fluxlocked

Fluxlocked, often written flux-locked in reference to the flux-locked loop (FLL), is a principle and mode of operation used in superconducting quantum interference device (SQUID) magnetometers and related sensors. In a flux-locked system, the external magnetic flux acting on a superconducting loop is actively canceled by a feedback current, keeping the net flux near a fixed operating point. This yields a linear, high-bandwidth response over a wide dynamic range, addressing the intrinsic periodic, highly nonlinear transfer function of a bare SQUID.

Operation: The SQUID converts magnetic flux to a change in voltage. This signal is compared to a reference, and a servo amplifier sends a current through a feedback coil to oppose the measured flux. The current required to cancel the flux is then proportional to the external magnetic field, enabling accurate measurement. The loop bandwidth is set by the electronics, and the design minimizes noise and flux trapping.

History and context: The flux-locked loop was developed in the 1960s–1970s as SQUID technology matured, enabling practical magnetometers with nearly linear response. Texts like The SQUID Handbook discuss implementations and performance.

Applications: FLL-based SQUIDs are used in magnetoencephalography (MEG), geophysical surveys, nondestructive evaluation, material characterization, and scanning SQUID microscopy. High-temperature superconductor versions extend operation toward liquid nitrogen temperatures but often with reduced sensitivity and bandwidth.

Characteristics and limitations: The method provides vast linear dynamic range and high sensitivity, but requires cryogenic cooling, careful shielding, and precise electronics. Bandwidth is limited by the feedback loop and readout chain, and flux trapping can degrade performance if the device is not cooled or magnetized properly.