用偏压限制器控制KT－5C托卡马克等离子体边缘电场的实验研究

在托卡马克ＫＴ－５Ｃ上，我们首次采用可灵活组合的多块限制器进行了施加偏压以控制等离子体边缘电场的实验，结果表明，在本装置上限制器加正偏压优于加负偏压，正偏压改变等离子体边缘电场效果更明显。偏压限制器收集面积、纵场和偏置电压的大小对偏压效果的影响都是敏感的。改变限制器收集面积和偏压导致的边缘电场的变化最终将趋于饱和，另外发现，偏压限制器与等离子体柱的相对位置对等离子体边缘电场的影响也是重要的。     

THE INFLUENCE OF ELECTRON OSCILLATION ON PLASMA TRANSPORT THROUGH A MAGNETIC DUCT

A magnetic duct is inserted between the cathodic arc plasma source and the chamber to eliminate the macroparticles. In this paper, the plasma output of the magnetic duct is determined as a function of the magnetic field and the bias voltage under the Bilek biasing mode and entire duct biasing mode. The computer simulation and the experimental result indicate that the ${\vec E}\times{\vec B}$ drift results in an extra diffusion flux under the Bilek biasing mode. The test verifying the electron oscillation was conducted in the magnetic duct biased in the Bilek mode. The electron behaviour under Bilek biasing mode is different from that under entire duct biasing mode. The Bilek biasing mode has a lower plasma output than the entire duct biasing mode.     

Sheath and plasma parameters in a magnetized plasma system

The variation of electron temperature and plasma density in a magnetized N₂ plasma is studied experimentally in presence of a grid placed at the middle of the system. Plasma leaks through the negatively biased grid from the source region into the diffused region. It is observed that the electron temperature increases with the magnetic field in the diffused region whereas it decreases in the source region of the system for a constant grid biasing voltage. Also, investigation is done to see the change of electron temperature with grid biasing voltage for a constant magnetic field. This is accompanied by the study of the variation of sheath structure across the grid for different magnetic field and grid biasing voltage as well. It reveals that with increasing magnetic field and negative grid biasing voltage, the sheath thickness expands.     

Plane Waves Propagating in Chiral Plasma and Chiral Ferrite Media

Plane wave propagation in chiral plasma and chiral ferrite media is studied in kDB coordinate system. General wave equations and characteristic equations of plane waves propagating along an arbitrary direction in chiral plasma and in chiral ferrites are derived in simple formulations respectively. Four wavenumbers and their corresponding dispersion characteristics are resulted for propagation both along and normal to the biasing magnetic field. When plane wave with negative helicity propagates along the biasing magnetic field in chiral ferrites, backward waves emerge. However backward waves occur with both positive and negative helicities when propagating along the biasing magnetic field in chiral plasma.     

Interactions of the directed plasma from a cathodic arc withelectrodes and magnetic fields

Curved magnetic ducts are frequently used to remove macroscopic-sized droplets from the plasma stream of cathodic vacuum arcs. The plasma of a cathodic vacuum arc in a magnetic filter is characterized by a strongly directional ion velocity (corresponding to 20-100 eV) and magnetized electrons. In the first section of this paper the effects of these features on the I-V characteristic curves of planar probes are identified and explained using a simple model. This is then used to interpret the interaction of the plasma with the walls of a biased quarter torus duct. Two small electrodes placed on the outer and inner sections of the curved duct wall show that the I-V characteristic is determined primarily by the electron-ion current balance at the wall on the outside of the curve. The application of a bias to a planar electrode on the outer wall section was found to give the same increase in throughput as a positive bias applied to the entire duct with the advantage of a much smaller electron current being drawn by the biasing power supply. The improvement in duct throughput achievable with positive-biasing of the duct wall was found to depend on both the configuration and strength of the magnetic field in the quarter torus filter. The plasma density profile and potential were unaffected by the application of the bias     

Characteristic behaviour of plasma fluctuations inside a plasma bubble in presence of a magnetic field due to the formation of a potential well

Experiments were carried out to observe the effect of a magnetic field and grid biasing voltage in presence of a plasma bubble in a magnetized, filamentary discharge plasma system. A spherical mesh grid of 80% optical transparency was negatively biased and introduced into the plasma for creating a plasma bubble. Diagnostics via an electrical Langmuir probe and a hot emissive probe were extensively used for scanning the plasma bubble. Plasma floating potential fluctuations were measured at three different positions of the plasma bubble. The instability in the pattern showed the dynamic transition from periodic to chaotic for increasing magnetic fields. Time scale analysis using continuous wavelet transform was carried out to identify the presence of non‐linearity from the contour plots. The mechanisms of the low‐frequency instabilities along with the transition to chaos could be qualitatively explained. Non‐linear techniques such as fast Fourier transform, phase space plot, and recurrence plot were used to explore the dynamics of the system appearing during plasma fluctuations. In order to demonstrate the observed chaotic phenomena in this study, characteristics of chaos such as the Lyapunov exponent were obtained from experimental time series data. The experimentally observed potential structure is confirmed with numerical analysis based on fluid hydrodynamics.     

