永磁磁共振系统盘式梯度线圈的设计

针对永磁磁共振成像系统极板的形状,提出了一种平面圆盘式梯度线圈的设计方法.将电流密度在极坐标下进行傅立叶级数展开,根据误差平方和建立目标函数,使目标函数最小化,得到电流密度的表达式.应用流函数技术离散电流密度,得到线圈的绕线形式.利用Biot-Savart积分方程计算线圈绕线产生的梯度磁场来验证所设计方法的正确性.结果表明,利用该方法设计的线圈可以产生成像所需要的线性度很好的磁场.该方法也可以用于双平面盘式自屏蔽梯度线圈的设计.     

Quench simulation results for a 12-T twin-aperture dipole magnet

A 12-T twin-aperture subscale dipole magnet is being developed for SPPC pre-study at the Institute of High Energy Physics (IHEP). The magnet is comprised of 6 double-pancake coils which include 2 Nb₃Sn coils and 4 NbTi coils. As the stored energy of the magnet is 0.452 MJ and the operation margin is only about 20% at 4.2 K, a quick and effective quench protection system is necessary during the test of this high field magnet. For the design of the quench protection system, attention was not only paid to the hotspot temperature and terminal voltage, but also the temperature gradient during the quench process due to the poor mechanical characteristics of the Nb₃Sn cables. With the adiabatic analysis, numerical simulation and the finite element simulation, an optimized protection method is adopted, which contains a dump resistor and quench heaters. In this paper, the results of adiabatic analysis and quench simulation, such as current decay, hot-spot temperature and terminal voltage are presented in details.     

Coil geometry models for power loss analysis and hybrid inductive link for wireless power transfer applications

This paper presents a hybrid inductive link for Wireless Power Transfer (WPT) applications. Achieving better power transfer efficiency over a relatively wider distance across coils is the prime objective in most of the WPT systems, but often suffers from power loss in the near field area of inductively coupled coils. One of the reasons for this power loss is the pattern of the magnetic field produced by the source coil used in the WPT system. Mostly the nature of magnetic field produced by the source coil is distributed radially over the coil, in which the produced magnetic field is not fully utilized. Achieving better efficiency and load current by reducing power loss is the main driving force of this work. One of the viable methods to reduce the power loss is by increasing the field intensity thereby redirecting the flux lines flow to be directional. With this aim, three coils such as solenoid, spiral and conical are designed and simulated to determine the magnetic field strength using Finite Element Method. The conical coil produces the highest self-inductance of 8.63 µH and a field strength of 1.542 Wb with the coil thickness of 3.20 mm. Then, WPT system is demonstrated with the inclusion of Maximum Power Point Tracking algorithm for improving efficiency. The schematic of flux generation of both in the transmitter and receiver sections are demonstrated and analyzed graphically. The efficiency of both simulation and experimental measurements are matched well with similar progression. The effect of parameters (angle, distance, and load resistance) on the efficiency is explored. The outcomes conclude that the inductive coupling has achieved 73% (average case) power transfer wirelessly over a distance of 5 cm with an input voltage of 5 V and 5 MHz frequency.     

Thermal performance of TF coils in HT-7U under plasma disruption

One of the most severe events for HT-7U TF coils is the plasma disruption event. Within a short period of the plasma disruption, the case and the coils will be heated up by the eddy current and coupling loss induced by the magnetic field variation. Certain amount of the deposited heat in coil cases would be conducted to the coil bundles. In this paper, the center plasma disruption is modeled and electromagnetic field is calculated by ANSYS software. The generated heat is thus directly coupled to the transient thermal analysis with the same software. It was found when the plasma current in normal condition is 1.5 MA and the disruption time constant is 3 ms, the eddy current will heat the case up to a temperature increase of about 15 K in the middle of the inner straight leg. The heat transferred from case to the cable can warm up the helium in the CIC bundle to a temperature increase of about 0.4 K 10 s after plasma disruption. The coupling loss in the coils is evaluated based on the magnetic field variation and the coupling time constant of the CICC cable. By applying GANDALF for the stability calculation, it is shown the coupling loss due to plasma disruption can reduce the temperature margin of coil by 0.63 K.     

