A compact permanent magnet array with a remote homogeneous field

We present the design and construction of a single sided magnet array generating a homogeneous field in a remote volume. The compact array measures 11.5 cm by 10 cm by 6 cm and weights approximately 5 kg. It produces a B(0) field with a 'sweet spot' at a point 1cm above its surface, where its first and second spatial derivatives are approximately zero. Unlike other sweet spot magnets of this general type, our array has B(0) oriented parallel to its surface. This allows an ordinary surface coil to be used for unilateral measurements, giving the potential for dramatic SNR improvement.     

A mobile one-sided NMR sensor with a homogeneous magnetic field: the NMR-MOLE

A new portable NMR sensor with a novel one-sided access magnet design, termed NMR-MOLE (MObile Lateral Explorer), has been characterised in terms of sensitivity and depth penetration. The magnet has been designed to be portable and create a volume with a relatively homogeneous magnetic field, 15,000 ppm over a region from 4 to 16 mm away from the probe, with maximum sensitivity at a depth of 10 mm. The proton NMR frequency is 3.3 MHz. We have demonstrated that with this approach a highly sensitive, portable, unilateral NMR sensor can be built. Such a design is especially suited for the characterisation of liquids in situations where unilateral or portable access is required.     

用于长城等古建筑探测的NMR探测器的磁体设计

以万里长城为代表的古建筑是世界瑰宝,更是中华民族的象征和骄傲.本文提出利用便携式核磁共振（NMR）装置来探测研究这类古建筑的建筑材料,在不对其造成损伤的基础上,发掘其隐含的科学、技术和工程相关的丰富信息.为此,作为第一步,设计了适合于探测这类古建筑的便携式单边NMR探测器组合式磁体.该探测器的磁体结构以semi-Halbach为基础,通过不同磁体模块间的组合得到对应移动探测模式、长距离探测模式和均匀磁场探测模式的磁体结构.随后根据优化结果,设计加工了磁体组件,并采用该磁体进行了流体、长城城砖和现代红砖的NMR实验,实测结果与模拟一致.该组合式磁体的优点在于通过不同磁体模块组合,实现了多种探测方式,适用于探测长城等这类古建筑物需要多种探测模式的科学研究.     

A unilateral NMR magnet for sub-structure analysis in the built environment: the Surface GARField

A new, portable NMR magnet with a tailored magnetic field profile and a complementary radio frequency sensor have been designed and constructed for the purpose of probing in situ the sub-surface porosity of cement based materials in the built environment. The magnet is a one sided device akin to a large NMR-MOUSE with the additional design specification of planes of constant field strength /B0/ parallel to the surface. There is a strong gradient G in the field strength perpendicular to these planes. As with earlier GARField magnets, the ratio G//:B0/ is a system constant although the method of achieving this condition is substantially different. The new magnet as constructed is able to detect signals 50mm (1H NMR at 3.2 MHz) away from the surface of the magnet and can profile the surface layers of large samples to a depth of 35-40 mm by moving the magnet, and hence the resonant plane of the polarising field, relative to the sample surface. The matching radio frequency excitation/detector coil has been designed to complement the static magnetic field such that the polarising B0 and sensing B1 fields are, in principal, everywhere orthogonal. Preliminary spatially resolved measurements are presented of cement based materials, including two-dimensional T1-T2 relaxation correlation spectra.     

便携式岩样核磁共振分析仪磁体的设计

核磁共振中的磁体设计有许多方案，但是要使磁体体积小、重量轻、均匀区大、稳定性好，将面临一系列的问题. 而这些问题的解决主要依赖于电磁场计算. 文中首先阐述这些问题，然后对方案进行优化分析计算，从中找出一个最佳的解决方案. 采用本方案研制出来磁体，我们研制成功了便携式核磁共振岩样分析仪. 经过多次的现场实验测试表明由该磁体构成的磁共振岩样分析仪的稳定性高、重复性好、满足现场恶劣的环境要求.     

单边磁共振成像仪磁体系统研究

单边磁共振成像（MRI）系统由于其良好的开放性引起了越来越多的关注,这种系统的成像样品区位于磁体外侧,其形状为薄片形,其磁体结构设计是一个全新的问题. 本文基于鞍点理论,探索这种单边磁体在其外侧产生均匀样品区的可行性,通过对几种单边磁体结构磁场特性的研究,指出适当设计的永磁磁体能够在磁体外侧产生鞍点. 最后给出一种符合成像要求的单边永磁MRI磁体结构. 单边MRI磁体结构的可行性分析及磁体结构的成功设计,为开展完全开放式磁共振成像装置的研究打下了坚实的基础.     

