Fig. 1. Monitoring experimental device

Fig. 2. Plasma spectrum of paint layer under different pulse numbers. (a) 1 pulse; (b) 2 pulses; (c) 8 pulses; (d) 15 pulses; (e) 16 pulses; (f) 18 pulses; (g) 20 pulses

Fig. 3. Relation between characteristic peak intensity and pulse number

Fig. 4. Relative intensities of characteristic peaks Ti (431.5, 455.5 nm) and Al (396.2 nm)

Fig. 5. Variation of plasma density and temperature with the number of pulses

Fig. 6. 1-20 laser pulse time domain sound signals

Fig. 7. Variation of frequency domain distribution of acoustic signal with pulse number. (a) 1 pulse; (b) 2 pulses; (c) 8 pulses; (d) 15 pulses; (e) 16 pulses; (f) 18 pulses; (g) 20 pulses

Fig. 8. Variation of different frequency intensities with the number of pulses

Fig. 9. Spectrum of laser plasma acoustic wave

Fig. 10. Acoustic ray simulation results