Radial Electric Field Shear Due to Neoclassical Effects in Transport Barriers

The E _r×B flow shear is simulated with a fully kinetic five-dimensional neoclassical Monte Carlo simulation for JET and ASDEX Upgrade plasma edge conditions. Here, E _r is the radial electric field and B is the magnetic field. It is shown that high enough shear for turbulence suppression can be driven at the Low (L) to High (H) transition conditions by pure neoclassical effects including ion orbit losses. Simulations indicate higher threshold shear for ASDEX Upgrade than for JET. Shear values are compared to different models and experimental results of critical shear, and isotope effect on shear is discussed. Formation of an internal transport barrier in FT-2 in the presence of lower hybrid waves is also studied.     

Electric biasing of the divertor tiles in compass-D

It has been suggested that by electrically biasing the divertor tiles in a tokamak, one could create a toroidally-asymmetric potential structure in the scrape-off layer (SOL) plasma. The resultingE×B convective motion should increase the plasma transport in the SOL and thereby reduce the heat load on the divertor plates. This paper describes plans for such experiments on COMPASS-D and preliminary theoretical modelling. The COMPASS-D divertor target consists of 24 graphite tiled toroidal segments. The inner and outer strike point tiles on all these segments will be insulated from each other, and cabling will be installed to allow for biasing of individual tiles in any combination. Theoretical analysis suggests that the electrostatic potential perturbations caused by the biasing will be confined to the divertor region below the X-point, and that substantial SOL broadening can be achieved by applying modest voltages. Aside from active control of SOL transport, it is hoped that the biasing will be useful to probe other divertor and SOL physics issues. Presented at the Workshop on the Role of Electric Fields in Plasma Confinement and Exhaust, Budapest, 18–19 June, 2000. This work was jointly funded by the UK Department of Trade and Industry and Euratom, and was supported by US DoE under contract W-7405-ENG-48 at LLNL.     

Poloidal inhomogenity of the particle fluctuation induced fluxes near the LCFS at lower hybrid heating and improved confinement transition at the FT-2 tokamak

This paper deals with the new spectral and microturbulence experimental data and their analysis, which show that the radial electric field E _r generated at the lower hybrid heating (LHH) in the FT-2 is high enough to form the transport barriers. The ETB is formed when LHH is switched off. The radial fluctuation-induced E × B drift flux densities near LCFS in SOL are measured at two different poloidal angles. For this purpose two Langmuir probes located at low and high field sides of the torus are used. Registration of the poloidal and radial components of the electric field and density fluctuations at the same time during one discharge permits to measure the poloidal asymmetry of the transport reduction mechanism of the radial and poloidal particle fluxes in the SOL. The absolute fluctuation levels show dependence on the sign of E _r shear. The modification of the microscale turbulence by the poloidal E _r × B rotation shear _{E × B} at the L-H transition near LCFS is also studied by X-mode fluctuation reflectometry. The new data were obtained by spatial spectroscopic technique.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.     

Theoretical Study of the Structure of the Electric Field in Helical Toroidal Plasmas

A set of transport equations is analyzed, including the bifurcation of the electric field. The structure of the electric field is studied by use of the theoretical model for the anomalous transport diffusivities. A steep gradient of the radial electric field (E _r) is obtained at the electric domain, where the electric domain is the structure of the electric field in which the ambipolar E _r spatially changes from the electron root (E _r>0) to the ion root (E _r<0). The suppression of the anomalous transport diffusivity is studied in the presence of a strong shear of the electric field. The hard transition between the multiple ambipolar solutions in the structure of the radial electric field is examined. The details of the structure of the electric domain interface are investigated.     

