Simulation on effect of poloidal power spectrum on lower hybrid wave propagation

The coupling of lower hybrid wave to the plasma is a crucial issue for efficient current drive in tokamaks. This paper establishes a new coupling model which assumes the antenna to be a curved face and the plasma to be a cylinder. Power spectrum considering the coupling between wave-guides in both poloidal and toroidal direction is simply estimated and discussed. The effect of the poloidal wave vector on wave propagation, power deposition and driven current is also investigated with the help of lower hybrid current drive code. Results show that the poloidal wave vector affects the ray tracing, and also has effect on power deposition and driven current. The effect of the poloidal wave vector on power deposition and driven current profile depends on plasma parameters. Preliminary studies suggest that it seems possible to control the current profile by adjusting the poloidal phase difference between the waveguide in poloidal direction.     

托卡马克中极向静电天线的离子Bernstein波的耦合计算

本文采用均匀的等离子体模型,对在托卡马克低场边由极向静电天线激发离子Bernstein波(IBW)的耦合问题作详细的理论分析。为了能更符合实验中的实际情况,文中考虑天线为有限长度以及非零的极向波数(k_y≠0)对耦合特性计算的影响。计算表明,极向静电天线可以有效地在等离子体中激发IBW。比较该天线与传统的环向IBW天线的辐射功率谱和阻抗等计算结果,表明两种天线对IBW的耦合有相似的特性。 关键词：     

Mirnov coil data analysis for tokamak ADITYA

The spatial and temporal structures of magnetic signal in the tokamak ADITYA is analysed using recently developed singular value decomposition (SVD) technique. The analysis technique is first tested with simulated data and then applied to the ADITYA Mirnov coil data to determine the structure of current peturbation as the discharge progresses. It is observed that during the current rise phase, current perturbation undergoes transition from m=5 poloidal structure to m=4 and then to m=3. At the time of current termination, m=2 perturbation is observed. It is observed that the mode frequency remains nearly constant (≈10 kHz) when poloidal mode structure changes from m=4 to m=2. This may be either an indication of mode coupling or a consequences of changes in the plasma electron temperature and density scale length.     

Contribution to Calculation of Ion Microfield Nonuniformity Effect on the Asymmetry of Lyman‐α Line in Dense Plasma

The high orders of Stark effects on spectral line shapes are examined in the ion‐static and electron‐impact ap‐proximations. At first the distribution functions of the spatial derivative of the ion microfield in He⁺ plasma are calculated for different plasma conditions when the coupling parameter is weak. We present new results about the spatial derivative ion microfield distributions and apply them to show the asymmetry of the Lyman‐α (Ly‐α) line in He⁺ plasma. At the second stage we show that asymmetry is affected by the spatial derivative tensor of the local ion electric field. We have used the Monte‐Carlo simulation (MCS) to compute the distribution functions for all tensor components and use them to solve the evolution equation of emitter whose solution serves to compute and therefore to show the line shape asymmetry. Good agreement of our distribution functions of ion microfield gradients and the line asymmetry with other results are obtained (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Measurement of the effects of Faraday shields on ICRH antennacoupling

Compact loop antennas are being applied to several fusion experiments, including DIII-D, the Advanced Toroidal Facility, Tore Supra, and the Tokamak Fusion Test Reactor. Although individual configurations vary, all of these antennas generally comprise a current strap in a recessed box and a Faraday shield. The coupling characteristics of cavity antennas that have current straps with the previously evaluated cross-sectional shapes are tested with several Faraday shields. The coupling is purely a measure of the magnetic flux linkage at relevant ion-cyclotron resonance heating (ICRH) frequencies and does not include plasma spectral effects. Impedances and relative fields are measured for various combinations of the current strap and Faraday shield. The experiments show that the fractional reduction in the magnetic flux linkage to the plasma resulting from the addition of any particular Faraday shield is virtually independent of the shape of the current strap. This is true in spite of the fact that the same mechanism which is responsible for the reduction in flux is also responsible for a significant redistribution of the antenna current on the current strap. Thus the process of optimizing antennas is reduced to that of separately optimizing the current strap and Faraday shield     

Antenna coupling study for ICWC plasma characterization in TEXTOR

Ion cyclotron wall conditioning (ICWC) discharges, in pulsed-mode operation, were carried out in the limiter tokamak TEXTOR to explore safe operational regimes for the experimental parameters for possible ICWC-discharge cleaning in International Thermonuclear Experimental Reactor (ITER) at half field. Antenna coupling properties obtained during the ion cyclotron range of frequencies (ICRF) wall conditioning experiments performed in helium–hydrogen mixture in TEXTOR were analysed in relation to the obtained ICWC-plasma characterization results. Satisfactory antenna coupling in the mode conversion scenario along with reproducible generation of ICRF plasmas for wall conditioning, were achieved by coupling radio frequency (RF) power from one or two ICRF antennas. The plasma breakdown results obtained in the TEXTOR tokamak have been compared with the predictions of a zero-dimensional RF plasma production model. The present study of ICWC emphasizes the beneficial effect of application of an additional (along with toroidal magnetic field) stationary vertical (B _V???B _T) or oscillating poloidal magnetic field (B _P???B _T) on antenna coupling and relevant plasma parameters.     

