Poloidal Flow Research Articles

The impact of flow induced by rotating magnetic fields on processes in a molten conductive medium

This paper studies a method of stirring liquid metal by the action of rotating magnetic fields using two ring inductors placed next to each other. These inductors generate magnetic fields rotating in opposite directions. The aim of this study is numerical investigation of the generated fluid flow and its impact on the homogenization of a two-phase medium, as well as on the crystallization process. The impact of these electromagnetic forces proves to cause the generation of intense poloidal flow component. The arising flow is accompanied by oscillatory motion of vortex structures and their interaction resulting in effective mixing of the liquid metal. The moderate values of the force parameter have been found to lead to the most homogeneous medium under stirring. Under non-stationary action, the force parameter modulations in a certain frequency range have practically no effect on the homogeneity occurrence time and the homogeneity value. The positive effect of stirring by magnetic fields of complex topology on the rate and homogeneity of metal solidification is stated. The obtained results are relevant for improving the quality of foundry ingots.

Study of turbulent transport in magnetized plasmas with flow using symplectic maps

Turbulent transport in a magnetized plasma of the kind found in tokamaks is modeled by a 2D wave spectrum that allows reduction to a symplectic map. The properties of particle transport when chaos sets in are analyzed in various circumstances including finite Larmor radius (FLR) effects and a background plasma flow. For large wave amplitudes, regular particle orbits become chaotic, which represents a type of Lagrangian turbulence. When chaos becomes global, it leads to the loss of particle confinement. Poloidal flows tend to decrease the chaos in some regions, and they can give rise to the formation of transport barriers. FLR effects not only reduce chaos but also give rise to non-local behavior. Thus, when the particles have a thermal distribution of Larmor radii, a non-Gaussian particle distribution function in space is obtained. However, the transport preserves its diffusive scaling when there is no flow. Previous results about the dependence of the diffusion coefficient with amplitude are re-derived analytically and numerically taking into account FLR effects. In the presence of general poloidal flows, the transport has to be described by a two-step map. They modify the nature of transport in the direction of the flow from diffusive to ballistic to super-ballistic, depending on the type of flow. The transverse transport, in turn, shows suppression of the oscillations with wave amplitude that are present in the absence of flow. When the plasma flow varies linearly with radius, the transport can be studied with a similar single-step map, and the transverse diffusion coefficient is reduced while parallel transport can become super-ballistic. For non-monotonic flows, there are accelerating modes that can produce ballistic-like particles while the bulk of the particles behaves diffusively.

Nonmodal kinetic theory of the stability of the compressed–sheared plasma flows generated by the inhomogeneous microscale turbulence in the tokamak edge plasma

The nonmodal kinetic theory of the stability of the two-dimensional compressed–sheared mesoscale plasma flows, generated by the radially inhomogeneous electrostatic ion cyclotron parametric microturbulence in the pedestal plasma with a sheared poloidal flow, is developed. It bases on the investigation of the temporal evolution of the compressed–sheared modes. The integral equation, which governs the temporal evolution of the electrostatic potential of the plasma species responses on the mesoscale compressed–sheared convective flows, is derived. The exceptional advantage of the derived integral equation, which uses the wavevector-time variables, is the ability to perform the analysis of the nonmodal evolution of electrostatic potential during any finite time domain and to investigate the transient processes which occurs at any definite time scales. The approximate nonmodal solution of this equation for the kinetic drift instability in the compressed flow is given.

Experimental study of the edge radial electric field in different drift configurations and its role in the access to H-mode at ASDEX Upgrade

