Boundary Plasma Turbulence Research Articles

Effect of triangularity on plasma turbulence and the SOL-width scaling in L-mode diverted tokamak configurations

The effect of triangularity on tokamak boundary plasma turbulence is investigated using global, flux-driven, three-dimensional, two-fluid simulations. The simulations show that negative triangularity (NT) stabilizes boundary plasma turbulence, and linear investigations reveal that this is due to a reduction of the magnetic curvature driven by interchange instabilities, such as the resistive ballooning mode (RBM). As a consequence, the pressure decay length L p , related to the scrape-off layer (SOL) power fall-off length λ q , is found to be affected by triangularity. Leveraging considerations on the effect of triangularity on the linear growth rate and nonlinear evolution of the RBM, the analytical theory-based scaling law for L p in L-mode plasmas, derived by Giacomin et al (2021 Nucl. Fusion 61 076002), is extended to include the effect of triangularity. The scaling is in agreement with nonlinear simulations and a multi-machine experimental database, which includes recent TCV discharges dedicated to the study of the effect of triangularity in L-mode diverted discharges. Overall, the present results highlight that NT narrows the L p and considering the effect of triangularity is important for a reliable extrapolation of λ q from present experiments to larger devices.

Thermal dynamics in the flux‐coordinate independent turbulence code GRILLIX

AbstractGRILLIX employs the flux‐coordinate independent approach (FCI), which allows us to study boundary plasma turbulence in realistic diverted configurations. Recently, the physical model in GRILLIX has been extended to a global drift‐reduced Braginskii model, without any separation between background and fluctuations. It includes electromagnetic and thermal dynamics with hot ions, relaxation of the Boussinesq approximation and non‐linear parametric dependencies. This contribution presents solutions to associated issues, that is, the ion diamagnetic polarization and the stiff parallel heat conduction. Simulations based on parameters characteristic for the Alcator C‐Mod tokamak were carried out. In circular geometry, the self‐consistent electric field contains zonal flows and geodesic acoustic modes in the confined region. In the scrape‐off layer, the electron parallel heat conduction and its boundary condition determine the temperature and electric field, leading to sheared flows at the last closed flux surface.

Simulations of tokamak boundary plasma turbulence transport in setting the divertor heat flux width

The BOUT + + code has been used to simulate edge plasma electromagnetic (EM) turbulence and transport, and to study the role of EM turbulence in setting the scrape-off layer (SOL) heat flux width λq. More than a dozen tokamak discharges from C-Mod, DIII-D, EAST, ITER and CFETR have been simulated with encouraging success. The parallel electron heat fluxes onto the target from the BOUT + + simulations of C-Mod, DIII-D, and EAST follow the experimental heat flux width scaling of the inverse dependence on the poloidal magnetic field. Further turbulence statistics analysis shows that the blobs are generated near the pedestal pressure peak gradient region inside the separatrix and contribute to the transport of the particle and heat in the SOL region. Transport simulations indicate two distinct regimes: drift dominant regime and turbulence dominant regime. Goldston's heuristic drift-based (HD) model yields a consistent divertor heat flux width in the drift dominant regime. For C-Mod enhanced Dα H-mode discharges, drifts and turbulence are competing in setting the divertor heat flux width, possibly due to its compact machine size and good pedestal confinement.The simulations for ITER and CFETR indicate that divertor heat flux width λq of the future machines may no longer follows the 1/Bpol,OMP HD-based empirical (Eich) scalings and the HD model gives a pessimistic limit of divertor heat flux width. The simulation results show a transition from a drift dominant regime to a turbulence dominant regime from current machines to future machines such as ITER and CFETR for two reasons. (1) The magnetic drift-based radial transport decreases due to large CFETR and ITER machine sizes. (2) The SOL turbulence thermal diffusivity increases due to larger turbulent fluxes ejected from the pedestal into the SOL when operating in a different pedestal structure, from an ELM-free H-mode pedestal regime to a small and grassy ELM regime.

Effect of resonant magnetic perturbation on boundary plasma turbulence and transport on J-TEXT tokamak

The effect of resonant magnetic perturbation (RMP) on boundary turbulence and transport in J-TEXT plasma is experimentally investigated. Edge plasma fluctuations in discharges with and without the (m/n = 3/1) RMP currents are diagnosed by using Langmuir probe arrays. It was found that fluctuations in the edge and scrape-off layer (SOL) regions decrease with the application of a 6 kA RMP. The broadband turbulence at the radial location of which has a characteristic frequency of 40–150 kHz was strongly suppressed when applying RMP, as was the radial turbulent particle flux and blob transport in the near-SOL region. These experimental findings make RMP a promising method of suppressing and controlling turbulence and particle transport in a plasma boundary.

