Edge Transport Barrier Research Articles

Off-Axis Driven Current Effects on ETB and ITB Formations based on Bifurcation Concept

This research studies plasma performance in fusion Tokamak system by investigating parameters such as plasma pressure in the presence of an edge transport barrier (ETB) and an internal transport barrier (ITB) as the off-axis driven current position is varied. The plasma is modeled based on the bifurcation concept using a suppression function that can result in formation of transport barriers. In this model, thermal and particle transport equations, including both neoclassical and anomalous effects, are solved simultaneously in slab geometry. The neoclassical coefficients are assumed to be constant while the anomalous coefficients depend on gradients of local pressure and density. The suppression function, depending on flow shear and magnetic shear, is assumed to affect only on the anomalous channel. The flow shear can be calculated from the force balance equation, while the magnetic shear is calculated from the given plasma current. It is found that as the position of driven current peak is moved outwards from the plasma center, the central pressure is increased. But at some point it stars to decline, mostly when the driven current peak has reached the outer half of the plasma. The higher pressure value results from the combination of ETB and ITB formations. The drop in central pressure occurs because ITB stats to disappear.

Studies of the pedestal structure and inter-ELM pedestal evolution in JET with the ITER-like wall

The pedestal structure of type I ELMy H-modes has been analysed for JET with the ITER-like Wall (JET-ILW). The electron pressure pedestal width is independent of ρ * and increases proportionally to √β pol,PED. Additional broadening of the width is observed, at constant β pol, PED, with increasing ν * and/or neutral gas injection and the contribution of atomic physics effects in setting the pedestal width cannot as yet be ruled out. Neutral penetration alone does not determine the shape of the edge density profile in JET-ILW. The ratio of electron density to electron temperature scale lengths in the edge transport barrier region, η e, is of order 2–3 within experimental uncertainties. Existing understanding, represented in the stationary linear peeling–ballooning mode stability and the EPED pedestal structure models, is extended to the dynamic evolution between ELM crashes in JET-ILW, in order to test the assumptions underlying these two models. The inter-ELM temporal evolution of the pedestal structure in JET-ILW is not unique, but depends on discharge conditions, such as heating power and gas injection levels. The strong reduction in p e,PED with increasing D 2 gas injection at high power is primarily due to clamping of half way through the ELM cycle and is suggestive of turbulence limiting the T e pedestal growth. The inter-ELM pedestal pressure evolution in JET-ILW is consistent with the EPED model assumptions at low gas rates and only at low beta at high gas rates. At higher beta and high gas rate the inter-ELM pedestal pressure evolution is qualitatively consistent with the kinetic ballooning mode (KBM) constraint but the peeling–ballooning (P–B) constraint is not satisfied and the ELM trigger mechanism remains as yet unexplained.

Modelling of transitions between L- and H-mode in JET high plasma current plasmas and application to ITER scenarios including tungsten behaviour

The dynamics for the transition from L-mode to a stationary high Q DT H-mode regime in ITER is expected to be qualitatively different to present experiments. Differences may be caused by a low fuelling efficiency of recycling neutrals, that influence the post transition plasma density evolution on the one hand. On the other hand, the effect of the plasma density evolution itself both on the alpha heating power and the edge power flow required to sustain the H-mode confinement itself needs to be considered. This paper presents results of modelling studies of the transition to stationary high Q DT H-mode regime in ITER with the JINTRAC suite of codes, which include optimisation of the plasma density evolution to ensure a robust achievement of high Q DT regimes in ITER on the one hand and the avoidance of tungsten accumulation in this transient phase on the other hand.As a first step, the JINTRAC integrated models have been validated in fully predictive simulations (excluding core momentum transport which is prescribed) against core, pedestal and divertor plasma measurements in JET C-wall experiments for the transition from L-mode to stationary H-mode in partially ITER relevant conditions (highest achievable current and power, H 98,y ~ 1.0, low collisionality, comparable evolution in P net/P L-H, but different ρ *, T i/T e, Mach number and plasma composition compared to ITER expectations). The selection of transport models (core: NCLASS + Bohm/gyroBohm in L-mode/GLF23 in H-mode) was determined by a trade-off between model complexity and efficiency. Good agreement between code predictions and measured plasma parameters is obtained if anomalous heat and particle transport in the edge transport barrier are assumed to be reduced at different rates with increasing edge power flow normalised to the H-mode threshold; in particular the increase in edge plasma density is dominated by this edge transport reduction as the calculated neutral influx across the separatrix remains unchanged (or even slightly decreases) following the H-mode transition.JINTRAC modelling of H-mode transitions for the ITER 15 MA / 5.3 T high Q DT scenarios with the same modelling assumptions as those being derived from JET experiments has been carried out. The modelling finds that it is possible to access high Q DT conditions robustly for additional heating power levels of P AUX ⩾ 53 MW by optimising core and edge plasma fuelling in the transition from L-mode to high Q DT H-mode. An initial period of low plasma density, in which the plasma accesses the H-mode regime and the alpha heating power increases, needs to be considered after the start of the additional heating, which is then followed by a slow density ramp. Both the duration of the low density phase and the density ramp-rate depend on boundary and operational conditions and can be optimised to minimise the resistive flux consumption in this transition phase. The modelling also shows that fuelling schemes optimised for a robust access to high Q DT H-mode in ITER are also optimum for the prevention of the contamination of the core plasma by tungsten during this phase.

