Study On Capillary Discharged Plasma State And X-Ray Laser Output At Low Pressure

X-ray laser has important application in many fields because of its predominant characteristics that can not be replaced by other coherent light sources. At the same time, the bulky pumping facility of x-ray laser for very high pumping energy brings the problems of high cost and low conversion efficiency, which limit its widespread use.Capillary discharge pumped soft x-ray laser is one of the most successful schemes for low cost small-scale x-ray laser. Since the first demonstration of Ne-like argon laser at 46.9nm in a capillary discharge in 1994, many countries started this investigation, but until 2000-2002 only three other groups succeeded by reason of complexity of the lasing conditions. Our group demonstrated successful experiment on independently developed capillary discharge device in 2004 and became the fifth researching group in the world that succeeded in this field. This dissertation reports the progress of theoretic research, device improvement and increasing output energy after the first observation of lasing.The active medium of capillary discharge pumped laser is the discharge-created plasma and the pumping energy is converted to lasing emission through Z-pinch, so that investigating the evolution of plasma and the plasma estate of lasing time is very important for deep understanding the physics process of lasing and the key determining factor for quality of laser beam. This dissertation investigates the two aspects. For the evolution of plasma an improvement of"snow-plow"model, that allows calculating the whole evolution process of plasma, has been made based on reasonable analysis of Z-pinch process. A simulation program has been worked out and the calculated result show that the plasma is pinched several times. This result is in good agreement with the time of lasing onset and other experimental phenomena. For the plasma estate of lasing time a ray tracing code, that can break through the limit of the analytic calculation and compute the spatial characteristic of laser by any gain and plasma density distributions, has been programmed. And through the comparison between the simulated results and the measurements the plasma state of lasing time can be estimated. In this dissertation the calculation for several typical plasma density distributions is reported and can serve as the base for estimating the plasma estate. The capillary discharge device is a very complex system that includes the sections of discharge pulse generation, current pulse transmission, predischarge pulse generation and detection. A lot of maintaining and improving works has been conducted for overcoming the disadvantage of the device for laser output and increasing the stability. A new predischarge trigger generator, that is more simple and stable than the former one, has been designed and fabricated using the car ignition loop DQ128. A Rogowski coil, that instead of the former device can much more accurately measures the current flowing through capillary, has been made and calibrated. The insulation switch of inherent prepulse of the Blumlein transmission line has been improved, that results in removing the influence of the inherent prepulse and the stability of laser output. The improvement of the main switch and the discharge chamber was also conducted, that leads to increasing the discharge current amplitude and restraining the unwished discharge paths.Firstly, a lot of basic experiments were conducted on 15cm-long capillary. The source of XRD disturbing signal has been found and deleted by a series of experiments. The characteristics of the gas switches in the device have been experimentally studied and the adjusting method of experimental system has been found. Moreover, the influence of the rising slope waveform of discharge current and the delay time between prepulse and main pulse on the laser output also has been experimentally studied, so that the best current waveform and the most suitable range of the delay time has been determined. On basis of preceding works the stability of laser output has been reached. Secondly, the laser output has been realized also on 20cm-long capillary and the laser energy has been increased much more. The relationship between the discharge parameters and the laser output has been investigated through changing the shape of the discharge electrode and the amplitude of prepulse current and the optimal range of the prepulse amplitude were determined. The origin of the multi-spike of XRD signal has been verified experimentally and the improved electrocircuit scheme of the XRD has been proposed. The laser properties including gain coefficient and directivity has been measured, that results in the gain of 0.45cm-1 and the gain-length product of 8.28. The measurements of the laser beam profile have been conducted and the measured divergence is 3.8mrad, furthermore, the plasma state has been estimated by comparing the measured profile with the theoretical simulations. Finally, the laser energy has been calculated according to experimental results and the calculated value proves that the Ne-like 46.9nm laser energy of per pulse obtained at low pressure (25Pa) is above 1μJ.The contents of this dissertation are concluded by closely combining the theoretical study and the experimental study. The effort of this dissertation completed the following achievements: the mechanism of capillary discharge pumped laser was understood more deeply, the laser energy has been increased, and the laser properties of amplification and spatial intensity distribution has been measured. And all these achievements indicate the clear direction for increasing the laser output up to a gain saturated regime.