Fig. 1. Schematic diagram of the experimental platform

Fig. 2. Simulations of electric field intensity at five voltages
(a): U=12 kV; (b): U=13.5 kV; (c): U=14.8 kV;(d): U=16 kV; (e): U=16.9 kV

Fig. 3. Emission spectrum in the early stage of discharge

Fig. 4. Emission spectrum of the discharge with deeper degree (U=16 kV)

Fig. 5. Emission spectrum of the critical breakdown stage of discharge (U=16.9 kV)

Fig. 6. Spectral comparison of different discharge stages

Fig. 7. Normalized spectral intensities at three characteristic wavelengths

Fig. 8. Line chart of Full-band photons’ number varying with voltage

Fig. 9. Normalized photons’ number in three bands at different discharge stages

Fig. 10. Normalized photons’ number in two bands at different discharge stages