spikes in Runaway Tokamak Discharges

The secondary runaway electrons generation is the process in which already existing high energy runaway electrons knock out thermal plasma electrons directly into the runaway region by close Coulomb collisions. Such knocked-on electrons are immediately accelerated to ultrarelativistic velocities, since in the runaway region the toroidal electric field force overcomes the collisional friction force with thermal plasma particles. The avalanche of runaway electrons with mega-electron-volt energy emerges, hit of which with the construction elements of large-scale tokamaks and future international tokamak ITER can lead to catastrophic consequences. Due to its importance, this phenomenon is being actively studied both theoretically and experimentally in leading thermonuclear fusion centers. It is known that during secondary generation, the value of the transversal component of knocked-on electrons momentum with respect to the confining magnetic field may be significantly higher than the longitudinal one: p⊥ >> p∥. Thus, conditions for knocked-on electron trapping in a non-uniform tokamak magnetic field occur (banana orbits). Such electrons can no longer be accelerated by the inducted toroidal electric field to high energies, avalanche formation is partially suppressed. The question is how long this population of knocked-on and trapped electrons exists. In the presented paper, it is shown the additional possibility of formation and existence of such long-lived banana orbits of suprathermal electrons under conditions of plasma MHD activity when MHD instability spikes induced the strong burst of the toroidal electric field that results in the abrupt growth in these knocked-on and trapped electrons. This phenomenon is considered for the recent low-density EAST (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China) tokamak quasistationary runaway discharges. Long-lived trapped electrons (p⊥ >> p∥) also have an influence on the intensity of ECE emission. The considered phenomenon is important for correct interpretation of the runaway experiments on present-day tokamaks.

Evolution of a Pinch Column During the Acceleration of Fast Electrons and Deuterons in a Plasma-Focus Discharge

Plasma in a pinch column, as produced by a plasma-focus discharge at the deuterium filling and the current intensity reaching 1 MA, was investigated at the total neutron yield reaching about 10 10 per discharge. The use was made of neutron diagnostics, laser interferometry, soft X-ray measurements, optical emission spectroscopy, magnetic probes, as well as electron and ion measurements with the temporal, spatial, and energetic resolutions. The detailed studies showed the ordered toroidal, helical, and plasmoidal structures which could contain currents with poloidal and toroidal components and their associated magnetic fields. Their spontaneous transformations were explained by changes in a topology of magnetic field lines due to magnetic reconnections. A nonthermal acceleration of fast electrons and ions (producing hard X-rays and fusion neutrons, respectively) corresponded to: 1) the formation of plasmoids in the pinch column and 2) a decay of pinch constrictions and secondary plasmoids during the evolution of instabilities. A filamentary structure of the current flow could explain the high energy density and fast transformations of the magnetic energy into kinetic energy of electron and ion beams (reaching energy of hundreds of kiloelectronvolt). This paper summarizes the results obtained with the PF-1000 facility in 2009-2017, and describes the internal transformations in a dense plasma column during the evolution of MHD instabilities.

The deteriorating effect of plasma density fluctuations on microwave beam quality

Turbulent plasma edge density fluctuations can broaden a traversing microwave beam degrading its quality. This can be a problem for scenarios relying on a high spatial localization of the deposition of injected microwave power, like controlling MHD instabilities. Here we present numerical estimations of the scattering of a microwave beam by density fluctuations over a large parameter range, including extrapolations to ITER. Two codes are used, the full-wave code IPF-FDMC and the wave kinetic equation solver WKBeam. A successful comparison between beam broadening obtained from DIII-D experiments and corresponding full-wave simulations is shown.

Helically localized ballooning instabilities in three-dimensional tokamak pedestals

Recent experimental observations have found toroidally localized MHD instabilities in the plasma edge during operation with applied magnetic perturbations on ASDEX Upgrade in H-mode with low collisionality . Large edge plasma displacements are induced by a stable kink response to the 3D magnetic perturbations. This kink response results in localized changes of geometric quantities, which in turn leads to the localization of MHD instabilities in the plasma edge. Infinite-n ideal MHD ballooning theory is shown to predict the existence of these instabilities, as well as the observed toroidal localization. Utilizing 3D VMEC equilibria, the local geometric parameters determining ideal stability, include the local magnetic shear, normal curvature, and geodesic curvature, are calculated for experimentally relevant conditions. It is found that these shaping parameters have significant levels of 3D variation, with the local magnetic shear being the dominant factor behind changes in the local geometry. This behavior leads to a significant decrease in the stabilizing line-bending energy for certain field lines, resulting in the localization of the ballooning instability. Furthermore, it is observed that a finite amount of magnetic perturbation (and subsequent edge perturbation) is necessary to modify the local geometry and excite the localized instability, leading to a threshold behavior.