The formation of the equilibrium radial electric field (Er) has been studied experimentally at ASDEX Upgrade (AUG) in L-modes of “favorable” (ion ∇ B-drift toward primary X-point) and “unfavorable” (ion ∇ B-drift away from primary X-point) drift configurations, in view of its impact on H-mode access, which changes with drift configurations. Edge electron and ion kinetic profiles and impurity velocity and mean-field Er profiles across the separatrix are investigated, employing new and improved measurement techniques. The experimental results are compared to local neoclassical theory as well as to a simple 1D scrape-off layer (SOL) model. It is found that in L-modes of matched heating power and plasma density, the upstream SOL Er and the main ion pressure gradient in the plasma edge are the same for either drift configurations, whereas the Er well in the confined plasma is shallower in unfavorable compared to the favorable drift configuration. The contributions of toroidal and poloidal main ion flows to Er, which are inferred from local neoclassical theory and the experiment, cannot account for these observed differences. Furthermore, it is found that in the L-mode, the intrinsic toroidal edge rotation decreases with increasing collisionality and it is co-current in the banana-plateau regime for all different drift configurations at AUG. This gives rise to a possible interaction of parallel Pfirsch–Schlüter flows in the SOL with the confined plasma. Thus, the different H-mode power threshold for the two drift configurations cannot be explained in the same way at AUG as suggested by LaBombard et al. [Phys. Plasmas 12, 056111 (2005)] for Alcator C-Mod. Finally, comparisons of Er profiles in favorable and unfavorable drift configurations at the respective confinement transitions show that also the Er gradients are all different, which indirectly indicates a different type or strength of the characteristic edge turbulence in the two drift configurations.

Development of the neutral model in the nonlinear MHD code JOREK: Application to E × B drifts in ITER PFPO-1 plasmas

The prediction of power fluxes and plasma-wall interactions impacted by MHD processes during ITER operation [disruption, Edge Localized Modes (ELMs), 3D magnetic fields applied for ELM control, etc.] requires models that include an accurate description of the MHD processes themselves, as well as of the edge plasma and plasma-wall interaction processes. In this paper, we report progress on improving the edge plasma physics models in the nonlinear extended MHD code JOREK, which has capabilities to simulate the MHD response of the plasma to the applied external 3D fields, disruptions and ELMs. The extended MHD model includes E × B drifts, diamagnetic drifts, and neoclassical flows. These drifts can have large influences, on e.g., divertor asymmetries. Realistic divertor conditions are important for impurity sputtering, transport, and their effect on the plasma. In this work, we implemented kinetic and fluid neutral physics modules, investigated the influence of poloidal flows under divertor conditions in the ITER PFPO-1 (1.8T/5MA) H-mode plasma scenario, and compared the divertor plasma conditions and heat flux to the wall for both the fluid and kinetic neutral model (in JOREK) to the well-established 2D boundary plasma simulation code suite SOLPS-ITER. As an application of the newly developed model, we investigated time-dependent divertor solutions and the transition from attached to partially detached plasmas. We present the formation of a high-field-side high-density-region and how it is driven by poloidal E × B drifts.

Internal Kink Instabilities Driven by Poloidal Flow via Non-Resonant Excitation in Tokamaks

Non-resonant excitation due to plasma inertia may be dominant in inducing internal kink (IK) instabilities. Poloidal rotation can effectively modify plasma inertia and cause non-resonant excitation to occur. An extended dispersion relation including poloidal rotation is established to study the IK mode and the fishbone (FB) mode. It is found that in rotating plasmas, even for a stable IK mode (i.e., the perturbed potential energy of background plasma δWc is positive) and in the absence of energetic particles (EPs), poloidal rotation can drive the IK mode via non-resonant excitation. Moreover, the IK mode is easy to be driven by poloidal rotation in weak magnetic shear plasmas. Similar to toroidal rotation, when poloidal rotation frequency exceeds a threshold, the FB mode can transform into a branch of a non-resonant mode. The real frequency of the mode, being independent of the precessional frequency of EPs, is just equal to the poloidal rotation frequency. Thus, the non-resonant mode is characterized by the long-lived mode (LLM) observed in toroidal rotating plasmas. The critical gradient of the poloidal rotation profile plays a crucial role in causing the resonant mode to evolve into a non-resonant one; for instance, only for a very peaked poloidal rotation profile can the FB mode transform into the LLM. In addition, the diamagnetic drift frequency of thermal ions can stabilize the FB and the IK modes.

