## 2 EMAT Technology  
EMAT is a non-contact technology of transducers that can be used for the inspection of moving structures or a specimen that operates at elevated temperatures. Their response can travel through a vacuum as well, making them even more suitable for this application compared to piezoelectric transducers. EMATs can excite/receive guided waves and thus they can be employed in GWT for either the inspection or the monitoring of large structures [27; 28; 29; 30]. Hence, high-temperature EMATs may be used for the inspection or monitoring of absorber tubes. Nevertheless, the high-temperature EMATs that have been reported so far have been designed for thickness measurements [31; 32; 33]. Therefore, a high-temperature EMAT that excites/receives guided waves for the long-term inspection of high-temperature structures is still required. A typical EMAT transducer is composed of either a permanent magnet or an electromagnet for the generation of a static magnetic field, and a coil. The coil is driven by an alternating current that generates a dynamic magnetic field. This dynamic magnetic field induces an eddy current in the specimen placed below the coil. If the specimen being tested is an electrical conductor and non-ferromagnetic, mainly Lorentz force is exerted upon the material particles; Equation (1) shows that Lorentz force is equal to the product of eddy current density and the overall magnetic field:  
\[F_{\mathrm{L}}=J_{\mathrm{e}}\times(B_{\mathrm{st}}+B_{\mathrm{dyn}}) \tag{1}\]  
where \(F_{\mathrm{L}}\) is Lorentz force, \(J_{\mathrm{e}}\) is the eddy current density, \(B_{\mathrm{st}}\) stands for the static magnetic field, and \(B_{\mathrm{dyn}}\) refers to the dynamic magnetic field. In this application, the absorber tubes are made of 316 L stainless steel, which is paramagnetic; therefore the main force generated in them would be Lorentz force. The EMAT configuration and the material properties of its main components affect EMAT performance at both room and high temperatures. The material the coil is made of influences the EMAT performance. The impedance of the coil affects the energy transmitted to the specimen and the SNR/quality of the signal received; Equation (2) shows how the resistance increases with temperature rise and Equation (3) demonstrates the relationship between the temperature and the noise level of the signal received:  
\[R(T)=R_{0}[1+\alpha(T-T_{0})] \tag{2}\]  
where \(R_{0}\) is the resistance of a single turn coil, \(\alpha\) is the temperature coefficient of resistance, and \(T_{0}\) is the room temperature. \[V_{\mathrm{Noise}}=(4K\beta TR_{\mathrm{EMAT}})^{2} \tag{3}\]  
where \(K\) is the Boltzmann constant, \(T\) is the temperature measured in Kelvin, \(\beta\) is the bandwidth, and \(R_{\mathrm{EMAT}}\) is the resistance of the EMAT coil per turn. Consequently, the coil should be preferably made of a low-resistance material such as copper. However, copper is known to oxidize at high temperatures and, therefore, other materials could be used for this application such as silver, platinum, nickel, or constantan. [PERSON] (2011) has already made an electromagnet EMAT that can operate at up to 600 \({}^{\circ}\)C without any cooling system for thickness measurements and apart from designing a high temperature electromagnet he has also tested several coil designs at high temperatures [32]. The permanent magnets also have limitations regarding their maximum operating temperature. The Maximum Operating Temperature (MOT) of a magnet is equal to half of its Curie Point; beyond Curie Temperature, the strength of the magnet decreases rapidly. Consequently, high Curie Point magnets are preferable for high-temperature applications. Nevertheless, the magnetic strength of the main two types of high-temperature magnets, Alnico (with a maximum operating temperature of 500 \({}^{\circ}\)C) and SmCo (with a maximum operating temperature of 300 \({}^{\circ}\)C), is smaller than the magnetic strength of Neo (NdFeB) magnets.