RF Magnetic Field Research Articles

Bi-chromatic adiabatic shells for atom interferometry

Free space atom-interferometry traditionally suffers from the large distances that atoms have to fall in order to achieve long interaction times. Trapped atom interferometry is emerging as a powerful way of achieving long interaction times in a reduced experimental volume. Here, we demonstrate bi-chromatic adiabatic magnetic shell traps as a novel tool for matterwave interferometry. We dress the magnetic hyperfine states of the F = 1 and F = 2 Rubidium 87 Bose–Einstein Condensates thus creating two independently controllable shell traps of which we use the and adiabatic states. Using microwave pulses, we put atoms originally loaded into one of the two shell-traps into a superposition between the two shell traps. Since the two traps can be manipulated independently, their position and vertical curvature can be matched, thus creating a good starting point for an atom interferometer. This interferometer can be sensitive to spatially varying electric or magnetic fields, which could be DC or RF magnetic fields or microwaves. We demonstrate that the trap-matching afforded by the independent control of the shell traps allows for a tenfold increase in coherence times when compared to adiabatic potentials created by a single RF-frequency. For large-radius shells the atoms are confined to a 2D surface enabling highly sensitive imaging matterwave interferometers.

A Ferrite-Filled Cavity Resonator for Electronic Article Surveillance on Metallic Packaging

Conventional electronic article surveillance (EAS) tags are ineffective on metallic packaging. The component of the RF magnetic field perpendicular to the surface of the packaging induces eddy currents that suppress the magnetic flux, linking the inductive element of the tag. In this article, an inductive quarter-wavelength planar cavity, formed by wrapping aluminum foil around a ferrite core, was extended by wrapping additional capacitive layers of foil/dielectric around the ferrite-filled central region. This so-called wrapped tag exhibits the frequency, $Q$ -factor, and read distance characteristics of existing EAS tags, but is instead driven by the RF magnetic fields parallel to the surface of the metallic packaging. In this article, we compare the observed frequency response of the wrapped tag with a simple LC resonator model that considers the tag’s geometrical features, and use the model to describe how the design and construction of the tag can be optimized. Finite-element method (FEM) modeling is used to reveal how the current flows in the wrapped foil of the tag. Prototype tags show good reproducibility, demonstrating the potential of the design as a solution to the problem of tagging metallic packaging in the EAS industry.

Simulation of 500 MHz Electromagnetic Interference Effect on Electrical Equipment with Various RF Grids and Enclosures

This paper presents the modelling and simulation of a protection system for equipment in the oil and gas industry with various RF grids and enclosures against 500 MHz electromagnetic interference (EMI). COMSOL Multiphysics®Modelling software was used in this study. Electric and magnetic fields distributions were determined by using the Generalized Minimal Residual Method (GMRES) which was integrated into COMSOL Multiphysics® Modelling software. Simulation results indicated that larger RF grid size contributed to the higher electric and magnetic field on equipment. Furthermore, without RF grid, electric and magnetic fields on the equipment were increased significantly (up to 100x). The maximum electric and magnetic fields were found to be near resonance enclosure size (299 mm for 500 MHz frequency source). The results showed that the presence of the RF grid for the EMI protection system was essential.

Harnessing Surface Plasmons for Magnetic Resonance Imaging Applications

Metamaterials have great potential to control near-field electromagnetic response at will. Lately metamaterial-based devices have been used to manipulate the rf magnetic field in magnetic resonance imaging (MRI) via Fabry-Perot resonances or subdiffraction lenses. This work shows that a negative-permeability metamaterial incorporated into an MRI setup supports magnetic surface plasmons, allowing local boosting of the magnetic field and an improved signal-to-noise ratio. These results could pave the way for high-performance MRI systems.

