Bitter Magnet Research Articles

Quasi-isothermal magnetocaloric effect in the DyAl2 alloy in magnetic field up to 14 T

Experimental and analytical studies were carried out on the magnetocaloric effect (MCE) in the rare-earth alloy of the Laves phase DyAl2, which has the second-order magnetic PT (PT) in the region of cryogenic temperatures. The measurements are carried out using an extraction magnetic calorimeter with a Bitter electromagnet. A polycrystalline DyAl2 sample was synthesized followed by heat treatment. X-ray diffraction and elemental chemical analyzes are provided. The magnetization of the sample was measured in magnetic fields up to 13.5T. Calculations of the entropy changes of the magnetic subsystem ΔSmag are provided. Measurements of the MCEs ΔT − effect in adiabatic and ΔQ − effect in quasi-isothermal conditions were carried out. The MCE investigated by direct method in the temperature range of 15–110 K and magnetic field up to 14 T. It was found that the maximum of the adiabatic MCE in the region of the Curie temperature of the DyAl2 alloy under adiabatic magnetization in a magnetic field of 14T is ΔT = 12.94K. The obtained values are well approximated by the ΔT=ΔH2/3, The maximum value of the quasi-isothermal MCE is ΔQ = 3.1kJ/kg. The value corresponds to the maximum of the entropy change of the magnetic subsystem ΔSmag = 32.3J/(kg*K). The influence of thermal contact resistance (TCR) on the results of measuring the MCE under quasi-isothermal conditions is assessed.

60 T pulsed magnetic field in Bitter magnet made of tungsten and coiled by liquid helium

60 T pulsed magnetic field in Bitter magnet made of tungsten and coiled by liquid helium

A proof-of-concept Bitter-like HTS electromagnet fabricated from a silver-infiltrated (RE)BCO ceramic bulk

A novel concept for a compact high-field magnet coil is introduced. This is based on stacking slit annular discs cut from bulk rare-earth barium cuprate ((RE)BCO) ceramic in a Bitter-like architecture. Finite-element modelling shows that a small 20 turn stack (with a total coil volume of < 20 cm3) is capable of generating a central bore magnetic field of > 2 T at 77 K and > 20 T at 30 K. Unlike resistive Bitter magnets, the high-temperature superconducting (HTS) Bitter stack exhibits significant non-linear field behaviour during current ramping, caused by current filling proceeding from the inner radius outwards in each HTS layer. Practical proof-of-concept for this architecture was then demonstrated through fabricating an uninsulated four-turn prototype coil stack and operating this at 77 K. A maximum central field of 0.382 T was measured at 1.2 kA, with an accompanying 6.1 W of internal heat dissipation within the coil. Strong magnetic hysteresis behaviour was observed within the prototype coil, with ≈30% of the maximum central field still remaining trapped 45 min after the current had been removed. The coil was thermally stable during a 15 min hold at 1 kA, and survived thermal cycling to room temperature without noticeable deterioration in performance. A final test-to-destruction of the coil showed that the limiting weak point in the stack was growth-sector boundaries present in the original (RE)BCO bulk.

Strongly nonlinear antiferromagnetic dynamics in high magnetic fields

Antiferromagnetic (AFM) materials possess a well-recognized potential for ultrafast data processing thanks to their intrinsic ultrafast spin dynamics, absence of stray fields, and large spin transport effects. The very same properties, however, make their manipulation difficult, requiring frequencies in THz range and magnetic fields of tens of Teslas. Switching of AFM order implies going into the nonlinear regime, a largely unexplored territory. Here we use THz light from a free electron laser to drive antiferromagnetic NiO into a highly nonlinear regime and steer it out of nonlinearity with magnetic field from a 33-Tesla Bitter magnet. This demonstration of large-amplitude dynamics represents a crucial step towards ultrafast resonant switching of AFM order.

