Numerical Modelling of Limiter Biasing Experiments on TEXTOR-94

The 2D multifluid code TECXY has been used to model the experimental biasing of the toroidal belt limiter ALT-II on the tokamak TEXTOR-94 with respect to the first wall. It is well known that the edge flow pattern can be influenced by the poloidal electric drifts from imposing radial electric fields. The modelling with TECXY introduces imprinted bias currents in the scrape-off layer for the case of negative (limiter) biasing, and imprinted bias potentials for the case of positive biasing. This allows to simulate sufficiently well the experimental current-voltage characteristics for either biasing and also reproduces the essential features and trends of the observed plasma profiles in the SOL. A moderate negative biasing is found to be optimal for the exhaust efficiency of the pump limiter.     

TOKAM-3D: A 3D fluid code for transport and turbulence in the edge plasma of Tokamaks

We present a new code aiming at giving a global and coherent approach for transport and turbulence issues in the edge plasma of Tokamaks. The TOKAM-3D code solves 3D fluid drift equations in full-torus geometry including both closed field lines and SOL physics. No scale separation is assumed so that interactions between large scale flows and turbulence are coherently treated. Moreover, the code can be run in transport regimes ranging from purely anomalous diffusion to fully established turbulence. Specific numerical schemes have been developed which can solve the model equations whether the presence of a limiter in the plasma is taken into account or not. Example cases giving an overview of the field of application of the code as well as verification results are also presented.     

Radial transport dynamics studies of SMBI with a newly developed TPSMBI code

In tokamak plasma fueling, supersonic molecule beam injection(SMBI) with a higher fueling efficiency and a deeper penetration depth than the traditional gas puffing method has been developed and widely applied to many tokamak devices.It is crucial to study the transport dynamics of SMBI to improve its fueling efficiency, especially in the high confinement regime. A new one-dimensional(1D) code of TPSMBI has also been developed recently based on a six-field SMBI model in cylindrical coordinate. It couples plasma density and heat radial transport equations together with neutral density transport equations for both molecules and atoms and momentum radial transport equations for molecules. The dominant particle collisional interactions between plasmas and neutrals, such as molecule dissociation, atom ionization and charge-exchange effects, are included in the model. The code is verified to be correct with analytical solutions and also benchmarked well with the trans-neut module of BOUT++ code. Time-dependent radial transport dynamics and mean profile evolution are studied during SMBI with the TPSMBI code in both slab and cylindrical coordinates. Along the SMBI path, plasma density increases due to particle fuelling, while plasma temperature decreases due to heat cooling. Being different from slab coordinate, the curvature effect leads to larger front densities of molecule and atom during SMBI in cylindrical coordinate simulation.     

Influence of seeded impurities on the fusion reactor operation

In order to study the influence of seeded impurities on ITER like reactor operation the COREDIV code has been extended to include the transport of several sputtered and/or injected impurities. In the COREDIV code the core plasma is treated in the frame of 1D radial transport model whereas in the edge the 2D multifluid code EPIT is used. The EPIT code solves in the slab geometry the MHD equations for densities and velocities for all ions species as well as for electron and ion temperatures. The iteration scheme in the code leads to steady state solution of coupled core and SOL system. The numerical results for Carbon and Nickel plate and seeded impurities Silicon and Neon are presented.     

Synergy of anomalous transport and radiation in the density limit

The critical plasma density n(cr) above which the edge anomalous transport in tokamaks is dominated by drift resistive ballooning instability is found analytically. In this transport regime, the drastic increase of particle losses and drop of the edge temperature provoke a strong increase in impurity radiation, and thermal equilibrium does not exist if the density is ramped up above the ultimate limit n(max). Because of the nonlinear character of impurity radiation, this density limit n(max) is very close to n(cr) and practically does not change with the ion effective charge. The importance of the synergy between the anomalous transport and impurity radiation for the density limit phenomenon is confirmed by the results of numerical simulations.     

New bipartition model of neutral particle transport in the HL-2A divertor region

A new bipartition neutral transport model has been developed for simulation of the hydrogenic neutral particle transport in the vicinity of HL-2A divertor target plate. The numerical calculation results on the basis of this model are fairly consistent with the results obtained with the “multi-generation method”. One possible application of this model is to provide a source term originating from neutral transport calculation for any other edge plasma transport code, for instance, B-2 code, which has been used to simulate edge plasma transport of the HL-2A divertor configuration. Especially it can be utilized to quickly classify the plasma in divertor region as high or low recycling regime.     

