等离子体电流反向平衡位形中带电粒子运动的数值模拟研究

本文采用数值方法求解Grad-Shafranov方程获得CT-6B交流放电实验总电流过零时的平衡位形和磁场分布,进而结合粒子在磁场中的运动方程,模拟氘离子在该平衡位形中的运动轨迹,统计氘离子的损失率与损失位置。结果表明：总电流过零时刻的平衡位形为内外两侧电流反向平衡位形,在强、弱场侧各存在1个磁岛,电流在磁岛附近取极值;位于强场侧的粒子几乎不损失,弱场侧的粒子在径向位置很大时存在损失,越靠近边界损失率越高;损失位置基本上位于赤道面以下并在最底部达到极值;随初始角变大,氘离子轨迹由通行轨迹变成损失轨迹再向香蕉轨迹演变。     

Best Magnetic Condition to Generate Hollow Cathode Glow Plasma in High Vacuum简

Based on magnetron hollow cathode discharge, the magnetic condition of glow plasma generation in high vacuum, including both direction and magnitude of the applied mag- netic field, is theoretically derived and experimentally evaluated in this paper. Single particle orbital theory is introduced to discuss the possibilities to generate glow plasma at gas pressure under 10-2 Pa when the magnetic field direction is parallel or perpendicular or oblique to the electric field direction. A quantitative estimation criterion of magnetic induction intensity is also proposed in theory. The comparison with experiments suggests that glow plasma in high vacuum will form more easily in oblique magnetic field condition and that the criterion is accurate enough to estimate magnetic induction intensity at a certain gas pressure.     

Effects of resonant magnetic perturbations on the loss of energetic ions in tokamak pedestal

Resonant magnetic perturbations (RMPs) are extensively applied to mitigate or suppress the edge localized mode in tokamak plasmas, but will break the axisymmetric magnetic field configuration and increase the loss of energetic ions. The mechanism of RMPs induced energetic ion loss has been extensively studied, and is mainly attributed to resonant effects. In this paper, in the perturbed non-axisymmetric tokamak pedestal, we analytically derive the equations of guiding center motion for energetic ions including the bounce/transit averaged radial drift velocity and the toroidal precession frequency modified by strong radial electric field. The loss time of energetic ions is numerically solved and its parametric dependence is analyzed in detail. We find that passing energetic ions cannot escape from the plasma, while deeply trapped energetic ions can escape from the plasma. The strong radial electric field plays an important role in modifying the toroidal precession frequency and resulting in the drift loss of trapped energetic ions. The loss time of trapped energetic ions is much smaller than the corresponding slowdown time in DIII-D pedestal. This indicates that the loss of trapped energetic ions in the perturbed non-axisymmetric pedestal is important, especially for the trapped energetic ions generated by perpendicular neutral beam injection.     

Study of nonlinear relationship between position and drift time incylindrical-type drift chamber and use of backgammon-shaped cathode pads

A cylindrical-type drift chamber consists of an appropriate array of cylindrical single wire proportional counters and has a simple structure. It is shown that a quadratic function reproduced the space-time relationship in the cylindrical-type drift chamber filled with PR gas (Ar:methane=90:10). The difference in the drift times in pair-cells is simply converted into the incident position of charged particles perpendicular to the anode wire using the quadratic function of the space-time relationship. Furthermore, the quadratic function improves the position resolution. A backgammon pattern is employed for the cathode pad of the cylindrical drift chamber. The cylindrical drift cell with a backgammon-shaped cathode pad gives the position resolution along the anode wire. The position resolution in one sigma along the anode wire is obtained to be 1.7 mm on average. The cylindrical-type drift chamber with a backgammon-shaped cathode pad may be applied to a two-dimensional position sensitive detector with a compact structure and simple signal-processing electronics     

Effects of the Hot Electron Interchange Instability on Plasma Confined in a Dipolar Magnetic Field

The Levitated Dipole Experiment (LDX) explores confinement and stability of plasma created within the dipole field of a strong superconducting magnet. During initial experiments, long-pulse, quasi-steady state discharges that last more than 10 s and have peak beta of more than 20% are studied. The plasma is created by multi-frequency electron cyclotron resonance heating (ECRH) at 2.45 and 6.4 GHz. A population of energetic electrons, with mean energies above 50 keV, dominates the plasma pressure. Creation of high pressure, high beta plasma is possible only when intense hot electron interchange (HEI) instabilities are stabilized by sufficient neutral gas fueling. The instabilities resonate with the magnetic drift motion of the energetic electrons and can cause rapid radial transport. Measurements of the electrostatic and magnetic fluctuations of the HEI instability are described along with observations of the instability’s spectral characteristics. Fluctuations of the outer poloidal field induced by the HEI show a rapid evolution of the perturbed pressure profile.      

