Mechanisms of Noise Generation in Superconducting Quantum Interference Device Based Gradiometers Under Vibrations

Abstract Superconducting quantum interference device (SQUID) gradiometers are sensitive instruments for detecting magnetic fields and are widely used in magnetocardiography (MCG). However, their performance is compromised by vibrations from foundations or refrigeration equipment. Vibration‐induced effects on SQUID gradiometers occur through two main mechanisms. First, an initial angular deviation between the pickup coils prevents cancellation of the reversed magnetic flux from external fields. Vibration converts this inherent interference into a time‐varying flux disturbance at the vibration frequency. Second, bubbles from boiling liquid helium passing through the gradiometer loops increase the gradiometer's noise floor in the frequency domain. Vibration alters the dynamic behavior of these bubbles. Finite element simulations are conducted to examine the influence of these mechanisms across various vibration frequencies, and the SQUID gradiometer's noise power spectral density is measured at six vibration frequencies. The noise contains both narrowband and broadband components: The narrowband noise primarily occurs at the vibration frequencies and their second harmonics when the pickup coil's angular displacement aligns with or is perpendicular to the vibration direction. Meanwhile, the introduction of vibrations elevates the broadband noise level relative to the non‐vibration state. Moreover, the broadband noise exhibits distinct peaks at the gradiometer's intrinsic resonance frequency of 15 Hz.