The Outgassing Characteristics of Silicon Carbide as an Intense-Field Cathode Material Under Long-Pulse Laser Irradiation

In this work, a Nd:YAG laser operating at a wavelength of 1064 nm with a pulse width of 750 μs was utilized to simulate pulsed thermal shock on silicon carbide (SiC) material, aiming to investigate its outgassing characteristics under ultra-high vacuum conditions (pressure of 8.5×10⁻⁷ Pa). The study primarily delved into the influence of laser energy, laser spot size, and the number of laser pulses on SiC gas release characteristics. Throughout the laser pulse thermal shock process, chamber pressure variations and outing gas composition (N₂⁺) changes were monitored by using a full-range vacuum gauge and a quadrupole mass spectrometer (QMS). The results showed that gas molecules (particles) began to release when the laser energy density was increased to 6.46 J/cm², with a noticeable increase when the laser energy density further increased. Additionally, it was found that an appropriate laser energy density could effectively induce gas release from the SiC material surface, while excessively high energy density could lead to material damage, with the experimentally determined damage threshold of SiC being 22.49 J/cm². Moreover, the laser spot size on the SiC surface was adjusted by moving the distance from the to the SiC surface with a fixed laser energy of 139 mJ to investigate the gas releasing characteristics. The maximum number of particles released per unit area is at a beam diameter of 0.68 mm, corresponding to a laser energy density of 38.04 J/cm², which was not the maximum laser energy density. This indicates that the number of particles released per unit area of SiC is not only related to the laser energy density but also to the size of the laser irradiation area. Furthermore, under a laser energy density of 25.76 J/cm², an increase in the number of pulses was found to incrementally elevate the gas release quantity of SiC. However, the quantity of particles released per pulse and the depth of ablation pits induced by each pulse demonstrated a gradual decline with increasing pulse count. This may be due to the decrease in material surface roughness caused by repeated laser irradiation, thereby reducing the material's absorption efficiency of laser energy.