Studies On Gas-Discharge For Optically-pumped Metastable Argon Laser

Optically-pumped gas laser combines the characteristics of high efficiency of semiconductor laser and high beam quality of gas laser.It is considered as a potential candidate for high-energy laser and has attracted world-wide attention in recent years.Typical representatives are optically-pumped alkali metal vapor lasers and optically-pumped rare gas lasers（OPRGLs）.OPRGLs have advantages in the chemical stability of the working medium and have attracted the attention of researchers in the field of high-energy lasers.A number of teams in the United States,Russia and China are engaged in relevant research.The research of OPRGL is still in the stage of low pump intensity of 1k W/cm² and small-volume discharge of less than 0.5 cm³,and its maximum output power is in the order of Watts.As one of the important components of OPRGLs,the discharge system is mainly used to produce metastable rare gas atoms.Hence,choosing a suitable discharge system is conducive to improve the power of laser and conversion efficiency for OPRGLs.Aiming at the discharge system of OPRGL,this thesis studied different kinds of discharge from the two perspectives of realizing large-volume discharge and obtaining high-density metastable particles,taking efficient conversion efficiency and practicability as the evaluation criteria.From the perspective of realizing large-volume discharge,the feasibility of radio-frequency（RF）capacitive discharge as OPRGL discharge system was studied.Ar-He mixture was the main research object.The dependence of metastable particle density on discharge conditions was studied by means of emission spectroscopy and absorption spectroscopy.The practicability of RF discharge was investigated,and the observation of long-term discharge was made.It was found in the experiments that glow discharge could be realized in the discharge volume of 4 cm³ under RF discharge.However,the density of metastable particles was in the order of 10¹¹ cm^-3 and decreased with the increase of gas pressure,which did not meet the requirements of high-efficiency laser system for the discharge schemes.In addition,a dynamic model based on RF capacitive discharge was established for Ar-He mixture,and the effectiveness of the model was verified by comparing the simulated results with the measured and reported results in the literature.The model reveals that the main reason for the low metastable particle density of RF capacitive discharge is mainly due to the voltage drop at the sheath.In order to produce metastable particles of the order of 10¹² cm^-3,a voltage of at least 650 V needs to be applied.From the perspective of obtaining high-density metastable particles,a high-voltage nanosecond direct current（DC）pulsed discharge device was designed.The dependence of metastable particle density on discharge conditions under high-voltage DC pulsed discharge was studied,and the characteristics of high-voltage pulsed discharge under different discharge volumes were compared.It was found that the density of metastable particles increased with the increase of voltage under DC pulsed discharge,and the metastable particles of the order of 10¹¹ cm^-3 and 10¹² cm^-3 were produced at the discharge volume of4 cm³ and 0.4 cm³,respectively.In the small-volume pulsed discharge,it was found that the laser output increased with the increase of gas pressure and reached the highest when the Ar-fraction is 1%.The laser temporal waveforms showed a bimodal structure at low pump intensity.Under the pump intensity of 730 W/cm²,the laser output with average power of225 m W and peak power of 9 W was realized.Schemes of preionized pulsed discharge and segmented pulsed discharge were applied to explore the methods of realizing bulky and dense plasma.It is found that RF preionization can expand the volume of pulsed discharge and the metastable particle density increases with the increase of RF deposited power,up to 1.65×10¹² cm^-3,but the plasma is not uniform.The segmented pulsed discharge with peaking capacitors and inductive isolation suppresses the influence of the jitters of the pulsed discharge and can generate uniform plasma in each discharge segment.The peak of metastable particle density is up to 1.0×10¹³ cm^-3.The addition of peaking capacitors can simplify the design of discharge device and expand the volume of discharge area by parallel connection.The above results show that RF discharge is easy to realize large-volume discharge,but it is necessary to further increase the discharge voltage to produce high-density metastable particles;Nanosecond DC pulsed discharge is easy to realize high-density discharge region,but the discharge volume supported by single power port is limited;The discharge volume is expanded by RF preionization and segmented parallel connection.In the segmented pulsed discharge with inductively isolated peaking capacitors,the peak value of metastable particle number density reaches 1×10¹³ cm^-3.These studies are preliminaries for the realization of high-power OPRGL laser system.