Maximum Field Gradient Research Articles

Identification of structural defects and their impact on the magnetic memory, static and cyclic strength of VNS9-SH thin sheet trip-steel

Identification of microstructure defects, which are stress concentrators (SC) during the operation of mechanical engineering products, is an important scientific and practical task relevant for manufacturing enterprises. This problem becomes especially urgent and difficult for critical helicopter parts made of sheet TRIP steel VNS9-Sh (23Kh15N5AM3-Sh) and operating under cyclic loads due to the complex microstructure of the steel and small thickness of strips and sheets. To assess the impact of structural defects on the cyclic strength of products made of the steel under study, the specimens were preliminary sorted proceeding from the results of their testing using the method of metal magnetic memory (MMM) and metallographic studies. The MMM method is a structure-sensitive procedure which provides information about the presence of structural defects that arise during the manufacture. Magnetic anomalies in the form of sharp local changes in the intrinsic stray magnetic field (SSMF) (H) and its gradient |ΔH| along the length of the controlled section Δx, were identified on the surface of sheets cut from five different batches. A conventional classification of the identified anomalies was made according to the magnitude of the magnetic field gradient. The specimens of two types were cut in zones of magnetic anomalies and outside them: type 1 — for cyclic tests and type 2 — for metallographic studies. The geometric parameters and field gradient values of magnetic anomalies on specimens of type 1 and type 2 were the same. Metallographic studies in zones of maximum magnetic field gradient on type 2 specimens revealed defects in the form of a strip at the boundary of different structures, which is a structural stress concentrator (SSC) and a source of the inhomogeneity and changes in the magnetic properties. Type 1 specimens with similar magnetic anomalies and Type 1 specimens cut from sheets outside zones of magnetic anomalies were then selected for cyclic testing. Comparative tests for cyclic strength of the specimens with and without specified SSC were carried out. It is shown that the presence of SSC zones in the specimens reduces the number of cycles to failure during cyclic tests by 1 – 2 orders of magnitude compared to the specimens free of SSC. Based on cyclic tensile tests of specimens, a limiting value of the magnetic field gradient was determined that corresponds to the acceptable level of stress concentration on structural defects. This value is recommended for use as a rejection criterion when examining a new tape by the MMM method.

Safety and function of programmable ventriculo-peritoneal shunt valves: An in vitro 7 Tesla magnetic resonance imaging study.

The quantity of ultra-high field MRI neuroimaging studies has rapidly increased. This study tests function, safety, and image artifacts of two frequently implanted programmable ventriculo-peritoneal (VP) shunt valves in a 7T MRI system. All tests were performed using a whole-body 7T MRI system. Three proGAV 2.0 and 3 CODMAN CERTAS® Plus programmable VP-shunt valves were tested in three steps. 1) Deflection angle tests close to the bore opening at the location of a static magnetic field gradient of 3-5 T/m. 2) Valves were fixed on a spherical phantom in 3 positions (a. lateral, b. cranial, c. cranial with 22.5° tilt anteriorly) and assessed for keeping the programmed pressure setting and reprogrammability. 3) Valves were fixed on the phantom and positioned lateral in a radiofrequency head coil. MRI scans were performed for both models, including MPRAGE, GRE and SE sequences. Deflection angles were moderate (13°, 14°, 13°) for the proGAV valves and close to critical (43°, 43°, 41°) for the CODMAN valves at the test location. Taking a scaling factor of 2-3 for the maximum spatial magnetic field gradient accessible to a patient within the magnet bore into account renders both valves MR unsafe regarding ferromagnetic attraction. The proGAV valves kept the pressure settings in all positions and were reprogrammable in positions a. and b. In position c., reprogrammability was lost. The CODMAN valves changed their pressure setting and reprogrammability was lost in all positions. MR image signal homogeneity was unaltered in the phantom center, artifacts limit the assessability of structures in close vicinity to the valves. Both tested programmable VP-shunt valves are MR unsafe for 7T systems. Novel programming mechanisms using permanent magnets with sufficient magnetic coercivity or magnet-free mechanisms may allow the development of programmable VP-shunt valves that are conditional for 7T MR systems.

