Internal Transport Barrier Formation Research Articles

Impact of T i/T e ratio on ion transport based on EAST H-mode plasmas

At the EAST tokamak, the ion temperature (T i) is observed to be clamped around 1.25 keV in electron cyclotron resonance (ECR)-heated plasmas, even at core electron temperatures up to 10 keV (depending on the ECR heating power and the plasma density). This clamping results from the lack of direct ion heating and high levels of turbulence-driven transport. Turbulent transport analysis shows that trapped electron mode and electron temperature gradient-driven modes are the most unstable modes in the core of ECR-heated H-mode plasmas. Nevertheless, recently it was found that the T i/T e ratio can increase further with the fraction of the neutral beam injection (NBI) power, which leads to a higher core ion temperature (T i0). In NBI heating-dominant H-mode plasmas, the ion temperature gradient-driven modes become the most unstable modes. Furthermore, a strong and broad internal transport barrier (ITB) can form at the plasma core in high-power NBI-heated H-mode plasmas when the T i/T e ratio approaches ~1, which results in steep core T e and T i profiles, as well as a peaked n e profile. Power balance analysis shows a weaker T e profile stiffness after the formation of ITBs in the core plasma region, where T i clamping is broken, and the core T i can increase further above 2 keV, which is 80% higher than the value of T i clamping in ECR-heated plasmas. This finding proposes a possible solution to the problem of T i clamping on EAST and demonstrates an advanced operational regime with the formation of a strong and broad ITB for future fusion plasmas dominated by electron heating.

Formation, propagation and conversion of transport barriers triggered by dynamical critical gradient in tokamak plasmas

In this work, we propose a reduced model with a dynamical critical gradient to study the formation, propagation, and conversion of the transport barriers. In contrast to the commonly adopted static critical gradient, an evolving critical gradient self-consistently softens the profile stiffness, so as to facilitate the generation of transport barriers. This is especially crucial to the internal transport barrier (ITB) formation. Numerically, we show that the inhomogeneity of turbulent and neoclassical diffusivities can induce the global wave front propagation of the transport barrier. When the heating power ramps quickly, the ITB propagates unidirectionally to the edge region and converts into an edge transport barrier. For slow power ramping, the propagating ITB will bifurcate into bidirectional wavefronts and finally convert into a steady double transport barrier state. Our model uncovers the vital role of a dynamical ‘profile-stiffness’ in depicting the global dynamics of the transport barrier.

Observation of fast electron redistribution during saturated kink mode in high β_p H-mode discharge with central heating in EAST tokamak

Abstract In the EAST tokamak, we have developed an internal transport barrier (ITB) high- confinement mode (H-mode) scenario characterized by dominant electron heating and centrally peaked electron temperature profiles, facilitated primarily through the combustion of lower hybrid current drive (LHCD) and electron cyclotron radio heating (ECRH). Hard X-ray (HXR) diagnostics reveal a marked increase in the population of fast electrons with energy from 30 keV to 80 keV, concurrent with augmented ECRH power during H-mode plasma operations. This surge in fast electron population precedes the formation of the electron temperature ITB (Te-ITB). Within the Te-ITB H-mode discharge, a mild and long-lived m/n=1/1 mode (where m and n denote the toroidal and poloidal mode numbers, respectively) emerges proximal to the ITB region. This mode precipitates a redistribution of fast electrons, contributing to an increase in the safety factor near the magnetic axis and thereby promoting the stability of the Te-ITB. Furthermore, we explore the influence of fast electrons on plasma pressure and examine the effects of ion cyclotron radio heating (ICRF) on the distribution of fast electrons. Strategies to maintain the m/n=1/1 mode at a moderate amplitude are also discussed, highlighting their significance in the sustained management of Te-ITB.