Design and Preliminary Experiment of Room-Temperature Bitter Magnet for Compact Gyrotron

Magnetic field system is one of the most crucial components for gyrotron operation. A compact gyrotron with a custom-designed magnet would be a promising continuous radiation source for millimeter-wave power applications. In this article, a design and preliminary experiment of a room-temperature Bitter magnet for developing compact gyrotron are presented. To lengthen the magnetic field homogeneity region without dramatically improving power consumption, a special design that every Bitter segment with different outer radii is adopted. Both in simulation and experimental measurement, the length of the homogeneity region is 120 mm, and homogeneity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \textit{B}$</tex-math> </inline-formula> / <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{B}$</tex-math> </inline-formula> is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 0.9% is obtained. A 5 kV magnetron injection gun (MIG) was designed based on the experimental measurement of this Bitter magnet, in which a velocity ratio of 1.3 and transverse velocity spread of 12% were obtained at current 0.5 A in simulation.

Magneto-transport properties of Si1−xGex &amp;lt;B, Hf&amp;gt; whiskers

The field and temperature magnetoresistance dependences of the Si1−xGex (x = 0.05) whiskers at low temperatures were studied in the magnetic field range of 0−14 T with using of the Bitter magnet. The investigated whiskers with diameters 5−15 μm were grown by chemical vapour deposition with a doping concentration in the vicinity to the metal-insulator transition (Nc ≈ 7.8 ⋅ 1018 cm−3). The linear magnetoresistance effect of the solid solution Si1-xGex &amp;lt;B, Hf &amp;gt; whiskers was found in all range of the magnetic fields due to the surface coherence scattering of charge carriers as a result of conduction in the whisker core-shell structure.

The Effect on the Germination Vigour of Cucumber Seeds after Receiving Magnetic Field Treatment Pre-Sowing

In the experiment, the impact of magnetisation on cucumber seeds is examined with the use of Bitter magnets with a constant magnetic field. The magnetisation process is performed in three magnetic fields: low—200 mT, medium—1 T, and high—9 T for 15 and 60 min. After germination, the biometric parameters are determined. The results of this research show that cucumber after pre-treatment in a magnetic field of 1 T for 60 min has a similar germination capacity and root length as the control sample. However, cucumber seeds magnetised in a 1 T field for 60 min have a significantly higher dry weight than the control sample (5.50 ± 0.32 mg vs. 3.01 ± 0.18 mg). The magnetisation in 9 T for both 15 and 60 min shows that these samples have a significantly lower germination capacity (86.8 ± 4.4% and 81.4 ± 7.3% vs. 91.8 a ± 3.2%) and root length (1.78 ± 0.02 cm and 4.42 ± 0.83 cm vs. 8.21 ± 0.34 cm) compared to the control sample. The cucumber seeds pre-treated at 9 T have a significantly greater dry weight than the control sample. Additionally, our research shows that some magnetic field intensities and magnetisation durations inhibit root growth and limit germination. These results are also important as they indicate which values of magnetic fields should be avoided.

Startup and Quench Electromagnetic Simulation of a Bitter Plate TF Coil Concept For the Fusion Nuclear Science Facility (FNSF)

Current distributions during startup and quench of very large reactor relevant coil systems will vary significantly from the smaller prototype magnets. A reactor scale simulation is needed. In this study, the FNSF magnet from the 2017 study is used as an example of a full sized reactor. The intent of the study is to investigate use of HTS conductors in the FNSF and how sizing and performance might be affected. A pancake or Bitter magnet like winding pattern is investigated to allow inductive and resistive redistribution of currents. A portion of the winding pack retains some resistivity to aid in redistribution of current during a transient or quench. Resistive materials can be used to support energy absorption for stability or to provide partial current dump capability during a quench, however, the primary quench mitigation mechanism is inductive redistribution of currents around the quench zone. In this implementation, the Bitter plate is both structural - similar to radial plates in ITER and electrical, intended, to limit current concentration during ramp-up. Use of localized resistors and parallel current paths are used to encourage more uniform startup currents. The analysis is a transient electromagnetic simulation using ANSYS. It is a cyclic symmetry modeling of an individual plate and is limited by the assumption that a local quench is repeated by the cyclic symmetry. This is a concession to the model size. Results are discussed in the context of performance needed for the FNSF.

