Numerical analysis of high Mach flow and flow reversal in the experimental advanced superconducting tokamak divertor

The B2-Eirene (SOLPS 4.0) code package is used to investigate the plasma parallel flow, i.e., the scrape-off layer (SOL) flow, in the experimental advanced superconducting tokamak (EAST) divertor. Simulation results show that the SOL flow in the divertor region can exhibit complex behaviour, such as a high Mach flow and flow reversal in different plasma regimes. When the divertor plasma is in the detachment state, the high Mach flow with approaching or exceeding sonic speed is observed away from the target plate in our simulation. When the divertor plasma is in the high recycling state, the flow reversal with a small Mach number (|M|< 0.2) is observed near the X-point along the separatrix region. The driving mechanisms for the high Mach flow and the reversed flow are analysed theoretically through momentum and continuity equations, respectively. The profile of the ionization sources is shown to be a possible formation condition causing the complex behaviour of the SOL flow. In addition, the effects of the high Mach flow and the flow reversal on the impurity transport are also discussed in this paper.     

Study of multiple impurity seeding effect using SONIC integrated divertor code for JT-60SA plasma prediction

In order to study the potential impurity seeding operation regime of the future fusion devices, the first application of the integrated divertor code SONIC to the Ar + Ne mixed-impurity seeding operation of JT-60SA steady-state high-β plasma has been carried out. In the case, Ne is added to Ar-only seeding, the separatrix electron density has fell into the desired low separatrix electron density of the scenario. This is mainly because the D⁺ flow velocity towards the inner divertor has been increased by the Ne seeding. The resultant friction force transports Ar impurities towards the inner divertor region, while impurities are stagnated in the top of scrape-off layer (SOL) in the Ar-only seeding case. The higher impurity radiation power in the divertor regions and lower one in the SOL region above the X point have been obtained in mixed-impurity seeding cases, which show similar tendency as the Ar + Ne mixed-impurity seeding experiment in JT-60 U. At the core edge, Z_eff has been slightly increased and the radiation power has been decreased as the Ne seeding rate increases. The core plasma/impurity transport has been also evaluated by the TOPICS code using the impurity density at the core edge computed by the SONIC as a boundary parameter. The results show lower Z_eff and radiation power, and higher electron temperature in the core in the mixed-impurity seeding cases. Above possible contributors to the better energy confinement indicate that the mixed-impurity seeding operation might be more effective than Ar-only seeding operation.     

The Role of Radial Electric Fields in the Tokamaks TEXTOR-94, CASTOR, and T-10

Radial electric fields (E _r) and their role in the establishment of edge transport barriers and improved confinement have been studied in the tokamaks TEXTOR-94 and CASTOR, where E _r is externally applied to the plasma in a controlled way using a biased electrode, as well as in the tokamak T-10 where an edge transport barrier (H-mode) is obtained during electron-cyclotron resonance heating (ECRH) of the plasma.The physics of radial currents was studied and the radial conductivity in the edge of TEXTOR-94 (R = 1.75 m, a = 0.46 m) was found to be dominated by recycling (ion-neutral collisions) at the last closed flux surface (LCFS) and by parallel viscosity inside the LCFS. From a performance point of view (edge engineering), such electrode biasing was shown to induce a particle transport barrier, a reduction of particle transport, and a concomitant increase in energy confinement. An H-mode-like behaviour can be induced both with positive and negative electric fields. Positive as well as negative electric fields were shown to strongly affect the exhaust of hydrogen, helium, and impurities, not only in the H-mode-like regime.The impact of sheared radial electric fields on turbulent structures and flows at the plasma edge is investigated on the CASTOR tokamak (R = 0.4 m, a = 0.085 m). A non-intrusive biasing scheme that we call "separatrix biasing" is applied whereby the electrode is located in the scrape-off layer (SOL) with its tip just touching the LCFS. There is evidence of strongly sheared radial electric field and E×B flow, resulting in the formation of a transport barrier at the separatrix. Advanced probe diagnosis of the edge region has shown that the E×B shear rate that arises during separatrix biasing is larger than for standard edge plasma biasing. The plasma flows, especially the poloidal E×B drift velocity, are strongly modified in the sheared region, reaching Mach numbers as high as half the sound speed. The corresponding shear rates ( 5×10⁶ s^-1) derived from both the flow and electric field profiles are in excellent agreement and are at least an order of magnitude higher than the growth rate of unstable turbulent modes as estimated from fluctuation measurements.During ECRH in the tokamak T-10 (R = 1.5 m, a = 0.3 m), a regime of improved confinement is obtained with features resembling those in the H-mode in other tokamaks. Using a heavy ion beam probe, a narrow potential well is observed near the limiter together with the typical features of the L-H transition. The time evolution of the plasma profiles during L-H and H-L transitions is clearly correlated with that of the density profile and the formation of a transport barrier near the limiter. The edge electric field is initially positive after the onset of ECRH. It changes its sign during the L-H transition and grows till a steady condition is reached. Similar to the biasing experiments in TEXTOR-94 and CASTOR, the experimentally observed transport barrier is a barrier for particles.     

