Modelling of magnetic fields to enhance the performance of an in-plane EMAT for laser-generated ultrasound.

A new magnetic arrangement is described for use with an in-plane electromagnetic acoustic transducer (EMAT) for detecting laser-generated ultrasound. The magnetic flux density was modelled and validated. Modelling was accomplished in 3D using finite element software to predict new magnet spatial distributions. A configuration was found which increased the magnetic flux density by a factor of 1.8+/-0.2, compared to magnetic configurations previously used in conventional designs. Model predictions were implemented and confirmed experimentally. As a result, laser ultrasound Rayleigh waves have been used to verify the performance of this sensor system. It was establish that the EMAT's in-plane sensitivity increased, while the frequency bandwidth improvement factor was about 1.9+/-0.2. The resonant frequency increased from 6.5 MHz and 16.4 MHz, with both exhibiting an extended frequency response well beyond the resonant values. For maximum frequency response, it was demonstrated that added elements such as cables may have a deleterious effect. In particular the length of the cable, which in turn adds capacitance to the overall circuit, will decrease the frequency response of the EMAT. The frequency response was compared with a previous sensor, to provide an increased resonant frequency factor of 2.5+/-0.2.