In this work, we investigate the kinetic entropy and its flux in fast electron systems under non-local thermodynamic equilibrium. Starting from the relativistic Boltzmann equation, we derive the kinetic entropy evolution equation and decompose the entropy density and entropy flux into the internal energy term and the relative energy term. By applying drifting bi-Maxwellian J & uuml;ttner distribution, we systematically compute these quantities as functions of the temperature anisotropy ratio and the dimensionless drift velocity, revealing that relativistic random thermal motion significantly balances the drift velocity and modifies the stress tensor, leading to a screening effect on drift-driven energy transport in the internal energy term within the relativistic regime. This work provides a theoretical foundation for characterizing non-equilibrium thermodynamics and the anisotropy-driven energy redistribution among distinct forms, such as internal energy and work, and kinetic entropy production of fast electrons, benefiting the understanding of their transport and thermalization in tokamaks.

The gas electron multiplier camera has been used to investigate the electron dynamics in the presence of turbulence and/or a magnetic island in disruptive plasmas on the Experimental Advanced Superconducting Tokamak. Interactions between the micro turbulence and the macroscopic island structure are observed to be mediated by electrons experiencing multiscale instabilities, which are accelerated to generate non-thermal radiation peaks. After comparisons of two typical H-mode disruptive shots, a statistical database is constructed which clearly shows that the amplitude of non-thermal peaks is linearly dependent on the plasma poloidal beta before the thermal quench. Thus, the driving-energy for the electron energization is actually due to the plasma thermal pressure. Although not quantitatively compared with simulations, this paper provides multiple diagnostic data demonstrating electron dynamics under the complex multiscale instability fields. This advances the mechanism causing thermal quench of H-mode plasmas.

Since the last IAEA-FEC in 2021, significant progress on the development of long pulse steady state scenario and its related key physics and technologies have been achieved, including the reproducible 403 s long-pulse steady-state H-mode plasma with pure radio frequency (RF) power heating. A thousand-second time scale (similar to 1056 s) fully non-inductive plasma with high injected energy up to 1.73 GJ has also been achieved. The EAST operational regime of high beta(P) has been significantly extended (H-98y2 > 1.3, beta(P) similar to 4.0, beta(N) similar to 2.4 and n(e)/n(GW) similar to 1.0) using RF and neutral beam injection (NBI). The full edge localized mode suppression using the n = 4 resonant magnetic perturbations has been achieved in ITER-like standard type-I ELMy H-mode plasmas with q(95) approximate to 3.1 on EAST, extrapolating favorably to the ITER baseline scenario. The sustained large ELM control and stable partial detachment have been achieved with Ne seeding. The underlying physics of plasma-beta effect for error field penetration, where toroidal effect dominates, is disclosed by comparing the results in cylindrical theory and MARS-Q simulation in EAST. Breakdown and plasma initiation at low toroidal electric fields (<0.3 V m(-1)) with EC pre-ionization is developed. A beneficial role on the lower hybrid wave injection to control the tungsten concentration in the NBI discharge is observed for the first time in EAST suggesting a potential way toward steady-state H-mode NBI operation.

A new hard x-r ay and soft gamma-ray spectrometer imaging system (HXS) has been built for two-dimensional measurements of plasma emitted photons in the Experimental Advanced Superconducting Tokamak. The system uses a 2D cadmium zinc telluride detector and integrated electronics, and is as a whole shielded by a tungsten box with a pinhole and tangential to the toroidal field. Three classes of typical energy spectra have been summarized in different experimental scenarios during the past campaigns. After performing tomography calculations, the local emissivity contours have been obtained in different energy ranges, which clearly show asymmetry of the plasma crosssection. The spatial perturbation structure is similar to the magnetohydrodynamic modes with low mode numbers. In particular, the runaway island found by an infrared camera (Jaspers et al 1994 Phys. Rev. Lett. 72 4093) is also measured by the HXS. There exists a reversal population in the energy spectra of both slide-away and strong neutral beam injection shots. It is consistently observed that the count rate is increased in the low-energy range before the plasma disruptions. Calculations in phase space indicate that the accelerated momentum flux can be deflected back to the low-energy region by the large pitch-angle scattering. In the post-disruption phase, the plasma current is not replaced by runaway electrons due to tearing modes or transiently bursting instabilities. This paper constructs the basics for the proper use of HXS for hard x-ray and soft gamma-ray measurements in future investigations of plasma disruptions.