Inter-ELM power decay length for JET and ASDEX upgrade: measurement and comparison with heuristic drift-based model

Experimental measurements of the SOL power decay length (λ(q)) estimated from analysis of fully attached divertor heat load profiles from two tokamaks, JET and ASDEX Upgrade, are presented. Data was measured by means of infrared thermography. An empirical scaling reveals parametric dependency λ(q) in mm = 0.73B(T)(-0.78)q(cyl)(1.2)P(SOL)(0.1)R(geo)(0), where B(T)(T) describes the toroidal magnetic field, q(cyl) the cylindrical safety factor, P(SOL)(MW) the power crossing the separatrix and R(geo)(m) the major radius of the device. A comparison of these measurements to a heuristic particle drift-based model shows satisfactory agreement in both absolute magnitude and scaling. Extrapolation to ITER gives λ(q) ? 1 mm.     

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     

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.     

核物质状态方程中动量相关作用可能的探针

研究了动量相关作用对于中子-质子比动能谱rb(Ek)的效应,发现rb(Ek)灵敏的依赖于动量相关作用而弱的依赖于介质中核子-核子碰撞截面和对称势.因此rb(Ek)是提取重离子碰撞中动量相关作用信息的可能探针.同时,对于丰中子弹核和相同质量稳定弹核在相同入射道条件下,丰中子碰撞系统明显加强了动量相关作用对rb(Ek)的效应.故两个碰撞系统rb(Ek)结果的比较为在重离子碰撞中提取动量相关作用的知识提供了另一个重要的判据。     

Ion cyclotron drift instability driven by cross-field current

An ion cyclotron drift instability is excited by a cross field current due to E_r × B drift of electrons and ion heating is observed in a plasma column in a magnetic field when a dc radial electric field E_r is applied.     

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.     

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.     

Resonant tunnelling in asymmetrical double barrier structures – complex coordinate approach

The complex coordinate approach of theoretical atomic spectroscopy is applied to the study of resonant tunnelling in (Ga,In)As/(Al,In)As-based asymmetrical double-barrier heterostructures within applied electric field. The method yields complex eigenvalues E = E _r – i/2 of the Hamiltonian matrix evaluated in a normalizable basis, where E _r is the resonant energy and = / is the tunnelling lifetime. The dependence of E _res on the field and barrier asymmetry is discussed.     

Resonant alteration of propagation in guiding structures with complex Robin parameter and its magnetic-field-induced restoration

Solutions of the scalar Helmholtz wave equation are derived for the analysis of the transport and thermodynamic properties of the two-dimensional disk and three-dimensional infinitely long straight wire in the external uniform longitudinal magnetic field B under the assumption that the Robin boundary condition contains extrapolation length Λ with nonzero imaginary part Λ_i. As a result of this complexity, the self-adjointness of the Hamiltonian is lost, its eigenvalues E become complex too and the discrete bound states of the disk characteristic for the real Λ turn into the corresponding quasibound states with their lifetime defined by the eigenenergies imaginary parts E_i. Accordingly, the longitudinal flux undergoes an alteration as it flows along the wire with its attenuation/amplification being E_i-dependent too. It is shown that, for zero magnetic field, the component E_i as a function of the Robin imaginary part exhibits a pronounced sharp extremum with its magnitude being the largest for the zero real part Λ_r of the extrapolation length. Increasing magnitude of Λ_r quenches the E_i − Λ_i resonance and at very large Λ_r the eigenenergies E approach the asymptotic real values independent of Λ_i. The extremum is also wiped out by the magnetic field when, for the large B, the energies tend to the Landau levels. Mathematical and physical interpretations of the obtained results are provided; in particular, it is shown that the finite lifetime of the disk quasibound states stems from the Λ_i-induced currents flowing through the sample boundary. Possible experimental tests of the calculated effect are discussed; namely, it is argued that it can be observed in superconductors by applying to them the external electric field E normal to the surface.     