Nearing final design of the ITER EC H&CD Upper Launcher

The ITER ECRH system consists of 24 gyrotrons with up to 24 MW installed millimeter wave heating power at 170 GHz, power supplies, control system, transmission lines, one Equatorial and the four Upper Launchers. With its high frequency and small beam focus the ECRH has the unique capability of driving locally current. While the Equatorial Launcher mainly acts for central heating and current profile shaping, the Upper Launchers aim on suppressing MHD instabilities, especially neoclassical tearing modes (NTM) triggering plasma disruptions. The Upper Launchers inject millimeter waves through a quasi-optical section consisting of three fixed and the front steering mirror set. The eight overlapping beams have focal points optimized for suppression of the q = 3/2 and q = 2/1 NTMs.Several project change requests required the redesign of the Upper Launchers and the connected ex-vessel system. This redesign includes a new boundary geometry of the launchers as well as a newly designed cooling system for the Blanket Shield Module (BSM), a modified flange of the BSM to the structural main frame and a refined optical design. Additionally shield blocks with integrated in-vessel waveguides were added and the closure plate with waveguide and supply line feedthroughs was adapted. Further changes, not all caused by PCRs, include newly designed ex-vessel waveguide components with a reduced aperture and redesigned ultra low-loss CVD diamond windows. Finally several components originally foreseen as off-the-shelf components have become part of the design scope. The new launcher design status is presented with selected results on numerical design validation.

3D non-linear MHD simulation of the MHD response and density increase as a result of shattered pellet injection

The MHD response and the penetration of a deuterium shattered pellet into a JET plasma is investigated via the non-linear reduced MHD code JOREK with the neutral gas shielding (NGS) ablation model. The dominant MHD destabilizing mechanism by the injection is identified as the local helical cooling at each rational surface, as opposed to the global current profile contraction. Thus the injected fragments destabilize each rational surface as they pass through them. The injection penetration is found to be much better compared to MGI, with the convective transport caused by core MHD instabilities (e.g. 1/1 kink) contributing significantly to the core penetration. Moreover, the injection with realistic JET SPI system configurations is simulated in order to provide some insights into future operations, and the impact on the total assimilation and penetration depth of varying injection parameters such as the injection velocity or fineness of shattering is assessed. Further, the effect of changing the target equilibrium temperature or q profile on the assimilation and penetration is also investigated. Such analysis will form the basis of further investigation into a desirable configuration for the future SPI system in ITER.

Access to pedestal pressure relevant to burning plasmas on the high magnetic field tokamak Alcator C-Mod

Experiments on the Alcator C-Mod tokamak have utilized reactor-relevant magnetic fields to sustain substantially higher pedestal pressure than in other devices and allow close approach to the ITER H-mode baseline target pedestal pressure of 90 kPa. The EPED model, which couples the physics of transport driven by kinetic ballooning modes and MHD instabilities arising from peeling-ballooning modes, predicts the pressure profile at the onset of edge-localized modes (ELMs), and yields to lowest order a critical-βN like behavior for the pedestal: ( for fixed edge q). C-Mod routinely accesses edge plasma pressure in excess of 30 kPa, often by using a high-density () approach to high confinement, taking advantage of a regime known as enhanced D-alpha (EDA) H-mode. In the EDA H-mode, plasma transport regulates both the pedestal profiles and the core impurity content, thus holding the pedestal stationary at just below the peeling-ballooning stability boundary. This stationary ELM-suppressed regime has approached the maximum pedestal predicted by EPED at these densities: 60 kPa. This in turn gives rise to volume-averaged core plasma pressure in excess of 0.2 MPa, a world record value for a magnetic fusion device. Another approach to achieving high pressure utilizes a pedestal limited by current-driven modes at low collisionality, in which pressure increases with density and which allows access to a higher EPED solution, termed ‘super-H’. C-Mod experiments at reduced density () and strong plasma shaping () accessed this regime, producing pedestals with pressures up to 80 kPa (approximately 90% of the ITER target) and temperatures of nearly 2 keV. In a number of these hot H-modes, we observe strong edge instabilities at low toroidal mode number (n = 1) when pedestal pressure approaches predicted values from EPED, showing that current-driven MHD modes can serve as a limit on the pedestal in a metal-walled tokamak at high pressure and low collisionality.