Micromagnet design for addressable fast spin manipulations in a 2 × 2 quantum dot array

The scaling up of semiconductor quantum dots to two-dimensional arrays is attracting considerable interest for use in large-scale spin-based quantum computation. One of the fundamental technologies to be realized in the two-dimensional arrays of quantum dots is the coherent manipulations of individual electron spins. In this work, we report on a micromagnet design that generates stray magnetic fields that can be used for spin manipulations via electric dipole spin resonance in a 2 × 2 quantum dot array. We consider a micromagnet with lower symmetry than that typically used in the case of linear dot arrays and optimize its dimensions to produce a maximum stray field gradient while maintaining Zeeman splitting differences sufficiently large among the four dots to permit qubit addressability. The optimized field gradient is around 60% smaller than that used in linear dot arrays, but it is still large enough for spin manipulation experiments. Our result represents an important step toward the experimental realization of spin-qubit operations in two-dimensional arrays of quantum dots.

Freeform shape optimization of a compact dc photoelectron gun using isogeometric analysis

Compact DC high-voltage photo-electron guns are able to meet the sophisticated demands of high-current applications such as energy recovery linacs. A main design parameter for such sources is the electric field strength, which depends on the electrode geometry and is limited by the field emission threshold of the electrode material. In order to minimize the maximum field strength for optimal gun operation, isogeometric analysis (IGA) can be used to exploit the axisymmetric geometry and describe its cross section by non-uniform rational B-splines, the control points of which are the parameters to be optimized. This computationally efficient method is capable of describing CAD-generated geometries using open source software (GeoPDEs, NLopt, Octave) and it can simplify the step from design to simulation. We will present the mathematical formulation, the software workflow, and the results of an IGA-based shape optimization for a planned high-voltage upgrade of the DC photogun teststand Photo-CATCH at TU Darmstadt. The software builds on a general framework for isogeometric analysis and allows for easy adaptations to other geometries or quantities of interest. Simulations assuming a bias voltage of -300 kV yielded maximum field gradients of 9.06 MV/m on the surface of an inverted insulator electrode and below 3 MV/m on the surface of the photocathode.

Single-spin scanning magnetic microscopy with radial basis function reconstruction algorithm

Exotic magnetic structures, such as magnetic skyrmions and domain walls, are becoming more important in nitrogen-vacancy center scanning magnetometry. However, a systematic imaging approach to mapping stray fields with fluctuations of several milliteslas generated by such structures is not yet available. Here, we present a scheme to image a millitesla magnetic field by tracking the magnetic resonance frequency, which can record multiple contour lines for a magnetic field. The radial basis function algorithm is employed to reconstruct the magnetic field from the contour lines. Simulations with shot noise quantitatively confirm the high quality of the reconstruction algorithm. The method was validated by imaging the stray field of a frustrated magnet. Our scheme had a maximum detectable magnetic field gradient of 0.86 mT per pixel, which enables the efficient imaging of millitesla magnetic fields.

Write head design for curvature reduction in heat-assisted magnetic recording by topology optimization

The reduction of the transition curvature of written bits in heat-assisted magnetic recording is expected to play an important role for the future areal density increase of hard disk drives. Recently, a write head design with flipped write and return poles was proposed. In this design, a large spatial field gradient of the write head was the key to significantly reduce the transition curvature. In this work, we optimized the write pole of a heat-assisted magnetic recording head in order to produce large field gradients as well as large fields in the region of the heat pulse. This is done by topology optimization. The simulations are performed with dolfin-adjoint. For the maximum field gradients of 8.1 mT/nm, 8.6 mT/nm, and 11.8 mT/nm, locally resolved footprints of an FePt-like hard magnetic recording medium are computed with a coarse-grained Landau-Lifshitz-Bloch model, and the resulting transition curvature is analyzed. Additional simulations with a bilayer structure with a 50% hard and 50% soft magnetic material are computed. The results show, that for both recording media, the optimized head design does not lead to any significant improvements in the written track. Thus, we analyze the transition curvature for the optimized write heads theoretically with an effective recording time window model. Moreover, we check how higher field gradients influence the curvature reduction. The results show that a simple optimization of the conventional head design is not sufficient for effective curvature reduction. Instead, new head concepts will be needed to reduce the transition curvature.

