Microwave ponderomotive action on the inhomogeneous collisionless and collisional plasmas

The nonlinear interaction of a high-power microwave (MW) with an unmagnetized inhomogeneous plasma is investigated in collisionless and collisional regimes. The electron density distribution and the nonlinear wave equation in an inhomogeneous plasma are obtained by taking into account the ponderomotive force due to the high-power MW. It is shown that the electron density distribution becomes very steepened in the presence of the ponderomotive force. In the collisional regime, the expression for electron temperature is also found by considering ohmic heating. It is indicated that the amplitude of oscillations of the electron temperature and dielectric permittivity increases and the wavelength of these oscillations decreases with increasing energy flux, hence modulation occurs.     

Losses and nonlinear steady-state particle distribution functions for fully ionized tokamak-plasmas in the collisional transport regimes

Fully ionized L-mode tokamak plasmas in the fully collisional (Pfirsch-Schlüter) and in the low-collisional (banana) nonlinear transport regimes are analyzed. We derive the expressions for particles and heat losses together with the steady-state particle distribution functions in the several collisional transport regimes. The validity of the nonlinear closure equations, previously derived, has been indirectly tested by checking that the obtained particle distribution functions are indeed solutions of the nonlinear, steady-state, Vlasov-Landau gyro-kinetic equations. A quite encouraging result is the fact that, for L-mode tokamak plasmas a dissymmetry appears between the ion and electron transport coefficients: the latter submits to a nonlinear correction, which makes the radial electron coefficients much larger than the former. In particular we show that when the L-mode JET plasma is out of the linear region, the Pfirsch-Schlüter electron transport coefficients are corrected by an amplification factor, which may reach values of order 10². Such a correction is absent for ions. On the contrary, in the banana regime, the ion transport coefficients are increased by a factor 2 and the nonlinear corrections for electrons are negligible. These results are in line with experiments.     

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.     

Effect of a dust component on the rates of elementary processes in low-temperature plasmas

Elementary processes in dusty, beam-driven plasma discharges are studied experimentally and theoretically for the first time. A theoretical model is constructed for a beam-driven plasma containing macroscopic particles. The effect of macroscopic particles on the electron energy distribution function is estimated assuming a Coulomb field for the particles. The resulting rate of electron-ion recombination on the macroscopic particles is compared with the electron loss constant calculated from the electron energy distribution function with an electron absorption constant in the orbital-motion approximation. This approximation, which is valid in the collisionless case, is found to work satisfactorily beyond its range of applicability. The distributions of the charged particles and electric fields created by macroscopic particles in a helium plasma are determined. The experimental data demonstrate the importance of secondary emission by high-energy electrons. Zh. éksp. Teor. Fiz. 115, 2020–2036 (June 1999)     

PIC Simulation of Current‐Driven Buneman Instability in Presence of Collisional and Thermal Effects

Collisional and thermal effects on the nonlinear evolution of Buneman instability in an unmagnetized current‐driven plasma are investigated by particle in cell simulations. These simulations show that, as the time passes, the electron distribution function profiles deviate from initial shifted Maxwellian distribution and the electron phase‐space holes can be formed. The electron distribution function profiles also indicate the counter‐streaming and plateau formation. Moreover, the contour of the electric field and profiles of the growth rate display the resonance condition for this instability. Finally, the time evolution of the electric field energy density and ion kinetic energy in the presence of collisional and thermal effects are presented. These evolutions illustrate that in the presence of collisions, the growth rate of the Buneman instability is smaller than the collisionless case. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Implicit Collisional Three-Fluid Simulation of the Plasma Erosion Opening Switch