This work presents a robust methodology for effectively distinguishing between the seed island and the onset of the neoclassical tearing mode (NTM) in the EAST tokamak. Unlike previously employed methods, the width of the seed island is carefully regulated by gradually ramping up the currents in the resonant magnetic perturbation (RMP) coils. Remarkably, the seed island phase can be sustained for several hundred milliseconds, providing ample time to examine the nonlinear dynamics of NTM threshold physics. This study investigates the plasma response in terms of various factors, including the plasma rotation, the electron density, and the electron temperature, from the initial formation of the seed island to the saturation of the NTM island. Through extensive statistical analysis of NTM triggering events, it is revealed that both the critical width of the seed island and the critical RMP currents are positively correlated with beta p. Moreover, the relationship of the transition time associated with mode penetration to beta p differs from the dependence of the transition time for NTM triggering on beta p. The growth rates associated with these two nonlinear phenomena show similar characteristics. Finally, reduced magnetohydrodynamic (MHD) modeling replicates the two nonlinear bifurcation states observed in the experiment. It is noteworthy that the RMP-induced NTM islands are locked to the static magnetic perturbation. This scenario differs from the natural excitation of NTMs, which occurs due to transient MHD phenomena, such as sawtooth crashes, where the triggering source is a rotating magnetic perturbation. This leads to the formation of rapidly rotating islands at a natural frequency. Furthermore, the stability of these islands is expected to be related to the polarization current effect. Nevertheless, this experiment sets a new course for the comprehensive investigation and understanding of NTM threshold physics, providing valuable insights for future tokamak design and operational strategies

The blob properties in I-mode and ELM-free H-mode plasmas compared to L-mode have been investigated on the EAST tokamak, including the blob detection rate N b , size δ b , lifetime τ b and radial velocity v r , b . The blob properties in L-mode and I-mode are similar, and those in ELM-free H-mode are different to them. The blob N b is smaller while τ b is larger in H-mode. The experimental blob size δ b and velocity scalings v r , b − δ b show a good agreement with the theoretical models. The variation in blob properties during the L-I and H-L transitions, and their relations to the scrape-off layer (SOL) density, edge and SOL turbulence, and SOL collisionality are discussed. The suppression of the edge (inside the last closed flux surface) turbulence is not reflected in the blob behavior, while the blob detection rate shows a correlation with the SOL density and its low-frequency (3–50 kHz) fluctuations. In addition, the blob detection rate is found to increase with the divertor collisionality Λ div , indicating a dependence of blob behavior on Λ div . The differences in blob detection rates among the three operating regimes might be due to their different SOL densities and collisionalities. The investigation contributes to understanding the influences of edge and SOL plasma parameters on the blob behavior.

Mastering nuclear fusion, which is an abundant, safe, and environmentally competitive energy, is a great challenge for humanity. Tokamak represents one of the most promising paths toward controlled fusion. Obtaining a high-performance, steady-state, and long-pulse plasma regime remains a critical issue. Recently, a big breakthrough in steady-state operation was made on the Experimental Advanced Superconducting Tokamak (EAST). A steady-state plasma with a world-record pulse length of 1056 s was obtained, where the density and the divertor peak heat flux were well controlled, with no core impurity accumulation, and a new high-confinement and self-organizing regime (Super I-mode = I-mode + e-ITB) was discovered and demonstrated. These achievements contribute to the integration of fusion plasma technology and physics, which is essential to operate next-step devices.

Recently, stationary plasma with a world-record pulse length of 1056 s was achieved on the Experimental Advanced Superconducting Tokamak (EAST). In this work, the core magnetohydrodynamics (MHD) events as well as mode coupling processes have been investigated in EAST long pulse operation with electron heating dominant and pure radio frequency wave heating low collisionality, by using several diagnostics and the nonlinear numerical code M3D together. A saturated m / n = 1/1 kink mode was observed in the core region, where a stable internal transport barrier was found in the electron temperature channel. The frequencies and 2D structures of these modes were studied by a combination of soft X-ray (SXR) imaging and electron cyclotron emission diagnostics. The frequency of the m / n = 1/1 mode exhibited a feature of chirping down with time, and this chirping rate corresponds to the rate of electron diamagnetic drift frequency change. A twisted pattern (‘Taichi structure in shape’) was reconstructed by SXR tomography of the m / n = 1/1 mode. The perturbations of electron temperature and density caused by m / n = 1/1 are different in size, the latter one being much smaller. The destabilization of m / n = 1/1 was due to the strong central heating combined with electron cyclotron resonance heating and lower hybrid current drive. In the presence of the m / n = 1/1 mode, a negative current was generated on the magnetic axis, which anomalously broadened the core current profile. A m / n = 3/2 tearing mode triggered by m / n = 1/1 mode was also observed. The m / n = 3/2 mode has a smaller frequency than the m / n = 1/1 mode, and carries an m / n = 3/2 island with detectable size. A novel 3D MHD model that separately evolves the plasma density and temperature is applied to the m / n = 3/2 triggered by m / n = 1/1. It is found that a toroidal current density at the q = 1.5 surface, caused by non-axisymmetric density perturbation during the m / n = 1/1 nonlinear growth phase, was generated and hence the destabilization of the m / n = 3/2 tearing mode. Both the modeled electron temperature and density perturbations agree well with experimental observations. In the last section of this paper, the interaction between the m / n = 1/1 mode and fast electrons as well as the active control of this mode are also presented.