Using an alpha magnet to compress the electron bunches from a photocathode RF gun

We report here the design, simulation, and characterization of a compact electron injector for the formation of high-current, femtosecond electron bunches from a photocathode RF gun without mandatory use of ultra-short laser pulses and a high power RF supply. The proposed injector consists of a 1.6-cell S-band photocathode RF gun to produce the electron beam and a small bunch compressor to increase the peak current. The RF gun is designed to operate at a 65 MV/m accelerating field gradient, and the bunch compressor is a small α-magnet with a maximum magnetic field gradient of 340 G/cm. The output beam from the gun has 100 pC charge per-bunch that is equivalent to a beam current of ∼15 A, and output beam from the injector has an RMS bunch length of ∼ 439 fs, a peak current of 0∼ 12 A and an RMS normalized horizontal emittance of 1.21 mm-mrad.

Yb− and 236UF5−—Two case studies of E/q and EM/q2 interferences in AMS

The Lalonde 3MV-AMS system uses one large injection magnet and one large high energy magnet, providing both sufficient bending power and resolution in both the low and high energy mass and charge systems for all heavy elements. This allows two demanding AMS subjects to be investigated conclusively: one is the search for the elusive atomic negative ion Yb−, and the other is the use of 236UF5− to achieve sub-10−13 abundance sensitivities with a single analyzing magnet in the high energy system. Both studies require the AMS system to be capable of sufficiently removing intense E/q and EM/q2 interferences to the rare signals. In the case of the Yb− search, the complication caused by the fragmenting ytterbium hydride beams must be well resolved; in the case of 236U detection, its relative mass difference from the abundant 235,238U is among the smallest of all cases in AMS. In carrying out these investigations, a generalized method of analyzing the potential E/q and EM/q2 interferences in AMS is illustrated. The conclusions are: (1) With 12 kV ion source extraction voltage and 0.5 MV accelerator terminal voltage, which correspond to a maximum electric field gradient of 2.8 kV/cm along the ion trajectory from the sputter target surface to the entrance of the accelerator terminal, Yb− has not yet been observed by AMS using a Cs+ sputter ion source. (2) A best-possible abundance sensitivity of 236U/U ∼ 4 × 10−14 has been indicated using UF5− with just one large magnet in the high energy system.

An analytical approach for beam loading compensation and excitation of maximum cavity field gradient in a coupled cavity-waveguide system

The critical process of beam loading compensation in high intensity accelerators brings under control the undesired effect of the beam induced fields to the accelerating structures. A new analytical approach for optimizing standing wave accelerating structures is found which is hugely fast and agrees very well with simulations. A perturbative analysis of cavity and waveguide excitation based on the Bethe theorem and normal mode expansion is developed to compensate the beam loading effect and excite the maximum field gradient in the cavity. The method provides the optimum values for the coupling factor and the cavity detuning. While the approach is very accurate and agrees well with simulation software, it massively shortens the calculation time compared with the simulation software.

Electrodeless dielectrophoresis: Impact of geometry and material on obstacle polarization.

Insulator-based (electrodeless) dielectrophoresis (iDEP) is a promising particle manipulation technique, based on movement of matter in inhomogeneous fields. The inhomogeneity of the field arises because the excitatory field distorts at obstacles (posts). This effect is caused by accumulation of polarization charges at material interfaces. In this study, we utilize a multipole expansion method to investigate the influence of geometry and material on field distortion of posts with arbitrary cross-sections in homogeneous electric fields applied perpendicular to the longitudinal axis of the post. The post then develops a multipole parallel or anti parallel to the excitatory field. The multipoles intensity is defined by the post's structure and material properties and directly influences the DEP particle trapping potential. We analyzed posts with circular and rhombus-shaped cross-sections with different cross-sectional width-to-height ratios and permittivities for their polarization intensity, multipole position, and their particle trapping behavior. A trade-off between high maximum field gradient and high coverage range of the gradient is presented, which is determined by the sharpness of the post's edges. We contribute to the overall understanding of the post polarization mechanism and expect that the results presented will help optimizing the structure of microchannels with arrays of posts for electrodeless DEP application.