Detection of Internal Transport Barrier in the T-10 Tokamak Using Thomson Scattering Diagnostics

At the T-10 tokamak, the Thomson scattering diagnostics was upgraded in 2016. The new system is based on the Nd:YAG laser with the pulse repetition frequency of 100 Hz; the measurements can be performed every 10 ms during the entire plasma discharge at the spatial resolution of up to 5 mm. Using the upgraded diagnostics, the electron temperature measurements were performed during experimental campaigns of the T-10 tokamak in 2016−2018. In the regimes with the electron cyclotron resonance heating, it was demonstrated that the regions with increased temperature gradients form in the plasma, which was interpreted as the formation of internal transport barrier. The ASTRA-code-based simulations of the current profile time evolution made it possible to correlate the positions of the barrier and the rational surface q = 1.

A simple model for internal transport barrier induced by fishbone in tokamak plasmas

Fishbone bursts have been observed to strongly correlate to internal transport barrier (ITB) formation in a number of tokamak devices. A simple model incorporating the fishbone dynamics and ion pressure gradient evolution is proposed in order to investigate the key physics parameters assisting the triggering of ITB. The time evolution of fishbone is described by the well-known predator–prey model. For each burst cycle, the energetic particles (EPs) resonantly interact with fishbone and are radially expelled from inner region leading to a radial current. A compensating bulk plasma return current and, hence, poloidal flow can be induced if the fishbone cycle frequency is greater than the poloidal flow damping rate. When the shear of the poloidal flow exceeds a critical value, the turbulent fluctuations are suppressed and the bulk ion pressure gradient transits to the high-confinement state. It is shown that this process is only sensitive to the deposition rate of the trapped EPs within the $q=1$ surface, but not sensitive to other parameters. A quantitative formula for the shearing rate of poloidal flow induced by fishbone bursts is derived and verified numerically.

Impact of avalanche type of transport on internal transport barrier formation in tokamak plasmas.

In magnetic fusion plasmas, a transport barrier is essential to improve the plasma confinement. The key physics behind the formation of a transport barrier is the suppression of the micro-scale turbulent transport. On the other hand, long-range transport events, such as avalanches, has been recognized to play significant roles for global profile formations. In this study, we observed the impact of the avalanche-type of transport on the formation of a transport barrier for the first time. The avalanches are found to inhibit the formation of the internal transport barrier (ITB) observed in JT-60U tokamak. We found that (1) ITBs do not form in the presence of avalanches but form under the disappearance of avalanches, (2) thesurface integral of avalanche-driven heat fluxe is comparable to the time rate change of stored energy retained at the ITB onset, (3) the mean E × B flow shear is accelerated via the ion temperature gradient that is not sustained under the existence of avalanches, and (4) after the ITB formation,avalanches are damped inside the ITB, while they remain outside the ITB.

A self-sustaining mechanism for internal transport barrier formation in HL-2A tokamak plasmas

The formation of Internal Transport Barrier (ITB) is studied in HL-2A plasmas by means of nonlinear gyrokinetic simulations. A new paradigm for the ITB formation is proposed in which different physics mechanisms play a different role depending on the ITB formation stage. In the early stage, fast ions, introduced by Neutral Beam Injection ion system, are found to stabilize the thermal-ion-driven instability by dilution, thus reducing the ion heat fluxes and finally triggering the ITB. Such dilution effects, however, play a minor role after the ITB is triggered as electromagnetic (EM) effects are dominant in the presence of established high pressure gradients. We define the concept of ITB self-sustainment, as the low turbulence levels found within the fully formed ITB are consequences of large scale zonal flows, which in turn are fed by a non-linear interplay with large scale high frequency EM perturbations destabilized by the ITB itself.

Turbulence stabilization in tokamak plasmas with high population of fast ions

This letter provides a new physical insight into the fast ion effects on turbulence in plasmas having a high fast ion fraction and peaked fast ion density profile. We elucidate turbulence stabilization mechanisms by fast ions that result in internal transport barrier formation in the fast ion regulated enhancement mode plasma. Both linear and nonlinear gyrokinetic simulations show that the dominant turbulence suppression mechanisms are the dilution effects. In particular, we find that turbulence can be sufficiently suppressed solely by an inverted main ion density gradient due to fast ions, for the first time. New physical findings reported here improve our understanding of fast ion effects on turbulence, essential for fusion energy production where . Moreover, they will open up a new methodology to control plasma turbulence applicable to a wide range of plasma confinement regimes.