Implementation of SOLPS-ITER code with new Grad–Zhdanov module for D–T mixture

A new Grad–Zhdanov module is implemented into the SOLPS-ITER code for calculation of the parallel kinetic coefficients. A complete, multi-ion generalization is performed, relaxing the heavy impurity assumption. A JET-like D+T+Ne test simulation is conducted to demonstrate the ability to model a 50/50 deuterium (D) and tritium (T) mixture in SOLPS-ITER. More than 30% T build-up with respect to D is observed in different parts of the simulation domain, in particular at the high field side. A T predominance over D is also observed near both the inner and outer targets. It is a result of the different effective diffusion, dominated by charge-exchange processes, for the D and T neutral species. A simple 1D model is proposed to describe this phenomenon. The contribution to the differing D and T distribution in different regions from the Grad–Zhdanov thermal force is also studied. Due to the thermal force and the D/T poloidal flow from the low field side to the high field side, the prevalence of the T species over D is found at the high field side at the X-point level. The latter leads to an inner–outer divertor asymmetry of . However, the effect is relatively small due to the close D and T masses. It is further shown that the infinite ion mass difference limit, which is used for the derivation of the Zhdanov-Yushmanov analytical expressions and applied for the thermal force coefficient calculation previously used in the SOLPS-ITER code, overestimates significantly. Thus, the old SOLPS-ITER model should not be applied for D–T simulations. Finally, possible experimental studies of the D and T spatial separation due to the new effects revealed by this modelling are discussed.

Effect of radial electric field on ion-temperature gradient driven mode stability

To understand the effect of given radial electric field on ion-temperature gradient driven mode (ITG) stability in tokamak plasmas, we derive the eigenmode equation for ITG including the poloidal rotation and density modulation associated with radial electric field by using nonlinear gyrokinetic theory. The equation is solved for the eigenfrequency, growth rate and parallel mode structure of ITG both in short- and long-wavelength limit with energetic-particle-induced geodesic acoustic mode (EGAM) as a specific form. The eigenmode equation is not only solved analytically, but also solved numerically to validate the analytic solutions. It is found that, the radial electric field induced poloidal rotation can significantly stabilize ITG, while the density perturbation of the radial electric field may slightly distort the ITG parallel mode structure, but has little effect on ITG stability. The result is consistent with the common picture of turbulence suppression by poloidal shear flow. The general model is also applicable to the investigation of the indirect interaction of ITG and energetic particle driven Alfvén instabilities via zonal structures generation, by introducing poloidal rotation and density modulation associated with zonal structures spontaneously excited by Alfvén instabilities. The indirect channel is supplement to the direct interaction of microturbulences and energetic particle driven Alfvén instabilities.

Introduction of reinforcing microparticles of TiB2 and BN into aluminium by electromagnetic stirring

The paper describes an experiment in which aluminum alloy was reinforced with TiB2 and BN particles and investigates the physical properties of the resulting aluminum composite. In the experiment, TiB2 and BN reinforcing particles were introduced into molten metal as part of pellets produced from aluminum micro powder in the presence of MHD mixing with subsequent directional crystallization. The pellets were thrown into a crucible with liquid aluminum placed in the working volume of an MHD stirrer, which generated separately controlled toroidal and poloidal flows. It was found that the dissolution of the pellets prepared from the mixture of aluminum micro particles (100–150 μm) and reinforcing TiB2 micro particles (1–5 μm) under MHD stirring proceeds much better compared to the pellets with BN micro particles (1–5 μm). The pellets in which the weight percentage of BN reinforcing particles exceeds 7% poorly “dissolve” in aluminum melt, while the pellets with 10% TiB2 micro particles “dissolve” satisfactorily. Due to a large number of pellets introduced into liquid aluminum in the course of the experiment and because of the strong cooling of the melt, it was not possible to obtain the weight percentage of reinforcing micro particles in an aluminum ingot exceeding 2.5% for TiB2 micro particles and 1.4% for BN micro particles. The results of the experiment showed that the hardness and tensile strength of aluminum increase with increasing content of TiB2 and BN micro particles in a similar way, but in the case BN particles to a lesser extent. The strength and electrical resistivity of liquid aluminum reinforced with TiB2 and BN particles increase with their rising fraction.The structural study of the specimens on electron and digital optical microscopes were carried out in compliance with the state budget subject AAAAA19-119012290101-5 of the Institute of Continuum Mechanics at the UralBranch of the Russian Academy of Sciences; the experiments on introduction of microparticles into molten aluminium were funded by the Government of the Perm Kray and the Russian Foundation for Basic Research under the Ural regional project No. [19-48-590001 р_а].