Dynamics of oscillatory plasma flows prior to the H-mode in the HL-2A tokamak

This paper discusses edge oscillatory plasma flows, geodesic acoustic mode (GAM) and limit cycle oscillations (LCOs), which have been measured by Doppler reflectometry prior to the high confinement mode (H-mode) in the HL-2A tokamak. The complex relations between the flows and background turbulence have been analyzed. It was observed that the GAM and LCO coexist, and these two flows and turbulence have strong nonlinear interactions during the intermediate confinement phase (I-phase). Dynamics of the shear flows and turbulence prior to the H-mode shows that the oscillatory flows quench the turbulence along with the increase of the mean E × B flow at the early stage of the I-phase, then the oscillatory flows are damped and the further increased mean flow takes over the role in turbulence suppression. The reduced turbulent transport results in the formation of a steep edge transport barrier. It suggests that the oscillatory flows can initiate the L–H transition through providing a positive feedback for the increase of the mean E × B flow strength.

Studies on radial and poloidal particle transport at the edge of SST-1 tokamak

The radial and poloidal particle fluxes occurring at the plasma edge are essential towards understanding the plasma confinement in the tokamak device. In tokamaks, the edge transport barriers play a critical role in the transitions from low confinement (L-mode) to high confinement (H-mode). Recently, the edge plasma profiles have been studied for steady-state superconducting tokamak-1 (SST-1) with the help of an array of Langmuir probes. The floating potential and ion saturation current fluctuations have been measured at different radial and poloidal distance at the edge of SST-1. Increases in magnetic fluctuations associated with enhanced magneto-hydrodynamic (MHD) activities have been found to increase the radial particle flux drastically. It indicates that the MHD activity leads to anomalous particle transport during the tokamak discharge at SST-1. It is found that the average poloidal velocity decreases from ∼4 km/s to ∼3.7 km/s whereas the average radial velocity decreases from ∼7.2 km/s to 4.7 km/s. Further, during MHD activity, it is found that as the magnetic island grows, the radial electric field changes from negative to positive. It is observed that the turbulent particle flux at SST-1 tokamak is predominantly positive and bursty. The local flux probability distribution function shows a clear non-Gaussian character, and it is skewed negatively.

Fast Low-to-High Confinement Mode Bifurcation Dynamics in a Tokamak Edge Plasma Gyrokinetic Simulation.

Transport barrier formation and its relation to sheared flows in fluids and plasmas are of fundamental interest in various natural and laboratory observations and of critical importance in achieving an economical energy production in a magnetic fusion device. Here we report the first observation of an edge transport barrier formation event in an electrostatic gyrokinetic simulation carried out in a realistic diverted tokamak edge geometry under strong forcing by a high rate of heat deposition. The results show that turbulent Reynolds-stress-driven sheared E×B flows act in concert with neoclassical orbit loss to quench turbulent transport and form a transport barrier just inside the last closed magnetic flux surface.

