High Magnetic Field Environment Research Articles

Validation of a novel fiber optic strain gauge in a cryogenic and high magnetic field environment

We report on the first operation of an easy to use low cost novel fiber optic strain gauge (FOSG) in cryogenic and magnetic field environments. The FOSGs were mounted on a superconducting coil and resin impregnated. The gauges detected resin shrinkage upon curing. On cooldown, the FOSG monitored the thermal contraction strains of the coil and the electromagnetic strain during energization. The coil was deliberately quenched, in excess of 175 times, and again the FOSG detected the quenches and measured the thermal expansion-induced strains and subsequent re-cooling of the coil after a quench. Agreement with FEA predictions was very good.

Sealed gaseous photomultiplier with CsI photocathode

A sealed gaseous photomultiplier tube (PMT) with a CsI photocathode has been developed using a micropattern gas detector (MPGD) made of Pyrex glass. A microblasting technique (MB) was employed for the production of a new hole-type MPGD. We investigated gaseous PMTs with a semi-transparent CsI photocathode for two different gas mixtures of Ne(90%)+i-C 4H 10 (10%) and Ne (90%)+CF 4 (10%) at a gas pressure of 700 torr. Gains of up to 10 5 were attained with the microblasted glass plate (MB-GP) for both neon gas mixtures. Quantum efficiencies of up to 0.5% were obtained for UV light at the wavelength of 170 nm. The results of the aging test and the characteristics in a high magnetic-field environment are described for the gaseous PMT filled with neon gas mixture.

Modified screening potential in a high density inhomogeneous quantum dusty magnetoplasma

The effect of strong ambient static magnetic field on Shukla–Nambu–Salimullah (SNS) potential in a dusty quantum magnetoplasma has been investigated using quantum hydrodynamic model. The potential is significantly modified by quantum statistical effects, density inhomogeneity, and dust polarization drift effect. The effective length of the modified SNS potential is found to be a sensitive function of external static magnetic field, E×B0 drift, and the scale length of inhomogeneity. Here E is the electric polarization vector produced via density inhomogeneity, and B0 is the ambient static magnetic field. It is found that dust polarization drift effect predominates the ion polarization drift effect in high magnetic field environments. It attracts our attention to the careful study of the underlying physics of dusty plasma environment of neutron stars and magnetars.

Integration of superconducting magnets with cryogen-free dilution refrigerator systems

There has been much recent research interest into “cryogen-free” dilution refrigerators. Cryogen-free systems have some advantages from a safety and convenience point of view as liquid cryogens are unnecessary. However, this also makes integrating the low-temperature system with a high magnetic field environment much more challenging. Here we shall describe recent successes of integrating superconducting magnets and dilution refrigerators into one system requiring a single pulse tube cooler. The resulting environment provides experimental temperatures between 7 mK and 30 K and magnetic fields up to 12 T. We shall describe the effects of AC loss heating in such systems on the pulse tube refrigerator when the field is ramped and also the effects of eddy current heating on the mixing chamber in sweeping fields.

An experiment for the precision measurement of the radiative decay mode of the neutron

The familiar neutron decay into a proton, electron, and antineutrino can be accompanied by photons with sufficient energy to be detected. We recently reported the first observation of the radiative beta decay branch for the free neutron with photons of energy 15–340 keV. We performed the experiment in the bore of a superconducting magnet where electron, proton, and photon signals were measured. A bar of bismuth germanate scintillating crystal coupled to an avalanche photodiode served as the photon detector that operated in the cryogenic, high magnetic field environment. The branching ratio for this energy region was measured and is consistent with the theoretical calculation. An experiment is under way to measure the branching ratio with an improved precision of 1% relative standard uncertainty and to measure the photon energy spectrum. In this paper, the apparatus modifications to reduce the systematic uncertainties will be described. Central to these improvements is the development of a 12-element detector based on the original photon detector design that will improve the statistical sensitivity. During data acquisition, a detailed calibration program will be performed to improve the systematic uncertainties. The development of these modifications is currently under way, and the second run of the experiment commenced in July 2008.

