建立了封闭水平管道中激光传输热效应物理模型，详细研究了降低气体压强对光束质量变化的影响，辨别了热传导、浮力对流传热机制的转换时间与光束质量、光斑质心漂移转换时间的关系；数值模拟了气体吸收不同激光能量，在不同压强下的温度分布、附加相差和远场光束质量、光斑质心漂移的变化。结果表明，气体温度分布与气体热光系数两种因素共同决定远场光束质量变化的大小，而光束质量转换时间与两种传热机制转换时间一致，光斑质心漂移转换时间略有滞后；降低管道中气体压强，可较大地改善远场光束质量。

A model of a thermal blooming gas in a closed horizontal tube is used to study the effects of gas pressure on laser propagation. It focuses on the variation of beam quality in far-field and the switch time of heat conduction and heat convection in gas. By numerical simulation, the distributions of gas temperature, additional phase and beam quality and drift of mass center in far-field are shown under different gas pressures and absorptions of laser energy. And it is found that the change of beam quality is decided by both of the distribution of gas temperature and the thermal-optical coefficient, and the switch time of beam quality accords with the one between the two thermal conduction mechanisms, but the switch time of drift of mass center delays. Reducing the pressure in the tube, the gas has a higher temperature, but more even distribution, and the thermal-optical coefficient of gas scales down, so the beam quality consequently improves.