Modeling subduction on the kitchen table

A simple analogue model home setup with white school glue and natural black magnetic sand, successfully reproduces many of the main processes associated to subduction on planet Earth. This homemade model reproduces the spontaneous subduction of a dense, magnetic sand layer into a viscous layer of white school glue in a plastic kitchen container, all in a few minutes. Preparation of this experimental setup is simple, the materials are inexpensive or easy to obtain, and quantitative observations can be performed using cellphone cameras and freely available software on a personal computer. This very visual model setup brings the concept of subduction closer to the student of geosciences, and can be performed and analyzed at home. Once all the materials are set, spontaneous subduction of the magnetic sand layer takes place in a single event punctuated by one or two slab-tearing episodes, where the trenches quickly retreat (consuming as much as 15 cm2/s), while leaving nearby slab segments unsubducted. Rapidly retreating trenches induce toroidal flow of glue around the torn subducting slab, and cause large vertical-axis rotations of the unsubducted segments above, dynamically supporting, extending, and eventually bringing them into the vigorous poloidal flow of the subducting slab. Black magnetic sand impurities within the otherwise white glue allow precise tracking of the toroidal and poloidal flows. Complete detachment from the edges of the container changes the kinematics of the experiment from rapid trench retreat, to an stationary trench that pulls the remainder of the black magnetic sand layer into the trenches and causes thinning and opening of an extensional axis, reminiscent of spreading centers.

Transport barrier in 5D gyrokinetic flux-driven simulations

Two ways for producing a transport barrier through strong shear of the E × B poloidal flow have been investigated using GYSELA gyrokinetic simulations in a flux-driven regime. The first one uses an external poloidal momentum (i.e. vorticity) source that locally polarizes the plasma, and the second one enforces a locally steep density profile that also stabilizes the ion temperature gradient (ITG) instability modes linearly. Both cases show a very low local turbulent heat diffusivity coefficient and a slight increase in core pressure when a threshold of (respectively the E × B shear rate and average linear growth rate of ITG) is reached, validating previous numerical results. This pressure increase and quench are the signs of a transport barrier formation. This behaviour is the result of a reduced turbulence intensity which strongly correlates with the shearing of turbulent structures as evidenced by a reduction of the auto-correlation length of potential fluctuations as well as an intensity reduction of the k θ spectrum. Moreover, a small shift towards smaller poloidal wavenumber is observed in the vorticity source region which could be linked to a tilt of the turbulent structures in the poloidal direction.

Investigation of the role of fishbone activity in the formation of internal transport barrier in HL-2A plasma

A tokamak scenario with q(0) close to 1 has been achieved on HL-2A tokamak, which is an internal transport barrier (ITB) at low central shear or a steady-state ITB combined with an H-mode edge barrier. In this scenario, the formation of an ITB with a steep ion temperature profile is observed to be closely linked to the q = 1 magnetic surface and magnetohydrodynamic (MHD) activities around it, such as long-lived mode (LLM) or fishbone activities. Experimental evidence and simulation analysis suggest that the fishbone activities can induce a poloidal flow, which is beneficial for the suppression of turbulence in the plasma core region. Furthermore, an application of central electron cyclotron resonance heating (ECRH) to such beam-heated weak shear ITB discharges leads to a substantial effect on central MHD stability, converting the LLM into fishbone activity and hence enhancing the strength of the ITB with a much stronger gradient. Moreover, ITBs in combination with an H-mode barrier were achieved for 10 confinement times with βN = 2.0.

Electro-vortex flows in a cylindrical cell under axial magnetic field

Liquid metal flows, generated in a cylindrical cell by a strong electric current flowing from the central bottom electrode to the sidewall, are studied experimentally and numerically. In the absence of an external magnetic field, the flow is caused by the interaction of the electric current with its own magnetic field and is known as electro-vortex flows (EVF), which has a pure poloidal configuration. It is also well known that even a relatively weak azimuthal force (which immediately arises if a vertical external magnetic field is applied) induces a swirl that completely changes the structure of the flow, suppressing the poloidal motion (Davidson et al., Eur. J. Mech. B Fluids, vol. 18, 1999, pp. 693–711). We study the steady-state modes and analyse the dynamics of transient modes, following the evolution of the flow structure at different ratios of azimuthal and poloidal Lorentz forces, governed by the applied current and vertical magnetic field. In the considered problem, the peculiarities of transient modes, leading to the suppression of the poloidal flow, are associated with the fact that the domain of acting forces is localized in the vicinity of the bottom electrode. The scenario of flow evolution strongly depends on the ratio of electro-vortex and rotational forces (whether the EVF has time to form before a noticeable swirl of the metal, or the swirl dominates from the beginning of flow evolution).