Fatigue Crack Growth Behavior of Metastable Austenitic Stainless Steel in Cryogenic High Magnetic Field Environments

This article studies the fatigue crack growth in a metastable austenitic stainless steel in cryogenic high magnetic field environments. Fatigue crack growth tests were performed with the compact tension (CT) specimens at liquid helium temperature (4 K) in magnetic fields of 0 and 6 T, and the crack growth rate data were expressed in terms of the J-integral range during fatigue loading. The J-integral range values were evaluated using an elastic-plastic finite element analysis. The measurement of martensite phase in the test specimens and the fractographic examination were also carried out. The high magnetic field effect on the fatigue crack growth rate properties at 4 K is discussed in detail.

Qualification Test Results of the KSTAR Superconducting Coils From the Construction to the Commissioning Steps

To achieve the first plasma of the Korea superconducting tokamak advanced research (KSTAR), the KSTAR superconducting coils were tested in advance. As they should operate in excessively low temperature of 4.5 K and high magnetic field environment of 7.5 T, it is crucial to monitor the cryogenic and the structural behaviors of KSTAR device during the commissioning period including a cool-down. The temperatures of the KSTAR toroidal field (TF) coil and the poloidal field (PF) coils were measured during the entire operating period. The mechanical stresses on the TF and PF structures were continuously monitored to check if they go beyond the limiting value calculated through the simulation. The alignment of the KSTAR device was checked by using displacement sensors. The TF coils were successfully supplied with 15 kA DC current for 8 hours, and the maximum 5 kA/s current variation of the PF coils were tested. For the main experiment, the interlock test of the quench detection system for the KSTAR coils was carried out at reduced currents of 1 kA. From these results the quench protection circuit, and the current-flow of the KSTAR superconducting coils proved to be well performed for the first plasma operation.

407 極低温・強砥場下における準安定オーステナイト系ステンレス鋼の疲労き裂進展挙動(材料力学(2))

407 極低温・強砥場下における準安定オーステナイト系ステンレス鋼の疲労き裂進展挙動(材料力学(2))

GEANT4 as a simulation framework in

GEANT4 is a Monte Carlo radiation transport toolkit which includes a complete range of functionalities required to build flexible simulation frameworks. Taking advantage of its open architecture and object-oriented design, we could develop a software suite, able to simulate μSR experiments and instrumentation. The versatility offered by this new tool has permitted us to model the existing instruments, thus allowing a fuller understanding of their operation. It has guided also the design and construction of new types of spectrometers, as those equipped for high-field μSR, where numerical simulations proved decisive in understanding the complex behaviour of the incoming muon beam and of the outgoing positrons in a high magnetic field environment. The developed μSR simulation framework, with its fully flexible and customizable design, will allow potential users not familiar with programming to focus exclusively on physics, by building and running their own applications without the need to modify the source code.

The Future of Spatially-Resolved Polychromatic Neutron and X-Ray Microdiffraction

Polychromatic microdiffraction is an emerging materials-characterization tool made practical by powerful X-ray and neutron sources, and by advanced optics and software. With polychromatic techniques, local crystalline properties including phase, texture (orientation), elastic strain, and defect density can be mapped with submicron spatial resolution in three dimensions. Here, we describe the evolving ability to nondestructively map local crystal structure in three dimensions and discuss how future advances will help address long-standing issues of inhomogeneous grain growth, deformation, fracture, and elastic strain. Current and future applications impact virtually all materials including electronic, solar, and light-emitting-diode (LED) materials, nanomaterials, structural materials, and joining materials. In addition, the ability to focus small beams on small samples dramatically increases signal-to-noise and greatly reduces the cost for extreme environmental chambers required for high-pressure, high-temperature, high-magnetic field or corrosive environments. Polychromatic techniques efficiently use source brilliance and minimize the required sample volume, which is essential for hard-to-make materials, irreplaceable materials, and for radioactive, toxic, or otherwise dangerous materials. New polychromatic neutron capabilities will significantly extend the range of samples that can be studied with neutrons and presents important new scientific opportunities for studies of magnetic materials, low Z elements, fragile crystal structures, and small samples in extreme environments.

