Neutron multiwave spin echo

The neutron multiwave interference mode is investigated using the spin echo technique. In this mode a neutron wave repeatedly splits in the magnetic field of resonance coils, which results in the appearance of additional maxima of a constructive interference being absent in the well-known classical and resonance neutron spin echo modes. Simple analytical expressions well describing the experimental data are presented. It is demonstrated that the multiwave part of a spin echo signal appears when the spin flip probability in radiofrequency coils of a resonance spin echo device is ρ < 1. The possibility to use the neutron multiwave spin echo mode for investigation of high-order correlation functions, spatial and time correlations of three and more particles, is discussed.     

Wave nature of moving striations in a longitudinal magnetic field

Wave nature of stationary moving striations in helium and neon discharges in a uniform longitudinal magnetic field is studied. With the increase of the magnetic field, the frequency of natural striations decreases, while the wave length increases, and they damp out at high field region. Artificial excitations in these gases show that the wave length is proportional to the excitation frequency for given magnetic field and the slope of linear lines increases with the field. These wave nature of striations is explained following the dispersion relation derived from the consideration of waves of ionization and including effects of the magnetic field on the ionization.     

电磁波与电离波面相互作用过程中的静磁场模和振荡电场

利用了匀速运动的均匀等离子体中电磁场的色散关系，对电磁波与由强激光驱动的运动电离波面的相互作用过程中出现的静磁模进行了研究，发现在特定的条件下振荡电场也会在该过程中出现，并确定了振荡电场产生的条件 关键词：     

A new method of modeling the working mode of the multipurpose isochronous cyclotron

The method of modeling the working mode is based on the selection of currents in concentric trim coils of the multipurpose isochronous cyclotron for the formation of the required magnetic field at a certain level of the main coil current. The traditional method of selecting currents is based on finding the solution for all concentric trim coils simultaneously. As a rule, after setting the calculated working mode, it is necessary to perform control measurement of the magnetic field and repeat the calculation to obtain the desired working mode. The new method of selecting currents is based on finding the solution for each separate concentric trim coil. The concentric trim coils are taken in turn in a specified order. A number of experiments on handling the proton beam in the range of working acceleration radii with an output energy between 45 and 60 MeV, performed using the multipurpose isochronous cyclotron AIC144 at the Institute of Nuclear Physics of the Polish Academy of Sciences in Krakov showed that the new method of obtaining the desired working mode does not require control measurements of magnetic fields, which testifies the high accuracy of the calculation.     

Propagation Characteristics of Wave Packet in Non-Uniform Magnetic Fields

Experimental observations on a wave packet in a positive column of helium discharge with magnetic fields are reported. The wave packet is a kind of ionization wave and is created by applying voltage pulses to a mesh grid. When an axial non-uniform magnetic field is applied to a positive column, the plasma parameters change inhomogeneously near the magnetic coil. So various characteristics (amplitude, frequency, wavelength and so on) of the wave packet are changed at the both sides of the coil. The wavelength of the wave making up the wave packet varies continuously with a magnetic field. On the contrary, its amplitude and frequency vary remarkable near the magnetic coil, as a strong magnetic field is applied.     

Effect of electron magnetic trapping in a plasma immersion ion implantation system

In this work we describe a two-dimensional computer simulation of magnetic field enhanced plasma immersion implantation system. Negative bias voltage of 10.0 kV is applied to a cylindrical target located on the axis of a grounded vacuum chamber filled with uniform nitrogen plasma. A pair of external coils creates a static magnetic field with main vector component along the axial direction. Thus, a system of crossed E×B field is generated inside the vessel forcing plasma electrons to rotate in azimuthal direction. In addition, the axial variation of the magnetic field intensity produces magnetic mirror effect that enables axial particle confinement. It is found that high-density plasma regions are formed around the target due to intense background gas ionization by the trapped electrons. Effect of the magnetic field on the sheath dynamics and the implantation current density of the PIII system is investigated. By changing the magnetic field axial profile (varying coils separation) an enhancement of about 30% of the retained dose can be achieved. The results of the simulation show that the magnetic mirror configuration brings additional benefits to the PIII process, permitting more precise control of the implanted dose.     