Determining the Anisotropy of Bit-Patterned Media for Optimal Performance

In conventional recording theory the transition width should be minimised by writing the transition at the location of the maximum head-field gradient, and this should be achieved by setting the coercivity equal to the head field at the location of maximum head field gradient (taking appropriate account of the variation of coercivity with field angle). In BPM it would therefore be natural to assume that the system could be optimised by setting the island switching field in a similar manner. In this paper we investigate what strategy of optimisation gives the minimum error rate, or maximum write-window, and whether different designs are more or less tolerant of anisotropy variations.

Simultaneous iterative reconstruction technique software for spectral–spatial EPR imaging

Continuous wave electron paramagnetic resonance imaging (EPRI) experiments often suffer from low signal to noise ratio. The increase in spectrometer time required to acquire data of sufficient quality to allow further analysis can be counteracted in part by more processing effort during the image reconstruction step. We suggest a simultaneous iterative reconstruction algorithm (SIRT) for reconstruction of continuous wave EPRI experimental data as an alternative to the widely applied filtered back projection algorithm (FBP). We show experimental and numerical test data of 2d spatial images and spectral–spatial images. We find that for low signal to noise ratio and spectral–spatial images that are limited by the maximum magnetic field gradient strength SIRT is more suitable than FBP.

Beam Heat Loads to Superconducting Quadrupoles for BigRIPS In-Flight Separator at RIKEN

The BigRIPS in-flight fragment separator in the RIKEN Radioactive Isotope Beam Factory (RIBF) project was commissioned in 2007 and has been operated for six years. The RIBF accelerator complex has provided a wide range of heavy ion beams from deuteron to uranium with the energy of 250-345 MeV/nucleon. More than 70 experiments of unstable nuclei have been performed using the RI beams produced by the BigRIPS fragment separator. The production target of the RI beams is in close proximity to the first superconducting triplet quadrupoles (STQs) in the BigRIPS fragment separator, so that the STQ is exposed to the high radiation, which causes “beam heat loads” to the cryogenic system. The STQ consists of three air-core NbTi coils with the maximum field gradients of 24 T/m, assembled in the liquid-helium bath cryostat with the warm bore radius of 90 mm. This paper presents measurements of the radiation heat loads to the STQ cryostat for a number of experienced heavy ion beams from oxygen to uranium with the beam power of 1-7 kW. Corresponding radiation transport calculations are presented, and radiation doses and damages to the STQ system are also discussed.

Characteristics of Spontaneous Magnetic Field of Artificial Crack Induced by Complex Stress in Large Components

In this study, the regularity of spontaneous magnetic field of artificial cracks in a seamless gas cylinder was investigated by metal magnetic memory technique during fatigue testing. The results showed that the regularity of magnetic signals had a strong relation with the degree of stress concentration around crack under complex stress. Moreover, the characteristic of normal component of spontaneous magnetic field Hp (y) and maximum magnetic field gradient, Kmax, were investigated when measured perpendicular as well as parallel to the crack. It is more difficult to quantitative the crack by Hp(y) signals using magnetic memory technique.

Quasi-monoenergetic and tunable X-rays from a laser-driven Compton light source

The maximum achievable photon energy of compact, conventional, Compton-scattering X-ray sources is currently limited by the maximum permissible field gradient of conventional electron accelerators1,2. An alternative compact Compton X-ray source architecture with no such limitation is based instead on a high-field-gradient laser–wakefield accelerator3,4,5,6. In this case, a single high-power (100 TW) laser system generates intense laser pulses, which are used for both electron acceleration and scattering. Although such all-laser-based sources have been demonstrated to be bright and energetic in proof-of-principle experiments7,8,9,10, to date they have lacked several important distinguishing characteristics of conventional Compton sources. We now report the experimental demonstration of all-laser-driven Compton X-rays that are both quasi-monoenergetic (∼50% full-width at half-maximum) and tunable (∼70 keV to >1 MeV). These performance improvements are highly beneficial for several important X-ray radiological applications2,11,12,13,14,15.