FEB-E动态气靶偏滤器的研究

在ＦＥＢ－Ｅ设计阶段,偏波器从开放式固定板靶优化为封闭式气体靶,以改善偏滤器的杂质控制和增加离子与气体的相互作用,通过喷气和注入杂质获得的部分脱靶等离子体形成了动态气体靶,喷气能降低删削层（ＳＯＬ）处等离子体温度,沪入的杂质增加了ＳＯＬ处的辐射功率,使靶板的负载降低,用ＮＥＷＴ１Ｄ编码模拟了ＳＯＬ处等离子体和杂质（硼杂质）的输运,得到了杂质、等离本温度和等离了体密度分布。着眼于杂质的滞留物辐射,优     

东方超环托卡马克高约束模式边界等离子体输运数值模拟研究

利用SOLPS5.0模拟研究东方超环托卡马克（EAST）高约束模式时的刮削层等离子体. 在高约束模式放电实验参数（第36291炮）的限制下,通过调整上游区径向反常输运系数来实现高约束模式模拟,在上游电子密度和温度与实验符合的条件下能够很好地进行下游区模拟. 在实现高约束模拟的基础上又分别研究了漂移项对偏滤器靶板能流不对称性的影响和上游能流衰减宽度对靶板能流密度峰值的影响. 通过模拟发现,漂移是导致EAST放电内外靶板不对称性的主要原因,增大上游能流衰减宽度可以明显降低入射到靶板的峰值热流,并且偏滤器区辐射以及与中性粒子的相互作用减小了能流的衰减宽度对达到靶板能流的影响. 关键词： 托卡马克 高约束模式 SOLPS5.0 漂移     

Significance of the Radial Electric Field to Divertor Load Asymmetries

JET divertor load measurements using embedded thermocouples have indicated a large target asymmetry and non-uniformities in the loads and that, under H-mode conditions, the load is dominated by the ion component. ASCOT simulations of the ion contribution to JET divertor loads are consistent with the presence of a large outward radial electric field E _r in the scrape-off layer (SOL) next to the separatrix. Such a field would provide an explanation for observed features of the divertor loads in JET. The required field for getting the correct in/out asymmetry is within the range of the estimated SOL E _r in JET, but reproducing the experimental heat deposition profiles requires higher field values.     