Velocity distributions remotely measured with a single-sided NMR sensor

The pulsed field gradient nuclear magnetic resonance (PFG NMR) method has proved to be a powerful non-invasive technique to measure molecular displacement in various systems. It has been largely implemented with conventional NMR magnets where the volume for housing the flow setup is restricted. In this work we present the first approach to measure velocity distributions ex situ implementing a pulsed field gradient sequence on a single-sided NMR sensor. The open geometry of these sensors provides access to NMR measurements of a large number of applications previously excluded by the geometry of conventional closed magnets. Both, the distortions to the displacement encoding observed when implementing a PFG sequence in the presence of strongly inhomogeneous B0 and B1 fields, and the performance of the modifications proposed to eliminate these distortions are shown by means of numerical simulations. An alternating stimulated spin-echo PFG sequence implemented to remotely measure velocity distributions was combined with a multi-echo acquisition scheme to significantly increase the sensitivity of the method. The technique was implemented to measure the velocity propagator in a fluid undergoing laminar flow and good agreement with the theoretical result is observed.     

An Efficacious Target-Field Approach to Design Shim Coils for Halbach Magnet of Mobile NMR Sensors

The main magnetic fields of mobile nuclear magnetic resonance (NMR) magnets differ from those of conventional NMR and magnetic resonance imaging (MRI) magnets. In the Halbach magnet, the main field B ₀ is perpendicular to the longitudinal axis, the symmetry of the current distribution with respect to the symmetry of the magnetic field differs from that in conventional target-field applications, and the current distribution on the coil surface cannot be expressed in terms of periodic basis functions. To obtain the winding pattern of the coil, an efficacious target-field approach. The surface of a coil is divided into small discrete elements, where each element is represented by a magnetic dipole. From the stream function of the elements, the resultant magnetic field is calculated. The optimization strategy follows an objective function defined by the power dissipation or efficiency of the coil. This leads to the optimum stream function on the coil surface, whose contour lines define the winding patterns of the coil. This paper shows winding patterns designed of shim coils for Halbach magnet and illustrates the craft of a shim coil using flexible printed circuit board. The performance of the coils is verified by simulating the fields they produce over the sensitive volume.     

An open magnet utilizing ferro-refraction current magnification

Ferro-refraction is the field magnification that is obtained when a current segment is near a high magnetic permeable boundary. It is shown that ferro-refraction may be used in the design of magnets for NMR or MRI to increase the efficiency of these magnets. The field may be modeled analytically with the Biot--Savart law and the inclusion of mirror image currents. Ferro-refraction is particularly useful in the design of monohedral magnets, magnets producing a remote homogeneous region which have the magnetic sources arranged to one side. These magnets have also been called planar magnets. Two designs for a monohedral magnet which produce good agreement between experimental and analytic results are presented.     

Single-sided sensor for high-resolution NMR spectroscopy

The unavoidable spatial inhomogeneity of the static magnetic field generated by open sensors has precluded their use for high-resolution NMR spectroscopy. In fact, this application was deemed impossible because these field variations are usually orders of magnitude larger than those created by the microscopic structure of the molecules to be detected. Recently, chemical shift resolved NMR spectra were observed for the first time outside a portable single-sided magnet by implementing a method that exploits inhomogeneities in the rf field designed to reproduce variations of the static magnetic field. In this communication, we describe in detail the magnet system built from permanent magnets as well as the rf coil geometry used to compensate the static field variations.     

A simple, small and low cost permanent magnet design to produce homogeneous magnetic fields

A new portable, pocket-size NMR probe based on a novel permanent magnet arrangement is presented. It is based on a Halbach-type magnet design which mimics the field of a spherical dipole by using cylindrical bar and ring magnets. The magnet system is made up of only three individual magnets, and most field calculations and optimisations can be performed analytically. A prototype system has been built using a set of small, off the shelf commercially available permanent magnets. Proton linewidths of 50 ppm FWHM could be achieved at a field strength of 1T. Calculations show that with custom-sized permanent magnets, linewidths of less than 1 ppm can be achieved over sample volumes of up to 1 mm3, which would in theory enable chemical shift resolved proton spectroscopy on mass-limited samples. But even with the achieved linewidth of 50 ppm, this can be a useful portable sensor for small amounts of liquid samples with restricted molecular mobility, like gels, polymers or high viscosity liquids.     

A portable Halbach magnet that can be opened and closed without force: the NMR-CUFF

Portable equipment for nuclear magnetic resonance (NMR) is becoming increasingly attractive for use in a variety of applications. One of the main scientific challenges in making NMR portable is the design of light-weight magnets that possess a strong and homogeneous field. Existing NMR magnets can provide such magnetic fields, but only for small samples or in small regions, or are rather heavy. Here we show a simple yet elegant concept for a Halbach-type permanent magnet ring, which can be opened and closed with minimal mechanical force. An analytical solution for an ideal Halbach magnet shows that the magnetic forces cancel if the structure is opened at an angle of 35.3° relative to its poles. A first prototype weighed only 3.1 kg, and provided a flux density of 0.57 T with a homogeneity better than 200 ppm over a spherical volume of 5mm in diameter without shimming. The force needed to close it was found to be about 20 N. As a demonstration, intact plants were imaged and water (xylem) flow measured. Magnets of this type (NMR-CUFF = Cut-open, Uniform, Force Free) are ideal for portable use and are eminently suited to investigate small or slender objects that are part of a larger or immobile whole, such as branches on a tree, growing fruit on a plant, or non-metallic tubing in industrial installations. This new concept in permanent-magnet design enables the construction of openable, yet strong and homogeneous magnets, which aside from use in NMR or MRI could also be of interest for applications in accelerators, motors, or magnetic bearings.     

