Enhancement of EMAT and eddy current using a ferrite back-plate

Electromagnetic acoustic transducers (EMATs) often have an electrically conducting surface behind the coil which we refer to as a back-plate, which could be there to act as an electromagnetic screen or could be a permanent magnet which is part of standard EMAT design. Eddy current probes can share many design features with EMATs, and both are designed to generate an eddy current in the electromagnetic skin-depth of a sample. In this paper we present an analytical solution for a quick and accurate calculation of the eddy current generated by a spiral coil and for the case of EMATs we also calculate the resultant Lorentz force which leads to ultrasonic generation. The Lorentz force can arise from an interaction of the eddy current with either a dynamic magnetic field from the coil or with a static magnetic field from a permanent of electromagnet. Theoretical predictions have shown that in the presence of a ferrite back-plate, the eddy current and the Lorentz force are enhanced greatly, and this has been verified experimentally. These developments will lead to improved EMAT and eddy current probe design. (C) 2006 Elsevier B.V. All rights reserved.