Frequency upshifting by an ionization front in a magnetized plasma

The interaction of a light wave with a relativistic ionization front in the presence of an applied DC magnetic field which is perpendicular or parallel to the incident wave is considered. In both cases, four transmitted modes are generated in the magnetized plasma by an incident linearly polarized wave. The frequency upshifts of the various modes are calculated and compared to the unmagnetized case. The corresponding reflection and transmission coefficients are also obtained. Finally, the density ripple associated with the free streaming mode in a magnetized plasma for the perpendicular case is discussed     

Lamb wave signal selective enhancement by an improved design of meander-coil electromagnetic acoustic transducer

In this paper, we investigate a method of selectively enhancing the single mode signal of a Lamb wave by using a meander-coil electromagnetic acoustic transducer(EMAT) with a new magnetic configuration. We use the Lamb antisymmetric(A0) mode and symmetric(S0) mode as an example for analysis. The analytical expression of the magnitude of the spatial Fourier transform of the Lorentz force generated by different meander coils is used to determine the optimal driving frequency for single mode generation. The numerical calculation is used to characterize the new magnetic configuration and the conventional EMAT magnet. Experimental examinations of each meander coil in combination with the conventional and new magnetic configuration show that the Lamb wave signal can be selectively enhanced by choosing the appropriate driving frequency and coil parameters through using the improved meander-coil EMAT.     

圆形复合式磁控溅射阴极设计及其放电特性模拟研究

基于二维有限元算法使用COMSOL软件对圆形复合式磁控溅射阴极的磁场进行了计算,结合Matlab优化工具箱分别采用遗传算法和模拟退火算法对圆形复合式磁控溅射阴极的结构进行优化,得到靶材利用率达到最大的最优结构.对得到的最优化磁控阴极,基于自洽粒子模拟方法,使用VSim软件对不同工况下的放电特性进行了模拟.研究发现随着磁场非平衡度的增加,阴极表面电势降落最大的位置和等离子体聚集的位置,沿着阴极表面外沿不断向阴极中心移动,阴极表面磁场的强度不断减小.随着磁场非平衡度的增加,等离子体密度先增加后减小,鞘层厚度先减小后增加,等离子体的密度和鞘层厚度不仅与磁场非平衡度有关,而且与磁场强度有关.最后根据粒子模拟的结果,对复合式磁控阴极的靶材刻蚀深度进行了研究.研究发现,在优化前后靶材的刻蚀范围从60 mm扩展至整个靶面,极大地提高了靶材利用率.     

Zum Charakter der teilkorrelierten Fluktuationserscheinungen in Argon-Niederdruck-Schwachstrom-Säulen im transversalen Magnetfeld

The elementary theory of ionization waves, given by Pekárek, is modified to include the influence of a magnetic field. The resulting dispersion ralation delivers an interpretation of the wave phenomena in argon low-pressure low-current columns under the influence of a local transversal field. The suppresion of ionization waves by weak magnetic fields as well as the excitation of wave-packet-like fluctuations in the case of strong magnetic fields can be shown.     

基于螺线盘聚焦径向电子束的理论分析与仿真研究

对径向电子束在浸没式聚焦条件下的传输特性进行了理论分析,得到了束流传输过程中轴向扩张幅值与外加引导磁场强度之间的解析表达式。阐述了螺线盘在空间中任意一点产生磁场的理论,给出了通以相反方向电流的螺线盘之间磁场的分布规律。设计了一种基于螺线盘聚焦径向电子束的引导磁场系统,并对电子束的传输特性进行了仿真研究,结果表明径向电子束能够在设计的引导磁场系统下实现束流的稳定传输。     

Tearing mode suppression by using resonant magnetic perturbation coils on J-TEXT tokamak

A series of experiments on the interactions between external resonant magnetic perturbations (RMP) and plasmas has recently been conducted, using static resonant magnetic perturbation (SRMP) coils on the Joint Texas Experimental Tokamak (J-TEXT). The tearing mode can be completely suppressed by applying SRMP. However, the locked mode is likely to be stimulated under a larger SRMP field even though the tearing mode has been first suppressed. A numerical simulation shows three typical regimes of RMP?s effects on the tearing mode, which are consistent with experimental results.     