EAST 四条带ICRF 天线的三维电磁场分析

采用3 维电磁场代码CST MWS 对EAST 4 条带的离子回旋(ICRF)天线性能进行模拟研究,分析了天线的耦合阻抗随频率的变化,计算了电流和波谱的分布,模拟研究了不同电流相位对天线的电压击穿打火和引入杂质的影响。计算结果表明：此EAST 的ICRF 天线耦合阻抗和波谱总体设计合理;但天线局部电场较高,当环向电流条带为(0, 0, 0, 0)相位时E_max 为3.61MV•m⁻¹ 超出击穿阈值,经过结构尺寸的优化后E_max 低于阈值。同时表明天线运行时限制器处有较高的RF 电势,当电流相位为(0, π, π, 0)时法拉第屏前10mm 处的RF 电势是(0, 0, π, π)相位时的1/3。     

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The performance of the four strap ion cyclotron range of frequency (ICRF) antenna in EAST was studied by three dimensional electromagnetic code CST MWS. The coupling impedance with various frequencies, the distribution of current on straps and the power spectra for the three types of phases were calculated. The voltage breakdown and ICRF-related impurity were also studied. Simulation results show that the coupling impedance and power spectrum of the antenna design were reasonable, however, highest electric field, which is 3.61MV•m⁻¹ for (0, 0, 0, 0) toroidal phasing at the local area, exceeds the empirical value of breakdown. After the structure size was optimized, the maximum of E-field was below the breakdown value, the results also show that the high radio frequency (RF) potential was close to the antenna limiter, the RF potential along total B-field in front of Faraday screens 10mm for (0, π, π, 0) phasing was one-third of that for (0, 0, π, π) phasing.     

The phase statistics of a multichannel radar interferometer

Abstract

The analysis in this paper is concerned with the problem of determining the phase statistics of the output of a multichannel coherent radar interferometer. The 2N channels of the radar consist of the outputs from N pairs of antennae. Each antenna receives a random electromagnetic wave field which has circular normal first-order statistics with an arbitrary coherence function. Each antenna in each pair receives a wave at a different time, the time difference Δt between each antenna in each pair being the same for all pairs. The signals received by each pair are independent. The signals from each pair are combined to give G(t, Δt)=Σ_k=1 ^N S_k(t) S_k*(t+Δt) where, for example, the signals from each antenna in the kth pair are S_k(t) and S_k(t+Δt).

The probability density function of the modulus and phase of G(t, Δt) is worked out. The joint density is shown to be a type of generalized K distribution, and the phase distribution is shown to be a hypergeometric function. The results show that it is possible to measure the phase of the coherence function of an electromagnetic wave field scattered from a randomly moving extended object (such as the ocean surface) using such a multichannel radar. This phase is related to asymmetry of the Doppler power spectrum. Furthermore, if this asymmetry is a result of surface currents on the ocean interacting with the surface waves which cause the electromagnetic scattering, then the surface currents may be measured in some sense.     

Investigation of lower hybrid current drive during H-mode in EAST tokamak

H-mode discharges with lower hybrid current drive (LHCD) alone are achieved in EAST divertor plasma over a wide parameter range. These H-mode discharges are characterized by a sudden drop in D_α emission and a spontaneous rise in main plasma density. Good lower hybrid (LH) coupling during H-mode is obtained by putting the plasma close to the antenna and by injecting D₂ gas from a pipe near the grill mouse. The analysis of lower hybrid current drive properties shows that the LH deposition profile shifts off axis during H-mode, and current drive (CD) efficiency decreases due to the increase in density. Modeling results of H-mode discharges with a general ray tracing code GENRAY are reported.     