Progress towards modeling tokamak boundary plasma turbulence and understanding its role in setting divertor heat flux widths

The heat flux distributions on divertor targets in H-mode plasmas are serious concerns for future devices. We seek to simulate the tokamak boundary plasma turbulence and heat transport in the edge localized mode-suppressed regimes. The improved BOUT++ model shows that not only Ip but also the radial electric field Er plays an important role on the turbulence behavior and sets the heat flux width. Instead of calculating Er from the pressure gradient term (diamagnetic Er), it is calculated from the plasma transport equations with the sheath potential in the scrape-off layer and the plasma density and temperature profiles inside the separatrix from the experiment. The simulation results with the new Er model have better agreement with the experiment than using the diamagnetic Er model: (1) The electromagnetic turbulence in enhanced Dα H-mode shows the characteristics of quasi-coherent modes (QCMs) and broadband turbulence. The mode spectra are in agreement with the phase contrast imaging data and almost has no change in comparison to the cases which use the diamagnetic Er model; (2) the self-consistent boundary Er is needed for the turbulence simulations to get the consistent heat flux width with the experiment; (3) the frequencies of the QCMs are proportional to Er, while the divertor heat flux widths are inversely proportional to Er; and (4) the BOUT++ turbulence simulations yield a similar heat flux width to the experimental Eich scaling law and the prediction from the Goldston heuristic drift model.

Modeling tokamak boundary plasma turbulence and its role in setting divertor heat flux widths

Modeling tokamak boundary plasma turbulence and its role in setting divertor heat flux widths

Comparisons between tokamak fueling of gas puffing and supersonic molecular beam injection in 2D simulations

Plasma fueling with high efficiency and deep injection is very important to enable fusion power performance requirements. It is a powerful and efficient way to study neutral transport dynamics and find methods of improving the fueling performance by doing large scale simulations. Two basic fueling methods, gas puffing (GP) and supersonic molecular beam injection (SMBI), are simulated and compared in realistic divertor geometry of the HL-2A tokamak with a newly developed module, named trans-neut, within the framework of BOUT++ boundary plasma turbulence code [Z. H. Wang et al., Nucl. Fusion 54, 043019 (2014)]. The physical model includes plasma density, heat and momentum transport equations along with neutral density, and momentum transport equations. Transport dynamics and profile evolutions of both plasma and neutrals are simulated and compared between GP and SMBI in both poloidal and radial directions, which are quite different from one and the other. It finds that the neutrals can penetrate about four centimeters inside the last closed (magnetic) flux surface during SMBI, while they are all deposited outside of the LCF during GP. It is the radial convection and larger inflowing flux which lead to the deeper penetration depth of SMBI and higher fueling efficiency compared to GP.

Effects of magnetic configuration on divertor power and particle deposition for long pulse operation in EAST

The magnetic configuration exhibits a strong influence on the dynamics of Edge Localized Modes (ELMs), as demonstrated in the EAST superconducting tokamak. We find that poloidal drifts play an important role in particle deposition during the ELMs, leading to a strong up/down asymmetry in the double null divertor configuration, favoring the upper divertor for normal toroidal field, Bt, i.e., with the ion ∇B drift towards the bottom, while the heat flux distribution appears to be rather uniform during ELMs. These observations are well reproduced by the boundary plasma turbulence code, BOUT++. As divertor pumping was only available at the bottom, the preferential particle flow towards the bottom divertor associated with reverse Bt led to a preferred scenario for long pulse operation in EAST.

2D simulations of transport dynamics during tokamak fuelling by supersonic molecular beam injection