Investigation of cathode spot characteristics in vacuum under transverse magnetic field (TMF) contacts

With the continuous improvement of current levels in power systems,the demands on the breaking capacity requirements of vacuum circuit breakers are getting higher and higher.The breaking capacity of vacuum breakers is determined by cathode spots,which provide electrons and metal vapor to maintain the arc.In this paper,experiments were carried out on two kinds of transverse magnetic field (TMF) contacts in a demountable vacuum chamber,the behavior of the cathode spots was recorded by a high-speed charge-coupled device (CCD) video camera,and the characteristics of the cathode spots were analyzed through the image processing method.The phenomenon of cathode spot groups and the star-shaped pattern of the spots were both discovered in the experiment.The experimental results show that with the condition of TMF contacts the initial expansion speed of cathode spots is influenced by some parameters,such as the tested current,contact gap,the structure of the contact,the contact diameter,the number of slots,etc.In addition,the influence of the magnetic field on the formation of the cathode spot groups,the distribution,and the dynamic characteristics of the cathode spots were analyzed.It is concluded that the characteristics of the cathode spots are due to the effect of the magnetic field on the near-cathode plasma.The study of the characteristics of cathode spots in this paper would be helpful in the exploration of the physical process of vacuum arcs,and would be of guiding significance in optimizing the design of vacuum circuit breakers.     

CYCIAE-100回旋加速器测磁仪自动控制系统的研制

为满足100 MeV回旋加速器磁场测量要求,设计研制了一套测磁仪自动化控制装置。该装置采用周向与径向相结合的运动方式,对目标点磁场进行测量。周向运动采用开环控制,在软件上通过算法实现间接闭环控制,整个测磁过程,只需完成1次周向运动,缓解了由硬件原因带来的周向定位震荡问题。径向运动基于光栅位置反馈,由运动控制器实现自动闭环控制,该方案使角度精度达到±5″以内,径向精度达±5 μm,达到并优于理论设计要求,解决了中能回旋加速器磁场测量时间较长、精度低的难题,对100 MeV回旋加速器的束流强度及品质的提高有重要意义。     

Parametric Study on Arc Behavior of Magnetically Diffused Arc

A model coupling the plasma with a cathode body is applied in the simulation of the diffuse state of a magnetically rotating arc.Four parametric studies are performed:on the external axial magnetic field (AMF),on the cathode shape,on the total current and on the inlet gas velocity.The numerical results show that:the cathode attachment focuses in the center of the cathode tip with zero AMF and gradually shifts off the axis with the increase of AMF;a larger cathode conical angle corresponds to a cathode arc attachment farther away off axis;the maximum values of plasma temperature increase with the total current;the plasma column in front of the cathode tip expands more severely in the axial direction,with a higher inlet speed;the cathode arc attachment shrinks towards the tip as the inlet speed increases.The various results are supposed to be explained by the joint effect of coupled cathode surface heating and plasma rotating flow.     

Symplectic Simulation of Fast Alpha Particle Radial Transport in Tokamaks in the Presence of TF Ripples and a Neoclassical Tearing Mode

A Hamiltonian guiding centre drift orbit code based on a symplectic integration algorithm, which enables the efficient calculation of particle trajectories and diffusion coefficients, is applied to fast alpha particle motion in magnetically perturbed tokamak plasmas. In particular, fast ion drift motion is examined in the presence of a stationary, low mode-number MHD magnetic perturbation in a toroidally rippled tokamak with circular flux surface. The main focus of our study is to investigate the dependence of the radial diffusion coefficient of energetic ions on their energy, on the perturbation strength and the localization of the perturbation. As expected, the resonance between bounce motion and toroidal field ripples plays a significant role in this context. For an ensemble of fast ions uniformly distributed in toroidal angle but with a given poloidal starting position their radial transport coefficient takes on higher values in the neighbourhood of resonance speeds and can exhibit there local minima, i.e. it shows an M-shaped speed dependence around resonances for sufficiently strong ripple perturbations. Expectedly, the addition of a modelled low-mode number neoclassical tearing mode perturbation will modify the pure ripple resonance structure of the radial diffusion coefficient. Depending on the strength and localization of the MHD mode it can cause enhancement or degradation of the radial ripple diffusion coefficient.     