NanoNeuroRFID: A Wireless Implantable Device Based on Magnetoelectric Antennas

A major obstacle during the design of brain–computer interfaces is the unavailability of a neural implantable device that is µ-scale in size and is wireless, self-powered, and long-lasting. The current state-of-the-art implantable devices suffer from various limitations. Electromagnetic-based wireless devices are big in size because of their large antenna, which must be larger than one-tenth of the wavelength of the operational frequency. Ultrasound-based wireless devices, in addition to their low data rate, have massive loss in the skull and need an intermediate electromagnetic transceiver under the skull. Furthermore, almost all state-of-the-art wireless devices use micro-electrodes for neuronal recording, which are not reliable in long-term monitoring applications because of the direct contact between the tissue and metal electrodes. In this paper, we propose a novel, wireless, and ultra-compact implantable device termed NanoNeuroRFID. At the core of this device, there is a magnetoelectric (ME) antenna array. ME antennas are smart and ultra-miniaturized (<200 μm diameter) and can perform multiple tasks. First, can harvest electromagnetic energy to power the NanoNeuroRFID system. Their limit of the detection for RF magnetic fields is 40 pT; second, they can sense quasi-static neuronal magnetic fields as small as 200 pT without direct contact to the tissue, allowing a long lifetime and reliable neural recording; and third, they can communicate with an external transceiver, and their operational frequency could be 10 to 100 s of MHz, where tissue loss is small.

Low-magnetic-field enhancement of thrust imparted by a stepped-diameter and downstream-gas-injected rf plasma thruster

A magnetic nozzle radiofrequency (rf) plasma thruster having a stepped-diameter source cavity is operated with upstream and downstream gas injection cases; the imparted thrust, the plasma density, and the rf magnetic field are measured. The plasma produced by a 13.56 MHz rf generator is sustained only with low magnetic field strength at the solenoid center of about 30–300 Gauss for the driving frequency of 13.56 MHz and the downstream gas injection; the larger thrust than the upstream gas injection case is obtained. The magnetic field strength giving the maximum thrust increases when changing the operating frequency to 40.68 MHz, implying the efficient plasma production by a low field helicon mode. The axial density measurements show that the maximum plasma density is located near the thruster exit for the downstream gas injection, while the maximum density location is observed at the upstream side of the source for the upstream gas injection. Large amplitude rf magnetic field near the thruster exit and in the magnetic nozzle is simultaneously observed for the low-magnetic field with the downstream gas injection, implying the propagation of the helicon wave. The measured axial wavenumber is qualitatively understood by a simple dispersion relation assuming a radial wavenumber determined by the plasma-vacuum boundary. It is shown that the low-field helicon mode contributes to an increase in a specific impulse since the high density plasma can be sustained for the low gas flow rate, i.e. in the low pressure condition.

MR Compatible Power Supply Module for PET Detectors of an Integrated PET/MR System

Power supply of positron emission tomography (PET) detectors is a serious issue for PET/magnetic resonance imaging (MRI) systems, due to mutual interaction between power supply electronics and MRI magnetic fields. We developed a prototype of a magnetic compatible power supply, that can be placed in an MRI system. The power supply is a compact ( $5.2\times 5.2$ cm2) iron-free switching dc–dc converter. The power supply works in the very high frequency (VHF) range, between 24.5 and 26.5 MHz, to allow using air core MR compatible inductors, and avoiding RF coupling between harmonics of power supply signal and RF magnetic field of a 1.5 T MRI scanner. The power supply has been designed as an upgrades for the TRIMAGE system and is able to convert 12-V dc input to 5-V dc output with a measured efficiency of 71.43%. Magnetic compatibility has been verified in the 1.5 T magnet of the IRCCS Stella Maris Foundation, Pisa, Italy, and in the 7 T MRI of the IMAGO7 Foundation, Pisa, Italy. Performance measurements of the power supply showed no variations during the magnetic compatibility tests. MRI scanner performance was similar with and without the power supply, and no artifacts were induced by the circuit in the acquired MRI images.

High-Frequency Nonlinear Response of Superconducting Cavity-Grade Nb Surfaces

Nb Superconducting Radio-Frequency (SRF) cavities are observed to break down and lose their high-Q superconducting properties at accelerating gradients below the limits imposed by theory. The microscopic origins of SRF cavity breakdown are still a matter of some debate. To investigate these microscopic issues temperature and power dependent local third harmonic response was measured on bulk Nb and Nb thin film samples using a novel near-field magnetic microwave microscope between 2.9K-10K and 2GHz-6GHz. Both periodic and non-periodic response as a function of applied RF field amplitude are observed. We attribute these features to extrinsic and intrinsic nonlinear responses of the sample. The RF-current-biased Resistively Shunted Junction (RSJ) model can account for the periodic response and fits very well to the data using reasonable parameters. The non-periodic response is consistent with vortex semi-loops penetrating into the bulk of the sample once sufficiently high RF magnetic field is applied, and the data can be fit to a Time-Dependent Ginzburg-Landau (TDGL) model of this process. The fact that these responses are measured on a wide variety of Nb samples suggests that we have captured the generic nonlinear response of air-exposed Nb surfaces.