The High Field Project at Dresden/Rossendorf: A Pulsed 100 T/10 ms Laboratory at an Infrared Free-Electron-Laser Facility

This article describes the project to build a pulsed magnetic field user laboratory at the Forschungszentrum Rossendorf near Dresden. Using a 50 MJ/24 kV capacitor bank, pulsed fields and rise times of 100 T/10 ms, 70 T/100 ms, and 60 T/1 s should be achieved. The laboratory will be built next to a free-electron-laser-facility for the middle and far infrared (5 to 150 µm, 2 ps, cw). We describe the work which has been performed until now to start the construction of the laboratory in 2003: coil concepts and computer simulations, materials development for the high field coils, and design of the capacitor bank modules. In addition, a pilot laboratory has been set up where fields up to 62 T/15 ms have been obtained with a 1 MJ/10 kV capacitor module. It is used to gain experience in the operation of such a facility and to test various parts of it. In this test laboratory special devices have been developed for measurements of magnetization and magnetoresistance, and have been successfully used to investigate various materials including semiconductors and Heavy Fermion compounds. In particular, metamagnetic transitions in intermetallic compounds and the irreversibility field of a high-T _c superconductor have been determined. Shubnikov–de Haas oscillations have been observed in the semimetallic compound CeBiPt. Resistance relaxation has been observed to start less than 1second after the field pulse. It could be shown for the first time that nuclear magnetic resonance (NMR) is detectable in pulsed fields.     

Formulas for the magnetic field of polygonal cross section currentcoils

An extension to the case of general polygonal cross section of the analytical expressions obtained previously by the author for the magnetic field of rectangular-cross-section current coils is presented. The polygonal-cross-section coils are decomposed in straight segments whose sides along the current direction are trapezoidal in general. By using surface source models, the field due to the volume current distribution is expressed in terms of that due to sources distributed over the segment side surfaces, for which simple formulas are derived. All these formulas contain elementary functions only, and numerical results, even for the special case of rectangular-cross-section coils, show a substantially higher computational efficiency than can be obtained with previous methods     

Quasi-static multi-domain inverse boundary element method for MRI coil design with minimum induced E-field

Boundary element methods represent a valuable approach for designing MRI gradient coils. The temporally varying magnetic fields produced by gradient coils induce electric currents in conducting tissues and so the exposure of human subjects to these magnetic fields has become a safety concern, especially with the increase in the strength of the field gradients used in MRI. Here we extend the boundary element method presented in Cobos Sanchez et al. (2010) [4], for the design of coils that minimize the electric field induced in prescribed multi-compartment volume conductor made of different homogeneous sub-domains. The multi-domain E-coil method is illustrated with the design of cylindrical head gradient coils, whose performance is numerically compared to that produced by conventional coils and single domain E-coils.     

Test Coils Made of Tl-1223 Tapes

Silver sheathed Tl-1223 tapes were prepared by a powder-in-tube process. The critical current density of short samples was 18 kA/cm² at 77 K. Longer tapes up to 1.2 m, prepared by sequential pressing, had a critical current density of 12 kA/cm². From these tapes we have wound two coils. A solenoid coil with 5 windings was made of 8 tapes with a total length of 4.5 m. At 77 K the critical current of the coil was 23 A in the self generated magnetic field (18 Gauss at the centre of the coil). Using an iron yoke the critical current remained at 22 A while the generated magnetic field increased to 120 Gauss. A pancake coil with 15 windings, made of 5 tapes with a total length of 5 m, generated a magnetic field of 149 Gauss at the critical current of 12 A. From measurements of the critical current density of our tapes in applied magnetic fields, we conclude that coils made of Tl-1223 tapes can be used to generate higher magnetic fields at 77 K.     

Tuning of tunnel resistance of nanogaps by field-emission-induced electromigration using current source mode

A newly investigated technique for the tuning of the tunnel resistance of nanogaps using electromigration method induced by a field emission current is presented to reduce the power consumption during the process. The method is called "activation" and is demonstrated with a current source. Planar-type initial nanogaps of Ni separated by 20-80 nm were defined on SiO2/Si substrates via electron-beam lithography and the lift-off process. Then, a bias current was applied to the initial nanogaps at room temperature, using a current source. The applied current was slowly ramped up until it reached the preset value. As a result, the process time of the current-source-based activation was 16 times shorter than activation using a voltage source. Furthermore, the tunnel resistance of the nanogaps was reduced from 100 T ohms to 70 M ohms by increasing preset current I(s) from 1 nA to 3.5 microA. Regarding the average power required for current-source-based activation, it can be successfully suppressed compared with that of voltage-source-based activation. These results imply that the current source directly and precisely tunes the field emission current passing through the nanogaps, and effectively causes the migration of atoms across the nanogaps, resulting in the successful control of the tunnel resistance of the nanogaps.     

Design of Power Supplies for the Pulsed High Magnetic Field Facility at HUST

Two types of pulsed power supply, a modular 12 MJ/25 kV capacitor bank and a 100 MVA flywheel pulsed generator, are under construction for the pulsed high magnetic field facility at the Huazhong University of Science and Technology (HUST) in Wuhan, China. The capacitor bank consists of 11 independent 1 MJ modules with a short circuit current of 40 kA each and 2 independent 0.5 MJ modules for 50 kA each. The bank is used to energize coils for magnetic fields in the 50–80 T range with pulse duration from 15 to 200 ms. The pulsed flywheel-alternator is used to energize a 50 T/100 ms long-pulse magnet via two 12-pulse power converter modules. Each converter module is designed to operate in the 95 to 66 Hz frequency operation range of the generator and can provide a no-load voltage of 4.6 kV and a full-load voltage of 3.4 kV at the rated current of 20 kA. In this paper the design of these two types of power supply is presented.     