In recent years magneto-hydrodynamic instabilities have been observed in different experimental scenarios by a gas electron multiplier (GEM) camera, since it was installed on the Experimental Advanced Superconducting Tokamak (EAST). The GEM camera is set up outside the EAST vacuum chamber to record x-ray photons through a beryllium window, which has obtained valuable data for analysis: (1) neoclassical tearing mode (NTM) onset and (2) plasma disruption dynamics in the presence of magnetic islands. Although not precisely compared with theory and simulations, the GEM, in synergy with other diagnostics, reveals the significance of electron dynamics in NTM and plasma disruption. This is meaningful for understanding and modeling the disruption instability of fusion plasmas.

In 2021,EAST realized a steady-state long pulse with a duration over 100 s and a core electron temperature over 10 keV.This is an integrated operation that resolves several key issues,including active control of wall conditioning,long-lasting fully noninductive current and divertor heat/particle flux.The fully noninductive current is driven by pure radio frequency(RF)waves with a lower hybrid current drive power of 2.5 MW and electron cyclotron resonance heating of 1.4 MW.This is an excellent experimental platform on the timescale of hundreds of seconds for studying multiscale instabilities,electron-dominant transport and particle recycling(plasma-wall interactions)under weak collisionality.

等离子体破裂是托卡马克运行中常见的一种现象,高参数等离子体运行过程中发生破裂会对实验装置安全运行产生威胁.通过对等离子体破裂的研究,可以在未来进一步做到破裂的避免和主动控制.首先介绍了 EAST装置上的等离子体破裂,通过GEM相机对EAST放电实验中几种类型的破裂炮进行二维成像分析,对破裂的发生过程进行了具体分析.并且在分析过程中展示了 GEM系统较传统多道系统的优越性,确认GEM可以为破裂机理研究提供有效的诊断数据.

A broadband (BB) mode is observed by collective Thomson scattering diagnostics in repeatable shots of EAST and analyzed for the first time. This BB mode usually grows during L–H transitions, featuring a BB quasi-coherent mode with increasing frequency. During H-mode operations, it is characterized by steady-state BB in the high-frequency range ( f ∼ 200–2000 kHz), at the electron scale ( k θ ρ s = 1‒2), mainly driven by the density gradient, and is sensitive to the value of η e in the region of interest ( ρ = 0.4‒0.8), where η e = R / L T e / R / L n e is the ratio of the normalized electron temperature gradient and density gradient, and the region ρ = 0.4‒0.8 usually has a relatively low collisionality ( v eff < 5). The frequency of BB is found to be dependent on the electron temperature and density gradient, which is a typical feature of electron-driven turbulence. A negative correlation between the energy confinement and the intensity of the BB turbulence during H-mode has been found, which indicates a strong electron thermal transport induced by the BB turbulence. The BB significantly decreases the electron temperature and causes flatter electron temperature profiles in the region of interest ( ρ = 0.4‒0.8), thus making η e decrease and the BB destabilize further. These characteristics of BB are related to the theoretical density gradient-driven trapped electron mode. It should be noted that this mode is not observed by other diagnostics in EAST, and shows very different features to the coherent modes in the edge.

