Analytic description of tokamak equilibrium sustained by high fraction bootstrap current

By using an expansion technique based on the tokamak ordering, the Grad－Shafranov equation is analytically solved for tokamak equilibrium sustained by a full bootstrap current. This approximate approach is suitable for equilibria with non-circular cross-sections of modest ellipse and triangular deformations. As an input parameter set, the plasma pressure profile and the electron and the ion temperature profiles can be selected as arbitrary functions of the averaged minor radius. Equilibrium properties of this plasma are discussed.     

Tokamak中自举电流的剖面准直性

利用Harris模型,通过求解等离子体平衡方程,计算俘获粒子份额,分别对常规剪切和中心负剪切下tokamak中的自举电流的大小和剖面准直性进行了计算和分析.自举电流分布与等离子体平衡电流分布之间的剖面准直性可以通过调整等离子体的密度、温度和电流分布参数,以及描述等离子体形状的拉长度k和三角变形因子d来获得.中心负剪切位形有利于自举电流产生,并有好的剖面准直性.通过计算比较,分别在常规剪切位形下和中心负剪切位形下获得了一组优化的等离子体参数,在这组参数下,自举电流有较大的份额和好的剖面准直性 关键词： tokamak 自举电流 剖面准直性     

Coherent spectral enhancement of carrier-envelope-phase stable continua with dual-gas high harmonic generation

Attosecond science is enabled by the ability to convert femtosecond near-infrared laser light into coherent harmonics in the extreme ultraviolet spectral range. While attosecond sources have been utilized in experiments that have not demanded high intensities, substantially higher photon flux would provide a natural link to the next significant experimental breakthrough. Numerical simulations of dual-gas high harmonic generation indicate that the output in the cutoff spectral region can be selectively enhanced without disturbing the single-atom gating mechanism. Here, we summarize the results of these simulations and present first experimental findings to support these predictions.     

Photonically enabled agile rf waveform generation by optical comb shifting

We present a photonically enabled rf arbitrary waveform generator that can rapidly switch between two output waveforms. This method is based on line-by-line shaping of an optical comb and then converting the optical pulses to rf waveforms with a fast photodetector. It uses a single diode laser as the optical source and selects different patterns preprogrammed into an optical pulse shaper by shifting the laser frequency. We demonstrate minimum update delay times of 0.45 ns.     

Tokamak equilibria with reversed current density

Observations of nearly zero toroidal current in the central region of tokamaks (the "current hole") raises the question of the existence of toroidal equilibria with very low or reversed current in the core. The solutions of the Grad-Shafranov equilibrium equation with hollow toroidal current density profile including negative current density in the plasma center are investigated. Solutions of the corresponding eigenvalue problem provide simple examples of such equilibrium configurations. More realistic equilibria with toroidal current density reversal are computed using a new equilibrium problem formulation and computational algorithm which do not assume nested magnetic surfaces.     

紧凑型聚变裂变混合堆自举电流的数值模拟研究

以平衡程序Jsolver 为基础开展了紧凑型聚变裂变混合堆先进等离子体平衡位形设计,重点研究了反剪切运行模式,并在此位形下研究了自举电流的计算、分布及份额。     

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以平衡程序Jsolver 为基础开展了紧凑型聚变裂变混合堆先进等离子体平衡位形设计,重点研究了反剪切运行模式,并在此位形下研究了自举电流的计算、分布及份额。     

Self-consistent diverted tokamak equilibria with nonzero edge current

The semi-analytical method, previously used to construct model double-null and single-null diverted tokamak equi- libria (Bingren Shi, Plasma Phys. Control Fusion 50 (2008) 085006, 51 (2009) 105008, Nucl. Fusion 51 (2011) 023004), is extended to describe diverted tokamak equilibria with nonzero edge current, including the Pfirsch–Schlüter(PS) cur- rent. The PS current density is expressed in a way suitable to describe a diverted tokamak configuration in the near separatrix region. The model equilibrium is expressed by only two terms of the exact separable solutions of the Grad– Shafranov equation, one of which is governed by a homogeneous ordinary differential equation, and the other by an inhomogeneous one. The particular merits of such a model configuration are that the internal region inside the separa- trix and a suitable scrape-off layer can be simultaneously described by this exact solution. To investigate the physics in the region near the X-point, the magnetic surfaces can be satisfactorily described by approximate hyperbolic curves.     

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研究了在托卡马克装置中中性束注入时快离子自举电流的产生。利用快离子分布函数在两小增量δ=ρp/a (ρp是极向拉莫尔半径,a是小半径)和δ*=τB/τs(τB是俘获粒子反弹周期,τs是慢化时间)下进行展开的方法求解漂移动力学方程;给出了快离子自举电流的一般表达式。计算了圆截面大纵横比托卡马克中快离子自举电流密度分布和总的快离子自举电流的大小,研究表明：在中性束垂直注入时快离子自举电流约占总电流10%;自举电流的大小既敏感地依赖于中性束注入的角度——平行注入时较小、接近垂直时迅速增大,同时也较强的依赖于快离子的产生速度与临界速度的比值,即vb/vc,而vb2∝E(束能量);自举电流的大小随注入束能量的增加而迅速增大。     

Theory of tokamak equilibria with central current density reversal

It is found that, with a model current profile, the Grad-Shafranov equation can be reduced to the Helmholtz equation, which can describe a variety of equilibrium configurations. With the eigenvalue problem solved in the toroidal coordinate system, an analytical solution to the Grad-Shafranov equation is found. It is demonstrated that current reversal equilibrium configurations exist with finite radial gradient of plasma pressure and continuous current density, and that current density reversal is accompanied by pressure gradient reversal.     