Numerical Analysis on Hysteresis Loss of HTS Bitter Magnet

The high temperature superconducting (HTS) Bitter magnet soldered by REBCO annular plates with grooves we proposed recently has the advantage of small size, strong magnetic field, etc. The current distribution of the Bitter magnet can be controlled by adjusting the area of the soldering joint. In this paper, the AC loss of the HTS Bitter magnet during the charging process is numerically analyzed. The results show that the AC loss of the HTS Bitter magnet during a field ramp depends strongly on the magnetic field and the magnetic field angle between the magnetic field and the annular plate thus leading to the maximum AC loss in the regions with the small radius of the HTS Bitter magnet.

Controllable Measures of Current Distribution of Annular Plates For HTS Bitter Magnet

The current distribution of high temperature superconducting (HTS) Bitter magnets soldered by REBCO annular plates can be controlled by adjusting the area of the soldering joint and the number of grooves on the plates. In this paper, the principle of controlling the current distribution of HTS Bitter magnets is analyzed. The magnetic field distribution and current saturation of HTS Bitter magnets under three different current distributions are analyzed. Finally, an approach of measuring the current distribution of the HTS Bitter magnet based on integral equation is proposed through which the current distribution of the model magnet is measured. The results show that the current control method of HTS Bitter magnet is effective. Besides, it is proved that the central magnetic field of the HTS Bitter magnet can be increased while decreasing the maximum current saturation of the magnet by a reasonable current distribution controlled by the suggested method.

Conceptual Design of HTS Bitter Magnet Above 25T Using a Fast Magnetic Field Computational Method

A high temperature superconducting (HTS) Bitter magnet consisting of REBCO annular plates with slits is proposed. The REBCO annular plates are soldered to each other at the edge of the slits. The resistance of the soldering joint, which can help control current distribution, is experimentally proven to be small enough to avoid producing excessive loss. Furthermore, a fast-computational method based on a convolution neural network (CNN) is used to calculate the magnetic field and stress distribution of the proposed magnet. With the help of this algorithm, a specific structure of a 25 T HTS Bitter magnet is designed and distribution of the magnetic field and stress are numerically computed. Finally, the feasibility of the structure of the magnet and the efficiency of the algorithm are experimentally verified on a model magnet.

Magnetic Sedimentation Velocities and Equilibria in Dilute Aqueous Ferrofluids.

Dilute ferrofluids have important applications in the separation of materials via magnetic levitation. However, dilute ferrofluids pose an additional challenge compared to concentrated ones. Migration of the magnetic nanoparticles toward a magnet is not well counteracted by a buildup of an osmotic pressure gradient, and consequently, homogeneity of the fluid is gradually lost. Here, we investigate this phenomenon by measuring and numerically modeling time-dependent concentration profiles in aqueous ferrofluids in the field of a neodymium magnet and at 10 T in a Bitter magnet. The numerical model incorporates magnetic, frictional, and osmotic forces on the particles and takes into account the polydispersity of the particles and the spatial dependence of the magnetic field. The magnetic sedimentation rate in our most stable ferrofluids can be understood in terms of the magnetophoresis of separate nanoparticles, a best-case scenario when it comes to applications.

Scaffold-free and label-free biofabrication technology using levitational assembly in a high magnetic field

The feasibility of magnetic levitational bioassembly of tissue-engineered constructs from living tissue spheroids in the presence of paramagnetic ions (i.e. Gd3+) was recently demonstrated. However, Gd3+ is relatively toxic at concentrations above 50 mM normally used to enable magnetic levitation with NdFeB-permanent magnets. Using a high magnetic field (a 50 mm-bore, 31 T Bitter magnet) at the High Field Magnet Laboratory at Radboud University in Nijmegen, The Netherlands, we performed magnetic levitational assembly of tissue constructs from living spheroids prepared from the SW1353 chondrosarcoma cell line at 0.8 mM Gd3+ containing salt gadobutrol at 19 T magnetic field. The parameters of the levitation process were determined on the basis of polystyrene beads with a 170 μm-diameter. To predict the theoretical possibility of assembly, a zone of stable levitation in the horizontal and vertical areas of cross sections was previously calculated. The construct from tissue spheroids partially fused after 3 h in levitation. The analysis of viability after prolonged exposure (1 h) to strong magnetic fields (up to 30 T) showed the absence of significant cytotoxicity or morphology changes in the tissue spheroids. A high magnetic field works as a temporal and removal support or so-called ‘scaffield’. Thus, formative biofabrication of tissue-engineered constructs from tissue spheroids in the high magnetic field is a promising research direction