Reinstallation of the COMPASS-D tokamak in IPP ASCR

The COMPASS-D tokamak, originally operated by UKAEA at Culham, UK, will be reinstalled at the Institute of Plasma Physics (IPP) AS CR. The COMPASS device was designed as a flexible tokamak in the 1980s mainly to explore the MHD physics. Its operation (with D-shaped vessel) began at the Culham Laboratory of the Association EURATOM/ UKAEA in 1992.The COMPASS-D tokamak will have the following unique features after putting in operation on IPP Prague. It will be the smallest tokamak with a clear H-mode and ITER-relevant geometry. ITER-relevant plasma conditions will be achieved by installation of two neutral beam injection systems (2 × 300 kW), enabling co-and counter-injections. Redeployment of the existing LH system (400 kW) is also envisaged. A comprehensive set of diagnostics focused mainly on the edge plasma will be installed.The scientific programme proposed for the COMPASS-D tokamak installed in IPP Prague will benefit from these unique features of COMPASS-D and consist of two main scientific projects, both highly relevant to ITER-Edge plasma physics (H-mode studies) and Wave-plasma interaction studies.The COMPASS-D tokamak will offer an important research potential as a small, flexible and low-cost facility with ITER-relevant geometry.     

Nonlinear collisional monte carlo simulations for high-temperature SOL plasma

A nonlinear Monte Carlo collisional model is applied to to investigate scrape-off layer (SOL) plasmas with high temperatures. In the proposed SOL modeling, A steady state SOL plasma, which satisfies the particle and energy balances and neutrality constraint, is determined in terms of total particle and heat fluxes across the separatrix, the edge plasma temperature, the secondary electron emission coefficient, and the SOL size. A conductive heat flux into the SOL is effectively modeled via random exchange of source particles and the SOL plasma particles. It is found that the potential drop and the electron transmission factor in the collisional SOL plasma are in good agreement with the theoretical prediction. The cooling effect of secondary electrons in the high temperature divertor operation is investigated. In such a collisionless plasma, the present nonlinear collision model is useful because the electron distribution function deviates far from a Maxwellian distribution. In the presence of strong secondary electron emission, the electron sheath energy transmission factor in the collisionless regime is found to be significantly smaller than that in the collisional regime. This fact suggests that a high-temperature divertor operation can be possible.     

Evolution of the high-field-side radiation belts during the neon seeding plasma discharge in EAST tokamak

Divertor detachment achieved by injecting impurities or increasing density is always accompanied with various local radiation phenomena in the boundary or core plasma. This paper presents the formation and evolution of the high-field-side (HFS) radiation belts during the neon seeding plasma discharge in upper single null configuration with two directions of toroidal magnetic field in EAST tokamak. The neon mixed with deuterium seeding can induce the divertor detachment with strong radiation belts in the HFS scrape-off layer (SOL) region. With the increase of the radiation power, the plasma discharge will transit from H-mode to L-mode, and meanwhile the radiation belts move away from the near X-point to HFS SOL. When the radiation power is high enough, the radiation belts begin to move further to the other X-point along the HFS SOL, and even cause plasma disruption. The results indicate that the behavior of the radiation belts is related to the radiation power, plasma confinement performance and state of divertor detachment, which is useful for developing better feedback control methods to achieve high-performance radiative divertor operation mode.     

偏滤器运行模式对托卡马克边缘区等离子体平行流的影响

我们使用一维流体模型,根据在不同偏滤器运行模式下静压强沿着磁力线方向的分布变化,讨论了偏滤器运行模式对托卡马克边缘区等离子体平行流的影响.低再循环模式下,静压强从X点 (X-point)附近的刮削层区域开始明显下降,变化趋势与密度变化趋势一致;等离子体平行流的马赫数在偏滤器区域逐步变大,变化从平缓到迅速.高再循环模式下,静压强在靶板附近的区域迅速下降,在其他区域变化非常小;等离子体平行流的马赫数仅在靠近靶板附近的区域迅速变大,在其他区域变化平缓.在弱脱靶模式下的静压强变化与高再循环模式下类似,不过静压强在X-point附近的刮削层区域开始出现下降的趋势,导致等离子体平行流的马赫数在X-point处的值比在高再循环模式下大.强脱靶模式下,静压强在刮削层区域开始明显下降,在远离靶板的偏滤器区域,静压强迅速下降的地方,观察到高马赫数等离子体平行流.静压强迅速下降引起动压强迅速上升来维持总的压强守恒是在强脱靶状态 下产生高马赫数平行流的一种可能驱动机理.     

