【作者】 谢耀；

【导师】 王骐； 赵永蓬；

【作者基本信息】 哈尔滨工业大学 ， 物理电子学， 2011， 博士

【摘要】 自1984年首次实现软X射线激光以来,软X射线激光作为一种相干光源,因其单色性好、瞬间亮度高、脉冲持续时间短和波长短等特点受到了越来越多科学家的重视。然而,通过使用大型的激光系统激发固体靶的方式实现的软X射线激光,由于庞大的体积和昂贵的运作成本,很大程度上限制了其应用。因而实现小型化的、低运转成本的软X射线激光也成为了各国科学家的研究重点。毛细管放电方案是实现台式的、小型化的软X射线激光最为有效的方案之一。自1994年,美国的Rocca小组首次成功实现毛细管放电泵浦类氖氩46.9nm软X射线激光输出以来,国际上已有包括日本、意大利等7个国家的研究小组采用该机制相继成功的实现了激光输出,极大的推动了软X射线激光的发展。本研究小组于2004年首次在自行研制的装置上实现了激光输出。在之前的研究基础上,本论文主要完成了46.9nm软X射线激光增益饱和输出研究,使得本课题组成为国际上第四个实现增益饱和输出的研究小组,并首次实现了小电流、低气压下的增益饱和输出,有利于装置的进一步小型化,在理论和实验研究领域对软X射线的发展起到一定的推动作用。而在小电流下实现激光的增益饱和输出,就是要在低的激发阈下实现最高能量的激光输出,电流幅值低必将导致工作气压低,产生激光的反转粒子数少,这就需要理论和实验中各实验参数的优化,围绕这一主题,本文主要包括了理论研究、实验装置的介绍和改造以及实验研究三个方面,致力于提高激光增益以实现稳定的、高能量的激光输出。在理论研究方面,本文第二章通过数值模拟和实验相结合的方式,通过雪耙模型Z箍缩理论的描述,系统的分析了激光的产生时间随初始Ar气压和主脉冲电流幅值的变化关系,为实验中放电参数的选择提供一定的参考。考虑到等离子体存在电子密度梯度影响了软X射线激光在等离子体中传输,通过光线传输方程的数值求解,详细的描述了不同的电子密度分布形式下光线在等离子体柱内的传输过程,并结合1mm差分孔对光线遮挡的计算分析和获得的激光4mrad左右的束散角的实验结果,推断出放电时毛细管中的等离子体柱内的电子密度接近抛物线形分布,而且抛物线形分布的电子密度有利于光线在传输过程中的增益放大,以获得更高增益,最终实现更高能量的激光输出。毛细管放电装置主要包括了主脉冲系统、预脉冲系统、探测系统、工作负载和充气及真空系统等五个主要组成部分。本文首先描述了放电装置各主要部件的工作原理,针对提高激光增益的实验要求,对装置进行了相应的改造。改造后的主开关可以通过更换附加电感的方式达到改变主脉冲电流上升沿的目的,以便研究主脉冲电流上升沿对激光增益的影响。充气系统的改造有利于研究掺杂气体对激光增益的影响。装置的改造为实验研究提供了可靠的保障。为了实现更高增益的激光输出,本文在第四章中进行了一系列旨在提高激光增益的实验研究。首先,系统的研究了主脉冲电流幅值对激光增益的影响,在给定气压下,找到了最佳的主脉冲电流幅值,以实现更高增益的激光输出。其次,研究了在Ar中掺入一定比例的He等气体对激光增益的影响,研究表明掺入一定比例的He有利于提高激光的增益,也就提高了激光的输出能量。再次,研究了主脉冲电流上升沿对激光增益的影响,实验结果表明,在其它放电条件不变的情况下,电流上升沿变大在一定范围内有利于获得更高的增益,从而获得更高能量的激光输出。预脉冲电流是实现软X射线激光输出的一个必要条件,它有利于等离子体均匀箍缩,本章最后分析了预主延时分别为2μs和7μs时预脉冲电流幅值对激光增益的影响,研究表明当预脉冲电流幅值为20A时,激光输出最为稳定,激光增益最高,相对输出能量也最强。在增益饱和输出和激光特性研究方面,首先,使用单色仪分析了激光的时间特性,使用平场光栅谱仪和Rowland圆谱仪准确的获得了毛细管放电软X射线激光的谱线信息,证实了X射线二极管获得的激光尖峰为类氖氩46.9nm软X射线激光。其次,采用较为简单的狭缝扫描的办法测得探测面XRD上的光斑直径为5.9mm和6.1mm,对应的激光的束散角在水平方向为4.0mrad,垂直方向为4.1mrad。再次,论文最主要的部分是实现了不同放电条件下的增益饱和输出,测量了不同等离子体长度时的激光相对强度,通过Linford公式拟合获得了不同放电条件下的增益系数,最大增益系数可达到0.68cm-1,并于国际上首次实现了增益系数为0.5cm-1的小电流、低气压下的增益饱和输出。论文最后通过标定过的X射线二极管近似计算了获得的激光能量为67.4μJ,并讨论了采用毛细管放电机制实现更短波长激光输出的可能性。更多还原

