Fig. 1. Schematic diagram of experimental equipment

Fig. 2. Schematic diagrams of passive electric probe device. (a) Side view; (b) top view

Fig. 3. Electrical signal waveform of the plasma under different heat inputs. (a) 75 J/mm; (b) 50 J/mm; (c) 37.5 J/mm

Fig. 4. Photoelectric synchronization signal of the plasma. (a) Electrical signal waveform of the plasma; (b)(c) electrical signal waveform of an plasma erupting period and its derivative waveform

Fig. 5. Variation of plasma erupting velocity with time under different heat input

Fig. 6. Probability density distribution of the plasma erupting velocity at different heat inputs

Fig. 7. Principle diagrams of the keyhole formation^[17](a: molten-pool keyhole diameter; r₀: laser spot radium; P₀: atmospheric pressure). (a) Variation of P_abl and P_λ with the keyhole radius; (b)variation of ΔP with the keyhole radius

Fig. 8. Time difference detection schematic diagram of characteristic bands of electrical signal waves

Fig. 9. Probability density distribution of k at different heat inputs

Fig. 10. Schematic diagrams of sequence of the signal start to increase. (a) Lower probe ahead; (b) lower probe behind

Fig. 11. Probability density distribution of the lower probe ahead time at different heat input