A new vibrator to stimulate muscle proprioceptors in fMRI

Studying cognitive brain functions by functional magnetic resonance imaging (fMRI) requires appropriate stimulation devices that do not interfere with the magnetic fields. Since the emergence of fMRI in the 90s, a number of stimulation devices have been developed for the visual and auditory modalities. Only few devices, however, have been developed for the somesthesic modality. Here, we present a vibration device for studying somesthesia that is compatible with high magnetic field environments and that can be used in fMRI machines. This device consists of a poly vinyl chloride (PVC) vibrator containing a wind turbine and of a pneumatic apparatus that controls 1-6 vibrators simultaneously. Just like classical electromagnetic vibrators, our device stimulates muscle mechanoreceptors (muscle spindles) and generates reliable illusions of movement. We provide the fMRI compatibility data (phantom test), the calibration curve (vibration frequency as a function of air flow), as well as the results of a kinesthetic test (perceived speed of the illusory movement as a function of vibration frequency). This device was used successfully in several brain imaging studies using both fMRI and magnetoencephalography.

Silicon photomultiplier as a position sensitive detector of Cherenkov photons

A novel photon detector, silicon photomultiplier, whose main advantage over photomultiplier tubes for RICH applications is the operation in high magnetic field environments, has been tested in view of measuring Cherenkov photons in a RICH counter. Results of the measurements of single photo-electron pulse height and timing distributions and of uniformity of the response are presented. Feasibility of a proximity focusing RICH counter using this type of photon detector is discussed.

Fiber Bragg Gratings for Strain Sensing in High Temperature Superconducting Magnet

Fiber Bragg grating (FBG) for measuring stress and strain in 77 K and high magnetic field environment for a high temperature superconducting (HTS) magnet has been studied. The sensing characteristics of FBG in the cryogenic environment were studied on the basis of a series of experiments. To study the strain sensing behavior of FBG at 77 K, the principle of the cantilever which used a aluminum plate clamped at one end and deployed as a cantilever was applied. The results demonstrated that the FBGs still possess good linear strain sensing property at liquid nitrogen temperature. The function of micro-strain and wavelength shift is determined as ??B = 1.23 muez at 77 K. Based on this equation, the hoop strain of a HTS magnet wounded by Bi2223 was measured. The hoop strain difference between the measured results by FBG and the calculated results by ANSYS is very small.

A method to capture six-degrees-of-freedom mechanical measurements of isometric shoulder and elbow torques during event-related fMRI

Functional magnetic resonance imaging (fMRI) experiments investigating cortical activity while controlling task performance are difficult to conduct due to the high magnetic field environment and a lack of compatible measurement tools. We describe a method to measure the generation of isometric shoulder and elbow torques with a six-degrees-of-freedom (DOF) load cell during an event-related fMRI study. Feasibility of this method is demonstrated by finding cortical activity on the motor cortices in a participant during an event-related study of shoulder abduction and elbow flexion. The described methodology permits researchers to control and measure intersubject and intrasubject motor task performance during event-related brain imaging.

Simulation of scattering and attenuation of 511 keV photons in a combined PET/field-cycled MRI system

Mixing the imaging modalities of positron emission tomography (PET) and magnetic resonance imaging (MRI) will offer the best soft tissue contrast (MRI) with information about metabolic function (PET). The high magnetic field environment of an MRI system makes the detection of annihilation photons difficult, as the response of standard photo-multiplier tubes is compromised. An approach using field-cycled MRI is discussed here, as field-cycled MRI makes it possible to have long periods of time available for nuclear imaging when there is no magnetic field present. This work focuses upon the effect of the field-cycled MRI upon the nuclear image due to the added material providing additional attenuation of the PET signal, and additional nuclei for scatter. These effects are studied using a Monte Carlo simulation based upon the GEANT libraries. Attenuation effects are shown to be significant, approximately 6% for the RF shield and coil and approximately 24% for the gradients. No significant effect is seen in image quality due to the scattering of the gammas. With these levels of attenuation it is concluded that open gradient coils and shim coils are required around the imaging volume.