Influence of the RF magnetic field on beam dynamics in SC200 cyclotron

Abstract The SC200 superconducting cyclotron is developed by the collaboration between ASIPP (Hefei, China) and JINR (Dubna, Russia). The SC200 cyclotron will deliver 200 MeV beam for proton therapy and research. The influence of the time-dependent magnetic field created by the acceleration system of SC200 should be taken into account in beam dynamics simulations and during the shimming procedure, as two conditions are present together: first — due to the spiral shape of the accelerating cavities, there is no vertical plane of symmetry through the middle of the dee; and second — second harmonic mode of acceleration in four-sector magnet structure imposes that the RF phase shift from beam entrance of the dee to its exit is not 180 degrees. Particle tracking was performed with 3D maps of electromagnetic fields obtained from computer modelling of the magnet system and the accelerating RF cavities using CST Studio Suite. We have calculated particle dynamics taking into account RF magnetic field and have estimated a value of additional mean field needed for the compensation of beam phase lag induced by RF magnetic field. And finally, we designed a shim that should form the required additional field.

A Systematic Review of the Safety and Efficacy of First- and Second- Generation Antipsychotics in Childhood and Early-Onset Schizophrenia

Transmit Array Spatial Encoding (TRASE) is a novel MRI technique that encodes spatial information by introducing phase gradients in the transmit RF (B1) magnetic field. Since TRASE relies on the use of multiple RF fields (B1 fields with different phase gradients) for k-space traversal, a TRASE pulse sequence requires RF pulses that are produced by switching between the transmit coils (B1 fields). However, interactions among the transmit RF coils can cause un-driven coils to produce unwanted B1 fields that impair the spatial encoding. Therefore, TRASE is sensitive to B1 field perturbations arising from inductive coupling among the RF transmit coils and any B1 field isolation (coil decoupling) technique requires an understanding of the effects of the B1 field interactions. The purpose of this study was to investigate the effects of B1 field coupling using Bloch equation based simulations and to determine the acceptable level of B1 field interactions for 2D TRASE imaging. The simulations show that 2D TRASE MRI (using a 3-coil setup) displays ideal performance for pairwise coupling constant lower than k = 0.01 while having acceptable performance up to k = 0.1. This translates into S12 measurements of range ~(− 50 dB to −30 dB) required for successful 2D TRASE MRI in this study. This result is of crucial importance for designers of practical TRASE transmit array systems.

Radiofrequency driving of coherent electron spin dynamics inn-GaAs detected by Faraday rotation

We suggest a new pump-probe method for studying semiconductor spin dynamics based on pumping of carrier spins by a pulse of oscillating radiofrequency (rf) magnetic field and probing by measuring the Faraday rotation of a short laser pulse. We demonstrate this technique on $n$-GaAs and observe the onset and decay of coherent spin precession during and after the course of rf pulse excitation. We show that the rf field resonantly addresses the electron spins with Larmor frequencies close to that of the rf field. This opens the opportunity to determine the homogeneous spin coherence time $T_2$, that is inaccessible directly in standard all-optical pump-probe experiments.

Towards predicting intracellular radiofrequency radiation effects.

Recent experiments have reported an effect of weak radiofrequency magnetic fields in the MHz-range on the concentrations of reactive oxygen species (ROS) in living cells. Since the energy that could possibly be deposited by the radiation is orders of magnitude smaller than the energy of molecular thermal motion, it was suggested that the effect was caused by the interaction of RF magnetic fields with transient radical pairs within the cells, affecting the ROS formation rates through the radical pair mechanism. It is, however, at present not entirely clear how to predict RF magnetic field effects at certain field frequency and intensity in nanoscale biomolecular systems. We suggest a possible recipe for interpreting the radiofrequency effects in cells by presenting a general workflow for calculation of the reactive perturbations inside a cell as a function of RF magnetic field strength and frequency. To justify the workflow, we discuss the effects of radiofrequency magnetic fields on generic spin systems to particularly illustrate how the reactive radicals could be affected by specific parameters of the experiment. We finally argue that the suggested workflow can be used to predict effects of radiofrequency magnetic fields on radical pairs in biological cells, which is specially important for wireless recharging technologies where one has to know of any harmful effects that exposure to such radiation might cause.