Role of poloidal E × B drift in divertor heat transport in DIII-D

Simulations for DIII-D high confinement mode plasmas with the multifluid code UEDGE show a strong role of poloidal E × B drifts on divertor heat transport, challenging the paradigm of conduction-limited scrape-off layer (SOL) transport. While simulations with reduced drift magnitude are well aligned with the assumption that electron heat conduction dominates the SOL heat transport, simulations with drifts predict that the poloidal convective E × B heat transport dominates over electron heat conduction in both attached and detached conditions. As poloidal E × B flow propagates across magnetic field lines, poloidal transport with shallow magnetic pitch angles can reach values that are of the same order as would be provided by sonic flows parallel to the field lines. These flows can lead to strong convection-dominated divertor heat transport, increasing the poloidal volume of radiative power front, consistent with previous measurements at DIII-D. Due to these convective flows, the Lengyel integral approach, assuming zero convective fraction, is expected to provide a pessimistic estimate for the radiative capability of impurities in the divertor. For the DIII-D simulations shown here, the Lengyel integral approach underestimates the radiated power by a factor of 6, indicating that, for reliable DIII-D divertor power exhaust predictions, full two-dimensional (2D) calculations, including drifts, would be necessary.     

Modeling of supersonic plasma flow in the scrape-off layer

The substantial drop of the plasma temperature along magnetic field lines with increasing plasma density is one of the main features in tokamak divertors. As a result the temperature gradient at the divertor plates becomes very steep and the boundary condition normally applied for the parallel Mach number M at the target, M_t = 1, cannot be satisfied. In this case the value of M_t based on the general form of the Bohm criterion, M_t  1, has to be determined from the continuity of plasma parameters.In the present paper a new approach to resolve the Mach number at the target for such a situation is proposed. This method avoids the singularity problem that arises by treating the particle balance and parallel motion equations in a differential form. Instead, the integral representation of the equations is formulated for an arbitrary form of particle and momentum sources. The approach can also take into account transport perpendicular to the magnetic field lines.The proposed method is demonstrated on the example of a one-dimensional stationary model for the scrape-off layer (SOL) plasma and includes the continuity-, parallel momentum- and heat transfer equations. The recycled neutrals are described in the diffusion approximation. In the case of low density the normal condition M_t = 1 is satisfied and the results are in agreement with the two-point model. At high enough plasma density solutions with the supersonic flow at the divertor plates, M_t > 1, are found. These states correspond to a partially detached plasma with a temperature of a few eV.     

Modelling of particle flows in a scrape-off layer with limiter biasing

The superconducting tokamak Tore Supra will be equipped with an actively cooled toroidal pump limiter (TPL), in the framework of the CIEL (Composants Internes Et Limiteurs) project, dedicated to plasma facing component design for steady state operation. The TPL is equipped with throats, located only on the high field side, for particle collection allowing the control of plasma density which is essential for long plasma discharges. The present design work of the CIEL includes a biasing system in order to enhance the particle pumping. A fluid model, based on the classical fluid equation, is used to estimate the effects of the electric field on the particle flows in the Scrape-Off Layer (SOL). The modifications of the density, the particle flow (toroidal and poloidal) and the position of the stagnation point are discussed as a function of the bias voltage. The model clearly illustrates the different resulting effects on particle pumping for a divertor and a limiter configuration which are designed respectively for poloidal or parallel particle collection. The model is used to interpret the ALT-biasing experiments recently carried out on TEXTOR-94. The pumping capability is shown to be improved by about (15–20)% for positive biasing while the experimental measurements of parallel Mach number are reproduced as a function of the applied voltage. The e-folding length of the edge density in the SOL is also shown to increase from 1.5 to about 2.0 cm for a biased voltage of −400 to 400 V, respectively, in accordance with the model. Finally, the model is used to extrapolate the TEXTOR-94 results to CIEL suggesting that pumping speed enhancement of 25 to 30% may be obtained. Partner in Trilateral Euregio Cluster Partner in Trilateral Euregio Cluster Presented at the Workshop on Role of Electric Fields in Plasma Confinement and Exhaust, Budapest, 18–19 June 2000.     