Oscillation dynamics of m/n= 3/1 double tearing mode

This study systematically investigates the roles of the plasma viscosity and resistivity in the oscillation dynamics during the decay phase of the m/n = 3/1 double tearing mode using the Ci-Liu-Ti (CLT) code. The primary objectives of this research are to examine the driving and suppressing mechanisms of the oscillation. The oscillation and steady-state are the result of the competition between the external injection and the reconnection annihilation of magnetic flux during the decay phase. In a regime with a higher viscosity (or a lower resistivity), the steady-state arises from the significant damping (weak generation) of plasma flows, resulting in the formation of saturated islands. In a regime with a lower viscosity (or a higher resistivity), the suppression of the oscillation amplitude can be attributed to a strong residual flow that quickly takes the injected magnetic flux away toward to the reconnection region, which caused no enough accumulated magnetic flux to drive oscillations and the system evolves toward a steady-state configuration. The steady-state condition results in the generation of a narrow radial vortex region which promotes formation of internal transport barriers. The upper threshold of the resistivity within the low-resistivity regime to achieve a steady-state decreases as the viscosity increases.

Self-generated vortex flows in a tokamak magnetic island with a background flow

We present a gyrokinetic theory of self-generated E × B vortex flows in a magnetic island in a collisionless tokamak plasma with a background vortex flow. We find that the long-term evolution of the self-generated vortex flows can be classified into two regimes by the background vortex flow potential Φ, with an asymptotic criterion given by , where T is temperature, ε is the inverse aspect ratio and r is the radial coordinate. We find that the background vortex flow above the criterion significantly weakens the toroidal precession-induced long-term damping and structure change of the self-generated vortex flows. That is, the finite background vortex flow is beneficial to maintain the self-generated vortex flows, favorable to an internal transport barrier formation. Our result indicates that the island boundary region is a prominent location for triggering the transition to an enhanced confinement state of the magnetic island.

The internal transport barrier formation on EAST tokamak during the fishbone instability

The internal transport barrier (ITB) which is related to the fishbone instability has been observed on the Experimental Advanced Superconducting Tokamak (EAST) in ELMy H-mode discharges. An interpretation of the formation of the ITB on EAST tokamak is provided, based on both analytical and numerical calculations. The fishbone instability induces the redistribution of fast ions and leads to the accumulation of fast ions in a local region where the ITB is going to appear. Correspondingly, the gradients of fast ions are enhanced, where the ion temperature gradient (ITG) mode exists. Fast ions can interact with the ITG mode through the dilution, Shafranov shift and wave-particle resonance mechanisms. It is found that the ITG mode is stabilized by fast ions and the stabilizing effects are determined mainly by the density, temperature and their gradients of fast ions. The enhanced density and temperature gradients of fast ions lead to a stronger stabilizing effect on ITG mode. Compared with the stabilizing effect before the appearance of fishbone instability, the stabilization on ITG mode is enhanced after the fishbone instability, which is beneficial to the formation of the ITB.

Investigation of key factors for ITB formation and maintenance in EAST high βN discharges

Long-pulse high βN operation scenario with internal transport barrier (ITB) and central flat q profile has been achieved on EAST. Analysis shows that higher heating power is needed to obtain ITBs in higher density discharges. Analysis of EAST high βN experiments shows that fishbone activities are accompanied with formation ITB and high performance. Experimental and simulation analysis demonstrates that the increasing E x B shear flow by the neutral beam injection (NBI) is important for the formation of ion temperature ITB (Ti-ITB). And, Ti-ITB disappears with the reduction of plasma rotation. While ITBs in channels of electron density (ne) and temperature (Te) are not achieved until fishbone activities are observed, which means that E x B shear flow mainly affects the formation of Ti-ITB in EAST high βN experiments. Experimental analysis shows that the fishbone is accompanied by the increasing E x B shear flow, leading to the formation of Ti-ITBs. A possible explanation for the formation of ITBs in all channels is that fishbone activities in high βN discharges with central flat q-profiles (closed to unity) induce the feedback process of the formation of ITBs, and leading to the increase of E x B shear flow as well as the formation and sustainment of ITBs.

