Detection of high-frequency gravitational waves using SRF cavities

TL;DR

This work targets high-frequency gravitational waves in the 10 kHz to 100 MHz band by reviving and optimizing a MAGO-era superconducting radio-frequency cavity with two nearly degenerate TE011 modes. The approach hinges on heterodyne energy transfer between a loaded and a quiet mode when $|\omega_\pi - \omega_0| \approx \omega_g$, with a deliberately weak cell-to-cell coupling $k_{cc} \sim 10^{-4}$ to maximize GW sensitivity. Through warm commissioning, surface treatment, and plastic tuning, the team reduced the initial large mode splitting, achieved near-degenerate TE011 modes, and demonstrated high-quality factors and stable resonance control in 2 K and 4 K cold tests, including mode suppression strategies. The results establish a practical pathway toward a high-frequency GW detector, and outline next steps such as DESY 2 K tests with Carrier-Suppression Interferometry and piezo-based actuation to push toward exclusion limits in this unexplored frequency domain.

Abstract

Today, apart from some isolated R&D efforts, there are no gravitational wave (GW) experiments, yet which explore a large part of the vast frequency range above the LIGO/Virgo band. It is planned to establish an experiment at Deutsches Elektronen-Synchrotron (DESY) and at the Superconducting Quantum Materials and Systems (SQMS) Center at Fermi National Accelerator Laboratory (Fermilab) to search for high-frequency GWs in the frequency range of 10 kHz to 100 MHz. The basic idea is to use superconducting radiofrequency (SRF) cavities to detect tiny harmonic deformations induced by GWs which change the boundary conditions of the oscillating electromagnetic field. This paper summarizes the challenging environmental boundary requirements, and the R&D to operate a cavity using a low level RF (LLRF) system which pushes beyond state-of-the-art accuracy and resolutions and a seismic noise mitigated cryostat at 1.8 K. The focus of this paper is the warm and cold commissioning of a prototype cavity, built 20 years ago during the MAGO collaboration, and its first measurement in our collaborative research project.

Figures (11)

• Figure 1: The PACO-2GHz-variable prototype niobium spherical cavity. In the picture we highlight which cell we refer as cell 1 vs cell 2.
• Figure 2: On the left: 3D Scan of the cavity, focused on the coupling cell. The colour code describes the deviation from the nominal cavity geometry. The dent in the tunable cell is clearly visible by the blue area. On the right: Close up image of the coupling cell. The outer surface was turned after deepdrawing to reduce the wall thickness and the force necessary to tune it.
• Figure 3: 3D visualisation of the cavity. The view is towards the two flanges at the side of each cell. The bending is mostly into one plane and clearly visible. The point $P_1$ on the rotational axis of the tube and the point $P_2$ on the nominal rotational axis of the cavity, highlighted on the right side of the cavity, are nominally expected to sit on the same plane, but are misaligned by 2.3 cm in the vertical plane. For reference, the cavity length, from flange to flange, is approximately 65 cm.
• Figure 4: Electric (left) and magnetic (right) field norm of the symmetric and anti-symmetric $\text{TE}_{011}$ modes which only differ in phase ($0$ and $\pi$) and have identical field patterns.
• Figure 5: $S_{21}$ measurements and simulation of the cavity at room temperature. Measurements at DESY/UHH (blue) have been performed with pin-antennas which couples to the electric field of the modes, while the measurement at Fermilab (green) used loop-antennas which couples to the magnetic field of the modes. The simulations (red) have been performed using the DESY antenna setup, using the scanned geometry and an averaged wall thickness.