Numerical Study of Instabiities in Magnetized Inhomogeneous Plasmas under the Effect of Ionization

Influence of ionization is studied on two stream instability (TSI) in an inhomogeneous plasma in the presence of obliquely applied magnetic field. In addition to the usual TSI, a new type of instability is found to occur in this system, which is driven by the magnetic field and survives for relatively longer wavelength of oscillations. The growth rates of both the instabilities are enhanced by the magnetic field but their magnitudes attain a minimum value at certain angles of the wave propagation depending upon the wavelength of oscillations. At a critical value of ionization rate there is a sudden fall in the growth of both the instabilities, the reason of which is understood as the Landau damping. A further enhancement in the ionization suppresses the usual TSI whereas the magnetic field‐driven instability attains much lower growth. This new type of instability grows faster in the plasma having heavier ions, but shows a weak dependence on the charge of the ions. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The properties of short-circuited HTSC coils

The properties of short-circuited multiturn superconducting coils have been studied; coils with nonsuperconducting contacts have been fabricated from a high-temperature superconducting (HTSC) tape made by Super Power Company. The magnetic flux captured by HTSC coils has been measured at different values of magnetic field of the magnetizing solenoid. the critical current in the coils have been experimentally determined based on the maximum values of the field they captured. It is ~50% of the nominal value for this HTSC tape. The range of external magnetic field, where HTSC coils keep the captured magnetic flux, has been experimentally found. The obtained results have demonstrated the possibility of designing magnet systems with levitating coils made of HTSC tape, in which levitation is controlled without using feedbacks.     

柱面内爆磁通量压缩实验中的磁扩散过程

利用一维磁流体力学程序MC11D,对套筒材料为不锈钢的柱面内爆磁通量压缩实验进行了数值模拟,研究了其中的磁扩散过程。计算结果表明:当套筒空腔中的磁场被压缩到350 T左右时,峭面磁扩散波开始形成,磁扩散波的波前从套筒内壁开始以0.75 km/s的平均速度向外快速推进,给磁场的压缩带来了不利影响;随着套筒内壁温度迅速升高,内壁附近会形成一个电阻率仅有0.3 mcm左右的等离子体保护层,又极大地减缓了空腔磁场向套筒中扩散的速度。在磁压缩过程中,峭面磁扩散波和等离子体层对于空腔磁场的扩散起着相反的作用,两者在发展的过程中相互竞争,在不同的阶段分别起着主导作用。     

霍尔推力器磁场位形及其优化的数值研究

基于麦克斯韦方程,在轴对称假设下建立了霍尔推力器磁场的数学模型.用有限差分方法对模型进行了离散.给出了数值求解模型的迭代法.通过对模型的数值求解,得到了相应的数值结果.通过对所得数值结果的分析,研究了磁场线圈电流变化对霍尔推力器磁场位形的影响.通过调整磁场线圈电流的大小找到了理想磁场位形.研究表明,对于理想磁场位形,内通道的磁镜比在3—3.5之间,外通道的磁镜比在0.4—0.9之间;增加磁场线圈的电流,出口的磁场强度随着增加,但不能增加磁镜比.通道内部的磁场强度几乎不随着磁场线圈电流的变化而变化. 关键词： 霍尔推力器 磁场位形 磁场线圈电流 磁镜比     

Eddy current simulation in thick cylinders of finite length induced by coils of arbitrary geometry

Eddy currents are inevitably induced when time-varying magnetic field gradients interact with the metallic structures of a magnetic resonance imaging (MRI) scanner. The secondary magnetic field produced by this induced current degrades the spatial and temporal performance of the primary field generated by the gradient coils. Although this undesired effect can be minimized by using actively and/or passively shielded gradient coils and current pre-emphasis techniques, a residual eddy current still remains in the MRI scanner structure. Accurate simulation of these eddy currents is important in the successful design of gradient coils and magnet cryostat vessels. Efficient methods for simulating eddy currents are currently restricted to cylindrical-symmetry. The approach presented in this paper divides thick conducting cylinders into thin layers (thinner than the skin depth) and expresses the current density on each as a Fourier series. The coupling between each mode of the Fourier series with every other is modeled with an inductive network method. In this way, the eddy currents induced in realistic cryostat surfaces by coils of arbitrary geometry can be simulated. The new method was validated by simulating a canonical problem and comparing the results against a commercially available software package. An accurate skin depth of 2.76 mm was calculated in 6 min with the new method. The currents induced by an actively shielded x-gradient coil were simulated assuming a finite length cylindrical cryostat consisting of three different conducting materials. Details of the temporal-spatial induced current diffusion process were simulated through all cryostat layers, which could not be efficiently simulated with any other method. With this data, all quantities that depend on the current density, such as the secondary magnetic field, are simply evaluated.     