Suppression of particle generation in a plasma process using a sine-wave modulated rf plasma

Sine-wave modulated rf plasma has been used to control particle generation and growth in a plasma-enhanced chemical vapor deposition of silicon dioxide thin films using TEOS/O₂. The density and the size of particles generated in the plasma are greatly reduced when the plasma is modulated with sine-wave modulation at low modulation frequency (<1000 Hz). In addition, particle contamination on the films is significantly reduced also for nanoparticles, and the film growth rates at the range of modulation frequencies where particle generation are greatly reduced do not decrease appreciably. Compared to its counterpart pulse-wave modulation plasma, the sine-wave modulation plasma has demonstrated a better performance in terms of reduction of particle generation and film contamination, and of film growth rate. Thus, the sine-wave modulation plasma has shown as a promising method to be applied in the production of thin film with a high deposition rate and a low particle contamination.     

On the bootstrap effect in large tokamaks with lower hybrid driven current

The amplification of lower hybrid (LH) driven current of various profiles has been found. The results strongly depend on the plasma temperature profile, on the LH current profile, and on the magnitude of the particle diffusion flux.This work was started during the visit of the author in the I. V. Kurchatov Institute of Atomic Energy, Moscow. The author thanks IAE for hospitality, and V. Parail, G. V. Pereverzev, A. Smoljakov and R. Klíma for many useful and critical discussions.     

Antiresonance induced spin-polarized current generation

According to the one-dimensional antiresonance effect (Wang X R, Wang Y and Sun Z Z 2003 Phys. Rev. B 65 193402), we propose a possible spin-polarized current generation device. Our proposed model consists of one chain and an impurity coupling to the chain. The energy level of the impurity can be occupied by an electron with a specific spin, and the electron with such a spin is blocked because of the antiresonance effect. Based on this phenomenon our model can generate the spin-polarized current flowing through the chain due to different polarization rates. On the other hand, the device can also be used to measure the generated spin accumulation. Our model is feasible with today's technology.     

Estimation of surface charges on dielectric materials for high power rf windows

The surface discharges observed at rf windows and vacuum circuit breakers (VCBs) are one of the difficulties faced when developing high-power rf windows or compact VCBs. The surface discharge is considered to take place due to the release of the surface charges. Despite the importance of the surface charging/discharging, these phenomena have not been well evaluated. In this paper, the surface charges are estimated using the multipulse method, where electron beam irradiates a sample up to the saturation condition of surface charges. The amount of surface charges on alumina and TiN coated alumina are compared and the charging mechanism is discussed.     

Validation of neoclassical bootstrap current models in the edge of an H-mode plasma

Analysis of the parallel electric field E(parallel) evolution following an L-H transition in the DIII-D tokamak indicates the generation of a large negative pulse near the edge which propagates inward, indicative of the generation of a noninductive edge current. Modeling indicates that the observed E(parallel) evolution is consistent with a narrow current density peak generated in the plasma edge. Very good quantitative agreement is found between the measured E(parallel) evolution and that expected from neoclassical theory predictions of the bootstrap current.     

COMPASS-D magnetic equilibria with lh and NBI current drive

Plasma equilibria are investigated numerically, using the ACCOME and ASTRA codes, on the COMPASS-D tokamak (R ₀ = 0:56 m, a = 0:17 m, B _T = 1:2 T, I _p = 200 kA, k = 1:7, δ_x = 0:4) for the planned Neutral Beam Injection (NBI) and Low Hybrid Current Drive (LHCD) systems. The LH system provides P _LH = 0:4 MW at n _∥ = 2:1 and f _LH = 1:3 GHz. The NBI system has two 40 keV deuterium beams in co-or counter-directions with a total power of 0.6 MW. The COMPASS-D tokamak can typically operate in two configurations-single null divertor (SND) and single null divertor with a higher triangularity (SNT). Higher triangularity provides access to higher confinement and improved stability, and leads to larger n _∥ up-shifts for better slow LH wave absorption.We investigate the range of densities n = 2 ÷ 6 × 10¹⁹ m^?3. Both the LH and NB driven currents decrease with density. The magnetic shear reverses with off-axis beam incidence. In the given plasma parameter range, typically up to 60 kA of bootstrap current is driven and with NB co-injection up to 80 kA of NB current is driven. LHCD is weak at f = 1:3 GHz and BT = 1:2 T, but at n = 3 × 10¹⁹ m^?3 the LH driven current is about 40 kA, so that the required plasma current of 200 kA is supported almost non-inductively.