Microscopic domain structures in unidirectional and isotropic exchange-coupled NiO/NiFe bilayers

The dependence on nickel oxide thickness in unidirectional and isotropic exchange-coupled NiO/NiFe bilayer films was investigated by magnetic force microscopy to better understand exchange biasing at microscopic length scales. As the NiO thickness increased, the domain structure of unidirectional biased films formed smaller and more complex in-plane domains. By contrast, for the isotropically coupled films, large domains generally formed with increasing NiO thickness including a new cross type domain with out-of-plane magnetization orientation. The density of the cross domain is proportional to exchange biasing field, and the fact that the domain mainly originated from the strongest exchange coupled region was confirmed by imaging in an applied external field during a magnetization cycle.     

Effects of radiation and thermal conductivity on MHD boundary layer flow with heat transfer along a vertical stretching sheet in a porous medium

A steady two-dimensional free convective flow of a viscous incompressible fluid along a vertical stretching sheet with the effect of magnetic field, radiation and variable thermal conductivity in porous media is analyzed. The nonlinear partial differential equations, governing the flow field under consideration, have been transformed by a similarity transformation into a systemof nonlinear ordinary differential equations and then solved numerically. Resulting non-dimensional velocity and temperature profiles are then presented graphically for different values of the parameters. Finally, the effects of the pertinent parameters, which are of physical and engineering interest, are examined both in graphical and tabular form.     

Cover Picture: Contrib. Plasma Phys. 6/2011

Images of fireballs on positively biased electrodes. (a) One‐sided fireball in an unmagnetized Ar plasma. (b) Spherical fireball concentric with small gridded electrode, B=0. (c) Fireball to a gridded electrode in the diverging magnetic field of a permanent magnet. (d) Neon fireball to a gridded electrode in the center of a mirror magnetic field between two permanent magnets. (e) Asymmetric fireball in a cusp magnetic field between two opposing permanent magnets. (f) Fireball elongated along the radial magnetic field lines of a cuspfield topology. Figure 3 of the paper by R.L. Stenzel et al (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Specific features in precession dynamics of magnetization of a uniaxial magnetic film

The precession dynamics of the magnetization of a film with in-plane uniaxial anisotropy in the case of its biasing along the hard magnetization axis has been analyzed by numerically solving the Landau-Lifshitz equations. It has been revealed that the ferromagnetic resonance spectrum near the magnetic anisotropy field exhibits an additional peak, which is associated with the angular bistability due to the presence of two symmetric angular equilibrium positions. The trajectories of precession motion during biasing along the hard magnetization axis differ substantially from the trajectories corresponding to the biasing along the easy axis.     

Analysis of the convective heat transfer characteristics in a two-dimensional cavity subjected to an external magnetic field

Natural convection heat transfer in the presence of a magnetic field has received considerable attention in the past few decades because of its various applications in industrial installations. In particular, a large number of numerical studies analyzing the effect of the magnetic field on natural convection in a two-dimensional cavity have been performed. In this work, we propose to study the main characteristics of the convective heat transfer of pure fluids and nanofluids in a two-dimensional cavity differentially heated and subjected to an external magnetic field. The scale analysis method is used first to obtain a correlation giving the heat transfer rate, which is then developed to predict the behavior of the heat transfer rate for pure fluids and nanofluids. To verify the reliability of the theoretical predictions, a numerical study is also carried out. The results show that the proposed correlation predicts well the convective heat transfer characteristics obtained numerically.     

Experimental observation of intermittent route to chaos in magnetized filamentary discharge plasma due to the cylindrical plasma bubble

We delineate an experimental observation of the effect of the magnetic field along with mesh grid biasing in the presence of a cylindrical plasma bubble in a filamentary discharge magnetised plasma system. The cylindrical mesh grid of 80% optical transparency has been negatively biased and introduced in the plasma for creating a plasma bubble. Plasma floating potential fluctuations have been taken outside (LP1) and inside (LP2) of the plasma bubble. It has been noticed that as the external magnetic field is increased the oscillation pattern shows intermittent route to chaos as the system evolved from regular type of relaxation oscillations (of larger amplitude) to an irregular type of oscillations (of smaller amplitude) We have used recurrence quantification analysis (RQA) to the observed intermittency to chaos in the plasma. The main measures of RQA are laminarity (LAM) and determinism (DET). The laminarity measure can be associated with the average time between the chaotic burst in the intermittency. It has also been observed that the DET depends on the control parameter and decreases exponentially, features like a dip in skewness and a hump in the kurtosis with the variation of control parameter have been noticed, which are the strong evidence of intermittent behaviour of the system. Further, a numerical model has been developed to the observed experimental analysis of the intermittent route to chaos.     

