Recent Advances and Challenges in the Development of Radiofrequency HTS Coil for MRI

Radiofrequency (RF) coils fashioned from high-temperature superconductor (HTS) have the potential to increase the sensitivity of the magnetic resonance imaging (MRI) experiment by more than a dozen times compared to conventional copper coils. Progress, however, has been slow due to a series of technological hurdles. In this article, we present the developments that recently led to new perspectives for HTS coil in MRI, and challenges that still need to be solved. First, we recall the motivations for the implementations of HTS coils in MRI by presenting the limits of cooled copper coil technology, such as the anomalous skin effect limiting the decrease of the electric resistance of normal conductors at low temperature. Then, we address the progress made in the development of MRI compatible cryostats. New commercially available low-noise pulsed-tube cryocoolers and new materials removed the need for liquid nitrogen-based systems, allowing the design of cryogen-free and more user-friendly cryostats. Another recent advance was the understanding of how to mitigate the imaging artifacts induced by HTS diamagnetism through field cooling or temperature control of the HTS coil. Furthermore, artifacts can also originate from the RF field coupling between the transmission coil and the HTS reception coil. Here, we present the results of an experiment implementing a decoupling strategy exploiting nonlinearities in the electric response of HTS materials. Finally, we discuss the potential applications of HTS coils in bio-imaging and its prospects for further improvements. These include making the technology more user-friendly, implementing the HTS coils as coil arrays, and proposing solutions for the ongoing issue of decoupling. HTS coil still faces several challenges ahead, but the significant increase in sensitivity it offers lends it the prospect of being ultimately disruptive.

THE USE OF FERROMAGNETIC AMORPHOUS MATERIALS 5BDSR IN HIGH-FREQUENCY OVERVOLTAGE PROTECTION DEVICES

The paper discusses the prospects of using nanocrystalline amorphous materials in the electric power industry. In particular, in the field of protection of electric power equipment of substations from high-frequency pulses. The effectiveness of using such materials to activate the anomalous skin effect in multilayer conductors is shown. The resulting conductors increase their resistance at lightning frequencies, thereby making it possible to create devices that effectively suppress high-frequency pulses. The authors give an example of a protective frequency-dependent device using nanocrystalline ferromagnetic material and the results of its use in trial operation. The paper shows the results of comparing round and flat frequency-dependent conductors.

Эллипсометрия металлических пленок в условиях аномального скин-эффекта

Inhomogeneous Fredholm’s integral equations of the second kind are formulated, which describe the fields of TE and TM polarized waves in metallic films with allowance for the anomalous skin effect. The equations are solved numerically by the quadrature method. The electric fields in gold and aluminum films located on a silicon substrate and the angular dependences of the polarization angles of light reflected from the films are investigated. It is found that the solution of the inverse problem of multi-angle ellipsometry for metallic films using the standard model of the normal skin effect is characterized by instability of the reconstructed complex refractive index of the metal with a change in the thickness of the metallic film.

The influence of fractal geometry on anomalous skin-effect in metal systems

The paper defines basic criteria of surface fractal geometry for 50Ω Au/i-GaAs{100} coplanar microwave transmission lines, which influence on active resistance of their skin-layer and inductivity L. The local approximation limit L for active resistance L(R) is ≈800 um and for inductivity L(L) is ≈400 um.