【Abstract】 Since the first reported in 1984, soft X-ray laser as a coherent light source, characterized by good monochromaticity, high instant brightness, narrow pulse duration and short wavelength, seizes more and more scientists’attention. Soft X-ray laser could be generated through hitting solid targets with energetic and bulky laser, which requires high operating costs and in turn limits its application. Therefore, realizing the miniaturized, low running-cost soft X-ray laser has become the research focus of worldwide scientists.The proposal of capillary discharge is one of the most effective solutions to achieve desktop, miniaturized soft X-ray laser. Since 1994, the first time when Rocca and his colleagues in US successfully realized capillary discharge Ne-like Ar 46.9nm soft X-ray laser, there has been 7 other countries such as Japan and Italy, that succeed in achieving soft X-ray laser using the same mechanism, which significantly promotes the development of soft X-ray laser. Our research team realized 46.9nm soft X-ray laser output in 2004, using the experimental setup developed by ourself, and becomes the third research group in the world that claims saturation output of 46.9nm soft X-ray laser after several years’endeavor, and it is the first time to realize saturation output under both low current and low pressure condition in 2009. Realization of saturation under both low current and Ar pressure is avail the proposed of miniaturization, which pushes forward the development of soft X-ray laser both experimentally and theoretically to some extent. Saturation output under low main current means obtain the highest laser energy under low threshold, and low main current lead to low Ar pressure, which means small inverted population density, all of this will need the optimization of character. Centre on this theme, this thesis consists of three parts, i.e.: theoretical research, introduction and improvement of experimental instruments and experimental investigation, all of which serves for augmentation of laser energy and realization of stable output of saturated laser.Theoretically, chapter 2 analyzes systematically dependence of laser generation time on the initial Ar pressure and the amplitude of main pulse current, using numerical simulations in combination with experiments recur to the description of Z-pinch theory using Snow plow model. The theory presented in chapter 2 provides reference for determinations of discharge parameters during experiments. The fact that the existence of electron density gradient during plasma formation affects the transmission of soft X-ray laser, necessitates the detail investigation on light transmission within the plasma column under different electron densities by solving the light transmission equation. Finally, associated with the analysis of the shelter from pinhole and the experimental result of about 4mrad angle of divergence, the results prove that the electron density of plasma is approximately parabolic distribution. And the parabolic distribution of electron density is be propitious to the gain amplification when the laser transmitting through the plasma, in order to obtain higher laser energy.Capillary discharge device is mainly constituted of five parts, i.e.: main pulse system, prepulse system, detection system, working load and inflation as well as vacuum system. This thesis first describes the operation principle of main components for discharge device, and improvements of the device with regarded to enhancing the laser gain as well. The improved main switch can alter the rising edge of main pulse current through switching different inductors. The improved inflation system is conducive to investigate on the influence of gas mixture on laser gain. All in all, improvements of discharge device guarantee the reliability of operation of experiments.Chapter 4 describs experimental research in order to obtain higher gain. Firstly, the impact of the amplitude for main pulse current on laser gain was studied, the optimal amplitude was found when the given Ar pressure, and this is in favor of realizing higher gain coefficient. Secondly, the influence of mixture ratio of He with Ar on laser gain was studied, and the results show that the incorporation of a certain proportion of He will help to improve the laser gain, finally increase the laser output energy. This thesis also investigated the effect of main pulse rising edge on laser gain. Experiments indicate that the slow rising edge is conducive to produce higher laser gain, and accordingly improve the laser energy, providing that other conditions remain unchanged. Finally, we also analyzed the impact of amplitude for prepulse current on laser output under delay times of 2μs and 7μs between main pulse and prepulse, which serves to produce high laser gain and stable laser output. It is found that under an amplitude of 20A for prepulse current, laser tends to have the most stable output, and a relatively higher gain, and higher laser energy.In the aspect of the laser character and saturation research, firstly, monochromator was employed to analyze the time characteristics of laser. Apart from that, application of both flat-field grating spectrometer and Rowland spectrometer gives accurate spectroscopic information of capillary discharge soft X-ray laser, which confirms that the spike detected by X-ray diode is indeed Ne-like Ar 46.9nm soft X-ray laser. Secondly, the employment of slit-scan method shows that the spot diameters of 5.9mm and 6.1mm in the position of XRD, which corresponds to horizontal and vertical divergence angle of the laser were 4mrad and 4.1mrad respectively. The most important research of the thesis, by fitting with the Linford formula, gain coefficients under different discharge conditions were achieved. The maximum gain coefficient was 0.68cm-1. Gain coefficient of 0.5cm-1 was achieved under both low current and low pressure condition, which is the first time realization in the world. The laser energy was estimated to be 67.4μJ through calibrating the X-ray diode. Finally, the thesis discussed the possibility of reaching shorter wavelength laser using mechanism of capillary discharge.更多还原