低杂波多结波导阵天线耦合特性的数值分析

通过数值模拟研究了低杂波多结波导阵天线的耦合特性。采用线型耦合理论,用艾黎函数计算了等离子体阻抗,推导出常规波导阵对等离子体的散射矩阵,并利用它和天线自身的散射矩阵研究了HT-7装置低杂波多结波导阵天线的耦合特性;计算了其平均功率反射系数、方向性系数和功率谱与等离子体边缘密度和主波导相位差等参数的关系。结果表明,与常规波导阵相比,由于多结波导阵天线存在自匹配特性,其天线功率反射系数小,且对相位不敏感。但由于存在多余副瓣,驱动效率较低。     

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通过数值模拟研究了低杂波多结波导阵天线的耦合特性。采用线型耦合理论,用艾黎函数计算了等离子体阻抗,推导出常规波导阵对等离子体的散射矩阵,并利用它和天线自身的散射矩阵研究了HT-7装置低杂波多结波导阵天线的耦合特性;计算了其平均功率反射系数、方向性系数和功率谱与等离子体边缘密度和主波导相位差等参数的关系。结果表明,与常规波导阵相比,由于多结波导阵天线存在自匹配特性,其天线功率反射系数小,且对相位不敏感。但由于存在多余副瓣,驱动效率较低。     

托卡马克中ICRH天线的优化设计

采用等离子体平板模型和三维天线模型数值模拟了托卡马克中ICRH天线与等离子体的耦合过程, 模拟结果表明在其它实验条件相同的条件下,与反对称电流和馈线端长度短的天线相比,对称电流分布和馈线端长度长的发射天线可获得更有效的功率耦合。     

HL-2A 装置ECRH天线系统的结构设计

HL- 2A 装置电子回旋共振加热的天线系统主要由天线外壳、椭球镜、平面镜、转动机构和推动机构组成。转动机构可以推动平面镜环向、极向转动, 进行不同区域的等离子体的加热和电流驱动。平面镜环向转动的角度范围为0°~ 37°, 极向转动的角度范围为0°~ 15 °。给出了驱动杆与平面镜转动角度之间的关系和对应曲线。介绍了椭球镜的安装和推动机构的结构, 给出了椭球镜的椭球面方程。整个天线系统结构紧凑, 安装和拆卸容易, 调节方便。     

Soliton modulation of the turbulence envelope and plasma rotation

The space-uniform amplitude envelope of the ion-temperature-gradient driven turbulence is unstable to small perturbations and evolves to nonuniform, solitonlike modulated profiles. The induced poloidal asymmetry of the transport fluxes can generate spontaneous poloidal spin-up of the tokamak plasma.     

离子回旋共振加热的耦合计算

托卡马克离子中回旋共振加热（ＩＣＲＨ）的平板模型在计算沿极向尺度较长的天线时误差较大，针对这一问题提出了一种将天线进行分段处理的改进算法，该算法提高了计算精度，计算结果表明，天线越长，改进算法的计算精度改善越明显。     

快波加热的天线设计

托卡马克中离子回旋频段的天线耦合效率主要取决于天线的体制和天线的尺寸。根据等离子体对不同谱分量的吸收性能差别,以及天线激励电流的谱应与等离子体对射频波吸收谱相匹配,对快波加热天线设计问题进行了细致地探讨。     

Poloidal Structure of the Turbulence and the Turbulent Flux During Edge Plasma Biasing in the Castor Tokamak

The poloidal structure of the turbulence and the turbulent particle flux in the scrape-off layer (SOL) of the CASTOR tokamak are analysed by means of a poloidal ring of 124 probes distributed uniformly along the whole poloidal circumference. Fluctuation measurements are performed in standard ohmic regime as well as in discharges when a biased electrode is inserted either into the SOL or deeper into the confinement region. It is found that in both cases a strongly sheared radial electric field is created in the SOL, which de-correlates the density and radial velocity fluctuations and reduces their levels. Consequently, the turbulent flux is reduced. However, no phase shift is observed between density and radial velocity fluctuations. When the electrode is localized in the confinement region, all the above effects are less pronounced because of a smaller shear. In addition, the increase of the E × B velocity at biasing leads in both cases to the formation of oscillations in the temporal correlation function. These oscillations are specifically associated to the poloidal mode (m = q), which is created for a limited time and rotates poloidally. This mode does not modify the phase between density and radial velocity fluctuations and has no further effect on the turbulent flux.     

Investigation of Plasma Equilibrium in the Saskatchewan Torus‐Modified (STOR‐M) during Alternating Current Operation

An experiment to investigate the source of plasma equilibrium of the residual plasma at zero cross over of plasma current is presented. The role of limiter in providing this equilibrium by short circuiting the electric field developed due to the toroidal drifts is examined by measuring the current flowing through the limiter. The role of a fast poloidal rotation observed by a set of Mach probes, during zero cross over is also examined in providing the equilibrium. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theoretical analysis of ion cyclotron range of frequency antenna array for HT-7U

This paper considers the coupling analysis of phased antenna array designed to excite fast wave in the ion cyclotron range of frequency. The coupling of the antenna is calculated in slab geometry. The coupling code based on the variational principle gives the self-consistent current flowing in the antenna, this method has been extended so that it can be applied to a phased antenna array. As an example, this paper analyses the coupling prosperities of a 2 × 2 phased antenna array. It gives the optimum geometry of antenna array. The fields excited at plasma surface are found to more or less correspond to the antenna current phasing.