Internal transport barriers in plasmas with reversed plasma flow

Evidences of internal particle transport barriers have been observed in plasma discharges with reversed plasma flow. To investigate the influence of the radial electric field profile on these barriers, we apply a drift wave map that describe the plasma particle transport and allows the integration of particle drift in the presence of a given electrostatic turbulence spectrum. With this procedure we show that transport barriers due to the shearless flow invariant lines are created inside the plasma. Moreover, by varying the radial electric field profile, we observe the formation and destruction of internal transport barriers constituted by shearless invariant lines, as well as its effects on the transport in the map's phase space. Applicability of our results are discussed for the Texas Helimak, a toroidal plasma device in which the radial electric field can be changed by application of bias potential.     

High-Q Thermally Stable Operation of a Tokamak Reactor

Thermally stable subignited operation of a tokamak reactor, sustained in operation by a feedback-controlled supplementary heating source, is discussed. One-dimensional (radial) thermal stability analyses of model transport equations, together with numerical results from a one-dimensional (1-D) transport code, are used in studying the heating of deuterium-tritium (D-T) plasmas in the thermonuclear regime. The establishment of stability depends on a number of radially nonuniform nonlinear processes whose effect is analyzed. Nonuniform heat deposition resulting from plasma core supplementary heating is found to be a thermally more stable process than bulk heating. In the presence of impurity line radiation, however, core-heated temperature profiles may collapse, contracting inward from the limiter, the result of a radiation-induced instability. The effect of nonuniform transport coefficients is also discussed. Conditions are established for the realization of a subignited high-Q (Q ? 50) toroidal reactor plasma with appreciable output power (?2000 MW thermal).     

Two-Dimensional MHD Simulations of Magnetic Field Penetration in the Plasma Opening Switch

Three mechanisms for anomalous magnetic field penetration have been investigated in computer simulations of Sandia's plasma opening switch (POS). The POS simulations have been performed using the two-dimensional (2-D) two-temperature single-fluid magnetohydrodynamic (MHD) code HAM [1], [2]. The three penetration mechanisms considered are 1) the Chodura model based on the ion-acoustic instability where the saturated value of the anomalous collision frequency is approximated by the ion plasma frequency; 2) a model based on the lower hybrid instability in which the anomalous collision frequency is proportional to both the ion plasma frequency and the electron drift velocity; and 3) a model that limits the ion drift velocity to the plasma influx velocity. Two-dimensional MHD calculations of the POS will be presented which show these models to be qualitatively similar for densities above a few 1013 cm-3, though at lower densities they can be quite different. The calculations are compared to experiments, and some agreement is seen with the lower hybrid model. The other models compare only marginally to experimental results.     

HL-2A边缘等离子体中捕获电子模的线性数值模拟

利用GTC程序和HL-2A装置的实验数据,对托卡马克边缘等离子体中的漂移波微观不稳定性进行了线性数值模拟。模拟结果表明,在HL-2A边缘等离子体中捕获电子模(TEM)是不稳定的,它均匀且规则地分布在托卡马克的弱磁场区域,而且其增长率随着等离子体的温度梯度和密度梯度的增加而增加。另外,模拟结果还表明,TEM的实频率远远小于离子的漂移频率,这一点与理论研究结果是一致的。     

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     

Observation of Radial Propagation of Electrostatic Fluctuations in Edge Plasma of the Sino United Spherical Tokamak

Radial propagation of electrostatic fluctuations in the edge plasma of Sino-United Spherical Tokamak (SUNIST) has been measured using Langmuir probes. The propagation characteristics of the floating potential fluctuations are analysed by the two-point correlation technique. The results show radially outward propagation of the turbulent fluctuations at all measured radial positions. The power-average wavenumber profile is approximately constant in plasma edge region and suddenly increases to the limiter. These results are in good agreement with the model predictions proposed by Mattor which suggests that the drift wave propagation may be a source of edge turbulence.     