Dynamics of MHD instabilities near a ferromagnetic wall

Prospective fusion component testing and DEMO power reactor concepts are expected to employ low-activation ferritic steels because of their ability to withstand the high neutron flux of the reactor environment. However, theory suggests that ferromagnetic material may amplify certain external MHD instabilities. Using its ferromagnetic-resistive wall mode (FRWM) upgrade, the High Beta Tokamak-Extended Pulse (HBT-EP) experiment has observed approximately doubled growth rates when operating with a close-fitting ferromagnetic first wall, compared to operation with a stainless steel first wall. The presence of a ferromagnetic wall correlates with earlier disruptions, and FRWM growth rates increase with decreasing mode rotation, as expected due to the increased skin depth allowing greater mode interaction with the bulk ferromagnetic material. It is also seen that introducing low-n asymmetries into the toroidal distribution of ferromagnetic material, similar to the ITER test blanket module toroidal asymmetry, changes the phase preference of rotating modes; meanwhile, a similar change in purely conducting material does not significantly change the mode’s phase preference.

DIII-D Research to Prepare for Steady State Advanced Tokamak Power Plants

We review progress made on the advanced tokamak path to fusion energy by the DIII-D National Fusion Facility (Luxon et al. in Nucl Fusion 42:614, 2002). The advanced tokamak represents a highly attractive approach for a future steady state fusion power plant. In this concept, there is a natural alignment between high pressure operation, favorable stability and transport properties, and a highly self-driven (‘bootstrap’) plasma current to sustain operation efficiently and without disruptions. Research on DIII-D has identified several promising plasma configurations for fully non-inductive operation with potential applications to a range of future devices, from ITER to nuclear science facilities, to compact or large scale fusion power plants. Significant progress has been made toward realizing these scenarios, with the demonstration of high β access, off-axis current drive techniques, model based profile control, and stability and ELM control in reactor relevant physics regimes. Radiative techniques have also been pioneered to develop improved compatibility with divertor requirements, and simultaneous access to high performance pedestals. Research has also developed major advances in physics understanding, validating concepts of kinetic damping of ideal MHD instabilities that enable high β operation, identifying how current profile and β influence plasma turbulence in order to validate and improve turbulent transport models, and understanding the physics of energetic particle redistribution due to Alfvénic and other instabilities. These advances have been partnered with development of a rigorous integrated modeling framework used to interpret and validate individual physics models of the various aspects of plasma behavior, and to guide development of improved regimes and upgrades. These tools are also being used to develop and validate concepts for future reactors directly. Having established these foundations, DIII-D is now undergoing a substantial upgrade to raise power, current drive, electron heating and 3-D field capabilities in order to validate this physics and test conceptual solutions in reactor-relevant physics regimes, with a goal to resolve the key scientific and technology questions to enable a decision on a future steady state fusion power plant.

Progress of the KSTAR Research Program Exploring the Advanced High Performance and Steady-State Plasma Operations

Korea Superconducting Tokamak Advanced Research (KSTAR) program is strongly focused on solving the scientific and technological issues in steady-state high performance plasma operation in preparation for ITER operation as well as the design basis for DEMO. In this regards, KSTAR has made significant advances in developing long pulse and high performance plasma scenarios utilizing the advantage of the fully superconducting tokamak. Ten-year of KSTAR operation showed the outstanding progress in the plasma control extending the operation window of the plasma discharges achieving the H-mode up to 1 MA in plasma current, up to 72 s in flat top duration, and up to 2.16 in elongation. In addition to the long pulse discharge, high performance discharges with high betas (βN ~ 3) could be achieved in the broad range of edge safety factor (q95) without external error field correction. The unique features of the KSTAR device (magnetic accuracy with extremely low error fields, steady-state capable heating systems, in-vessel control coils, and advanced imaging and profile diagnostics) has been fully exploited to explore the unveiled physics as well as to exploring the systematic solution for suppression of edge localized mode (ELM) crash. Achieved examples are the record long pulse of H-mode operation without an ELM crash (~ 30 s up to date), and progress in the fundamental transport physics through systematic study using these unique capabilities. Based on the previous research results, intensive research will be followed to explore the advanced high beta operation (βN ~ 4) with fully suppressed harmful MHD instabilities aiming the integrated solution for DEMO. In this regards, an additional current drive systems and in-vessel structures will be upgraded.