Modeling of Small Size Divertor Tokamak in Discharge withEdge Transport Barrier (ETB)

The article is the simulation of small size divertor tokamak edge transport barrier “ETB”. The modeling carries out with the multifluid transport code B2SOLPS5.0 2D with drifts and currents which was specially developed for simulation of tokamak edge transport barrier“ETB”. The emphasis is made on edge transport barrier “ETB”. The simulation demonstrated the following results: The “ETB” width has strong influence on the radial electric field. The E×B drift shear is function of “ETB” width. The ion parallel (toroidal) velocity has cocurrent direction and quite different for different “ETB” width. This difference is connected with the contribution of large E×B drift velocity in toroidal torque driven by parallel viscosity which strong influence on toroidal rotation. The plasma density, electron and ion temperatures are typical feature of small size divertor tokamakwhen the particle and heat fluxes from core plasma are low. The sizes of the barrier have ability to prevent neutral particles penetrate the barrier edge in this tokamak. The ETB width has influence on the poloidal velocity in the edge plasma of this tokamak

Modeling of radiative divertor experiments with argon seeding for H-mode plasma in EAST

To obtain a quantitative understanding of the recent radiative divertor experiments and to give an instructive prediction for the future relative work performed on EAST, the 2D numerical tool SOLPS has been used to investigate the argon seeded EAST H-mode plasmas. The simulations were mainly based on a typical H-mode discharge with lower single null divertor configuration, in which the partially detached divertor plasma has been achieved due to the argon seeding. First the perpendicular particle and energy transport coefficients for particle density diffusivity D⊥, electron and ion conduction, χ⊥i,e were radially varied to determine the edge transport barrier in H-mode plasma by comparing the simulated upstream profiles of electron temperature (Te) and electron density (ne) with the edge Thomson scattering data. Then the reduction of the particle flux, static pressure, and the peak heat load onto the lower outer divertor target have been reproduced by radiative divertor simulations in agreement with the experimental measurements, which demonstrated that the argon seeding is effective to mitigate the heat load onto the divertor target and achieve partially detached plasma during the H-mode discharge. The simulations also revealed that the strongest radiation occurred near the X-point and in the divertor region for short pulsed argon seeding with a relatively low puffing rate in agreement with bolometer measurements.

Turbulence in edge and core transport barriers: new experimental results and modeling

In this paper, recent progressive studies on experimental analysis and theoretical models for turbulence phenomena around the transport barriers in high-performance magnetic confined fusion plasma are reviewed. The linkage of radial electric fields and turbulence, the importance of radial electric field curvature, and observations of spatiotemporal turbulence structures are described with related theoretical models.

Fitting of the Thomson scattering density and temperature profiles on the COMPASS tokamak.

A new technique for fitting the full radial profiles of electron density and temperature obtained by the Thomson scattering diagnostic in H-mode discharges on the COMPASS tokamak is described. The technique combines the conventionally used modified hyperbolic tangent function for the edge transport barrier (pedestal) fitting and a modification of a Gaussian function for fitting the core plasma. Low number of parameters of this combined function and their straightforward interpretability and controllability provide a robust method for obtaining physically reasonable profile fits. Deconvolution with the diagnostic instrument function is applied on the profile fit, taking into account the dependence on the actual magnetic configuration.

Experimental validation of non-uniformity effect of the radial electric field on the edge transport barrier formation in JT-60U H-mode plasmas.

The turbulent structure formation, where strongly-inhomogeneous turbulence and global electromagnetic fields are self-organized, is a fundamental mechanism that governs the evolution of high-temperature plasmas in the universe and laboratory (e.g., the generation of edge transport barrier (ETB) of the H-mode in the toroidal plasmas). The roles of inhomogeneities of radial electric field (Er) are known inevitable. In this mechanism, whether the first derivative of Er (shear) or the second derivative of Er (curvature) works most is decisive in determining the class of nontrivial solutions (which describe the barrier structure). Here we report the experimental identification of the essential role of the Er-curvature on the ETB formation, for the first time, based on the high-spatiotemporal resolution spectroscopic measurement. We found the decisive importance of Er-curvature on ETB formation during ELM-free phase, but there is only a low correlation with the Er-shear value at the peak of normalized ion temperature gradient. Furthermore, in the ELMing phase, the effect of curvature is also quantified in terms of the relationship between pedestal width and thickness of the layer of inhomogeneous Er. This is the fundamental basis to understand the structure of transport barriers in fusion plasmas.