Propagation of vacuum arc plasma beam in a toroidal filter

An analytical solution to the problem of plasma beam transport in a toroidal magnetic filter for unmagnetized ions is derived. A two-fluid model taking into account electromagnetic and pressure forces, electron-ion collisions, magnetic force line curvature, and radial dependence of centrifugal force is used. From comparison with experimental data it is shown that the obtained solution describes well the main properties of plasma beam behavior in the filter, e.g. (1) the relative value of the ion current along the torus decreases exponentially, (2) the deflection of the plasma beam from the center of the torus correlates with the centrifugal drift of the plasma beam across a magnetic field, and (3) experiment and theory agree well on the weak correlation between magnetic field strength and filter efficiency     

Investigation of Ion Drift Waves in the PSI‐2 Using Langmuir‐Probes

The properties of on drift waves observed in the rotating magnetized plasma column of the PSI‐2 are analysed. Their parallel wave numbers are found to be nearly zero, the measured azimuthal dispersion relation is approximately linear, and the azimuthal phase velocity is nearly equal to the azimuthal on drift velocity. Furthermore, the potential fluctuations always lag behind the density fluctuations with a phase shift slightly below π. A simple analytical model, describing the potential‐and density distribution of the ion drift wave, shows the underlying mechanism of the on drift instability. Finally, a classification of the observed on drift instability is given.     

Plasma position control in SST1 tokamak

For long duration steady state operation of SST1, it would be very crucial to maintain the plasma radial and vertical positions accurately. For designing the position controller in SST1 we have adopted the simple linear RZIP control model. While the vertical position instability is slowed down by a set of passive stabilizers placed closed to the plasma edge, a pair of in-vessel active feedback coils can adequately control vertical position perturbations of up to 1 cm. The shifts in radial position arising due to minor disruptions would be controlled by a separate pair of poloidal field (PF) coils also placed inside the vessel, however the controller would ignore fast but insignificant changes in radius arising due to edge localised modes. The parameters of both vertical and radial position control coils and their power supplies are determined based on the RZIP simulations.     

Drift-wave instability excited by field-aligned ion flow velocity shear in the absence of electron current

The drift wave is observed to be destabilized by a magnetic-field-aligned ion flow velocity shear in the absence of field-aligned electron drift flow in laboratory experiments using a concentrically three-segmented plasma source. The fluctuation amplitude increases with increasing a shear strength, but the instability is found to be gradually stabilized when the shear strength exceeds a critical value. The destabilizing and stabilizing mechanisms are well explained by a plasma kinetic theory including the effect of radial density gradient.     

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.     

Quasiparticle approach to the modulational instability of drift waves coupling to zonal flows

The interaction between broadband drift mode turbulence and zonal flows has been studied through the wave-kinetic approach. Simulations have been conducted in which a particle-in-cell representation is used for the quasiparticles, while a fluid model is employed for the plasma. The interactions have been studied in a plasma edge configuration which has applications in both tokamak physics and magnetopause boundary layer studies. Simulation results show the development of a zonal flow through the modulational instability of the drift wave distribution, as well as the existence of solitary zonal flow structures about an ion gyroradius wide, drifting towards steeper relative density gradients.     

Modeling ExB drift transport in conceptual slot divertor configurations

At DIII-D, a slot divertor concept, called small-angle-slot (SAS), is under development, aiming to enable detachment at relatively low plasma edge density. We report on simulations using the SOLPS-ITER two-dimensional edge code to examine the performance of conceptual “SAS 2” slot configurations. The focus of the analysis is on E  ×  B drift effects on upstream density at detachment (UDD), with detachment marked by electron temperature T_e ≤ 3 eV at the outer strike point (OSP). With toroidal field such that radial E  ×  B drift carries particles from the OSP towards the private flux region (PFR), placing the OSP near the inner slot wall gives ≈20% lower UDD than having the OSP near the outer wall. The inner wall effectively traps the radial E  ×  B drift flux, resulting in low T_e and associated radial electric field in the PFR, and thus small losses from the slot to the inner target via poloidal E  ×  B drift flux. With toroidal field reversed such that radial E  ×  B drift is reversed, OSP placement near the inner wall gives ≈10% lower UDD than OSP placement near the outer wall. Although radial E  ×  B flux is from the OSP towards the outer wall, this flux largely escapes the slot, raising the UDD. A change in the slot shaping is suggested with the goal of eliminating such E  ×  B -driven particle losses from the slot.