Studies On High Repetition Rate Pulsed Discharge Of Atmospheric He-Ar Gas Mixture

Optically pumped metastable rare gas lasers have received widespread attention because of their chemical inertness,which overcomes the chemically active defect of optically pumped alkali metal vapor laser.As a new type of gas laser,it has the potential of high beam quality and high-power laser output.This kind of laser has the characteristics of a three-level scheme,in which the lower energy level of the laser is a metastable state,which is mainly produced by discharge.A sufficiently high metastable particle number density is the key to the efficient operation of the laser system.High voltage direct current pulse discharge can produce the required metastable particle number density,but the effect of discharge conditions on the characteristic parameters of plasma needs further study.Therefore,under the condition of atmospheric pressure,this paper takes He-Ar mixture gas as the research object to study the plasma generated by DC pulse discharge.In order to reduce the energy loss caused by ballast resistance,and to obtain high amplitude pulse DC high voltage with lower charging voltage,this thesis proposed a charging and discharging circuit scheme based on resonant charging loop and LC generator,and connected to the discharge zone by a high voltage coaxial cable.The differential equations of the charge-discharge circuit were obtained by approximating the discharge zone with a resistor with resistance varying with the voltage and the high-voltage coaxial cable with the single-ring or four-ring RLC circuit.Fourth-order Runge-Kutta method was used to obtain the voltage and current waveforms at the output port of discharge circuit and the high voltage electrode of the discharge zone.The current sensor and high voltage prober were used to test at the output port,and the discharge voltage and current waveforms under different working pressure,argon content and charging power supply voltage were obtained.These voltage and current waveforms are basically consistent with the results of circuit simulation,revealing that the discharge process is mainly composed of the charging of the high-voltage coaxial cable,the main discharge,and the secondary discharge caused by the overcharging of the energy storage capacitor.Further analysis of the discharge voltage and current waveforms shows that the discharge current will increase with the increase of the charging voltage,working pressure and argon content.The breakdown voltage also increases with the increase of charging power supply voltage and argon content of working gas.These results are consistent with the phenomenon of unstable discharge under high pressure and high argon content.The characteristic parameters of He-Ar mixed gas discharge plasma were diagnosed by emission spectroscopy.The gas temperature was obtained by fitting the Q₁ emission spectrum of the OH radical;the electron excitation temperature was obtained by using Boltzmann mapping to the characteristic emission lines of argon;the electron density was obtained by Stark broadening of the emission lines of Ar atom at 696.54 nm.According to the time-average experimental results,the gas temperature under experimental conditions is350?400 K and the electron excitation temperature is greater than 1 e V.The electron density is on the order of 10¹⁵ cm^-3.The plasma gas temperature,electron excitation temperature and electron density will increase with the increase of discharge voltage.The increase of gas pressure leads to the increase of electron density,but the gas temperature and electron excitation temperature decrease.When the argon content in the working gas increases,the gas temperature increases,while the electron excitation temperature decreases,and the electron density increases slightly.Time-resolved emission spectroscopy by a photo multiplier tube was used to study the time-resolved characteristics of mixed gas discharge plasma.It was found that the evolution of time-resolved emission spectrum can be divided into three processes,the first two are synchronized with the change of the main peak and the secondary peak of the current pulse,and the third is the plasma recombination process after the discharge current pulse,in which the maximum electron excitation temperature appears.It is found that the increase of argon content will weaken the plasma recombination,that is,the radiation of the third process will be weakened.The maximum time-resolved electron excitation temperature is about 2?5times of the time-averaged.The effect of discharge parameters on the time-resolved excitation temperature of electrons is the same as that of the time-averaged,and the excitation temperature decreases with the increase of gas pressure and argon content.