Cryogen-free Magnet Research Articles

Design and manufacture of an ultra-compact, 1.5 T class, controlled-contact resistance, REBCO, brain imaging MRI magnet

Abstract Brain imaging MRI comprises a significant proportion of MRI scans, but the requirement for including the shoulders in the magnet bore means there is not a significant size reduction in the magnet compared to whole-body magnets. Here we present a new design approach for brain imaging MRI magnets targeting ±20 kHz B 0 variation over the imaging volume rather than the more usual ±200 Hz making use of novel high-bandwidth MRI pulse sequences and distortion correction. Using this design approach, we designed and manufactured a 1.5 T class ReBCO cryogen-free magnet. The magnet is dome-like in form, completely excludes the shoulders and is <400 mm long. The magnet was wound using no-insulation style coils with a conductive epoxy encapsulant where the contact resistance of the coils was controlled so the emergency shut-down time of the magnet was less than 30 s. Despite acceptable coil testing results ahead of manufacture, during testing of the magnet, several of the epoxy coils showed signs of damage limiting stable performance to <55 A compared to the designed 160 A. These coils were replaced with insulated paraffin encapsulated coils. Subsequently the magnet was re-ramped and was stable at 81 A, generating 0.71 T as several other coils had sustained damage not visible in the first magnet iteration. The magnet has been passive shimmed to ±20 kHz B 0 variation over the imaging volume and integrated into an MRI scanner. The stability of the magnet has been evaluated and found to be acceptable for MRI.

Conceptual Design of a 33 T Cryogen-Free Magnet REBCO Insert: Mechanical Aspects and Protection Against Thermal Runaway

Conceptual Design of a 33 T Cryogen-Free Magnet REBCO Insert: Mechanical Aspects and Protection Against Thermal Runaway

A novel STM for quality atomic resolution with piezoelectric motor of high compactness and simplicity

Scanning tunneling microscope (STM) is a renowned scientific tool for obtaining high-resolution atomic images of materials. Herein, we present an innovative design of the scanning unit with a compact yet powerful inertial piezoelectric motor inspired by the Spider Drive motor principle. The scanning unit mainly consists of a small 9 mm long piezoelectric tube scanner (PTS), one end of which is coaxially connected to the main sapphire body of the STM. Of particular emphasis in this design is the piezoelectric shaft (PS), constructed from piezoelectric material instead of conventional metallic or zirconium materials. The PS is a rectangular piezoelectric stack composed of two piezoelectric plates, which are elastically clamped on the inner wall of the PTS via a spring strip. The PTS and PS expand and contract independently with each other to improve the inertial force and reduce the threshold voltage. To ensure the stability of the PS and balance the stepping performance of the inertial motor, a counterweight, and a matching conical spring are fixed at the tail of the PS. This innovative design allows for the assessment of scanning unit performance by applying a driving signal, threshold voltage is 50 V at room temperature. Step sizes vary from 0.1 to 1 µm by changing the driving signal at room temperature. Furthermore, we successfully obtained atomic-resolution images of a highly oriented pyrolytic graphite (HOPG) sample and low drift rates of 23.4 pm/min and 34.6 pm/min in X-Y plane and Z direction, respectively, under ambient conditions. This small, compact STM unit has the potential for the development of a rotatable STM for use in cryogen-free magnets, and superconducting magnets.

The CW-EPR Capabilities of a Dual DNP/EPR Spectrometer Operating at 14 and 7 T

High-field electron paramagnetic resonance (EPR) measurements are indispensable for a better understanding of dynamic nuclear polarization (DNP), which relies on polarization transfer between electron and nuclear spins. DNP experiments are typically performed at high > 7 T magnetic fields and low ≤ 100 K temperatures, while EPR instrumentation capable of EPR measurements under these conditions is scarce. In this paper, we describe the CW EPR capabilities of a dual DNP/EPR spectrometer that is designed to carry out EPR experiments under “DNP conditions” at 14 and 7 T. In the first part, we present the design of this instrument, highlighting the choices made to allow for both DNP and EPR operations. The spectrometer uses a sweepable cryogen-free magnet with NMR-grade homogeneity, a closed-cycle cooling system, a quasi-optical induction mode bridge, and a superheterodyne receiver system. The probe design is optimized for low heat load and fast sample exchange under cryogenic conditions. The spectrometer can operate in frequency and field sweep modes, including wide field sweeps using the main coil of the magnet. In the second part, we present EPR spectra acquired over a wide range of samples and operating conditions, illustrating the CW EPR capabilities of the instrument.

