Femtosecond intense laser pulses will show a strong nonlinear self-focusing effect when they propagate in the air. The self-focusing light intensity will cause field ionization of molecules in the air and form a plasma with a certain lateral density gradient. When the laser self-focusing effect and the plasma defocus effect to achieve dynamic equilibrium, it will form a stable self-guiding transmission, this transmission is also known as "wire." Filamentous femtosecond lasers produce narrow, long ionized channels in the air that can be extended to the kilometer-scale length. The filaments of femtosecond lasers in the atmosphere have been receiving a great deal of attention since their discovery in 1995 [A. Braun, et al., Opt. Lett. 20, 73 (1995)], mainly due to some of the ionization channels Special properties can lead to many potential practical applications. Such as using its conductivity, it is possible to achieve laser-guided lightning or electromagnetic radiation [LM Ball, Appl. Opt. 13, 2292 (1974); M. Chateauneuf, et al., Appl. Phys. Lett. 92, 091104 (2008) ]; The use of superconducting white light radiation generated during filament formation can achieve long-distance remote sensing and monitoring of air pollution [J. Kasparian, et al., Science 301, 61 (2003)]; The ultrafast interaction of the plasma produces a broad spectrum of THz radiation [CC Cheng, et al., Phys. Rev. Lett. 87, 213001 (2001)]; femtosecond laser filaments in water vapor-rich air A large number of ions generated will play the role of condensation nuclei, resulting in condensation of water vapor droplets, in the low temperature environment will generate snow, so some scholars have suggested that the use of femtosecond laser to increase the cloud of water droplets or ice crystals, thereby increasing precipitation Volume [P. Rohwetter, et al., Nat. Photonics 4, 451 (2010); J. Ju, et al., Opt. Lett. 37, 1214 (2012)]. Some important applications of ionization channels have high requirements on their life expectancy, such as laser lightning, long-distance conduction of electromagnetic radiation, laser artificial snow precipitation and so on. However, after the ionization channel is formed, the electron density decays rapidly due to the recombination of electrons and ions and the adsorption of electrons by the neutral molecules. Experiments show that the ionization channel generated by the monopulse femtosecond laser has a lifetime of only a few nanoseconds. This situation severely restricts the practical use of various technologies based on channel conductivity and ionization. Therefore, how to extend the life of ionization channels has become a very important research topic. In order to maintain the electron density of the ionization channel, energy must be continuously injected into the channel to prevent or retard the recombination and adsorption of electrons. At present, it is feasible to prolong the life of the ionization channel by repeatedly refreshing the ionization channel with a very short femtosecond pulse sequence. Researchers and their collaborators in the L05 group at the Institute of Physics, Chinese Academy of Sciences / Beijing Condensed State Laboratory (preparatory) conducted many years of unrelenting exploration in generating long-lived plasma channels. For the first time in 2012, After the "natural" high repetition rate pulse train output by the mode-locked oscillator is broadened, the multi-stage amplification without a menu succeeds in obtaining a pulse train consisting of femtosecond pulses of the order of 20 mJ with a pulse interval of 14.8 ns , Resulting in a significant breakthrough in the number of effective pulses [XL Liu, et al., Opti. Express, 20, 5968 (2012)]. Subsequently, in order to further improve the quality of the femtosecond laser pulse sequence, the original seed source of the "Aurora II" system was replaced by a 350 MHz femtosecond oscillator independently developed by the L07 group through the cooperation between the research groups. They multistage multi-pass amplification of the newly generated ultra-high repetition femtosecond seed pulse sequence, and succeeded in suppressing the gain competition between pulses by pumping two laser pulses in the main amplifier , And finally for the first time in the world, a high-quality femtosecond pulse sequence with a mean energy distribution of only 2.9 ns was obtained (see Figure 1e). Experiments in which such a sequence of femtosecond pulses are focused in the air by a telephoto lens give rise to ionization channels with a lifetime of 60 to 80 ns (see Fig. 2, 30-40 times longer lifetime compared to a single pulse ionization channel). This research has opened up a practical technical way for producing practical long-distance air plasma channels. On the other hand, this ultra-high repetition frequency femtosecond laser pulse sequence is a new type of light source, which has potential advantages in the fields of laser micro-machining, strong THz radiation generation and laser remote sensing. Relevant research papers were published in the journal Scientific Reports in October 2015. The work has been the Ministry of Science and Technology Project, 973 projects, the National Natural Science Foundation of China and the Chinese Academy of Sciences. Figure 1. Schematic diagram of the laser system and photodiode signals at different magnification stages of a femtosecond pulse train. (A) an oscillator signal; (b) a first preamplifier; (c) a second preamplifier; (d) a pulse train output when two pump lasers of the main amplifier are simultaneously pumping; Of the two pump laser pump, and its relative delay to optimize the uniform pulse sequence. Figure 2. Current signal obtained by applying DC voltage across the plasma channel. The duration of the current reflects the lifetime of the plasma channel (about 80 ns).