Nonlinear Fokker-Planck collision operator in Rosenbluth form for gyrokinetic simulations using discontinuous Galerkin method

A gyroaveraged nonlinear collision operator is formulated based on the Fokker-Planck operator in the Rosenbluth-MacDonald-Judd (RMJ) potential form and implemented for the gyrokinetic simulations with the discontinuous Galerkin scheme. The divergence structure of the original RMJ form is carefully preserved throughout the formulation to guarantee the density conservation while neglecting the finite Larmor radius effect. The B-spline finite element method is used to calculate the Rosenbluth potentials for the nonlinear collision operator. In addition to the nonlinear collision operator, linear and Dougherty collision models are also implemented to assess the benefits and drawbacks of each model. For the conservation of the parallel momentum and energy, we adopt a simple advection-diffusion model which numerically enforces the conservation of physical quantities. From bump-on-tail relaxation tests, the monotonically increasing entropy in time and conservation properties are demonstrated for the developed collision operator. Also, a few theoretical predictions for the neoclassical physics such as the neoclassical heat flux, poloidal flow and collisional damping of zonal flow are successfully reproduced by numerical simulations.

Characterizing the flow and turbulence structure near the last closed flux surface in L-mode plasmas of ASDEX Upgrade

Since high density operation is advantageous for building an efficient fusion reactor, understanding the density limit in tokamaks has been seen as one of the most important issues. This paper reports a series of measurements around the last-closed flux surface (LCFS) in L-mode plasmas by using a thermal helium beam diagnostic. Fluctuation analysis has been employed to characterize the poloidal flow and the turbulence structure. A reversal of the poloidal flow in the scrape-off layer and concomitant cooling of the outer divertor plasma are observed as the density is raised. While, in the confined region, the change in the density barely affects the poloidal flow, a higher density shifts the fluctuation power spectral densities toward lower frequencies and wave numbers. The eddy tilting of this region is consistent with what is expected from the magnetic shear effect. A radially coherent low frequency mode appears in the case of the highest density investigated in this study (n¯e/ne,GW = 0.51), and higher frequencies near the LCFS are modulated by this mode.

Numerical investigation of the fishbone instability effect on thermal pressure in EAST tokamak

The formation of the internal transport barrier (ITB) is observed after the emergence of fishbone instabilities on the Experimental Advanced Superconducting Tokamak (EAST). The kinetic–magnetohydrodynamic hybrid code M3D-K has been applied to investigate the fishbone instability effect on thermal pressure based on EAST Shot No. 71320. Without fluid nonlinearity, it is found that when the central gradient of the total pressure profile is above a threshold, the thermal pressure profile becomes more peaked due to the nonlinear evolution of the fishbone instability, which confirms that the fishbone instability can transport the thermal pressure radially inward and promote the ITB formation. When fluid nonlinearity is included, the poloidal zonal flow prevents the thermal pressure to become more peaked in the core region. As the neoclassical effect can cause the damping of the poloidal zonal flow and is neglected in our simulation, the actual promotion of ITB formation due to the fishbone instability is expected to be between that without fluid nonlinearity and with fluid nonlinearity.

Anomalous convective transport of the tokamak edge plasma, caused by the inhomogeneous ion cyclotron parametric turbulence

In this paper, we develop the kinetic and hydrodynamic theories of the convective mesoscale flows driven by the spatially inhomogeneous electrostatic ion cyclotron parametric microturbulence in the pedestal plasma with a sheared poloidal flow. The developed kinetic theory predicts the generation of the sheared poloidal convective flow and of the radial compressed flow with radial flow velocity gradient. The developed hydrodynamic theory of the convective flows reveals the radial compressed convective flow as the dominant factor in the formation of the steep pedestal density profile with density gradient exponentially growing with time. This gradient density growth is limited by the formation of the radial oscillating with time ion outflow of pedestal plasma to the scrape-off layer.