ITB formation in gyrokinetic flux-driven ITG/TEM turbulence

The formation mechanism of internal transport barriers (ITBs) in flux-driven turbulence is studied by means of the full-f gyrokinetic code GKNET. In the adiabatic electron case with a weak magnetic shear configuration, toroidal momentum injection can change the radial mean electric field through radial force balance, leading to a kind of driven ITB formation in which the ion thermal diffusivity by ion temperature gradient (ITG) turbulence decreases to the neoclassical transport level. Only cocurrent toroidal rotation in the outer core region can benefit the ITB formation, and this mechanism is identified to originate from a positive feedback loop between the radial shear and resultant momentum flux. On the other hand, in the kinetic electron case with a reversed magnetic shear configuration, robust co-intrinsic rotation is driven near the surface in ITG turbulence and sustains the shear through the radial force balance, leading to the spontaneous reduction of ion turbulent thermal diffusivity, while this is not observed in the adiabatic electron case. In the presence of electron heating, counter-intrinsic rotation by trapped electron mode turbulence is selectively driven in the negative magnetic shear region, which provides steeper shear formation and a resultant larger reduction of ion turbulent thermal diffusivity. This indicates that the co-existence of different modes can trigger the ‘discontinuity’ of mode structure, intrinsic rotation, and resultant mean near , leading to spontaneous ITB formation.

Experimental study of the core instability before and after internal transport barrier formation in EAST

In a reversed shear discharge in the Experimental Advanced Superconducting Tokamak (EAST), an internal transport barrier (ITB) was formed. For the first time, the turbulence in the plasma core before and after the formation of the ITB in EAST was investigated by microwave reflectometry. It was found that during the formation of the ITB, the turbulence inside the barrier was not completely suppressed. The density fluctuation level decreased in the early phase of ITB and significantly increased later. It was found that the density fluctuation level increased with the density gradient after the appearance of reversed shear Alfvén eigenmodes (RSAEs). The change in turbulence, measurable by the reflectometer, did not affect the rate of increase in core density. Through the measurement of RSAEs, it was found that the formation of this ITB may be related to the minimum q (q min). q min was close to 2.

Multiple interactions between fishbone instabilities and internal transport barriers in EAST plasmas

Fishbone instabilities and internal transport barriers (ITBs) are frequently and sequentially observed in tokamak plasmas. Recently, the relationship between fishbone instabilities and ITBs was numerically studied, mainly on the basis of experimental results (Liu et al 2020 Nucl. Fusion 60 122001). It was identified that a radial electric field can be generated by the fishbone itself, which may act as a trigger for ITB formation. To gain a deeper understanding of this subject, in this work we further demonstrate the multiple interactions between fishbone instability and ITBs in Experimental Advanced Superconducting Tokamak (EAST) experiments (discharge #56933) using the hybrid kinetic-MHD code M3D-K. In multiple-n simulations, it is found that a zonal electric field can be induced in the nonlinear fishbone stage, leading to a relatively large E × B zonal flow that is sufficient to suppress the dominant microinstability before ITB formation; this should account for ITB triggering. After the ITB is triggered, the equilibrium pressure gradient increases and fast ions from the neutral beam injection accumulate in the ITB region. Linear simulations are performed to analyze the effect of ITB formation on fishbone instability. It is shown that due to the change of the pressure gradient during ITB expansion, the change in the bootstrap current density profile modifies the q-profile and then stabilizes the fishbone mode. Additionally, the accumulation of the fast ions leads to a broadening of fast ion distribution around the ITB region, which also has a stabilizing effect on the fishbone mode.