MHD Instabilities in Accretion Disks and Their Implications in Driving Fast Magnetic Reconnection

Abstract Magnetohydrodynamic instabilities play an important role in accretion disk systems. Besides the well-known effects of magnetorotational instability (MRI), the Parker–Rayleigh–Taylor instability (PRTI) also arises as an important mechanism to help in the formation of the coronal region around an accretion disk and in the production of magnetic reconnection events similar to those occurring in the solar corona. In this work, we have performed three-dimensional magnetohydrodynamical (3D-MHD) shearing-box numerical simulations of accretion disks with an initial stratified density distribution and a strong azimuthal magnetic field with a ratio between the thermal and magnetic pressures of the order of unity. This study aimed at verifying the role of these instabilities in driving fast magnetic reconnection in turbulent accretion disk/corona systems. As we expected, the simulations showed an initial formation of large-scale magnetic loops due to the PRTI followed by the development of a nearly steady-state turbulence driven by both instabilities. In this turbulent environment, we have employed an algorithm to identify the presence of current sheets produced by the encounter of magnetic flux ropes of opposite polarity in the turbulent regions of both the corona and the disk. We computed the magnetic reconnection rates in these locations, obtaining average reconnection velocities in Alfvén speed units of the order of 0.13 ± 0.09 in the accretion disk and 0.17 ± 0.10 in the coronal region (with mean peak values of order 0.2), which are consistent with the predictions of the theory of turbulence-induced fast reconnection.

Instability induced by recollimation in highly magnetised outflows

In the paradigm of magnetic acceleration of relativistic outflows, a crucial point is identifying a viable mechanism to convert the Poynting flux into the kinetic energy of the plasma, and eventually into the observed radiation. Since the plasma is hardly accelerated beyond equipartition, MHD instabilities are often invoked to explain the dissipation of the magnetic energy. Motivated by the fast variability that is shown by the gamma-ray flares of both AGN and PWNe, different authors have proposed the Poynting flux to be dissipated in a region where the flow is converging. Here we perform a linear stability analysis of ultra-relativistic, highly magnetised outflows with such a recollimation nozzle, showing that MHD instabilities are indeed induced by the convergence of the flow. The amplitude of the perturbations increases while recollimation gets stronger, and eventually diverges when the flow is focused to a single point. Hence, depending on the geometry of the outflow, instabilities excited while the flow is converging may play an important role to dissipate the magnetic energy of the plasma.

Data mining for vortices on the Earth's magnetosphere – algorithm application for detection and analysis

Abstract. Unsteady processes in the solar wind–magnetosphere interaction, such as vortices developed at the magnetopause boundary by the Kelvin–Helmholtz instability, may contribute to the process of mass, momentum and energy transfer into the Earth's magnetosphere. The research described in this paper validates an algorithm to automatically detect and characterize vortices based on velocity data from simulations. The vortex identification algorithm (VIA) systematically searches the 3-D velocity fields to identify critical points where the magnitude of the velocity vector vanishes. The velocity gradient tensor is computed and its invariants are used to assess vortex structure in the flow field. We use the Community Coordinated Modeling Center (CCMC) Runs on Request capability to create a series of model runs initialized from the conditions observed by the Cluster mission in the Hwang et al. (2011) analysis of Kelvin–Helmholtz vortices observed during southward interplanetary magnetic field (IMF) conditions. We analyze further the properties of the vortices found in the runs, including the velocity changes within their motion across the magnetosheath. We also demonstrate the potential of our tool to identify and characterize other transient features (e.g., flux transfer events, FTEs) with vortical internal structures. We find that the vortices are associated with flows on the magnetosheath side of the magnetopause that reach speeds greater than the solar wind speed at the bow shock. Keywords. Magnetospheric physics (MHD waves and instabilities; solar wind–magnetosphere interactions) – space plasma physics (numerical simulation studies)

Statistical study of magnetic reconnection in accretion disks systems around HMXBs

AbstractHighly magnetized accretion disks are present in high-mass X-ray binaries (HMXBs). A potential mechanism to explain the transition between the High/Soft and Low/Hard states observed in HMXBs can be attributed to fast magnetic reconnection induced in the turbulent corona. In this work, we present results of global general relativistic MHD (GRMHD) simulations of accretion disks around black holes that show that fast reconnection events can naturally arise in the coronal region of these systems in presence of turbulence triggered by MHD instabilities, indicating that such events can be a potential mechanism to explain the transient non-thermal emission in HMXBs. To find the zones of fast reconnection, we have employed an algorithm to identify the presence of current sheets in the turbulent regions and computed statistically the magnetic reconnection rates in these locations obtaining average reconnection rates consistent with the predictions of the theory of turbulence-induced fast reconnection.