轴对称高场超导磁体电磁应力有限元分析方法

核磁共振成像和核磁共振谱仪是高场超导磁体的主要应用领域.高场超导磁体通常具有较高的磁场和运行电流,在运行过程中超导线会产生较高的电磁应力,其临界特性将发生退化,影响磁体的稳定性.开展高场超导磁体的电磁应力精确分析显得尤为必要.本文发展了一种快速有效的有限元分析方法,第一步,为整个超导磁体系统建立平均有限元模型,采用传统的电磁-结构耦合方法求解电磁应力,获得最大应力位置;第二步,对最大应力所在的超导线圈建立详细有限元模型,采用单积分-结构分析方法精确求解每一组分中电磁应力.基于该模型研究了500 MHz NMR超导磁体的电磁应力.该分析方法也可以用于超导磁体冷却过程中的热应力分析.为高场超导磁体设计和建造提供有益的理论依据.     

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)     

CSNS/RCS引出系统快脉冲冲击磁铁的磁场仿真和结构设计

中国散裂中子源快循环同步加速器引出系统快脉冲冲击磁铁,由分组安装于三个真空箱内的8台磁铁组成。对冲击磁铁进行磁场仿真和结构设计,并使用仿真软件进行优化。仿真结果表明:磁铁中心场60%宽度内的磁场均匀性达到0.7%,满足物理设计要求。同时,对两台磁铁间距大小与互感的关系进行模拟计算和分析。磁铁结构设计中,针对原样机中出现的铁芯不能可靠固定问题,进行修改和完善,并且设计出了一种简单实用的多螺栓滑动支撑结构,可使多台磁铁平稳推入真空箱内,同时完成多台磁铁的准直和固定。     

Permanent magnet modelling for dynamic applications

Rare earth permanent magnets are very sensitive to temperature as an overheating can lead to a partial or a total demagnetization. It is then necessary to determine the losses in the permanent magnets to design with accuracy an electromagnetic system. In fact, if the permanent magnet experiences a high frequency magnetic field, significant losses appear. In the literature, these losses are calculated assuming the permanent magnet to be a bulk conductive material. In this work, we aim at verifying this assumption and we give a method to determine the equivalent conductivity. Then, we present a comparison between calculated and measured characteristics of two permanent magnet samples.     

Permanent magnet applications

Rare-earth permanent magnets are ideally suited to generate magnetic fields comparable to their spontaneous polarization J_S. Near-square hysteresis loops and large values of the coercivity and anisotropy fields greatly simplify magnet design, as each magnet block is effectively transparent to the magnetic fields produced elsewhere in the magnet assembly. The fields generated by compact and efficient magnet structures requiring no continuous expenditure of energy can be static or variable, uniform or nonuniform. Permanent magnets are fully competitive with electromagnets for fields up to 2 T, and fields as high as to 5 T can be produced in a small volume. When a field with a rapid spatial variation is required, permanent magnets may offer the only practicable solution. Both permanent magnet structures and the uses to which they are put are reviewed, classifying the magnet applications in terms of the nature of the field, the effect on the magnet and the physical effect exploited.     

Profiles with microscopic resolution by single-sided NMR

A single-sided NMR sensor to produce depth profiles with microscopic spatial resolution is presented. It uses a novel permanent magnet geometry that generates a highly flat sensitive volume parallel to the scanner surface. By repositioning the sensitive slice across the object one-dimensional profiles of the sample structure can be produced with a space resolution better than 5 microm. The open geometry of the sensor results in a powerful testing tool to characterize arbitrarily sized objects in a non-destructive way.     

Device for aeration and mixing of cell and organelle suspensions during NMR experiments

A device for aeration and mixing of cell or organelle suspensions in a vertical bore NMR magnet is described. Multiple external sensors (e.g., ion-selective electrodes) may be immersed in the suspension within the bore of the magnet. The sensors are positioned to avoid noise due to contact with gas bubbles and proximity to the probe head. The required sample volume is minimised. The modular design of components permits the use of the device in magnets of various internal dimensions, or with probe heads of different sample tube diameter, by modification of the simpler components of the assembly.     

Magnet design with high B(0) homogeneity for fast-field-cycling NMR applications

The design, construction, and performance of a low-inductance solenoidal coil with high B(0) homogeneity for fast-field-cycling NMR is presented. It consists of six concentric layers. The conductor width is varied to minimize the B(0) inhomogeneity in the volume of the sample. This is done using an algorithm which takes the real shape of the conductor directly into account. The calculated coil geometry can be manufactured easily using standard computerized numeric control equipment, which keeps the costs low. The coil is liquid cooled and produces a B(0) field of 0.95 T at 800 A. The field inhomogeneity in a cylindrical volume (diameter 5 mm, length 10 mm) is about 10 ppm, and the inductance is 190 microH. Switching times below 200 micros can be achieved. During 6 months of operation the coil has shown good stability and reliability.