Glovebox-assisted magnetic force microscope for studying air-sensitive samples in a cryogen-free magnet.

Most known two-dimensional magnets exhibit a high sensitivity to air, making direct characterization of their domain textures technically challenging. Herein, we report on the construction and performance of a glovebox-assisted magnetic force microscope (MFM) operating in a cryogen-free magnet, realizing imaging of the intrinsic magnetic structure of water and oxygen-sensitive materials. It features a compact tubular probe for a 50 mm-diameter variable temperature insert installed in a 12T cryogen-free magnet. A detachable sealing chamber can be electrically connected to the tail of the probe, and its pump port can be opened and closed by a vacuum manipulator located on the top of the probe. This sealing chamber enables sample loading and positioning in the glove box and MFM transfer to the magnet maintained in an inert gas atmosphere (in this case, argon and helium gas). The performance of the MFM is demonstrated by directly imaging the surface (using no buffer layer, such as h-BN) of very air-sensitive van der Waals magnetic material chromium triiodide (CrI3) samples at low temperatures as low as 5K and high magnetic fields up to 11.9T. The system's adaptability permits replacing the MFM unit with a scanning tunneling microscope unit, enabling high-resolution atomic imaging of air-sensitive surface samples.

Gradient coil design with enhanced shielding constraint for a cryogen-free superconducting MRI system

The relatively fragile low-temperature stability of cryogen-free superconducting magnetic resonance imaging (MRI) magnets requires the careful management of exogenous heat sources. A strongly shielded gradient magnetic field is important for the optimal operation of cryogen-free MRI systems. In this study, we present an enhanced shielding method incorporating a regionalized stray field constraining strategy. By optimizing the constraint parameters, we could develop engineering-feasible gradient coil schemes without increasing system complexity but with the stray field intensity reduced by half. In real measurement in an integrated MRI system, the developed gradient assembly demonstrated good performance and supported to output images of excellent quality. Our findings suggested that the proposed method could potentially form a useful design paradigm for cryogen-free MRI magnets.

Anisotropic NMR data acquisition with a prototype 400 MHz cryogen-free NMR spectrometer.

High-temperature superconducting (HTS) materials have recently been incorporated into the construction of HTS cryogen-free magnets for nuclear magnetic resonance (NMR) spectroscopy. These HTS NMR spectrometers do not require liquid cryogens, thereby providing significant cost savings and facilitating easy integration into chemistry laboratories. However, the optimal performance of these HTS magnets against standard cryogen NMR magnets must be evaluated, especially with demanding modern NMR applications such as NMR in anisotropic media. The stability of the HTS magnets over time and their performance with complex pulse sequence experiments are the main unknown factors of this new technology. In this study, we evaluate the utility of our prototype 400 MHz cryogen-free power-driven HTS NMR spectrometer, installed in the fumehood of a chemistry laboratory, for stereochemical analysis of three commercial natural products (artemisinin, artemether, and dihydroartemisinin) via measurement of anisotropic NMR data, in particular, residual dipolar couplings. The accuracy of measurement of the anisotropic NMR data with the HTS magnet spectrometer is evaluated through the CASE-3D fitting protocol, as implemented in the Mestrenova-StereoFitter software program.