Understanding of Hysteresis Behaviors at the L‐H‐L Transitions in Tokamak Plasma Based on Bifurcation Concept

AbstractThe hysteresis behaviour at the L‐H‐L transitions in tokamak plasma is investigated based on bifurcation concept. The formation of an edge transport barrier (ETB) is modeled via thermal and particle transport equations with the flow shear suppression effect on anomalous transport included. The anomalous transport is modeled based on critical gradients threshold and the flow shear is calculated from the force balance equation, couples the two transport equations leading to a non‐linear behaviour. Analytical investigation reveals that the fluxes versus gradients space exhibits bifurcation behaviour with s ‐curve soft bifurcation type. Apparently, the backward H‐L transition occurs at lower values than that of the forward L‐H transition, illustrating hysteresis behaviour. The hysteresis properties, i.e. locations of threshold fluxes, gradients and their ratios are analyzed as a function of neoclassical and anomalous transport values and critical gradients. It is found that the minimum heat flux for maintaining H ‐mode depends on several plasma parameters including the strength of anomalous transport and neoclassical transport. In particular, the hysteresis depth becomes larger when neoclassical transport decreases or anomalous transport increases. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stationary Zonal Flows during the Formation of the Edge Transport Barrier in the JET Tokamak.

High spatial resolution Doppler backscattering measurements in JET have enabled new insights into the development of the edge Er. We observe fine-scale spatial structures in the edge Er well with a wave number krρi≈0.4-0.8, consistent with stationary zonal flows, the characteristics of which vary with density. The zonal flow amplitude and wavelength both decrease with local collisionality, such that the zonal flow E×B shear increases. Above the minimum of the L-H transition power threshold dependence on density, the zonal flows are present during L mode and disappear following the H-mode transition, while below the minimum they are reduced below measurable amplitude during L mode, before the L-H transition.

Understanding roles of E × B flow and magnetic shear on the formation of internal and edge transport barriers using two-field bifurcation concept

A set of heat and particle transport equations with the inclusion of E × B flow and magnetic shear is used to understand the formation and behaviors of edge transport barriers (ETBs) and internal transport barriers (ITBs) in tokamak plasmas based on two-field bifurcation concept. A simple model that can describe the E × B flow shear and magnetic shear effect in tokamak plasma is used for anomalous transport suppression with the effect of bootstrap current included. Consequently, conditions and formations of ETB and ITB can be visualized and studied. It can be seen that the ETB formation depends sensitively on the E × B flow shear suppression with small dependence on the magnetic shear suppression. However, the ITB formation depends sensitively on the magnetic shear suppression with a small dependence on the E × B flow shear suppression. Once the H-mode is achieved, the s-curve bifurcation diagram is modified due to an increase of bootstrap current at the plasma edge, resulting in reductions of both L-H and H-L transition thresholds with stronger hysteresis effects. It is also found that both ITB and ETB widths appear to be governed by heat or particle sources and the location of the current peaking. In addition, at a marginal flux just below the L-H threshold, a small perturbation in terms of heat or density fluctuation can result in a transition, which can remain after the perturbation is removed due to the hysteresis effect.

Collisionality dependence of edge rotation and in–out impurity asymmetries in ASDEX Upgrade H-mode plasmas

The poloidal and toroidal impurity flows in the edge transport barrier of H-mode plasmas have been studied over a wide range of pedestal top ion collisionalities. A comparison of the edge poloidal rotation measurements to neoclassical predictions shows good agreement in all cases. The measured edge impurity toroidal rotation is observed to change sign from co-current to counter-current with decreasing collisionality. The switch occurs at the same collisionality at which neoclassical theory predicts the main ion poloidal rotation to change from the electron to the ion diamagnetic direction. The behaviour of these two species, when used to calculate the main ion toroidal rotation via the radial force balance equation, leads to fairly constant co-current main ion toroidal rotation. Hence, at low collisionality, due to a reduced frictional coupling, the main ion-impurity differential rotation can be quite large. The behaviour of impurity ion flows on a flux surface has also been investigated in detail and it was found that the measurements are consistent with the continuity equation only if the poloidally asymmetric impurity density distribution is taken into account. The asymmetry is found to be the result of the interplay of all forces in the parallel momentum balance, with the friction force providing the dominant drive. Close to the separatrix the poloidal centrifugal force, which is usually neglected, also gives an additional contribution to the impurity density asymmetry. Within the experimental uncertainties the ion temperature and the electrostatic potential are simultaneous flux functions, despite the presence of a poloidally asymmetric impurity density profile.

Simulations of the L–H transition dynamics with different heat and particle sources**Project supported by the Funds of the Youth Innovation Team of Science and Technology in Sichuan Province, China (Grant No. 2014TD0023), the National Natural Science Foundation of China (Grant Nos. 11447228 and 11205053), and the China National Magnetic Confinement Fusion Science Program (Grant No. 2013GB107001).