Time-dependent transport of both plasma and neutrals is simulated during supersonic molecular beam injection (SMBI) yielding the evolution of edge plasma and neutral profiles. The SMBI model is included as a module, called trans-neut, within the original BOUT++ boundary plasma turbulence code. Results of calculations are reported for the realistic divertor geometry of the HL-2A tokamak. The model can also be used to study the effect of gas puffing. A seven-field fluid model couples plasma density, heat, and momentum transport equations together with neutral density and momentum transport equations for both molecules and atoms. Collisional interactions between molecules, atoms, and plasma include dissociation, ionization, recombination and charge-exchange effects. Sheath boundary conditions and particle recycling are applied at both the wall and the divertor plates. A localized boundary condition of constant molecular flux (product of density times speed) is applied at the outermost flux surface to model the SMBI. Steady state profiles with and without particle recycling are achieved before SMBI. During SMBI, the simulation shows that neutrals can penetrate several centimetres inside the last closed (magnetic) flux surface (LCFS). Along the SMBI path, plasma density increases while plasma temperature decreases. The molecule penetration depth depends on both the SMBI flux and the initial plasma density and temperature along its path. As the local plasma density increases substantially, molecule and atom penetration depths decrease due to their higher dissociation and ionization rates if the electron temperature does not drop too low. Dynamic poloidal spreading of the enhanced plasma density region is observed due to rapid ion flow along the magnetic field (parallel) driven by a parallel pressure asymmetry during SMBI. Profile relaxation in the radial and poloidal directions is simulated after SMBI termination, showing that the plasma returns to pre-SMBI conditions on a time scale of 60 ms.

Boundary plasma turbulence simulations for tokamaks

The boundary plasma turbulence code BOUT models tokamak boundary-plasma turbulence in a realistic divertor geometry using modified Braginskii equations for plasma vorticity, density (ni), electron and ion temperature (T{sub e}; T{sub i}) and parallel momenta. The BOUT code solves for the plasma fluid equations in a three dimensional (3D) toroidal segment (or a toroidal wedge), including the region somewhat inside the separatrix and extending into the scrape-off layer; the private flux region is also included. In this paper, a description is given of the sophisticated physical models, innovative numerical algorithms, and modern software design used to simulate edge-plasmas in magnetic fusion energy devices. The BOUT code's unique capabilities and functionality are exemplified via simulations of the impact of plasma density on tokamak edge turbulence and blob dynamics.

Boundary plasma turbulence simulations for tokamaks

The boundary plasma turbulence code BOUT models tokamak boundary-plasma turbulence in a realistic divertor geometry using modified Braginskii equations for plasma vorticity, density (ni), electron and ion temperature (T{sub e}; T{sub i}) and parallel momenta. The BOUT code solves for the plasma fluid equations in a three dimensional (3D) toroidal segment (or a toroidal wedge), including the region somewhat inside the separatrix and extending into the scrape-off layer; the private flux region is also included. In this paper, a description is given of the sophisticated physical models, innovative numerical algorithms, and modern software design used to simulate edge-plasmas in magnetic fusion energy devices. The BOUT code's unique capabilities and functionality are exemplified via simulations of the impact of plasma density on tokamak edge turbulence and blob dynamics.

Magnetic presheath in a weakly turbulent multicomponent plasma

A fluid model of the magnetic presheath in a weakly turbulent multicomponent plasma consisting of electrons and several positive ion components is presented. Turbulent particle source terms in the continuity equations and turbulent momentum source terms in the momentum balance equations for the ion components are derived from basic principles [Plasma Phys. Controlled Fusion 47, 685 (2005)], i.e., by means of the ensemble averaging procedure from the statistical theory of plasma turbulence. Then the unknown correlations between the fluctuating quantities occurring in the turbulent source terms derived are replaced with phenomenological expressions based on analogy with classical transport. The remaining correlation terms in the momentum balance equations, which are responsible for explicit coupling of the equations for different ion components, reduce to the corresponding classical terms when the turbulent transport corrections are negligible (i.e., for “quiescent” plasmas). Under certain conditions regarding the nature of the boundary plasma turbulence, the analysis of the planar magnetic presheath performed in this study yields results for the ion component fluid velocities and particle flux densities at the magnetic presheath entrance that are directly applicable to the boundary conditions of multifluid plasma transport and turbulence codes. A fluid formulation of the Bohm criterion, valid for weakly turbulent multicomponent plasmas as treated in this study, is also derived.

Experimental and theoretical study of quasicoherent fluctuations in enhanced D(alpha) plasmas in the Alcator C-Mod tokamak.

A comparison of experimental measurements and theoretical studies of the quasicoherent (QC) mode, observed at high densities during enhanced D(alpha) (EDA) H mode in the Alcator C-Mod tokamak, are reported. The QC mode is a high frequency ( approximately 100 kHz) nearly sinusoidal fluctuation in density and magnetic field, localized in the steep density gradient ("pedestal") at the plasma edge, with typical wave numbers k(R) approximately 3-6 cm(-1), k(theta) approximately 1.3 cm(-1) (midplane). It is proposed here that the QC mode is a form of resistive ballooning mode known as the resistive X-point mode, in reasonable agreement with predictions by the BOUT (boundary-plasma turbulence) code.