Performance of Cryogen-Free Superconducting Magnet in Isochronous Cyclotron for Proton Therapy

A superconducting azimuthally varying field (AVF) cyclotron, SC230, was developed for proton therapy by Sumitomo Heavy Industries, Ltd. in 2021. This isochronous cyclotron is compact owing to the high magnetic field generated by NbTi superconducting coils. The average magnetic field is 3.9 T at the outer region, and the extraction radius is 0.6 m. Its yoke weight is 65 t. Currently, it is the most compact isochronous cyclotron for this purpose. The coils are cryogen-free and cooled by conduction cooling using four 4 K-GM cryocoolers. The system is highly safe and easy to maintain, which reduces the effects of unstable helium supply. Thus, cryogen-free magnets have various advantages; however, in isochronous cyclotrons, the heat inputs to the coils during beam operation are larger owing to the leakage radio frequency (RF) and beam loss. The magnet is designed to have a sufficient temperature margin for stable operation. The coil temperature during the RF excitation and its dependence on the beam loss were measured to verify the stability of operations. Subsequently, it was confirmed that the coil cools sufficiently at the critical temperature.

Post-acquisition correction of NMR spectra distorted by dynamic and static field inhomogeneity of cryogen-free magnets

Post-acquisition correction of NMR spectra is an important part of NMR spectroscopy that enables refined NMR spectra to be obtained, clean from undesirable out-phasing, broadening and noising. We describe analytical and numerical mathematical methods for post-acquisition correction of NMR spectra distorted by static and dynamic magnetic field inhomogeneity caused by imperfections of main superconducting coils and the cold head operation, typical for cryogen-free magnets. For the dynamic inhomogeneity, we apply a variant of the general reference deconvolution method, complemented with our mathematical analysis of spectral parameters. For the static inhomogeneity, we apply the method of a delayed Fourier transform, also supported with our analytical calculations. We verify our approach by correction processing of high-field experimental liquid-state 1H NMR spectra of water and ethanol as well as solid-state 13C MAS NMR spectra of adamantane and obtain good results for both static and dynamic field distortions. This work complements our previous work on instrumental suppression of dynamic distortions caused by the cold head operation. The results presented contribute well to the general field of processing NMR spectra and serve towards a more extensive use of cryogen-free magnets in high-resolution NMR spectroscopy.

Cryogen-free 400 MHz (9.4 T) solid state MAS NMR system with liquid state NMR potential

We show that the temporal magnetic field distortion generated by the Cold Head operation can be removed and high quality Solid-State Magic Angle Spinning NMR results can be obtained with a cryogen-free magnet. The compact design of the cryogen-free magnets allows for the probe to be inserted either from the bottom (as in most NMR systems) or, more conveniently, from the top. The magnetic field settling time can be made as short as an hour after a field ramp. Therefore, a single cryogen-free magnet can be used at different fixed fields. The magnetic field can be changed every day without compromising the measurement resolution.

Design, construction and performance testing of a 1.5 T cryogen-free low-temperature superconductor whole-body MRI magnet

This work describes the design, construction and testing of a 1.5 T cryogen-free low-temperature superconductor niobium–titanium whole-body magnet with a bore diameter of 850 mm that is suitable for clinical magnetic resonance imaging (MRI) applications. The magnet is actively shielded and passively shimmed to achieve confinement within a 0.5 mT stray magnetic field at 2.5/4 m radial/axial positions and 12.1 ppm field inhomogeneity over a 45 mm diameter of spherical volume. The magnet is conductively refrigerated using a two-stage Gifford–McMahon cryocooler and can be maintained at a steady temperature below 5.7 K during MRI scanning. The MRI scanner assembled with the designed cryogen-free magnet demonstrated stable performance and provided MRI images with good quality and high contrast.

Simultaneous Determination of Multicomponent Dosage Forms Using Benchtop NMR Spectroscopy: Application to Phenytoin-Phenobarbital Combination.

The use of nuclear magnetic resonance (NMR) spectroscopy as a tool for determining pharmaceutical molecules in bulk drugs and their dosage forms is growing. New advancements in benchtop NMR spectrometers with cryogen-free magnets have made this technique more appealing and accessible. Herein, we developed a method using a benchtop NMR spectrometer to quantify phenytoin (PhT) and phenobarbital (PhB) in bulk and combined dosage forms. The results were compared to those obtained by high performance liquid chromatography (HPLC) as a well-characterized procedure. This method is simple, low cost, relatively fast, and non-inferior to HPLC in terms of figures of merit.

