By using Mach-Zehnder interferometer, we gain time series interference patterns of delayed double pulse laser and three- monopulse laser produced air plasmas. And then we gain electron density values in the centre of plasma region at different moments. We compare electron density values of the plasmas produced, respectively, by delayed double pulse laser and three-monopulse laser. The results show that the electron density of the plasma produced by the delayed double pulse laser is greater than by the three- monopulse laser at the same time after the second laser effects. The electron density time change processes of the plasma produced by delayed double pulse laser and monopulse laser of the same injection energy are analyzed theoretically. The analysis results show that when the same laser energy is injected, the delayed double pulse method can increase the plasma existing time effectively.%利用马赫-曾德尔干涉仪获得了延迟双脉冲激光和三种单脉冲激光产生大气等离子体的时间序列干涉图，得到了等离子体中心区域在不同时刻的电子密度．把延迟双脉冲激光与三种单脉冲激光产生的等离子体电子密度进行比较的结果显示：第二束光作用后的相同时刻，延迟双脉冲激光产生的等离子体电子密度大于三种单脉冲激光产生的等离子体电子密度．对注入相同能量的延迟双脉冲激光与单脉冲激光产生等离子体的电子密度时间变化进行理论分析的结果表明：延迟双脉冲激光的第二束光与等离子体相互作用，使得作用结束时等离子体的电子密度增加得很多，进而造成了第二束光作用后延迟双脉冲激光产生的等离子体电子密度大于单脉冲激光产生的等离子体电子密度．进一步的分析表明，注入能量相同时延迟双脉冲激光有效延长了等离子体的存在时间．
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