Design of atomic mirror for silicon atoms

Abstract

Cook and Hill suggested that the gradient of an optical field can be used to reflect atoms. To reflect atoms, the repulsive dipole force is used, which comes from the interaction between the electric dipole moment of atoms and the evanescent field. The evanescent wave is generated when light is totally reflected internally at the interface of different refractive indices. Later, the way to enhance the evanescent wave with a thin dielectric waveguide has been reported. We designed the atomic mirror for silicon atoms, whose structure enhances the evanescent field that is used to repel silicon atoms. We also set up the equations of motion for silicon atoms and derive trajectories of the atoms reflected by the atomic mirror. Optical intensity, incident angle of the light, and effective detuning are described in terms of controlling the trajectory of the atom.     

场致发射体的局域功函数研究

实际场致发射体表面不可能绝对光滑 ,而具有原子尺度的微小凸起会导致发射电流大大增加。基于发射电流主要来自于原子尺度微小凸起的假设 ,提出了局域半球镜像电荷模型 ,研究了场致发射功函数的降低 ,发现微小凸起处场致发射的局域功函数会降低而场增强因子将增大。将它代入F N公式计算了一个典型的单尖Spindt阴极的发射电流 ,所得结果与实验符合。在只考虑Nottingham效应的情况下 ,计算了一些材料Spindt型阴极每微尖的最大稳定发射电流     

Experimental launcher facility - ELF-I: Design and operation

The ELF-I system has recently become fully operational and the experimental program begun. This system uses a 36 kJ, 5 kV capacitor bank as a primary pulse power source. When used in conjunction with a 5 μH pulse conditioning coil, a 100 kA peak current and ∼10 ms wide pulse is obtained. The launcher barrel is 2 m long with a 12.7 mm square bore. With the launched mass in the range of 3 to 7 g velocities in excess of 1 km/s have been obtained. An extensive array of diagnostic instruments have been installed on the system to obtain measurements of current, breech and muzzle voltage, in-bore velocity profiles, and terminal velocity. Problems associated with electromagnetic interference and high common-mode voltages have been addressed and solved. A three station, 150 kV, flash x-ray system is operational for obtaining in-bore photographs of the projectiles. Details of the mechanical and electrical design and the instrumentation system will be presented. The experimental program, now underway, is directed towards obtaining information on armature-rail interactions at sliding velocities up to 1 km/s for both metallic and plasma armatures. One particular problem being addressed is that of current transfer in the breech where the sliding velocity is very low. Experimental results and their analysis are presented.     

Trapping Fermionic 40K and Bosonic 87Rb on a Chip

We demonstrate the loading of a Bose–Fermi mixture into a microfabricated magnetic trap. In a single-chamber vacuum system, laser-cooled atoms are transported to the surface of a substrate on which gold wires have been microfabricated. The magnetic field minimum formed near these current-carrying wires is used to confine up to 6 × 10⁴ neutral ⁴⁰K atoms. In addition, we can simultaneously load 2 × 10^{5 87}Rb atoms, demonstrating the confinement of two distinct elements with such a trap. In a sequence optimized for ⁸⁷Rb alone, we observe up to 1 × 10⁷ trapped atoms. We describe in detail the experimental apparatus, and discuss prospects for evaporative cooling towards quantum degeneracy in both species.     

A target-field method to design circular biplanar coils for asymmetric shim and gradient fields

The paper presents a method for designing circular, shielded biplanar coils that can generate any desired field. A particular feature of these coils is that the target field may be located asymmetrically within the coil. A transverse component of the magnetic field produced by the coil is made to match a prescribed target field over the surfaces of two concentric spheres (the diameter of spherical volume) that define the target field location. The paper shows winding patterns and fields for several gradient and shim coils. It examines the effect that the finite coil size has on the winding patterns, using a Fourier-transform calculation for comparison.     

Development of a high harmonic gyrotron with an axis-encircling electron beam and a permanent magnet

A gyrotron with an axis-encircling electron beam is useful for high frequency operation, because the high beam efficiency is kept even at high harmonic of electron-cyclotron resonance. We have designed and constructed such a gyrotron with a permanent magnet. The gyrotron has already succeeded in operation at the third harmonic and the fourth harmonic resonances. The operation frequencies are 89.3 and 112.7 GHz, respectively. Operation cavity modes are TE₃₁₁ and TE₄₁₁. The permanent magnet system consists of many magnet elements made of NbFeB and additional coils for controlling the field intensities in cavity and electron gun regions. The magnetic field at the cavity region can be varied from 0.97 to 1.18 T. At the optimum condition of the magnetic field intensity, the output power at the third harmonic operation is 2.5 kW. The operation is pulsed, the pulse length is 1 ms and the repetition frequency is 1 Hz. The beam energy and current are 40 kV and 1.2 or 1.3 A. Starting current, beam efficiency and emission pattern also have been measured. In this paper, the operation results of the gyrotron and comparison with the computer simulation results are described.     