It is crucial to increase the total stored energy by realizing the transition from a low confinement (L-mode) state to a high confinement (H-mode) state in magnetic confinement fusion. The L–H transition process is simulated by using the predictive transport code based on Weiland’s fluid model. Based on the equilibrium parameters obtained from equilibrium fitting (EFIT) in the experiment, the electron density ne, electron temperature Te, ion temperatures Ti, ion poloidal Vp, and toroidal momenta Vt are simulated self-consistently. The L–H transition dynamic behaviors with the formation of the transport barriers of ion and electron temperatures, the electron density, and the ion toroidal momenta are analyzed. During the L–H transition, the strong poloidal flow shear in the edge transport barrier region is observed. The crashes of the electron and ion temperature pedestals are also observed during the L–H transition. The effects of the heating and particle sources on the L–H transition process are studied systematically, and the critical power threshold of the L–H transition is also found.

Stochastization and pump-out in edge plasma caused by edge localized modes

Mechanisms of current generation in edge localized mode (ELM) filaments are discussed. It is shown that dipole type currents should flow in the filaments and that these currents are short-circuited through the ambient plasma of the scrape-off layer. The steady-state monopole currents in the filaments cannot be closed through the ambient plasma and oscillatory currents with Alfven frequency should emerge instead. The dipole type currents in the filaments are sufficient to produce magnetic field perturbations which create a stochastic layer inside the edge transport barrier (ETB) region. The dynamic of penetration of the magnetic field perturbations is analyzed and it is shown that the corresponding time is comparable with the life time of the filaments. At the stage when the ETB region is stochastized by the currents of the filaments the radial electric field becomes less negative or even positive inside the ETB and the onset of outward convective flux takes place. It is shown that this outward flux could lead to the significant pump-out effect—the density decrease in the pedestal region.

Fuelling and density control for DEMO

Plasma fuelling and density control are an open issue regarding EU DEMO studies and solutions may be different from present day experiments. The present paper addresses through JINTRAC core transport code simulations the feasibility of different fuelling methods such as gas puff and pellet injection and the influence of neoclassical and anomalous inward pinch in the edge transport barrier in order to achieve and control the target DEMO density. Given the expected high fusion power production, He accumulation in the plasma core is a critical issue, and an estimation of the influence of impurities (He, Ar, and W) on core fuelling and plasma dilution is given together with a discussion on D–T core balance. The DEMO reference scenario investigated in this work is characterized by a peaked density profile, which requires a careful core fuelling. Due to the large pedestal temperature gradient, gas puff may not be a feasible option for core density control, unless assuming a large anomalous inward pinch in the edge transport barrier of more than ~2 m s−1. Pellet injection from the high field side of the torus, on the contrary, may represent a viable solution for core fuelling and D–T ratio control. The effect of pellet mass, speed, and injection geometry is also discussed in the present paper. Regardless, core fuelling efficiency with pellet injection is almost entirely determined by the presence of E × B drift.

Thermal ion orbit loss and radial electric field in DIII-D

A relatively simple model for the generation of the radial electric field, Er, near the outboard boundary in a tokamak is presented. The model posits that Er is established to supply the return current necessary to balance the thermal ion orbit loss current. Comparison with DIII-D data is promising. Features of the model that promote a more negative edge Er are higher ion temperature, lower density, lower impurity ion content, and a shorter pathlength for orbit loss. These scalings are consistent with experimentally established access to the high-confinement mode edge transport barrier.

Ion temperature and toroidal velocity edge transport barriers in KSTAR

The structure and evolution of the ion temperature () and toroidal rotation () profile have been investigated in neutral beam injection (NBI)-heated KSTAR H-mode plasmas, both without and with resonant magnetic pertubations (RMPs). A clear disparity between the width of the -pedestal and that of the -pedestal was observed. Also, it was found that there exists a close correlation and weak relative hysteresis between the pedestal and during both L → H and H → L transitions. During the L → H transition, the -pedestal is observed to form ahead of the -pedestal, and build inward from the separatrix. Linear gyrokinetic stability analysis of these KSTAR profiles was performed. The results indicate that parallel velocity shear is a relevant drive for pedestal turbulence and transport. This was largely ignored in previous studies of the pedestal micro-stability. Pedestal ion temperature and rotation profiles were also measured during edge localized mode (ELM) suppression experiments on KSTAR using an n = 1 RMPs. It was found that the top values of the ion temperature and toroidal rotation pedestal drop with RMPs when ELMs are suppressed.