Mantle dynamics of the North China Craton: new insights from mantle transition zone imaging constrained by P-to-S receiver functions

SUMMARY Cratons are generally defined as stable continental blocks with a strong cratonic root that typically is at least ∼200 km thick. Many cratons have undergone little internal tectonism and destruction since their formation, but some of them, such as the eastern part of the North China Craton (NCC), the Dharwar Craton and the Wyoming Craton, have lost their thick cratonic root and become reactivated in recent geological history, leading to widespread Meso-Cenozoic volcanisms. The mechanisms responsible for such decratonization remain debated. To provide new constraints on models leading to decratonization, in this study we stack 612 854 source-normalized P-to-S conversions (receiver functions or RFs) from the 410 and 660 km discontinuities (d410 and d660, respectively) bordering the mantle transition zone (MTZ) recorded at 1986 stations in the NCC. Both the number of RFs and the number of stations are unprecedented in the study area. The average apparent depths of the d410 and d660 and the thickness of the MTZ are 413 ± 6, 669 ± 8 and 255 ± 6 km, respectively. A depression of up to 37 km and mean 11 km of the d660 are clearly observed beneath the eastern NCC, mainly caused by the possible existence of a relatively large amount of water in the MTZ. Our study provides strong observational evidence for geodynamic modelling that suggests water in the MTZ can be driven out into the upper mantle by poloidal mantle flow induced by the subduction and retreat of subducted oceanic slabs. The results are consistent with the weakening of the lithosphere beneath the eastern NCC by the release of water (in the form of structurally bound H/OH) brought down to the MTZ by subduction of the Pacific slab. Continuous slab dehydration and the ascent of fluids would have triggered intraplate volcanism and mantle upwelling in the eastern NCC, as evidenced by the spatial correspondence among the lower-than-normal upper-mantle seismic velocities, unusually large depressions of the d660, Cenozoic basaltic volcanism and thinning of the cratonic lithosphere.

Feasibility study of heavy ion beam probe in CFQS quasi-axisymmetric stellarator

The world’s first quasi-axisymmetric stellarator, CFQS, is now under construction. The CFQS will be dedicated to studies on the interaction between flow and turbulence, and confinement improvement by suppression of turbulence in connection with proof-of-principle experiment of quasi-axisymmetry. In order to conduct this experimental research, a heavy ion beam probe (HIBP) system is planned to be installed and utilized to measure the radial electric field and its fluctuation in a CFQS plasma. In this paper, an orbit calculation for a probe beam is performed to verify feasibility of the HIBP in the CFQS. The required beam energy, possible ion species, and the observable region in a CFQS plasma are investigated. The beam attenuation by a CFQS plasma is also estimated for different beam ion species. If we use 133Cs+ as a primary probe beam, the required beam energy is expected to be 30∼50 keV, which is relatively easy to handle. In this case the beam attenuation, evaluated by the ratio between the injected and detected beam currents, is 10−3∼10−2 in a CFQS plasma with a line-averaged electron density of <1.0 × 1019 m−3. For a higher density plasma, usage of 85Rb+ is better in terms of low-beam-attenuation, and a high signal-to-noise ratio. The HIBP in the CFQS will provide a great opportunity to study physics experimentally, related to the radial electric field, poloidal flow, and turbulence suppression.

Development of dual X-mode Doppler reflectometry system in Heliotron J

A dual X-mode Doppler reflectometry system is developed to measure the radial electric field in a stellarator/heliotron device, Heliotron J. The system is designed to have dual channels where the observation points are placed symmetrically to the equatorial plane, enabling the poloidal flow velocity measurement at two different positions in the same toroidal section, which is useful for the search for a zonal flow. In the system, an RF source generates the microwave frequency of 8.25–12.5 GHz, upconverted by an intermediate frequency of 27.5 MHz and transmitted with a coaxial cable to a transmitter located near the Heliotoron J vacuum vessel. After quadrupling the RF waves at the transmitter, the microwaves of 33–50 GHz are injected in X-mode into a plasma using a spherical focusing mirror installed inside the vacuum vessel. The local wavenumber of the probing microwaves, k ⊥, is 1.56–1.66 cm−1. The Doppler-shifted reflected wave is downconverted to a 110 MHz signal by mixing with the LO at the receiver, amplified, and then detected by an I/Q detector. In a tabletop test, we have confirmed that the phase estimated by the I/Q detector is proportionally changed as a function of the horn antenna distance. We have successfully measured the Doppler-shifted spectra of the I/Q signals and estimated the radial electric field in an electron cyclotron heated (ECH) plasma.