Superconducting magnet for K-500 cyclotron at VECC, Kolkata

K-500 superconducting cyclotron is in the advanced stage of commissioning at VECC, Kolkata. Superconducting magnet is one of the major and critical component of the cyclotron. It has been successfully fabricated, installed, cooled down to 4.2 K by interfacing with LHe plant and energized to its rated current on 30th April, 2005 producing magnetic field of 4.8 T at median plane of cyclotron. The superconducting magnet (stored energy of 22MJ) consists of two coils (α and β), which were wound on a sophisticated coil winding machine set-up at VECC. The superconducting cable used for winding the coils is multi filamentary composite superconducting wire (1.29 mm diameter) having 500 filaments of 40 μm diameter Nb-Ti in copper matrix which is embedded in OFHC grade copper channel (2.794 mm × 4.978 mm) for cryogenic stability. The basic structure of coil consists of layer type helical winding on a SS bobbin of 1475 mm ID × 1930 mm OD × 1170 mm height. The bobbin was afterwards closed by SS sheet to form the LHe chamber. The total weight of the coil with bobbin was about 6 tonne and the total length of the superconducting cable wound was about 35 km. Winding was done at very high tension (2000 PSI) and close tolerance to restrict the movement of conductor and coil during energization. After coil winding, all four coils (two each on upper and lower half of median plane of cyclotron) were banded by aluminium strip (2.7 mm × 5 mm) at higher tension (20,000 PSI) to give more compressive force after cool down to 4.2 K for restricting the movement of coil while energizing and thereby eliminating the chances of quench during ramping of current.After completion of coil winding by October, 2003, cryostat assembly was taken up in house. The assembly of cryostat (13 tonne) with support links (9 Nos.) refrigeration port, instrumentation port, helium vapour cooled current loads, etc. was completed by June, 2004. Meanwhile assembly of magnet frame was taken up and the cryostat was positioned in the magnet frame with proper alignment by August, 2004. After installation of cryostat on magnet, the cryostat was connected to the helium refrigerator/liquefier, having refrigeration capacity of 200 W and 100 l/h in liquefier mode with LN2 pre-cooling. The cryogenic delivery system supplying the liquid helium and liquid nitrogen to the superconducting magnet was successfully commissioned in November, 2004. The cool down of the cryostat to 10 K took around 8 days following which the LHe was filled in the cryostat (300 l) on 15th January, 2005. Subsequently the superconducting coils (α and β) were energized by two DC current regulated power supplies (20 V, 1000 A, 10 ppm stability) with slow and fast dump resistors connected externally across the superconducting coils for protection of coils at the time of power failure and quench.The paper describes the intricacies involved in coil winding, winding set-up, assembly of cryostat, cooling down the superconducting coils, filling by LHe and energization to rated current. The paper also highlights the operating experience of superconducting magnet and related test results.     

Scattering of an induced surface electromagnetic field by fatiguecracks in ferromagnetic metals

In crack detection and sizing by the alternating current field measurement technique, U-shaped wires or coils excited by a high-frequency AC current source can be used to induce the surface field in the workpiece. The authors present a modeling technique for the interaction of a fatigue crack in a ferromagnetic metal with the surface field resulting from an inducer with two U-shaped wires. This work is an extension of a previous modeling technique to have developed for infinitely long (one-dimensional) cracks. In the present technique, the boundary of the fatigue crack is approximated by a circular arc, leading to a formulation for an efficient computation of the field-flaw interaction. Various numerical and experimental results supporting the modeling and illustrating the behavior of the magnetic field and electric potential at the metal surface around circular-arc cracks are presented     

Design of atomic mirror for silicon atoms

Cook and Hill suggested that the gradient of an optical field can be used to reflect atoms. To reflect atoms, the repulsive dipole force is used, which comes from the interaction between the electric dipole moment of atoms and the evanescent field. The evanescent wave is generated when light is totally reflected internally at the interface of different refractive indices. Later, the way to enhance the evanescent wave with a thin dielectric waveguide has been reported. We designed the atomic mirror for silicon atoms, whose structure enhances the evanescent field that is used to repel silicon atoms. We also set up the equations of motion for silicon atoms and derive trajectories of the atoms reflected by the atomic mirror. Optical intensity, incident angle of the light, and effective detuning are described in terms of controlling the trajectory of the atom.