The plasma erosion opening switch (PEOS) has been studied with the aid of the ANTHEM implicit simulation code. This switch consists of fill plasma injected into a transmission line. The plasma is ultimately removed by self-electrical forces, permitting energy delivery to a load. Here, ANTHEM treats the ions and electrons of the fill plasma and the electrons emitted from the transmission-line cathode as three distinct Eulerian fluids-with electron inertia retained. This permits analysis of charge separation effects, and avoids the singularities that plague conventional MHD codes at low density. E and B fields are computed by the implicit moment method, allowing for time steps well in excess of the electron plasma period ?t >> ?p-1, and cells much wider than a Debye length, ?x >> ?D. Switch dynamics are modeled as a function of the driving electrical pulse characteristics, the fill plasma parameters, and the emission properties of the transmission line walls-for both collisionless and anomalously collisional electrons. Our low-fill-density (ne ? 4 × 1012 electrons/cm3) collisionless calculations are in accord with earlier particle code results. Our high-density computations (ne ? 2 × 1013 electrons/cm3) show the opening of the switch proceeding through both ion erosion and magnetic pressure effects. The addition of anomalous electron collisions is found to diffuse the driving B field into the fill plasma, producing broad current channels and reduced magnetic pressure effects, in some agreement with NRL experimental measurements.     

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     

Thomson scattering off a pair （electron-positron） plasma

Thomson scattering off a pair (electron--positron) plasma is theoretically investigated in the collisionless and collisional limits respectively. Our calculations show that the power spectrum of the Thomson scattering off a collisionless pair plasma is just proportional to the velocity distribution function of the particles in the plasma. Collective modes in the plasma do not have any effects on the Thomson scattering spectrum because of the correlation between the negatively- and positively-charged particles. In the collisional limit, the power spectrum of the Thomson scattering presents three spikes: two peaks correspond to two contra-propagating sound waves and one peak corresponds to an entropy wave.     

Dust particles in collisionless plasma sheath with arbitrary electron energy distribution function

Dust particles often appear in industrial plasmas as undesirable product of the plasma-wall interactions. Large particles of several micrometers in diameter are concentrated in a thin layer (the sheath) above the lower electrode of the rf driven parallel plate device, where the electric force is strong enough to compensate particle’s gravity. Experimental and theoretical uncertainties are significantly increased in the plasma sheath. Common models of dust charging in the plasma sheath suppose the Maxwellian electron energy distribution function (EEDF) in conjunction with a flux of cold ions satisfying classical Bohm criterion at the sheath edge. In this paper we generalize this model to arbitrary EEDF with adapted Bohm criterion. We limit our considerations to collisionless or slightly collisional plasma, where the EEDF inside the sheath is expressed through the EEDF in the plasma bulk. Derived theoretical formulas are incorporated into numerical model, describing collisionless radio frequency (rf) plasma sheath together with the electrical charge, various kinds of forces, balancing radius and oscillation frequency of particles.     

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.     

High-density operation in tokamaks

Conclusion  In this paper we have examined various aspects regarding high-density operation in tokamaks and in particular the density limit, the plasma detachment, the MARFE formation and the fuelling efficiency. As regarding the density limit, both experimental findings and theoretical model indicate that the plasma current and the total input power are relevant in limiting the edge density that can be sustained in a tokamak discharge: radiation losses and SOL momentum and energy conservation are the underlying mechanisms. In the latest divertor experiments, operating in the detached regime, the influence of the input power seems to vanish or even disappear. Edge phenomena such as plasma detachment, occurring beyond a density threshold that can be lowered by means of impurity injection, can lead to the almost complete exhausting of the heating power by radiation which is greatly helpful for the design of the divertor plates. The compatibility of H-mode operation with this regime is still under investigation. The MARFE phenomenon, sometimes precursor of a major disruption, is now understood in terms of a radiation induced thermal instability. Finally, experiments performed in order to investigate the fuelling efficiency of the gas puffing technique have shown that at high density this technique becomes rather inefficient, thus indicating that pellet injection still remains an essential requirement to fuel the reactor plasma. The drop of the fuelling efficiency of gas-puffing at high density can be accounted for by collision phenomena taking place in the SOL.     

Modelling of Secondary Electron Impact on the Self-Consistent RF Bulk Plasma Description

The impact of a secondary electron beam, generated at the electrodes and accelerated in the sheaths, on the self-consistent treatment of the electron behaviour in an rf bulk plasma has been investigated by a parametric study. Source of electrons in the plasma are collisional ionization and secondary electron injection. Electrons are lost by ambipolar diffusion to the electrodes of a parallel plate rf discharge configuration. The non-stationary Boltzmann equation is used to determine self-consistently the rf field amplitude necessary for maintaining the steady-state rf bulk plasma as well as the time resolved behaviour of the electron energy distribution function and of all contributions to the electron particle and power balance, at given source rate and energy distribution of secondary electron injection.     