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Beside the point

Metamaterial Zeroth-Order Resonator RF Coil for Human Head: Preliminary Design for 10.5 T MRI

The objective is to develop a highly efficient RF head coil on a thin substrate for the ultra-high magnetic field MRI systems. The metamaterial zeroth-order resonator is investigated for this purpose. Simulation and experimental results are provided for an eight-channel M-ZOR-based RF coil in comparison with a standard high performance eight-channel dipole-based RF coil for the 10.5 T MRI system. Each element is 18 cm (approximately a quarter of a wavelength λ0) long, identical, evenly spaced along the circumference of the cylindrical phantom, loaded with dielectric material, and referred to as an inverted metamaterial zeroth-order resonator. The resonator elements are open circuited, matched, and tuned to 447.06 MHz with the phantom. An unloaded to loaded Q-factor ratio of 2.97 is obtained from the scattering matrix of the proposed design. The length independent nature of the proposed design and the flexibility of the lumped elements have provided an optimized element with a substrate thickness of roughly 3 mm (λ0/200). With the proposed design, there is a similar RF magnetic field strength (B1+) to SAR ratio with a reduced 10 g averaged SAR of 2.892 for the same input power compared to that of a dipole coil. This could make the coil acceptable for the clinical high-quality imaging.

Imaging collective behavior in an rf-SQUID metamaterial tuned by DC and RF magnetic fields

We examine the collective behavior of two-dimensional nonlinear superconducting metamaterials using a non-contact spatially resolved imaging technique. The metamaterial is made up of sub-wavelength nonlinear microwave oscillators in a strongly coupled 27 × 27 planar array of radio-frequency Superconducting QUantum Interference Devices (rf-SQUIDs). By using low-temperature laser scanning microscopy, we image microwave currents in the driven SQUIDs while in non-radiating dark modes and identify the clustering and uniformity of like-oscillating meta-atoms. We follow the rearrangement of coherent patterns due to meta-atom resonant frequency tuning as a function of external dc and rf magnetic flux bias. We find that the rf current distribution across the SQUID array at zero dc flux and small rf flux reveals a low degree of coherence. By contrast, the spatial coherence improves dramatically upon increasing the rf flux amplitude, in agreement with simulation.

Time minimal saturation of a pair of spins and application in Magnetic Resonance Imaging

In this article, we analyze the time minimal control for the saturation of a pair of spins of the same species but with inhomogeneities of the applied RF-magnetic field, in relation with the contrast problem in Magnetic Resonance Imaging. We make a complete analysis based on geometric control to classify the optimal syntheses in the single spin case to pave the road to analyze the case of two spins. The Bocop software is used to determine local minimizers for physical test cases and Linear Matrix Inequalities approach is applied to estimate the global optimal value and validate the previous computations. This is complemented by numerical computations combining shooting and continuation methods implemented in the HamPath software to analyze the structure of the time minimal solution with respect to the set of parameters of the species. Symbolic computations techniques are used to handle the singularity analysis.

First commissioning results of the waveguide RF Wien filter

The JEDI (Julich Electric Dipole Investigations) (http://collaborations.fz-juelich.de/ikp/jedi/) Collaboration aims to carry out a long term project for the measurement of the permanent electric dipole moments of charged particles in a storage ring. As a proof-of-concept, the COoler SYnchrotron (COSY) was equipped with a waveguide RF Wien filter designed to operate at some harmonics of the spin precession frequency ranging from 0.1 to 2 MHz. This device maintains the corresponding ratio between the RF electric and magnetic fields necessary not to induce any beam excitation and most importantly acts as a spin flipper. In the course of 2017, the waveguide RF Wien has been successfully commissioned and tested. The ability of the device to produce a Lorentz Force compensation and to rotate the particles’ polarization vector has been verified. Driven vertical spin oscillations and vertical polarization build-up has been observed. This short article briefly discusses the results of the Lorentz force measurements at 871 kHz.