Dynamical simulations of boundary plasma turbulence indivertor geometry

Directcomparisons between numerical simulations and the measuredplasma fluctuations and transport are presented by performingnonlinear two-fluid simulations with the BOUT code (Xu X Q andCohen R H 1998 Contrib. Plasma Phys. 38 158). BOUTmodels boundary-plasma turbulence in a realistic divertorgeometry using modified Braginskii equations for plasmavorticity, density (ni), electron and ion temperature (Te, Ti) and parallel momenta. The BOUT code solves for the plasmafluid equations in a 3D toroidal segment, including the regionsomewhat inside the separatrix and extending into the scrape-offlayer; the private flux region is also included. In this paper,the physics of resistive X-point turbulence and its relation toflow shear generation is discussed. We present comparisonsbetween the boundary plasma turbulence observed in the BOUT codeand experiments on DIII-D (Luxon J L et al 1986 Int. Conf. onPlasma Physics and Controlled Nuclear Fusion (Vienna: IAEA)p 159), the National Spherical Torus Experiment (Peng Y-K M 2000 Phys. Plasmas 7 1681), and C-Mod (Hutchinson I H et al 1994Phys. Plasmas 1 1511). In an L-mode discharge in the DIII-Dtokamak, both BOUT simulations and beam emission spectroscopy showa similar flow pattern and blob size across the last closed fluxsurface. In an L-mode discharge, both BOUT simulations and gaspuff imaging show similar filament structures along the fieldline and similar frequency spectrum at the outboard midplane. Insimulations of the quasi-coherent mode in the EDA regime ofC-Mod, the particle flux measured from BOUT simulation isconsistent with Langmuir probe measurements on C-Mod at themidplane near the separatrix. The qualitative comparisons thusindicate that BOUT contains much of the relevant physics forboundary plasma turbulence in the experimentally relevantX-point divertor geometry of present-day tokamaks and sphericaltori.

The BOUT Project; Validation and Benchmark of BOUT Code and Experimental Diagnostic Tools for Fusion Boundary Turbulence

A boundary plasma turbulence code BOUT is presented. The preliminary encouraging results have been obtained when comparing with probe measurements for a typical Ohmic discharge in HT-7 tokamak. The validation and benchmark of BOUT code and experimental diagnostic tools for fusion boundary plasma turbulence is proposed.

Turbulence in boundary plasmas

We simulate boundary plasma turbulence using a 3D turbulence code BOUT and a linearized electromagnetic instability shooting code BAL. The code BOUT solves fluid equations for plasma vorticity, density, ion temperature and parallel momentum (along the magnetic field), electron temperature, and parallel momentum. A realistic DIII-D X-point magnetic geometry is used. The focus is on the possible local linear instability drives and turbulence suppression mechanisms involved in the L–H transition and on the consistency of the computed turbulence with observed temperature and density profiles. Comparison is made with data from the DIII-D tokamak where probe measurements provide turbulence statistics in the boundary plasma and transport modeling.

Fluid simulations of conducting-wall-driven turbulence in boundary plasmas

It is clear that the edge plasma plays a crucial role in global tokamak confinement. This paper is a report on simulations of a new drift wave type instability driven by conducting wall (also originally named as a ∇Te instability) [Phys. Fluids B 3, 1364 (1991)]. A 2d(x,y) fluid code has been developed in order to explore the anomalous transport in the boundary plasmas. The simulation consists of a set of fluid equations (in the electrostatic limit) for the vorticity ∇⊥2φ, and the temperature Te in a shearless plasma slab confined by a uniform, straight magnetic field Bz with two divertor (or limiter) plates intercepting the magnetic field. The model has two regions separated by a magnetic separatrix: In the edge region inside the separatrix, the model is periodic along the magnetic field while in the scrapeoff layer (SOL) region outside the separatrix, the magnetic field is taken to be of finite length with model (logical sheath) boundary conditions at diverter (or limiter) plates. The simulation results show that the observed linear instability agrees well with theory, and that a saturated state of turbulence is reached. In saturated turbulence, clear evidence of the expected long-wavelength mode penetration into the edge is seen, an inverse cascade of wave energy (toward both long wavelengths and low frequencies) is observed. The simulation results also show that amplitudes of potential and the electron temperature fluctuations are somewhat above and the heat flux are somewhat below those of the simplest mixing-length estimates. A full inverse cascade of the turbulence indicates that the cross-field transport is not diffusive. A self-consistent simulation to determine the microturbulent SOL electron temperature profile has been done, the results of which reasonably agree with the experimental measurements.