Induced current density in the foetus of pregnant workers in high magnetic field environments

There are moves to limit by legislation the amount of electric and magnetic fields that workers and the general public are exposed to. In work locations near wiring, cables & equipment carrying high electric currents, there are situations in which the proposed magnetic field limits could be exceeded. Since the limits for the general public are more conservative than those for workers and since the foetus or a pregnant worker should be afforded the status of a member of the general public, it is important to assess a worst-case scenario for the purposes of a general code of practice. Three different magnetic field exposures are modelled, which include the worst case - the body of a pregnant woman at a smallest distance of 30 cm to the conductor. All computations were done by using Multiple Multipole Program (MMP), which is based on the Generalized Multipole Technique (GMT) from ETH (Swiss Federal Institute of Technology), Zurich, Switzerland. In a worst-case scenario the proposed basic restrictions would be exceeded slightly in both maternal and foetal tissue. With appropriate pre-placement assessment, these over-exposures can be avoided.

Fracture and deformation properties of Ni–Fe superalloy in cryogenic high magnetic field environments

The present paper includes experimental and analytical data on the fracture properties of a nickel–iron superalloy, a ferromagnetic austenite, at 4 K in magnetic fields of 0 and 6 T. The tensile, notch tensile and small punch tests are employed. A finite element analysis is also performed to convert the experimentally measured load–displacement data into useful engineering information. To interpret the results we review the available theory of the influence of magnetic field on the stress intensity factor for a crack in ferromagnetic materials.

Remote control of catheter tip deflection: an opportunity for interventional MRI.

This study seeks to exploit the high magnetic field environment of a clinical MRI scanner and demonstrate the technical feasibility of developing a catheter whose tip can be remotely oriented within the magnetic field by applying a DC current to a coil wound around the catheter tip to generate a magnetic moment and consequent deflection. To achieve arbitrary three-dimensional deflections, a three-axis coil was wound on a 1.5 Fr cylindrical catheter. By applying DC currents in the 100 mA range, this catheter was successfully guided through a 3D phantom maze, mimicking the vasculature, under MR imaging guidance. Feasibility was demonstrated that the strong ambient magnetic field of the MR scanner offers a special opportunity to develop simple devices that can be remotely steered to sites of clinical interest.

CHEMICAL EVOLUTION OF STRONGLY MAGNETIZED QUARK CORE IN A NEWBORN NEUTRON STAR

The chemical evolution of nascent quark matter core in a newborn compact neutron star is studied in presence of a strong magnetic field. The effective rate of strange quark production in degenerate quark matter core in presence of strong magnetic fields is obtained. The investigations show that in presence of strong magnetic field the quark matter core becomes energetically unstable and hence a first/higher order deconfinement transition to quark matter at the centre of a compact neutron star under such circumstances is absolutely impossible. The critical strength of magnetic field at the central core to make the system energetically unstable with respect to dense nuclear matter is found to be ~4.4 × 1013 G. This is the typical strength at which the Landau levels for electrons are populated. The other kind of phase transformations at such high density and ultra strong magnetic field environment are discussed.

Digital-image capture system for the IR camera used in Alcator C-Mod

An infrared imaging system, based on an Amber Radiance 1 infrared camera, is used at Alcator C-Mod to measure the surface temperatures in the lower divertor region. Due to the supra-linear dependence of the thermal radiation with temperature it is important to make use of the 12-bit digitization of the focal plane array of the Amber camera and not be limited by the 8 bits inherent to the video signal. It is also necessary for the image capture device (i.e., fast computer) to be removed from the high magnetic field environment surrounding the experiment. Finally, the coupling between the digital camera output and the capture device should be nonconductive for isolation purposes (i.e., optical coupling). A digital video remote camera interface (RCI) coupled to a PCI bus fiber optic interface board is used to accomplish this task. Using this PCI-RCI system, the 60 Hz images from the Amber Radiance 1 camera, each composed of 256×256 pixels and 12 bits/pixel, are captured by a Windows NT computer. An electrical trigger signal is given directly to the RCI module to synchronize the image stream with the experiment. The RCI can be programmed from the host computer to work with a variety of digital cameras, including the Amber Radiance 1 camera.