Searching for dark matter with plasma haloscopes

We summarize the recent progress of the Axion Longitudinal Plasma Haloscope (ALPHA) Consortium, a new experimental collaboration to build a plasma haloscope to search for axions and dark photons. The plasma haloscope is a novel method for the detection of the resonant conversion of light dark matter to photons. ALPHA will be sensitive to QCD axions over almost a decade of parameter space, potentially discovering dark matter and resolving the strong CP problem. Unlike traditional cavity haloscopes, which are generally limited in volume by the Compton wavelength of the dark matter, plasma haloscopes use a wire metamaterial to create a tuneable artificial plasma frequency, decoupling the wavelength of light from the Compton wavelength and allowing for much stronger signals. We develop the theoretical foundations of plasma haloscopes and discuss recent experimental progress. Finally, we outline a baseline design for ALPHA and show that a full-scale experiment could discover QCD axions over almost a decade of parameter space

Extending a Birdcage Coil for Magnetic Resonance Imaging of a Human Head with an Artificial Magnetic Shield

In magnetic resonance imaging, a birdcage coil is the most commonly used volumetric resonator creating a highly homogeneous radiofrequency magnetic field in a conductive sample. An artificial magnetic radiofrequency shield was recently shown to improve the magnetic field amplitude per unit power (transmit efficiency) of a preclinical birdcage coil by reducing the intrinsic losses in the coil and increasing power absorbed by the sample. In this paper, we propose a new application of an artificial shield in clinical MRI. Thanks to the proposed artificial shield a birdcage coil for human brain imaging operating at 300 MHz (Larmor frequency of protons at static fields of 7 T) can be expanded to increase free space. As a result, the coil becomes more comfortable for the patient and keeping comparable transmit efficiency. The same extended coil with a conventional copper shield would have at least 10% lower efficiency. The proposed artificial shield is implemented as an annular-ring cavity-backed slot in a copper cylinder that tightly surrounds the birdcage. To demonstrate the effect, radiofrequency magnetic field and specific absorption rate distributions were compared numerically and experimentally for the initial and extended coils with different shields.Peer reviewe

Wideband coil based on microstrip line for multiheteronuclear magnetic resonance spectroscopy

Funding Information: This work was supported by the Russian Foundation for Basic Research (Grants No. 19-29-10038). Publisher Copyright: © 2021 Institute of Physics Publishing. All rights reserved.Magnetic Resonance Spectroscopy of X-nucleus is a method for mapping metabolite quantity in-vivo in the desired region of the human body. However, this method has a lot of difficulties. Since natural abundance of X-nucleus is much lower than for hydrogen, to improve SNR of the spectrum we need to work in higher static magnetic fields. But, even at such high fields, X-nuclei imaging still a very difficult process. One of the problems are RF-coils, which are required for transmission and reception of signals. Traditionally, for human body X-nuclei MRS multi-tuned loop coils are used. However, such coils suffer from additional losses in the circuits required for double-frequency tuning and high complexity. In this work, we propose an alternative approach, based on a recently introduced leaky-wave antenna for MRI, that allows creating wideband excitation which provides wideband transmit and receive. This wideband frequency range covering 13C, 23Na and 31P Larmor resonant frequencies.Peer reviewe