A high temperature superconductor dc SQUID planar gradiometer measurement system for routine inspections

We have developed a high temperature superconductor (HTS) dc SQUID gradiometric measurement system for routine applications in non-destructive evaluations. Low values of the white flux noise and white field gradient noise were measured for gradiometers operating at 77 K. Low frequency noise was suppressed using an ac bias technique, even in a magnetically unshielded environment. First-order planar dc SQUID flip-chip gradiometers were used in the measurement system for stable operation under electromagnetically noisy conditions. A synchronous filter was employed to remove all harmonic components of parasitic line frequency interference. This improved the resolution of the system in a typical laboratory or industrial environment showing strong magnetic gradients of the line frequency signal due to nearby metallic constructions. Test scans of contaminations with magnetic particles were performed.