Investigation of the dependency of JET midplane separatrix density as a function of engineering parameters

Midplane separatrix density is a crucial parameter in tokamaks since it strongly impacts divertor conditions. Scaling midplane separatrix density, n_{e, SEP} , and pedestal density, n_{e, PED} , as function of engineering parameters such as auxiliary heating P_injected, toroidal magnetic field B_T, and plasma poloidal current I_p are relevant to observe the effect of tuning these parameters on, for example, quality of confinement and divertor regime governed by n_{e, PED} and n_{e, SEP} , respectively. Thus, a dataset of JET H-mode pulses performed with Iter like wall (ILW) has been analysed. Midplane density data are collected from an HRTS (high-resolution Thomson scattering) diagnostic and n_{e, SEP} is determined using the power balance method. Parallel heat flux model is chosen using transport code SOLEDGE2D (S2D) applying power balance method over a simulated n_{e, SEP} and T_{e, SEP} profiles to obtain separatrix positions. The parameters are averaged over time windows with order of (85–185 ms ) and the magnetic configuration has been fixed to avoid divertor geometrical effect on n_{e, SEP} determination, configuration chosen is corner–corner. A ratio between separatrix density and pedestal density at outer midplane ranges between 0.3 and 0.7 on the data set. A scaling law of n_{e, SEP}/n_{e, PED} is obtained as function of P_injected, B_T, and I_P.     

Ehrenfest’s Paradox and Radial Electric Field in Quasi-Neutral Tokamak Plasma

A relation between physical consequences of the so-called Ehrenfest’s Paradox and the radial electric field E _r (r) in the classical quasi-neutral tokamak plasma is shown. Basic author’s approach to the relativistic nature of the tokamak E _r (r) has been described in Romannikov (J. Exp. Theor. Phys. 108:340–348, 2009). The experiment which can resolve the Ehrenfest’s Paradox is presented.     

Experimental measurements and modelling of the structure of the radial electric field in RFX

The experimental determination of the radial electric fieldE _r and the associatedE ×B drift velocity at the edge of RFX is presented and possible mechanisms responsible for its generation are discussed.E _r measurements by means of an array of Langmuir probes and those deduced from Doppler spectroscopy of impurity lines agree fairly well. In particular the rotation velocity of the plasma edge has been determined from the Doppler shift of a C III emission line. The observation of other ions characterised by different radial positions, such as B IV and C V, has allowed an estimate of the velocity shear. Typical values of plasma rotation at the edge are around 10 km/s while the velocity shear is of the order of (10⁵−10⁶)s⁻¹ in the spontaneous layer, a few cm thick, usually observed in standard discharges. Plasma rotation has been artificially modified by both positive and negative edge biasing and the associated increase or decrease of the fluid velocity is well in agreement with the radial electric field change. The modification ofE _r during edge biasing and Pulsed Poloidal Current Drive (PPCD) are also reported and interpreted within a momentum balance model. Analytical and self-consistent Monte Carlo models at the plasma edge suggest that impurities have a relevant role in the generation of the radial electric field, due to their relatively large Larmor radius. Presented at the Workshop on Role of Electric Fields in Plasma Confinement and Exhaust, Budapest, 18–19 June 2000.     

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     

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.     

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.     

L–H power threshold studies with tungsten/carbon divertor on the EAST tokamak

The power threshold for low (L) to high (H) confinement mode transition achieved by radio-frequency heating and molybdenum first wall with lithium coating has been experimentally investigated on the EAST tokamak for two sets of divertor geometries and materials: tungsten/carbon divertor and full carbon divertor. For both sets of divertors, the power threshold was found to decrease with gradual accumulation of the lithium wall coating, suggesting the important role played by the low Z impurities and/or the edge neutral density on the L–H power threshold. When operating in the upper single null configuration, with the ion grad-B drift direction away from the primary X-point, a lower normalized power threshold is observed in EAST with the tungsten/carbon divertor, compared to the carbon divertor after intensive lithium wall coating. A newly installed cryopump increasing the pumping efficiency also plays an important part in the observed lower threshold. In addition, the H-mode in the Quasi-Snowflake divertor configuration has been obtained on EAST, exhibiting higher L–H power threshold compared to the lower single null configuration with similar I_P/B_T pairs.