Redistribution of Gaseous Phase of Liquid Metal Two-Phase Flow in a Strong Magnetic Field

In the previous paper, the authors pointed out the motion of bubble or gaseous phase in the direction to the both side walls due to the pinch effect caused by the induced magnetic field in the liquid metal two-phase flow under the strong magnetic field. In the present paper, to clarify the existence of the pinch effect experimentally, an experimental study was performed.

Firstly the distributions of the void fraction in the cross section perpendicular to the flow were measured at three locations in the flow direction for the various strength of the applied magnetic field. Secondly a magnetic field was superposed on the induced magnetic field by the outer coil to disturb the pinch effect by the cancel of the induced magnetic field with the superposed one, resulting in the evident redistribution of the void-fraction profile obtained above.

From these experiments it is concluded that the pinch effect will play an important role to redistribute the bubble or gaseous phase in the liquid metal two-phase flow under the strong magnetic field and that the effect is more promoted with increasing magnetic interaction number defined as a ratio of the electromagnetic force to the inertia of the fluid.     

脉冲四极磁铁峰值梯度测量

一、测量原理质子直线加速器中的漂移管四极透镜是由近似半正弦波的脉冲电流励磁的。脉冲底宽约为3.5毫秒,重复频率为每秒10次。四极透镜孔径内的磁场,若只考虑其空间分布,它是中心为零的梯度场,如图1所示。图中B_y、B_x分别表示磁场在y和x方向的分量。     

Magnetic field measurement of large aperture FFAG magnet for muon-phase-rotation ring

The PRISM-FFAG ring, which consists of 10 scaling-radial-sector magnets, has been designed to store muons with large emittance and momentum spread. The FFAG magnets differ from conventional dipole magnets in that they have a magnetic field that varies strongly with radius and the vertical direction of the magnetic field alternates. Each magnet is composed of three sets of coils to produce a defocussing (outward bending)-focussing-defocussing triplet. After careful design, the PRISM-FFAG magnets were constructed and magnetic field measurements were made. The field maps in the fiducial volume show satisfactory agreement with the design fields based on OPERA 3D/TOSCA calculations.     

A novel self-biased linear silicon drift detector

A novel linear silicon drift detector (SDD) is proposed in which the proper potential profile is established by the voltage drop along a unique p⁺ cathode implanted across the surfaces. This p⁺ implant, arranged in a zigzag shape, acts at the same time as voltage divider and field cathode and allows us to increase the sensitive area, improving also the uniformity of the thermal distribution and thus minimizing the fluctuation of the electron mobility on the sensitive zone of the SDD. The perturbations of the drift field due to the asymmetry of the strips constituting the zigzag cathode have been evaluated by solving analytically Poisson's equation for a simplified model of the structure. Three-dimensional numerical simulations have been carried out to prove the negligible amount of the perturbation and the effectiveness of the proposed structure. Based on this principle, a prototype has been manufactured at Canberra Semiconductor Company. Dynamic measurements of the time-of-flight of an injected charge prove that the linearity of the prototype and the drift uniformity in the anode direction are very high     

The Influence of the Negative Radial Electric Field on the Orbit of Banana Particles in Tokamak

This paper explores the patterns of influence of the negative radial electric field on the drift displacement and trajectory of charged particles, for it is essential for further investigation into the transitional mechanism of L-H Mode. In the light of superposition between the poloidal velocity of charged particles and the E ×B drift caused by the negative radial electric field, the paper offers a theoretical analysis and value simulations. Under the action of different radial electric fields, results have been obtained in regard to changes in the velocity of charged particles (mainly ions), patterns of changes in drift displacement, regional change of banana particles, and features of transition and change between trajectories of transiting particles and banana particles.     

Perpendicular Conductivity and Self-Consistent Electric Fields in Small Size Divertor Tokamak Plasma Edge

The radial electric field of small size divertor tokamak in the vicinity of separtrix is simulated by using B2-SOLPS5.0 2D code, in which the most complete system of transport equations (Rozhansky et al., Nucl Fusion, 41:4, 2001) is solved including all the important perpendicular current and E × B drifts. Simulations demonstrated the following results: (a) It is shown that in the vicinity of the separatrix, the radial potential profile is determined by perpendicular currents (b) since, due to the pressure asymmetry, radial diamagnetic current integrated over the closed flux surface is not automatically zero, additional radial currents balance the diamagnetic current and make the average net current zero. (c) On the closed flux surfaces far from separetrix, where the pressure is almost constant, the calculated parallel currents (toroidal current) agree with Pfirsch–Schlueter currents.     