Measurement of natural plasma flow along the field lines in the scrape-Off layer on the JT-60U divertor tokamak

The plasma profile and parallel plasma flow in the scrape-off layer (SOL) were systematically measured using reciprocating Mach probes installed at the outer midplane and near the divertor magnetic null (x point) in the JT-60U tokamak with a single null divertor. For the ion vertical drift due to the toroidal magnetic field gradient (ion nablaB drift) directed towards the divertor, SOL plasma flow along the magnetic field lines away from the divertor ("flow reversal") was discovered at the midplane far from the divertor. A quantitative evaluation of the ion "Pfirsch-Schluter flow," wherein the parallel flow is naturally produced in a toroidal plasma, was consistent with the measurement.     

边界局域模对EAST钨偏滤器靶板腐蚀程度的数值模拟研究

在高约束模式下发生的边界局域模会释放高能量等离子体,其中主要部分会辐照到面积相对较小的偏滤器靶板,偏滤器钨靶板发生热腐蚀的可能性最大.本文建立了包括了熔化、汽化和热辐射效应的一维热传导模型,采用数值模拟的方法,研究了EAST未来偏滤器钨靶板在边界局域模作用下的热腐蚀程度.根据现有的边界局域模热流数据和多种未来可能的高能量边界局域模热流数据,计算了钨靶板的表面温度分布.结果显示当前的第一类边界局域模作用在钨靶板上,在高约束模式运行时间取32 s情况下,靶板表面温度从350 K增加到373 K,表明在当前的参数范围内,只要避免其他更严重的瞬时事件如破裂的发生,边界局域模还不会带来严重的威胁;如果边界局域模的能量增加到接近未来托卡马克边界局域模的能量范围1 MJ/m2,沉积时间为600μs,表面最大熔化厚度将达到6.8—6.9μm.     

Limiter Biasing Experiments in TEXTOR-94

Particle collection by the ALT-II belt limiter in the TEXTOR-94 tokamak is exclusively limited to the parallel outflux because the scoop walls that are oriented parallel to the field lines obstruct the poloidal E×B mass flow. With normal B _tor direction (E×B towards the scoops), a threefold decrease of plenum pressure is measured during negative biasing, while with reversed B _tor (E×B away from the scoops), a 60% pressure increase is observed. This behaviour is exactly opposite to that observed in X-point divertors. A simple fluid model explains this apparent contradiction, and gives good quantitative agreement with measurements of the parallel Mach number in the SOL. The essential physics is governed by the Bohm-Chodura criterion.     

Relaxation phenomena induced by edge biasing experiments in the CASTOR tokamak

In this paper we present the first experimental results obtained in the CASTOR tokamak by using a segmented biasing electrode, which is composed of five segments radially separated by 3 mm. The basic idea of choosing such configuration was to allow a spatial distribution of the biasing voltage in the radial direction. In the described experiments, one or more selected segments can be biased up to +300V, while the remaining segments can be either grounded or floating. Two rake probes measure the edge radial profiles of the floating potential and the ion saturation current at the top and low field side of the torus. A Gundestrup probe, located at the top of the torus, measures the parallel and perpendicular Mach numbers together with the local electron temperature and density. A clear and reproducible transition to a regime with improved particle confinement is routinely observed, if the electrode is inserted deep enough into the plasma (r/a ≈ 0.5) and if one or two segments are biased up to +250V (the remaining segments are floating). The steepening of the radial profiles of the plasma density and potential demonstrate the formation of a transport barrier just inside the last closed flux surface. Fast relaxations of the edge profiles, with a frequency of about 10 kHz or higher, are observed when the value of the average radial electric field within the barrier reaches values of about 20 kV/m and the density gradient increases up to a factor 3 with respect to the ohmic phase. A detailed analysis of the spatial-temporal behavior of these relaxations is presented. Presented at the Workshop “Electric Fields, Structures and Relaxation in Edge Plasmas”, Tarragona, Spain, July 3–4, 2005.     