A recent EAST experiment has successfully demonstrated long pulse steady-state high plasma performance scenario and core-edge integration since the last IAEA in 2018. A discharge with a duration over 60 s with β P ∼ 2.0, β N ∼ 1.6, H 98y2 ∼ 1.3 and an internal transport barrier on the electron temperature channel is obtained with multi-RF power heating and current drive. A higher β N ( β N ∼ 1.8, β p ∼ 2.0, H 98y2 ∼ 1.3, n e / n GW ∼ 0.75) with a duration of 20 s is achieved by using the modulated neutral beam and multi-RF power, where several normalized parameters are close or even higher than the phase III 1 GW scenario of CFETR steady-state. High-Z impurity accumulation in the plasma core is well controlled in a low level by using the on-axis ECH. Modeling shows that the strong diffusion of TEM turbulence in the central region prevents tungsten impurity from accumulating. More recently, EAST has demonstrated compatible core-edge integration discharges in the high β p scenario: high confinement H 98y2 > 1.2 with high β P ∼ 2.5/ β N ∼ 2.0 and f bs ∼ 50% is sustained with reduced divertor heat flux at high density n e / n GW ∼ 0.7 and moderate q 95 ∼ 6.7. By combining active impurity seeding through radiative divertor feedback control and strike point splitting induced by resonant perturbation coil, the peak heat flux is reduced by 20–30% on the ITER-like tungsten divertor, here a mixture of 50% neon and 50% D 2 is applied.

The excitation of an initially stable neoclassical tearing mode (NTM) is investigated in Experimental Advanced Superconducting Tokamak (EAST) low- β p H-mode plasmas ( β p is defined as the ratio of the thermal pressure to the poloidal magnetic pressure). Using similar plasma parameters, n = 1 resonant magnetic perturbation (RMP) cannot always successfully excite the m / n = 2/1 NTM with the same RMP coil current setup ( n and m are the toroidal and poloidal mode numbers, respectively). Data from a gas electron multiplier camera shows that NTM destabilization is related to RMP-induced crashes at the q = 1 resonant surface during the RMP ramp-up phase. The second RMP-induced crash amplitude decays exponentially as β p increases. There is a critical value β p ≈ 0.76 above which the crash amplitude (or seed island width) is too small (below the critical island width) to trigger an NTM. Observation and analysis indicate that the m / n = 2/1 NTM is not forcibly driven by the n = 1 RMP (such as the m / n = 2/1 component), but is probably due to electron heat transport between the q = 1 and the q = 2 resonant surfaces. This paper describes experimental observations of NTM excitation which also have implications for further investigations of NTM locking and disruptions.

An internal transport barrier (ITB) can be formed on EAST in exploring high-parameter operation. Previous studies show that safety factor ( q ) profiles, Shafranov shift and magnetohydrodynamic behaviors could be helpful in ITB formation by suppressing anomalous transport. Recently, electron density evolution with high resolution demonstrates that fishbone could be dominant in electron density ITB formation and sustainment. The power threshold is low in the fishbone condition and the electron density profile is determined by traits of fishbone. Simulation shows that the low- k ion mode is suppressed by fishbone. Direct measurement of turbulence in the inner region shows that the internal kink mode could sustain an electron temperature ITB by suppressing the trapped electron mode. The multi-scale interaction between the kink mode and turbulence by current could be key in sustaining high-electron-temperature long-pulse operation.

High-β_{θe} (a ratio of the electron thermal pressure to the poloidal magnetic pressure) steady-state long-pulse plasmas with steep central electron temperature gradient are achieved in the Experimental Advanced Superconducting Tokamak. An intrinsic current is observed to be modulated by turbulence driven by the electron temperature gradient. This turbulent current is generated in the countercurrent direction and can reach a maximum ratio of 25% of the bootstrap current. Gyrokinetic simulations and experimental observations indicate that the turbulence is the electron temperature gradient mode (ETG). The dominant mechanism for the turbulent current generation is due to the divergence of ETG-driven residual flux of current. Good agreement has been found between experiments and theory for the critical value of the electron temperature gradient triggering ETG and for the level of the turbulent current. The maximum values of turbulent current and electron temperature gradient lead to the destabilization of an m/n=1/1 kink mode, which by counteraction reduces the turbulence level (m and n are the poloidal and toroidal mode number, respectively). These observations suggest that the self-regulation system including turbulence, turbulent current, and kink mode is a contributing mechanism for sustaining the steady-state long-pulse high-β_{θe} regime.

