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长脉冲激光辐照强场阴极材料碳化硅释气特性研究

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

  • 摘要: 本文在超高真空(8.5×10−7 Pa)条件下采用波长1064 nm,脉宽750 μs的激光模拟脉冲热冲击,对碳化硅(SiC)材料的释气特性进行了实验研究。主要探讨了激光能量、激光光斑大小和激光脉冲数对SiC释气特性的影响。通过全量程真空规和四极杆质谱仪(QMS)监测了激光脉冲热冲击过程中真空腔室内气压和质谱检测的气体成分(N2+)的变化。研究表明,增加激光能量密度为6.46 J/cm2时,SiC表面开始有气体分子(粒子)释放;随着激光能量密度的进一步增加,释气量也进一步增加。当激光能量密度达到22.49 J/cm2时,SiC表面释放的气体分子(粒子)数明显增多。研究还表明,适当的激光能量密度可以有效诱导SiC材料表面的气体释放,但过高的能量密度可能导致材料损伤,实验测定的SiC损伤阈值为22.49 J/cm2。在激光能量为139 mJ的条件下,通过调节聚焦透镜焦点到SiC表面的距离改变光斑直径,单位面积释放的粒子数在光斑直径为0.68 mm处达到最大值,此时的激光能量密度为38.04 J/cm2,并不是最大的激光能量密度。这说明SiC单位面积释放的粒子数不仅与激光能量密度相关,还与激光辐照区域面积大小相关。在激光能量密度为25.76 J/cm2的条件下,随着脉冲数的增加,SiC的释气量逐渐增加,但每次脉冲引起的粒子释放量和烧蚀坑深度随脉冲数逐渐减小。其原因可能是由于激光重复辐照导致材料表面粗糙度降低,从而降低材料对激光能量的吸收效率。

     

    Abstract: 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.

     

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