瞬态电子碰撞激发类氖锗19.6nm x光激光的研究

【作者】 乔秀梅；

【导师】 张国平；

【作者基本信息】 中国工程物理研究院 ， 等离子体， 2005， 博士

【摘要】 自从1984年美国利弗莫尔实验室第一次成功演示了电子碰撞激发软x光激光后,x光激光的实验和理论的研究都取得了很大的进展,最近提出的瞬态电子碰撞激发机制大大节省了驱动激光的能量,提高了实验的重复频率,瞬态电子碰撞激发的最短波长已经做到了7.3nm。我们介绍了新开发的类氖锗瞬态电子碰撞激发的程序,并用系列程序模拟了卢瑟福实验室在2000年做的瞬态电子碰撞激发类氖锗的实验,与实验结果的比较表明,在实验误差的范围内,我们的模拟结果与实验较符合。设计并模拟了一系列瞬态电子碰撞激发类氖锗的实验,采用两个250ps能量为10J的钕玻璃激光做预脉冲,间隔一定的时间后续一个1ps能量为10J的短脉冲横向辐照长0.9cm的锗平板靶,产生的焦线长1cm和宽100μm,模拟表明峰值增益系数为～60cm^-1,利用二维几何光学旁轴近似下x射线激光传播和小讯号放大程序XBPA计算了x光激光在等离子体中的传播,得到了x光激光的小信号增益系数为19.5cm^-1,折射角～6.8mrad,表明折射效应是影响输出x光激光增益等的重要因素。为此,我们采用2ω1ω泵浦驱动方案,即预脉冲采用二倍频钕玻璃激光,短脉冲采用基频钕玻璃激光,模拟结果表明增益区的电子密度从小于～0.6×10²¹cm^-3增加到到临界面附近非常平缓的区域,小信号增益系数增加为33cm^-1,输出x光激光的折射角～12mrad,表明折射效应对输出x光激光的影响仍然不可忽略,这主要是因为增益区宽度较窄。模拟和实验都表明,瞬态电子碰撞激发类氖锗19.6nm x光激光的峰值增益系数出现在基频光的临界面附近,即电子密度为～1×10²¹cm^-3,在此临界面以上更高的电子密度区域能否产生更高的增益系数是我们关心的一个问题,因为如果能够得到更高的增益系数,用几个毫米甚至更短的靶长就可以做到饱和x光激光,可以大大减少x光激光在增益区的传播距离,从而可以降低折射对增益的影响,为此,我们设计并模拟了一系列实验,研究在基频钕玻璃激光的临界而以上的区域中是否能够产生高增益,我们采用3ω2ω泵浦方案,且短脉冲与靶面法线成45°角入射,模拟表明,在电子密度～2×10²¹cm^-3附近区域,产生了高达111cm^-1的高增益系数,增益维持的时间非常短,只有～1ps,增益区的宽度也很窄,只有～10μm。利用同样的方法还可以研究更高电子密度区域类氖锗19.6nm瞬态电子碰撞激发x光激光增益系数的情况,但是,随着电子密度的增加,电离过程会变快,这样,增益系数维持的时间将会更短。亚稳态电子碰撞激发机制的发展经历了从高的预脉冲强度驱动,预脉冲产生电离合适的等离子体到用10%左右的预脉冲驱动,预脉冲低电离,主脉冲既要得到高的电子温度并要将等离子体电离到类镍离子占优势的状态,最后,发展到用1%左右的低预脉冲驱动,通过适当的时间延迟再用主脉冲驱动。最近有许多实验仅仅采用一个脉宽为几个ps的短脉冲就获得了TCE x光激光的高增益,他们的分析指出,短脉冲的本底噪音起了关键作用,这个低强度的本底产生预等离子体,其中电离度和电子温度都很低,短脉冲即电离又加热等离子体从而产生高增益的,这表明瞬态电子碰撞激发机制出现了与亚稳态电子碰撞更多还原

【Abstract】 Since the first demonstration of soft x-ray laser in 1984 in LLNL, extensive experimental and theoretical work has been done and significant advances have been made. The recently proposed TCE （Transient Collisional Excitation） scheme greatly reduces the driving energy and lasing wavelength down to 7.3 nm x-ray laser has been obtained .In this thesis, we introduced the newly developed serie codes for TCE Ne-like Ge in the IAPCM, and modeled the x-ray laser experiment done by the RAL in 2000, comparing our results with experimental data shows agreement within experimental uncertainty. And with this serie codes we simulated Ne-like Ge 19.6 nm X-ray laser driven by two 250 ps Nd:glass laser pulses with 10 J total energy and after some delay time a 1ps pulse at 1.053μm under different pumping parameters producing a focus line 1cm length and 100μm width, and simulations indicate that under the optimized pumping condition, the local gain greater than 60cm^-1 could be obtained. Calculations of the propagation of X-ray laser, including refraction effects, shows small signal gain of ¹9.5 cm^-1, and the deflection angle is ⁶.8 mrad which suggests that refraction is an important factor that affects the x-ray laser output. And x-ray laser pumped by double-frequency laser is modeled and simulations show that the electron density in the gain region moves from less than ⁰.6×l0²¹cm^-3 to near the critical density surface where the electron density is ¹×10²¹cm^-3 , and the small signal gain increases to 33cm^-1, with ¹2 mrad deflection angle which again means that refraction effect can not be neglected . One possible way to suppress the effect of refraction is to sharply reduce the distance that x-ray laser propagates in plasmas, but on the other hand, to obtain saturation, one must have much higher gain in the gain region. Previous simulations and experiments show that gain for the Ne-like Ge 19.6nm x-ray laser driven by Nd:glass laser peaks at critical density for the pumping laser, one problem arises that whether it is possible to produce higher gain above this electron density region. We have designed and simulated a series of experiments for Ne-like Ge 19.6nm x-ray laser to study gain above this electron density region, we adopt 3ω2ω pumping scheme, and the short pulse incidents with a 45° angle relative to the normal of the target surface, with less than 2.5J total pumping energy, simulations predict high gain up to 111cm^-1, and a ¹0 μm width narrow flat region with electron density larger than 1.7×10²¹cm^-3, the gain’s life time is very short, it is only lps. Looking back at the development of QSS scheme, one can find that the prepulse pumping technique has experienced the following stages: The prepulse with almost the same intensity as that of the main pulse, was firstly adopted to produce plasma with proper ionization, and later it was found that a 10% or even less than 1% prepulse was more advantageous, in the later method, the low intensity prepulse produce plasma with low ionization, the main pulse not only heats but ionizes the plasma to produce high gain. Recent TCE experiments have obtained amplified x-ray laser with only one single short pulse, their analysis indicates that the pedestal of the short pulse produces preplasma with low ionization and low electron temperature, the short pulse bothionizes and heats preplasma to proper ionization with proper electron temperature to produce nigh gain, which suggests that there appears the same tendency in TCE scheme as in QSS, and according to this idea, we designed a series of experiments expecting to get high gain? hydrodynamic simulation was made and analysis show that as for the TCE Ne-like Ge 19.6 nm x-ray laser , it is possible to get high gain by this pumping method.更多还原