## 1 Introduction
The high-power thyristor integrated circuit is generally used in conventional high-power phase EMATs as an acoustic excitation source, such as a four-channel IGBT excitation source. Due to the shortcomings of thyristors, such as the thermal effects, low switching frequency, and low withstand voltage, the existing transducer can only excite low-frequency acoustic waves. Moreover, it is difficult to excite thousands of amperes of tone-burst excitation current in the electromagnetic acoustic excitation coil. Above all, the transduction efficiency of conventional EMATs is low, especially at a higher lift-off distance. There is an upper limit in the bias magnetic field strength, which limits the maximum ultrasonic power, and ferromagnetic particles can be attracted by the magnets of conventional EMATs, the measurement results can be affected by which [27]. In order to solve the above problems, in this paper, a high-energy electromagnetic acoustic excitation system was designed without a static bias magnetic field, and a high-power LC oscillation circuit was used as an excitation source. A transient high voltage is applied to the excitation coil. Then a large current is generated in the coil to excite a dynamic strong magnetic field. The dynamic magnetic field interacts with the induced eddy current to generate a powerful electromagnetic force that can generate the acoustic wave. This method solves the problem that the excitation source power output is limited. The natural frequency of the LC oscillation circuit can be changed to achieve acoustic excitation in each frequency band, by simultaneously adjusting the LC circuit capacitance and the electrical parameters of the excitation coil. The magnetic field excited by the high-energy pulse coil in the eddy current layer of the tested material is horizontal. In order to study the main transduction mechanism, an EMAT lift-off characteristic experiment was carried out, and the relationship between the EMAT lift-off distance and the main action mechanism of ultrasonic waves was analyzed. The EMAT without permanent magnets avoids the effect of the bias magnetic fields, and Lamb waves are very sensitive to the change of the external and internal acoustic impedance of the propagation medium. The requirement for sensitivity is satisfied for the detection of the ferromagnetic material. Electromagnetic Acoustic Excitation Transduction Mechanism of Ferromagnetic Materials Excited by the High-Energy Pulse  
The three transductions mechanism of Lorentz force, magnetostrictive force, and magnetizing force exists simultaneously in ferromagnetic metal materials. The magnetostrictive force and the magnetizing force change nonlinearly with the applied magnetic field. Therefore, they were combined with the applied magnetic field during analysis. The Lorentz force transduction mechanism of ferromagnetic materials is consistent with non-ferromagnetic metal materials [16]. Due to the different electromagnetic parameters of materials, the magnitude of the Lorentz force is different. In metal materials, the Lorentz force is generated by the interaction between induced eddy current and applied dynamic magnetic field, and the induced pulsed eddy current field is expressed as,  
\[\frac{1}{\mu}\
abla^{2}\mathbf{A}-\sigma\frac{\partial\mathbf{A}}{\partial t}=-\mathbf{J} \tag{1}\]  
where \(\mathbf{A}\) is the magnetic vector potential, \(\sigma\) is the material conductivity, \(\mu\) is the material permeability, and \(\mathbf{J}\) is the current density in the coil. The magnetic flux density \(\mathbf{B}\) and induced pulse eddy \(\mathbf{J_{e}}\) meet:  
\[\mathbf{B}=\
abla\times\mathbf{A} \tag{2}\]  
\[\mathbf{J_{e}}=-\sigma\frac{\partial\mathbf{A}}{\partial t}. \tag{3}\]  
In metal materials, the Lorenz force is expressed as,  
\[\mathbf{F_{L}}=\mathbf{J_{e}}\times\mathbf{B} \tag{4}\]  
where, \(\mathbf{F_{L}}\) is the Lorentz force in the metal material, \(\mathbf{J_{e}}\) is current density in the coil, \(\mathbf{B}\) is dynamic magnetic flux density.