HL-2A װ�ó���������ע�뻺��ƫ�����а��ϱ�Ե����ģ��ͨ���о�

在HL-2A 装置上优化和发展了偏滤器靶板上的红外测温系统,并利用该系统分析了高约束模放电期间边缘局域模的热沉积分布特性。在高约束模式放电期间,超声分子束注入使边缘局域模所引起的偏滤器靶板上瞬间热通量峰值下降了~60%,并伴随着边缘局域模爆发频率增加了2~3 倍,而等离子体储能仅下降了~8%。分析结果表明,大幅度的丝状结构在超声分子束注入之后得到了有效抑制,沉积到偏滤器靶板上的瞬间热通量峰值也随之下降。此外,在超声分子束注入之后偏滤器室内的热辐射损失大幅度增加,从而耗散了热输运所携带的部分能量,进一步分散了沉积到偏滤器靶板上的能量,有效地保护了偏滤器靶板。     

HL-2A 装置超声分子束注入缓解偏滤器靶板上边缘局域模热通量研究

在HL-2A 装置上优化和发展了偏滤器靶板上的红外测温系统,并利用该系统分析了高约束模放电期间边缘局域模的热沉积分布特性。在高约束模式放电期间,超声分子束注入使边缘局域模所引起的偏滤器靶板上瞬间热通量峰值下降了~60%,并伴随着边缘局域模爆发频率增加了2~3 倍,而等离子体储能仅下降了~8%。分析结果表明,大幅度的丝状结构在超声分子束注入之后得到了有效抑制,沉积到偏滤器靶板上的瞬间热通量峰值也随之下降。此外,在超声分子束注入之后偏滤器室内的热辐射损失大幅度增加,从而耗散了热输运所携带的部分能量,进一步分散了沉积到偏滤器靶板上的能量,有效地保护了偏滤器靶板。     

Kinetic vortex chain solution in the drift-wave plasma regime

A fully nonlinear Bernstein-Greene-Kruskal stationary solution is found in the form of a quasi-three-dimensional chain of electron holes coupled to hydrodynamic vortices. This new coherent structure is enabled by the trapping and depletion of resonant particles, and the cyclotron dissipation of the singular current sheets. It is expected to play an important role in the collisionless magnetic field line reconnection in the drift-wave plasma regime, where it represents a plausible saturated state.     

非均匀磁场磁镜粒子约束的Monte-Carlo模拟

本文在碰撞区和无碰撞区之间的过渡区,就LAMEX和抛物线轮廓两种空间变化磁场磁镜,用Monte-Carlo模拟研究了粒子约束问题。在过渡区,粒子约束时间高于其它两区公式的外推值,并与磁镜比成正比关系。这一模拟还用自洽解的方法得到了密度轮廓,并研究了这一轮廓和等离子体参数的关系。 关键词：     

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.     

HL-2A装置偏滤器位形超声分子束注入深度观测

用22道Hα辐射测量阵列观测了HL-2A装置偏滤器位形超声分子束的注入过程。观测结果表明：在HL-2A装置偏滤器位形下,当分子束注入气源压强大于1MPa时,Hα辐射沿径向分布存在两个峰值,其第二个Ha辐射峰值位于r/a~0.8附近,表明大部分氢粒子在等离子体边缘(SOL区域)即被离解或电离,但仍有部分氢粒子沉积在分界面内5cm。     

Ultrashort-laser-pulse absorption with spatial dispersion and nonlocal transport effects

We study ultrashort-laser-pulse absorption and plasma heating at a sharp plasma–vacuum interface using advanced models for all-range plasma permittivity and nonlocal heat transport. The electron response includes both collisional and collisionless dissipative effects in the plasma of an arbitrary ion charge. We show that nonlocal electron heat transport is important for correct determination of the value of electron temperature and spatial temperature profile. Nonlocal electron heat flux comes into play after electrons heat up to temperatures of the order of 1 keV and to temperatures several times less for low-density targets.