Design and Development of High Current Pulse Shaping Inductor Based on Bitter Magnet Configuration for Electro Magnetic Launcher

A unique pulse shaping inductor was designed and developed using a modified Bitter magnet configuration, to operate at peak current of 150 kA for duration of 5-10 ms in a Pulse Forming Network (PFN). The inductor was designed with a stack of parallel copper plates in the form of circular discs with a central hole, with GRP insulation plates in between. The design is optimized by electromagnetic finite element techniques for inductance and structural stability. The conducting discs and insulators are clamped together with tie rods between two endplates to provide adequate contact pressure and sufficient mechanical support. The conducting, inter connecting and insulating components were modeled and optimised parametrically for the desired performance and structural integrity. The device was fabricated, assembled and tested with 24 bitter plates for 28 μH inductance. The inductor was integrated with 400 kJ capacitor bank module and tested under static and dynamic condition. Current pulse of peak above 100 kA was generated, with consistent arc-free performance, high reliability and structural stability.

Direct measurement of shape memory effect for Ni54Mn21Ga25, Ni50Mn41.2In8.8 Heusler alloys in high magnetic field

Abstract Recently the new scheme of layered composite with shape memory effect (SME) for thermal and magnetic actuation was suggested and proved to demonstrate the reversible actuation on micro- and nanoscale. The principles of magnetic field induced martensitic transition (MFIMT) and magnetic field controlled shape memory effect (MFCSME) have the advantages of magnetic-field-controlled actuation at constant temperature, extremely small size of an actuator and compatibility with modern nanotechnologies. The present paper is devoted to experimental study of MFCSME in two ferromagnetic Heusler alloys Ni54Mn21Ga25 and Ni50Mn41.2In8.8 with positive and negative shift of the martensitic transition temperature in a magnetic field. The three points bending device for dilatometric tests of plate-like samples of the alloys was designed and placed in the field of Bitter coil magnet. The bending deformations versus temperature were measured at various magnetic fields up to 10 T and at mechanical stresses up to 45 MPa. The temperature shifts of thermoelastic martensitic phase transition were found to be 0.55 K/T and −0.95 K/T for Ni54Mn21Ga25 and Ni50Mn41.2In8.8 respectively. For direct MFCSME study the dependences of the bending deformation on magnetic fields up to 14 T were also obtained at constant temperatures for these alloys. Practically complete recoverable deformation of 0.2% due to MFIMT was obtained at 14 T in Ni54Mn21Ga25 alloy in temperature range 316–318 K and stress 8.6 MPa. The new variant of the layered functional composite actuator scheme based on the alloys with MFCSME is suggested. The proposed functional composite combines the layers of the alloys with positive and negative temperature shifts of martensitic transition temperature in magnetic field. It is argued that this combination of the alloys can improve the performance of the composite actuators driven by MFCSME, particularly, it can provide higher generated force, actuation stroke, sensitivity to magnetic field and frequency of actuation.

An ultra-compact low temperature scanning probe microscope for magnetic fields above 30 T.