Transport current and magnetization of Bi-2212 wires above liquid Helium temperature for cryogen-free applications

Since the discovery of high temperature superconductors, a possible cryogen-free scenario has always been wished. Nowadays, liquid Helium is running out, and it is likely that the cooling by will be a large part of the costs of any superconducting system. Bi-2212 wires at temperature higher than 4.2 K still show a very high irreversibility field and thus a deep investigation of their properties in such a range of temperature is very useful in order to assess the applicability in high field cryogen-free magnets. Here electrical transport and magnetic properties characterization at variable temperature and magnetic field on our “GDG—processed” wires are reported together with a well-described original approach to calculate the irreversibility field Hirr. This study is devoted to provide reference data on the behaviour of the only isotropic wire for high field application with an eye to the performances at temperatures above 4.2 K.

A new method to measure the temporal magnetic field instabilities in cryogen-free magnets for magnetic resonance

Despite the obvious advantages of cryogen-free magnets for NMR such as independence of liquid helium supply and the possibility to use the same magnet at different fields, the practical application of those magnets remains limited because of temporal magnetic field distortions associated with cryogen-free cold head operation. A new experimental method for the simple and reliable detection of the temporal field distortions is described in this paper. The accuracy of the magnetic field measurements by this method is two orders of magnitude higher than by conventional MetroLab Tesla meter. This has enabled us to make improvements in the design of cryogen-free magnets by reducing the amplitude of such field distortions down to sub ppb level. This then results in cryogen-free magnets that are suitable for MRI and MAS NMR applications.

Detection and Protection Against Quench/Local Thermal Runaway for a 30 T Cryogen-Free Magnet

The upgrade to 30 T of the existing 25 T cryogen-free magnet (25 T CSM) at the High Field Laboratory for Superconducting Materials, Tohoku Univ., will consist in replacing the existing High Temperature Superconductor insert made of Bi-2223 by one wound using REBCO tape. The new insert is designed to produce 16 T instead of 11 T. In order to have good dynamics and allow sweeping mode experiment, the insert will be conventionally isolated, as it was the case for the previous Bi-2223 version. In previous work we modelled thermal runaway phenomenon and showed that a sensitive dissipative voltage detection technique can give sufficient warning to protect an HTS coil using external dump resistor even in the case of sharp local critical current drop. For the 30 T insert, the conductor will consist of a two tape bundle so as to reduce the chance of such sharp Ic drop due to tape performances inhomogeneities or local damage. We simulate the quench dynamics in the LTS outsert coupled with the HTS insert to demonstrate that the existing detection/protection system of the 25 T CSM can be re-used and determine the current dumping dynamics. We then use our thermal runaway modelling tool to study the behavior of the future 16 T REBCO insert in case of local sharp defect and determine realistic detection threshold for its safe protection.

Cryogen-free one hundred microkelvin refrigerator.

A temperature below 100 µK is achieved in a customized cryogen-free dilution refrigerator with a copper-nuclear demagnetization stage. The lowest temperature of conduction electrons of the demagnetization stage is below 100 µK as measured by using a pulsed platinum nuclear magnetic resonance thermometer, and the temperature can remain below 100 µK for over 10 h. A demagnetization magnetic field of up to 9 T and a research magnetic field of up to 12 T can be controlled independently, provided by a coaxial room-temperature-bore cryogen-free magnet.

A method for fast field settling in cryogen-free superconducting magnets for NMR

We propose a fast algorithm to energise a cryogen free magnet to a highly persistent state. A decay rate as low as 0.021 ​ppm/h can be achieved in less than an hour after reaching the target field. The decay rate drops further to 0.0004 ​ppm/h in the following 48 ​h. This procedure can be applied at different values of target field, which makes it feasible to use a single magnet for study of various NMR lines at different fields. The mechanism of establishing a highly stable magnetic field can be understood on the basis of the magnetic properties of the superconducting wire, which were studied using a vibrating sample magnetometer. The results confirm the high quality of the superconducting wire and joints.