Numerical analyses on liquid-metal magnetohydrodynamic flow in sudden channel expansion

Three-dimensional numerical calculations have been performed on the magnetohydrodynamic (MHD) flows through a rectangular channel with sudden expansion, particularly in order to estimate the pressure drop through the sudden expansion. The sudden expansion is in the directions both perpendicular and parallel to the applied magnetic field. The Hartmann number, the Reynolds number and the magnetic Reynolds number are set to ～100, ～1000 and ～0.001, respectively, in simulating laboratory conditions. The continuity equation, the momentum equation and the induction equation were solved numerically by the finite difference method as discretization following the MAC method as solution procedure. On the whole, in the sudden expansion in the direction perpendicular to applied magnetic field, the loss coefficient is estimated to be nearly zero or small. In particular, the loss coefficient becomes negative for small aspect ratios. The reason of negative loss coefficient is attributable to decrease in the induced current just upstream of the expansion. On the other hand, in the sudden expansion in the direction of applied magnetic field, all the cases give positive and large loss coefficients, meaning that the pressure drop through the expansion becomes large. In particular, the loss coefficient becomes considerably large when the Hartmann number increases.     

IMPROVEMENT ON PROPERTY AND WORKING LIFE OF FREEMAN SOURCE

The magnetic mirror field caused by a pair of mild steel circular in 15mm diameter,2mm thickness placed outside the discharge chamber can increase the atom ion ratio of nitrogen from 66% to 81% ,and the heating filament current can be decreased from 130A down to 100A for a new filament of 2mm in diameter,The Mo shield cylinders in 14mm diameter put behind the reflect cathode surrounding the BN insulator prevent the metallization of BN insulators,and result in increasing the stability of arc discharge till to the end of filament working life.Because it restricts the deposition of the sputtered W and Mo atoms on the BN insulator,and maintains a high resistance of 70 MΩ between the anode and cathode.The combination of cathode shield cylinder and forming magnetic mirror field improves the running property,life of the Freeman source being about 40h.     

Cyclotron with a magnetic field traveling in the radial direction

In this paper we consider the design of a cyclotron with a magnetic field which travels in the radial direction; a machine of this kind had been proposed by the author. By means of circular windings which are fed by ac generators it is possible to produce one or more concentrated magnetic fields which travel in the radial direction in the gap.Two versions are proposed. In the first, the traveling wave is the field in-which the particles are accelerated. In the second, the traveling wave is superimposed on the usual fixed magnetic field and acceleration takes place by virtue of the combined fields. The spatial distribution of the field in the wave makes it possible to obtain a stability region(1 > n > 0) which is displaced in the radial direction with the radial velocity of the particles. A stability region is also obtained in the case in which the absolute value of the magnetic field of the acceleration region increases in the radial direction.In principle, the system proposed here makes it possible to build cyclotrons with energies as high as desired. In spite of the cyclic-action characteristics of the acceleration, because of the improved focusing, there is reason to believe that the mean intensity can be larger than that which is obtained in the fm cyclotron.Typical calculations are given for accelerators of various energies. These calculations indicate that the weight and size of these accelerators may be much smaller than other accelerators of the same energy.     

Reversal of radial glow distribution in helicon plasma induced by reversed magnetic field

In this work,the reversal of radial glow distribution induced by reversed magnetic field is reported.Based on the Boswell antenna which is symmetric and insensitive to the magnetic field direction,it seems such a phenomenon in theory appears impossible.However,according to the diagnostic of the helicon waves by magnetic probe,it is found that the direction of magnetic field significantly affects the propagation characteristic of helicon waves,i.e.,the interchange of the helicon waves at the upper and the lower half of tube was caused by reversing the direction of magnetic field.It is suggested that the variation of helicon wave against the direction of magnetic field causes the reversed radial glow distribution.The appearance of the traveling wave does not only improve the discharge strength,but also determines the transition of the discharge mode.     

3D-simulation studies of the modified magnetic multipole structure for an electron cyclotron resonance ion source

Experiments have shown that efficient operation of an electron cyclotron resonance ion source requires that the magnetic field fulfills the so-called scaling laws. In most cases the requirements for the radial magnetic field, i.e. the strength of the magnetic multipole are the most difficult to satisfy. This is due to the fact that the multipole is usually produced from permanent magnets, which makes a value of 1.3 T feasible. One possible solution to increase the multipole field is the so-called Modified MultiPole Structure (JYFL-MMPS). This new idea makes it possible to increase the magnetic field at the places where the plasma flux is in contact with the plasma chamber wall. In this article we will present detailed three-dimensional magnetic field simulations of the JYFL-MMPS.