In the 2018 EAST experimental campaign, a very high central electron heating, fully-non-inductive discharge with the core electron temperature over 9 keV has been achieved. Such high central electron heating was realized by injecting radio frequency waves, including 1.8 MW lower hybrid wave (LHW) and 0.8 MW electron-cyclotron waves (ECW). Experimental diagnosis indicates two different time scales characterizing the electron heating process, a rapid and a slow rise of the central electron temperature after the injection of ECW. In this work, integrated modeling is performed to investigate the physical mechanisms of such high electron heating. Five characteristic phases during the increase of the electron temperature are chosen for modeling. In phase 1, the electron heating is by LHW alone. The modeling confirms that the LHW can only sustain the core electron temperature T (e) approximate to 5.5 keV, which is consistent with the experiment. In phase 2, the electron temperature increases rapidly after the first 0.4 MW ECW is injected. The result shows that the rapid increase of the electron temperature is due to the interaction between the ECW and the electrons. With the increase of the electron temperature, the electron flux induced by the trapped electron modes (TEMs) and the electron temperature gradient driven modes (ETGs) is enhanced in the core region. In phase 3, the electron temperature increases slowly after phase 2. It is found that the slow increase is mainly due to the flattening of the density profile. The flattening of the density profile can decrease the thermal diffusivity of the electrons mainly induced by the TEMs leading to a higher electron temperature for a given heating source. In phase 4, the electron temperature again increases rapidly after the second 0.4 MW ECW is injected. The physical mechanism is similar to that in phase 2. In phase 5, the LHW power deposition of the LHW remains almost unchanged compared to that in phase 4 since the electron temperature is sufficiently high. The slow rise of the electron temperature is caused by the improvement of the electron energy confinement as thermal diffusivity of the electrons is decreased due to the flattening of the electron density profile, which is similar to the main reason in phase 3.

The loss of beam ions due to magnetic islands is investigated in a tokamak. The perturbed guiding-center drifts of passing particles including the effect of the finite orbit width are demonstrated. The widths of the drift islands under resonant conditions are studied theoretically and numerically. The ORBIT code is used to simulate the action of the neoclassical tearing mode with a toroidal mode number n = 1 and poloidal mode number m = 2 on passing fast ions generated by neutral beam injection in the Experimental Advanced Superconducting Tokamak. Two loss channels for passing fast ions are identified as the resonant interaction and the stochastic interaction. The lost fast ions in the loss detector zone (LDZ) to simulate the fast-ion loss detector assemble around two regions in phase space, namely, (i) a pitch angle of θ = 28° both with and without the mode and (ii) θ = 59° when the mode amplitude is large enough, where θ=arccosv∥/v. The number of these lost ions in the LDZ evolves in the period of the mode. The fraction of the total lost ions evolves in the period of the n = 1 oscillation in the toroidal direction. The fraction of lost beam ions has a linear relationship with the mode amplitude in first 10 µs and a quadratic one thereafter. The corresponding characteristics of the lost beam ions in phase space are also discussed.

A toroidal Alfvén eigenmode (TAE) is excited by electron cyclotron resonance heating (ECRH) induced barely trapped energetic electrons in experimental advanced superconducting tokamak . This TAE appears in the low density EAST discharges under pure off-axis ECRH heating. After analysing the ECRH power modulation induced local density and temperature oscillations, the location of this TAE mode and the power deposition of ECRH are determined. This edge localized TAE mode drifts in ion-diamagnetic direction may be driven by barely trapped energetic electrons considering the contribution of poloidal bounce effect in the general wave-particle resonance condition. The experimental observations also demonstrate that ECRH power modulation with fixed frequency could be used as an effective diagnostic tool to study the internal properties of MHD modes as well as particle and heat transports.

We observed for the first time a helical m/n = 1/1 (mis the poloidal mode number andnis the toroidal mode number) saturated steady mode (SSM) in the center of the EAST electron heating dominant plasma where the core safety factor was close to but slightly below unity and the T-e profile had an extreme peak at the plasma center ( T-e is the electron temperature). An internal crash can be caused by the dynamics of the SSM, and its influence on the electron temperature profile is as large as that of a typical sawtooth crash (SC) in magnitude. Due to the weak magnetic shear in the core, the SSM regularly exhibits an m/n = 2/2 harmonic component. For low shear q <= 1 auxiliary heated plasma, the SSM cannot cause the degradation of plasma confinement (beta(p) T-e), but an SC is indeed a harmful factor. Three-dimensional resistive magnetohydrodynamic simulations with a realistic EAST strong elongated magnetic configuration, a high Lundquist number (S = 10(8)) and a strong central peaked pressure profile, have been made to demonstrate the formation of the SSM in the case of q(0) <= 1 and the destabilization of the SC in another case with q(0) < 1. These nonlinear simulation results agree well with the observations of both the SSM and SC in EAST electron heating dominant discharges with an extreme central peaked T-e. An M3D simulation found that the SC is caused by additional newly developed harmonics of the m/n = 1/1 helical instability. Nonlinear simulation also predicted a modulation of the SSM on current in the plasma core, which is favorable for maintaining q(0) <= 1 in the plasma core. Furthermore, nonlinear simulation also showed that a small toroidal plasma flow can be generated by the SC.