Design and construction of hot limiter biasing system for the tokamak

A hot limiter biasing system with a simplified fast switch circuit was designed, constructed, and installed on the IR-T1 tokamak, and then the negative voltage applied to a hot limiter inserted inside the tokamak fixed limiter and the plasma current, poloidal, and radial components of the magnetic fields, loop voltage, diamagnetic flux, and the ion saturation currents in the absence and presence of the biased limiter were measured. Results of measurements of biasing effects on the plasma equilibrium behavior and edge plasma rotation are compared and discussed.     

Partial magnetization reversal using laser annealing in patterned NiFe/FeMn film

We have studied local magnetization reversal by laser annealing in exchange biased NiFe/FeMn bilayer. Local magnetization reversal was performed by using the Nd:YAG laser under external magnetic field. When the laser illuminated the patterned film with the power of above 300 mW during 15 min, a magnetoresistance (MR) curve with symmetric peaks at the opposite field was obtained due to the local reversal of exchange biasing. A similar result was observed in NiFe/FeMn/NiFe trilayer. As the exposed area expanded, the intensity of opposite MR peak increased. The direction of exchange anisotropy in the partially reversed region can be restored by local laser annealing under alternating magnetic field, even if its MR peak was reduced by the damage and interdiffusion. The magnetic new domain structures of the partially reversed region was generated by laser annealing near the exposed area.     

Influence of plasma extraction on a cylindrical low-pressure RFdischarge: a PIC-MC study

A model is presented for plasma extraction from a cylindrical RF discharge. Even though the extraction geometry is two-dimensional, the model proposed here allows to discuss extraction in a one-dimensional geometry. Specific results are obtained for a low-pressure RF discharge between cylindrical electrodes with a high asymmetry. These are calculated with the help of a PIC-MC computer simulation. The influence of extraction on the plasma characteristics, the ionization efficiency, and the influence of an axial magnetic field are calculated and discussed     

利用离子声波朗道阻尼测量氧化物阴极放电中的离子温度

作为等离子体重要参数之一, 特别是在低温等离子体中离子温度的测量一直较为困难. 在磁化线性等离子体装置氧化物阴极脉冲放电条件下, 利用栅网激发离子声波, 通过测量波幅在朗道阻尼作用下随空间的演化, 利用阻尼长度是离子温度和电子温度的函数, 计算得到离子温度为0.3 eV. 测量值与国外类似装置利用光谱诊断所得结果基本相同.     

Enhancement of convective heat transfer by electrically-induced swirling effect in laminar and fully-developed internal flows

A computational and experimental approach is undertaken to study the enhancement of convective heat transfer in fully-developed internal flows by an electrically-induced secondary flow field. Using longitudinal flat electrodes along a parallel-plate configuration, the corona discharge is employed to generate an electrically induced secondary flow on the cross section of the flow passage. The electrically-induced secondary flow forms a swirling flow field in the fully-developed condition and enhances the heat transfer significantly. The flow field was solved computationally and the results were verified and validated by grid refinement study and computational error analysis.     

水平圆管内磁性潜热型功能流体对流换热特性的数值模拟

建立磁场作用下水平圆管内磁性潜热型功能流体对流换热的数学物理模型,分析磁场强度、磁性相变微胶囊体积分数、流体质量流量等因素对流体对流换热的影响.磁场对磁性潜热型功能流体的对流换热具有显著的强化作用,磁场强度愈大强化作用愈明显,强化原因是磁性相变微胶囊受到磁力作用产生扰动.     

Magneto-ballistic transport through micro-structured junctions on a curved two-dimensional electron gas

We investigate theoretically the ballistic transport in a two-dimensional electron gas, which is rolled up as a tube and is micro-structured into a Hall bar. A uniform magnetic field applied to such a curved surface results in a non-uniform perpendicular magnetic field. The bend resistances become asymmetric with respect to the orientation of the magnetic field due to the varying magnetic field along the junction. The resistance asymmetry is strongly affected by corrugation due to the varying mobility along different crystallographic directions. We compare our results with a recent transport measurement.     

Magnetoquantum transport in a modulated 2D electron gas with spin-orbit interaction

We investigate the effects of spin-orbit interaction (SOI) and plane-perpendicular magnetic field on the conductivity of a two-dimensional electron system in the presence of one-dimensional electrostatic modulation. The calculations are performed when a low-intensity, low-frequency external electric field is applied. The Kubo formula for the conductivity is employed in the calculation. The single-particle eigenstates which depend on the strengths of the magnetic field, the SOI and modulation potential, are calculated and then used to determine the conductivity. We present numerical results for the conductivity along the channels as well as the tunneling conductivity perpendicular to the constrictions as functions of the modulation potential, the SOI and the magnetic field. We demonstrate that the effect of finite frequency is to related to the reduction of both the longitudinal and transverse conductivities.