Effects of rf magnetic field on upstream dielectric multipactor

A theoretic model of upstream dielectric multipactor in TEM microwaves, taking both the rf electric field and rf magnetic field into account, is proposed. In this theory, the growth rate of the window multipactor is predicted. The multipactor is approximately independent of the rf frequency, and much more serious than Kishek’s model (1998 Phys. Rev. Lett. 80 193) in the regions of weak dc electric fields and strong rf electric fields. Then, an electromagnetic Particle-In-Cell method is carried out to simulate the multipactor in TEM microwaves. Simulation results show a good agreement with the theoretical results. Furthermore, the lower boundary of multipactor is derived analytically, while there is no upper boundary any more. Finally, a gradually transition between Kishek’s model and our model is found by examining microwaves of TE modes. It is found that the upper boundary reappears when the rf frequency of TE modes trends to the cutoff frequency of waveguide.

Fast quantitative MRI using controlled saturation magnetization transfer.

PurposeThis study demonstrates magnetization transfer (MT) effects directly affect relaxometry measurements and develops a framework that allows single‐pool models to be valid in 2‐pool MT systems.MethodsA theoretical framework is developed in which a 2‐pool MT system effectively behaves as a single‐pool if the RMS RF magnetic field (B1rms{\ ext{B}}_{1}^{{{\ ext{rms}}}}) is kept fixed across all measurements. A practical method for achieving controlled saturation magnetization transfer (CSMT) using multiband RF pulses is proposed. Numerical, Phantom, and in vivo validations were performed directly comparing steady state (SS) estimation approaches that under correct single‐pool assumptions would be expected to vary in precision but not accuracy.ResultsNumerical simulations predict single‐pool estimates obtained from MT model generated data are not consistent for different SS estimation methods, and a systematic underestimation of T2 is expected. Neither effect occurs under the proposed CSMT approach. Both phantom and in vivo experiments corroborate the numerical predictions. Experimental data highlights that even when using the same relaxometry method, different estimates are obtained depending on which combination of flip angles (FAs) and TRs are used if the CSMT approach is not used. Using CSMT, stable measurements of both T1 and T2 are obtained. The measured T1 (T1CSMT)) depends on B1rms{\ ext{B}}_{1}^{{{\ ext{rms}}}}, which is therefore an important parameter to specify.ConclusionThis work demonstrates that conventional single pool relaxometry, which is highly efficient for human studies, results in unreliable parameter estimates in biological tissues because of MT effects. The proposed CSMT framework is shown to allow single‐pool assumptions to be valid, enabling reliable and efficient quantitative imaging to be performed.

Multi-functional RF coils for ultra-high field MRI based on 1D/2D electromagnetic metamaterials

Electromagnetic metamaterials have already proven very valuable for the enhancement and molding of RF magnetic fields within ultra-high field MRI scanners at 7T. We report on our development of coil elements based on composite right-/left-handed (CRLH) 1D electromagnetic (EM) metamaterial transmission lines (metalines) operating in the zeroth order resonance (ZOR) to foster uniform RF magnetic field distributions along the scanner axis. Tailored EM metalines supporting full-wave or quarter-wave resonances are used either as metamaterial ring antenna or as dual-band coil elements for simultaneous 1H/23Na imaging. The EM metalines are key to the MetaBore, which is a fully adaptive RF field control scheme based on a periodic axial arrangement of conformal metamaterial ring antennas in the framework of high-field traveling-wave MRI. With the 2D EM metamaterials (metasurfaces) we realized high-impedance surfaces (HIS) in order to enhance the uniformity and directivity of the RF magnetic field from e.g. overlaid (elongated) dipole elements towards the probe volume. The designs include simulation studies of the overall multichannel coil systems, which are carried out with our home-made, open source electromagnetic 3D EC-FDTD solver openEMS supporting conformal cylindrical inhomogeneous meshing. Experimental verifications of our coils have been carried out within 7T MRI scanners (Siemens Magnetom).