We present the design of a highly compact high field scanning probe microscope (HF-SPM) for operation at cryogenic temperatures in an extremely high magnetic field, provided by a water-cooled Bitter magnet able to reach 38 T. The HF-SPM is 14 mm in diameter: an Attocube nano-positioner controls the coarse approach of a piezoresistive atomic force microscopy cantilever to a scanned sample. The Bitter magnet constitutes an extreme environment for scanning probe microscopy (SPM) due to the high level of vibrational noise; the Bitter magnet noise at frequencies up to 300 kHz is characterized, and noise mitigation methods are described. The performance of the HF-SPM is demonstrated by topographic imaging and noise measurements at up to 30 T. Additionally, the use of the SPM as a three-dimensional dilatometer for magnetostriction measurements is demonstrated via measurements on a magnetically frustrated spinel sample.

Design and experimental results of the 1-T Bitter Electromagnet Testing Apparatus (BETA).

The Bitter Electromagnet Testing Apparatus (BETA) is a 1-Tesla (T) technical prototype of the 10 T Adjustable Long Pulsed High-Field Apparatus. BETA's final design specifications are highlighted in this paper which include electromagnetic, thermal, and stress analyses. We discuss here the design and fabrication of BETA's core, vessel, cooling, and electrical subsystems. The electrical system of BETA is composed of a scalable solid-state DC breaker circuit. Experimental results display the stable operation of BETA at 1 T. These results are compared to both analytical design and finite element calculations. Experimental results validate analytical magnet designing methods developed at the Dusty Plasma Laboratory. The theoretical steady state maxima and the limits of BETA's design are explored in this paper.

Energy and Material Efficient Noncircular Bore Bitter Magnets

Resistive Bitter magnets are still widely used to produce very high continuous magnetic fields. There exist a number of experiments/applications where the second dimension of the bore of Bitter magnets is not fully utilized and thus can be minimized accordingly. Evidently, a significant reduction of one of the dimensions of the bore should result in significant decrease in consumed power and/or coil material, typically with no reduction in applicable science. However, no efforts were mounted before to quantify the benefits, likely owing to the problem intricacy. Using an analytical solution for magnetic field and current density in homothetic elliptical bore coils and finite element analysis solutions for other shapes as well, we make up for the deficiency to a degree. Relevant structural mechanical aspects are viewed to complete the picture. In closing, possibilities to enhance the cooling efficiency of Bitter magnets are discussed, albeit very briefly.

A low-temperature scanning tunneling microscope capable of microscopy and spectroscopy in a Bitter magnet at up to 34 T.

We present the design and performance of a cryogenic scanning tunneling microscope (STM) which operates inside a water-cooled Bitter magnet, which can attain a magnetic field of up to 38 T. Due to the high vibration environment generated by the magnet cooling water, a uniquely designed STM and a vibration damping system are required. The STM scan head is designed to be as compact and rigid as possible, to minimize the effect of vibrational noise as well as fit the size constraints of the Bitter magnet. The STM uses a differential screw mechanism for coarse tip-sample approach, and operates in helium exchange gas at cryogenic temperatures. The reliability and performance of the STM are demonstrated through topographic imaging and scanning tunneling spectroscopy on highly oriented pyrolytic graphite at T = 4.2 K and in magnetic fields up to 34 T.

The world's smallest capacitive dilatometer, for high-resolution thermal expansion and magnetostriction in high magnetic fields.

For the characterization of novel quantum phases of matter, it is often required to study materials under multi-extreme conditions, in particular down to very low temperatures and in very high magnetic fields. We developed the world's smallest high-resolution capacitive dilatometer suitable for temperatures down to 10 mK and usage in high magnetic fields up to 37.5 T. Despite the extreme miniaturization, the capacitive dilatometer can resolve length changes down to 0.01 Å. This is an unprecedented resolution in a capacitive dilatometer of this compact size. Many cryogenic devices have limited space. Due to the extremely reduced cell size (3 cm3, 12 g), implementation or new applications in many of these sample space lacking devices are now possible. As an important example, the minute device can now be rotated in any standard cryostat, including dilution refrigerators or the commercial physical property measurement system. The present super compact design provides also for high resolution thermal expansion and magnetostriction measurements in a 15.2 mm diameter tube, enabling its use in the 32 mm bore, 37.5 T Bitter magnet at the High Field Magnet Laboratory in Nijmegen down to a temperature of 300 mK.