Levitation Force Characteristics of High-Temperature Superconducting Bulks in a High Magnetic Field

The levitation force of high temperature superconducting (HTS) bulks can be enhanced by raising the strength of the external magnetic field. As to the field of HTS magnetic levitation (maglev), many researchers have also obtained this conclusion by levitation experiments above a permanent magnet guideway (PMG). However, the experimental results have limitations because the magnetic field above the PMG is relatively low, around 0.4 T at the 15 mm working height. In order to obtain more comprehensive levitation force characteristics of HTS bulks, it is necessary to carry out more experiments in high magnetic field. In this paper, the effects of magnetization angle and external field gradient on the levitation force of an HTS YBCO bulk which was manufactured with a top-seeded multi-seeded melt-textured process were studied. We set up a levitation force measurement system based on a 5-T Cryogen-Free Magnet (CRY-CFM-5 T-300-H3, Cryogenic Limited). The HTS bulk was cooled by liquid nitrogen in zero-field-cooling condition, and the external magnetic field was enhanced from 0 to 3 T. The HTS bulk was rotated 30° each time and finally rotated to 90°. Experiments show that the levitation force decreases from θ = 0° to 90°, where θ is the angle between the upper surface of the HTS bulk and the horizontal plane. The conclusion that the levitation force increases with the increase of the external magnetic field gradient in high magnetic field was also further confirmed.

30 T scanning tunnelling microscope in a hybrid magnet with essentially non-metallic design

We report on the construction and performance of the first hybrid resistive-superconducting magnet (HM) based scanning tunnelling microscope (STM) above 30 T. This custom-design HM-STM features a novel design of the STM head unit, whose tip-sample approach is implemented using a slender piezoelectric tube (PZT). The scanner shares part of PZT by fixing a sapphire frame onto the front quarter of PZT to construct a compact tip-sample loop, realising an outer diameter of 8.8 mm, which makes it compatible with a narrow sample space. Its main components are made of non-metallic materials of sapphire, which allows it to be immune from eddy currents and to operate under the condition of strong magnetic field fluctuation from a hybrid magnet, as well as cryogen-free cryocooler magnet systems. To analyse the stiffness of the STM head unit, the eigenfrequencies with 11 kHz and 12 kHz in bending modes, 25 kHz in a torsional mode, and 67 kHz in a longitudinal mode were simulated by finite element analysis; also, the drifting rates of the STM in ambient conditions in the X-Y plane and Z direction were measured at 25.5 and 38.2 pm/min, respectively. We present the first atomic images in magnetic fields up to 30.1 T in an HM. The raw data show the stable and distinguished performance while ramping up to maximum fields, indicating the new device's potential capability of operating in the future 45T-hybrid magnet and hundred-field pulsed magnet. Meanwhile, our compact and concentric cylindrical STM insert can operate in the low-temperature tubular sample space housed by the HM bore to develop low-temperature and extreme high-magnetic field STM.

On the magnetic field stability of cryogen-free magnets for magnetic resonance applications

The temporal magnetic field variation associated with Cold Head operation in cryogen-free magnets was studied. Three different mechanisms for such variations were tested separately and rated by their importance. It was found that mechanical displacement of the magnet inside the cryostat is the main issue of magnetic field perturbation. In a cryostat with the Gifford- McMahon type of cryocooler, motion of the displacer with magnetic material inside also produces significant field modulation. The temperature variation of the magnet, although noticeable, leads to smaller field distortions compared to the previous two factors. It was shown that the temporal magnetic field variation could be reduced down to below 20 ​ppb level that could be acceptable for MRI and MAS NMR applications. It was also shown that a single cryogen-free magnet could be easily used at different fields on a day-to-day basis without compromising the field stability unlike magnets housed in a liquid helium reservoir.