Radiative energy exhaust by sputtered and seeded impurities in fusion reactor

The combine effect of the seeded and sputtered impurities on the power load to the divertor plate and operation of fusion reactor is investigated in the paper. Since the energy balance depends strongly on coupling between core and scrape of layer (SOL) regions the modelling requires solving the transport problem in both region with the coupling took into account. The energy and particle transport is analyzed numerically with the help of COREDIV code treating self-consistently both regions. In COREDIV the radial 1D energy and particle transport of core plasma is coupled to 2D model of the SOL. The two types of transport model have been used: described by local transport coefficients proposed by Mandrekas and Stacey and transport model proposed by Tokar and determined by several types of drift instabilities. The 2D model is based on Braginskij-like equations for densities and velocity components parallel to magnetic fields and electron and ion temperatures (it was assumed that all ions has the same temperature). The steady states of ITER-FEAT reactor have been studied numerically. Several wall and plates materials and Argon as seeded impurities has been considered. The numerical results show that the effect of seeded impurity radiation is strongly mitigated by decreasing of sputtered impurity density and its radiation due to lower energy flowing to the SOL.     

偏滤器靶板附近氦输运的蒙特卡洛模拟

本文用Monte Carlo方法模拟了偏滤器扁平抽气收集板附近区域的氦原子输运。考虑了氦原子被电子碰撞电离,氦原子与等离子体离子之间的弹性散射等原子过程以及离子的热运动和沿磁力线的流动。计算表明当偏滤器等离子体密度在7cm距离内从里向靶板逐渐增加约4.7倍,而温度分布保持不变时,氦灰返回靶板的几率比密度在7cm内向靶板递减4.7倍的通常正分布提高约45％。磁力线与靶板的交角、靶板的温度,边缘等离子体鞘层电位以及不同的靶材料对氦返回靶板几率的影响作了比较。得到的结果对托卡马克聚变裂变增殖堆孔栏和偏滤器工程设计有参考价值。     

On confinement characteristics of the newly developed sawtooth-free plasmas in KSTAR tokamak

Development of advanced scenarios, an important experimental goal of the Korea superconducting tokamak advanced research (KSTAR) project, has just begun. The safety factor (q) profile is a key to achieve these advanced scenarios. Particularly the hybrid scenario, one of the advanced scenarios, can be established generally with low magnetic shear (s) at the center with central q-value above unity so to avoid sawtooth instability. This q-profile was successfully produced using early divertor formation during a plasma current ramp-up phase in KSTAR. Auxiliary heating was also employed during the current ramp-up phase to delay the inductive current diffusion to the center of the plasma. In addition to the early divertor formation method, the target q-profile was attempted to be achieved by modifying the plasma current waveform using the so-called, ‘current-overshoot’ method and the timing of L-mode to H-mode transition. In this work, the confinement characteristics of these sawtooth-free regimes are investigated. The global energy confinement time is calculated and compared with that of conventional H-modes in KSTAR. The confinement enhancement factor reveals that the newly developed discharges are not improved over H-modes contrary to results of other tokamaks. To investigate the reason, transport modeling is performed self-consistently with an integrated simulation package incorporating plasma equilibrium, transport, and heating and current drive. The current ramp-up phase is simulated and impact of early divertor formation, current-overshoot, and early L–H transition on the target q-profile and s/q profile is addressed. The s/q profile is found to be not improved in these discharges compared with hybrid scenarios reported in other tokamaks. Based on these results, future experimental directions are addressed to access the hybrid regimes in KSTAR.