Alkali-metal spin maser for non-destructive tests

Abstract

Radio-frequency atomic magnetometers offer attractive alternatives to standard detection methods in nondestructive testing, which are based on inductive measurements. We demonstrate a magnetometer in the so-called spin maser configuration, which addresses two important challenges of the technique: shifts in the radio frequency resonance position caused by magnetically permeable samples and the sensor bandwidth. Key properties of the self-oscillating sensor are presented in both a magnetically shielded and an open environment. Demonstration of defect detection via magnetic induction tomography in a ferromagnetic carbon steel sample is presented. The configuration discussed paves the way for a simple, rapid, and robust nondestructive material defect detection system based on an atomic magnetometer.

Inductive Imaging of the Concealed Defects with Radio-Frequency Atomic Magnetometers

“…The measurements are performed in a magnetically unshielded environment [22][23][24][25][26]. The bias magnetic field is actively stabilised by three pairs of nested, orthogonal, square Helmholtz coils.…”

“…Our previous studies have been focused on measuring object thinning in the form of a recess [22][23][24][25][26]. Analysis of the amplitude and phase images validated this method as a tool for monitoring defect (recess) depth.…”

“…In particular, it enables operation at low frequencies, where the change in the penetration depth of the rf field is most significant. This category of sensors includes giant magnetoresistance (GMR) magnetometers [12][13][14], superconducting quantum interference devices (SQUIDs) [15,16] and atomic magnetometers [17][18][19][20][21][22][23][24][25][26].…”

“…In particular, the semi-vector mapping of the secondary field components generated by a defect was recently demonstrated in NDT measurements with an rf atomic magnetometer, although this sensor is typically regarded as scalar in nature [23,24]. The operation of a magnetometer in the so-called spin maser mode (self-generated drive for rf magnetic field) addresses the issue of the sensor's narrow bandwidth [25,26]. In this mode, spontaneous fluctuations of the atomic spins are detected, amplified, and fed back to the primary coil to create an rf primary field whose frequency self-tunes to the rf resonance frequency.…”

“…For the uncovered defect, the amplitude of the defect signature increases with frequency, which is a simple consequence of the growth of induction with rf field frequency. Most of the observations reported in [22][23][24][25][26] have been performed with the rf primary field frequency in a range of tens of kHz. One of the main challenges for existing NDT methods is the detection of concealed defects; e.g., defects covered by an insulating material, buried within, or located on an inner surface of the object.…”