Laser Beam Diagnosis And Realization Of Double-pulse Laser Irradiating Scheme For Laser Ion Sources

In order to better understand the mechanism of laser ion sources, the laser beam,one of the key components in a laser ion source, was studied in this thesis, which iscomposed of two parts. The first part is about laser beam diagnosis. Based on thefundamental theory of laser beam and the specific parameters of the Nd-YAG laserapplied on the laser ion source at IMP，a laser beam diagnostic system was established,with which several critical laser parameters could be measured, such as laser beamM-square factor, far-field divergence angle, focal spot size, pointing stability and nearfield parameters involving modulations and contrast ratios. It is significant for us tostudy how to improve ion beam intensity, pulse width and repeatability from theinformation about power density and pointing stability on the target. At the same time,the laser beam conditions of different energy and different operation modes weremeasured and the effect of lens focal lengths and lens types on laser ion sources,especially power density at the focal spot was investigated.Experimental results showed that:(1) Laser beam quality was nearly identicalwhen laser output energy was tuned between0.5J and3J, which means that only theeffect of laser energy on laser ion sources needs considering.(2) Operation mode had alimited effect on energy, energy stability and laser beam quality, but the pointingstability in1Hz mode was better than that in single shot mode while the one inhorizontal direction was better than that in vertical direction; the pointing stability oflaser beam could be improved with preheating laser at the beginning of operating.(3)Lens focal lengths and lens types had an important effect on intensity distribution and power density of focal spot on the target; to reach high laser peak power density ontarget, short focal length of lens should be used and astigmation should be eliminated,and achromatic lens might be a good choice.(4) Interclass variance method was usedas the criterion of calculating laser beam near field information for laser ion sources.The oddity spot in laser near field could be judged based on whether the intensity wasgreater than Iavg(1+4σ) or less than Iavg(1-4σ). Then the image smoothing technique ofeight-neighbor regions was used to process the oddity spots. The modulation andcontrast ratio evaluated by two methods were compared and discussed. Finally, thelaser near field modulation was evaluated as1.80±0.03and contrast ratio as0.195±0.005, which could be used as a reference to judge the laser operation safety andchoose the appropriate optical components.The second part of the thesis is about the realization of double-pulse laserirradiating scheme. To improve the laser-plasma coupling efficiency and depress therecombination process during the very beginning stage of laser plasma expanding, adouble-pulse laser irradiating scheme for the laser ion source has been designed andestablished with optical method. Its feasibility was verified by conducting a series ofpreliminary experiments. Comparing the results with the same conditions from theprevious single-pulse irradiating scheme, the differences in the time structure andcharge state distribution of the ion pulse were observed, but its mechanisms andoptimization require further studies.