用磁探针反演磁岛二维结构的方法及其在HL-2A装置上的应用

介绍了HL-2A装置中利用磁探针数据反演磁岛极向二维结构的新方法,以及在反演基础上建立的撕裂模动态分析方法。在实验中通过磁探针测量确定作为扰动磁场来源的扰动电流。将扰动磁通与由EFIT重建的平衡磁通叠加反演出磁岛的结构,并给出磁岛的宽度。然后,按时间顺序建构二维结构图并依次记录,之后依次展现图像就可以对磁岛进行动态分析。应用此方法进行撕裂模分析,得出了磁岛旋转与电子抗磁漂移方向相同,验证了磁岛宽度与扰动场的关系及ECRH对磁岛的抑制作用。这显示了磁探针反演磁岛结构方法的直观性,对观察并控制MHD不稳定性非常有利。     

Anfachung und Dämpfung magneto-akustischer Wellen in MHD-Kanälen und ihr Einfluß auf die mittlere Stromdichte und die mittlere Hall-Feldstärke

Magneto-acoustic waves generated by fluctuations in the Hall parameter, the electric conductivity and the stream velocity are theoretically investigated in a weakly ionized plasma streaming across a strong external magnetic field and bearing a current flowing perpendicular to both magnetic field and stream velocity. The investigations hold for seeded rare gas plasmas at any degree of seed ionization but are resticted to waves propagating in parallel or antiparallel direction to the current density vector and in parallel or antiparallel direction to the stream velocity vector and to wave lengths which are small in comparsion to the interaction length which occurs as a characteristic wave length. The influence of these waves on the mean current density and the mean Hall field intensity is calculated in case of small amplitudes and low degree of seed ionization up to second order terms. Omitting Ohmic heating the dispersion equation can be solved exactly. A phase shift exists between the fluctuations in gas density and gas velocity. The phase velocity and the amplification rate depend on the wave length. Typical results are represented in a diagram. For both types of waves the phase velocity slightly rises with increasing wave length, while the amplification rate decreases. Waves propagating in opposite direction to the current density vector are amplified, if the electron velocity exceeds a critical value. They reduce the mean current density and the mean Hall field intensity. Waves propagating in opposite direction to the stream velocity vector are also amplified except for very high degrees of seed ionization. The threshold current density is greater than that for the waves of the first type approximately by the Hall parameter as factor. At extremely high degree of seed ionization the phase velocity is directed opposite to the direction occuring at weakly ionized seed. Waves of the second type decrease the mean current density, but increase the mean Hall field intensity.     

RIKEN超导ECR离子源磁体系统的设计

Superconducting magnet system for a 28GHz ECR ion source has been designed.The maximum axial magnetic fields are 4T at the rf injection side and 2T at the beam extraction side,respectively.The hexapole magnetic field is about 2T on the inner surface of the plasma chamber.The superconducting coils consist of six solenoids and six racetrack windings for a hexapole field.Two kinds of coil arrangements were investigated:one is an arrangement in which the hexpole coil is located in the bore of the solenoids,and another is the reverse of it.The coils use NbTi-Copper conductor and are bath-cooled in liquid helium.The six solenoids are excited with individual power supplies to search for the optimal axial field distribution.The current leads use high Tc material and the cryogenic system is operated in LHe re-condensation mode using small refrigerators.The thermal insulated supports of the cold mass have also been designed based on the calculated results of the magnetic force.The heat loads to 70K and LHe stages were estimated from the design of the supports,the current leads and so on.