A linear helicon plasma device with controllable magnetic field gradient

Current free double layers (CFDLs) are localized potential structures having spatial dimensions - Debye lengths and potential drops of more than local electron temperature across them. CFDLs do not need a current for them to be sustained and hence they differ from the current driven double layers. Helicon antenna produced plasmas in an expanded chamber along with an expanding magnetic field have shown the existence of CFDL near the expansion region. A helicon plasma device has been designed, fabricated, and installed in the Institute for Plasma Research, India to study the role of maximum magnetic field gradient as well as its location with respect to the geometrical expansion region of the chamber in CFDL formation. The special feature of this machine consisting of two chambers of different radii is its capability of producing different magnetic field gradients near the physical boundary between the two chambers either by changing current in one particular coil in the direction opposite to that in other coils and/or by varying the position of this particular coil. Although, the machine is primarily designed for CFDL experiments, it is also capable of carrying out many basic plasma physics experiments such as wave propagation, wave coupling, and plasma instabilities in a varying magnetic field topology. In this paper, we will present the details of the machine construction, its specialties, and some preliminary results about the production and characterization of helicon plasma in this machine.

Micromagnetic specifications for recording self-assembled bit-patterned media

A design including exchange coupled composite (ECC) bits in hexagonal arrays and a multi-pole write head is proposed to meet the requirement of 9.7 × 10−4 bit error rate, 4 nm magnetic fly height, 5% switching field distribution, 5% synchronization error, and 5% jitter error to achieve 2.9 Tbits/in.2 bit-patterned recording. Our head design writes two staggered tracks in a single pass and has maximum perpendicular field gradients of 580 Oe/nm along the down-track direction and 476 Oe/nm along the cross-track direction.

Photoresponse enhancement by anisotropic shape of a microscale freestanding solid-state bridge of a polythiophene-based polymer

This study shows another way to enhance the photoresponse of a conjugated polymer. This is confirmed by the wavelength and voltage-dependent photocurrent study of an anisotropic freestanding polymeric solid-state bridge, which was made of sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate] between aluminum and copper wire electrodes. The field gradient induced by the anisotropic structure of the microscale polymer bridge enhanced the photoresponse by driving the photogenerated excitons toward the maximum field gradient region of the conical polymer bridge. The relatively high electric field strength at the narrow region efficiently broke excitons into electrons and holes. As a result, the photoresponse was enhanced.

Application of empirical mode decomposition in early diagnosis of magnetic memory signal

In order to eliminate noise interference of metal magnetic memory signal in early diagnosis of stress concentration zones and metal defects, the empirical mode decomposition method combined with the magnetic field gradient characteristic was proposed. A compressive force periodically acting upon a casing pipe led to appreciable deformation, and magnetic signals were measured by a magnetic indicator TSC-1M-4. The raw magnetic memory signal was first decomposed into different intrinsic mode functions and a residue, and the magnetic field gradient distribution of the subsequent reconstructed signal was obtained. The experimental results show that the gradient around 350 mm represents the maximum value ignoring the marginal effect, and there is a good correlation between the real maximum field gradient and the stress concentration zone. The wavelet transform associated with envelop analysis also exhibits this gradient characteristic, indicating that the proposed method is effective for early identifying critical zones.

Full Size Model Magnets for the FAIR SIS100 Synchrotron

The planned FAIR synchrotron SIS100 has to deliver high intensity beams. Following the JINR Nuclotron, GSI has chosen to build this machine using fast-cycling 4 T/s, 2 T superferric magnets. Cycle repetition rates of about 1 Hz cause high AC power losses in the magnet components. Eddy currents deteriorate the field quality. After millions of operation cycles and high radiation flux the coil support structure can loose mechanical stability. Therefore these effects were analyzed to warrant the magnet's performance over its lifetime. Following intensive experimental and computational investigations on short test models three full size dipoles and a 1.2 m long quadrupole with a maximum field gradient of 27 T/m in the aperture of 135 mm 65 mm will be manufactured until the end of 2007. Two of the dipoles are straight 2.7 m long 2.1 T magnets with a beam aperture of 130 mm 60 mm (width x height), while the third one is a curved 1.9 T dipole 3.0 m long with a reduced horizontal aperture of 115 mm. All the models use a superconducting hollow NbTi/Cu composite cable cooled with two-phase helium flow. The main design features and estimated operation parameters are given. In parallel preliminary tests of a 2.8 m SIS100 equivalent dipole system was performed. The cooling stability of the system at different SIS100 operation cycles was checked experimentally for the first time. These results are discussed as well as possible design options to fulfil additional operation requirements.