Electron thermal internal transport barriers triggered by the effect of ion shielding

The formation of electron thermal internal transport barriers (eITBs) is investigated by critical gradient threshold analysis for recent EAST experiments with dominant RF heating and low torque injection. The ratio of electron temperature to ion temperature, τ = T e/T i, is identified to be the key parameter that triggers eITB in the hot electron mode through electron turbulence suppression by the effect of ion shielding. The critical gradient of electron-temperature-gradient turbulence can be greatly increased with τ due to strong electron heating and weak electron–ion energy coupling. The formation and evolution of eITB is found to be determined by the critical temperature gradient of electron turbulence.

Staircase formation by resonant and non-resonant transport of potential vorticity

The E × B staircase is a quasi-periodic pattern of pressure profile corrugations. In this work, we present a new mechanism for E × B staircase formation that involves resonant transport versus non-resonant transport. We start from a potential vorticity evolution system and use quasi-linear theory, a model dispersion relation, and a bi-Lorentzian spectrum approximation, to construct the relation between the fluxes and the profiles. With these fluxes, we close the profile evolution equations and the extended turbulence intensity evolution equation, which together constitute a turbulence-profile evolution system. In this system, the Doppler effect from the E × B mean flow can cause resonance between trapped ion precession motion and the trapped ion mode, which drives a resonant transport contribution to the fluxes. The profiles will be flattened where the resonant transport is switched on. In contrast, for the regions of non-resonant transport, profiles are steeper. A quasi-periodic pattern of profile corrugation (the E × B staircase) spontaneously emerges in this system, which is the two states mentioned above, arranged as alternating layers in space. The feedback processes during the staircase pattern formation are identified. An estimate of the critical value of the boundary heat flux is obtained, above which the staircase formation will be triggered. An estimate scaling of the step size in the staircase pattern is obtained. The resonant turbulent transport is also a mechanism for collisionless saturation of zonal flow. This work is related to internal transport barrier formation and suggests some new scenarios, such as an enhanced confined L mode.

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.

Predictive modeling of operation scenarios with negative central magnetic shear and large-radius ITBs on EAST

Negative central magnetic shear or magnetic shear reversal is an attractive aspect for advanced operation scenarios in tokamaks as it suppresses turbulence and facilitates the formation of internal transport barriers (ITBs). The advanced operation scenario with negative magnetic shear, which is associated with high confinement quality, a large bootstrap fraction, ITBs, and so on, is one of the future goals of the experiments on EAST. In this work, modeling efforts have been made to find the operation regimes on EAST with negative central shear and large-radius ITBs. The modeling results indicate that negative central shear and large-radius ITBs can be achieved when the electron density is 〈ne〉/nG > 0.7 and the off-axis deposited electron cyclotron heating (ECH) power is 2 MW. Additionally, scenarios with lower ECH power (∼1.5 MW) have been studied since at present, the maximum ECH power that can be provided on EAST is 1.5 MW. With a lower off-axis ECH power of 1.5 MW, scenarios with negative central shear and large-radius ITBs but with a higher electron density of 〈ne〉/nG > 0.8 are obtained. The same as the first case, if the electron density is reduced to 〈ne〉/nG = 0.8 in this case, large-radius ITBs disappeared in the predicted scenario. This reveals that high electron density and large off-axis deposited ECH power are important for scenarios that have q-profiles with negative central shear and large-radius ITBs since lower hybrid current drive (LHCD) tends to peak near the axis at lower electron density, and hence, it helps to increase the temperature gradient near the axis, which will promote bootstrap in this region, leading to a centrally peaked total current density profile. Higher off-axis deposited ECH power helps to obtain scenarios with strong negative central shear and large-radius ITBs at a lower density. Scenarios predicted in this work will guide future experiments on EAST.

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.