Research on the Technology Issue of High Power Laser Amplifier

激光惯性约束核聚变(Inertial Confinement Fusion，ICF)是利用X射线(直接驱动)或者电子(间接驱动)驱动腔靶内的热核燃料，需要激光ICF驱动器在合适波长范围内产生兆焦耳级能量，纳秒脉冲宽度的激光束，从而达到内爆条件。为了激光驱动器更加高效、经济和安全地运行，主要从放大器小信号增益系数入手，研究提高放大器的电光转换效率，减小热恢复时间。泵浦反射腔作为氙灯和钕玻璃之间的耦合器，对小信号增益系数影响较大。因此，本文主要针对放大器的相关工艺问题，研究提高放大器反射腔转换效率的方法，以提升驱动器的工作效率。主要研究内容如下： 1、设计反射腔时，氙灯的发光模型主要包括：(1)氙灯中心点光源；(2)朗伯光源。模型一将氙灯简化为点光源，设计产生成像椭圆反射腔。氙灯和钕玻璃棒分别置于椭圆的两个焦点上，根据椭圆几何特性，能够将大部分氙灯光线传输至钕玻璃棒。模型二认为脉冲放电在氙灯灯管内形成高温等离子体，辐射和吸收两个过程同时存在，放电灯管几乎不透明，大部分辐射实际发生在近管壁的环形区域内。氙灯等离子体的自吸收与电流密度、氙灯直径和氙气气压均有关，因此，氙灯等离子体对自身辐射的不透明度显得尤为重要。为了确定氙灯辐射模型，研究了不同电流密度下，氙灯等离子体对不同波长的自吸收系数；不同氙灯直径下，等离子体的自吸收系数。测试了氙灯表面一点发光区域内任一波长的光谱强度，验证了数值计算的准确性，即氙灯与目标面距离较近时，辐射模型为朗伯光源。 2、已知放大器中氙灯辐射模型为朗伯光源。需要设计最大理论耦合效率下的反射腔结构。采用边缘光线原理和拉线法设计了多灯非成像反射腔，根据系统结构差异，设计了两种计算模型。在相同泵浦能量下，对非成像反射腔和成像椭圆反射腔中钕玻璃棒表面辐照度进行了数值模拟，结果表明：椭圆反射腔中，氙灯辐射光线存在自吸收和互吸收，影响了其增益性能；非成像反射腔避免了氙灯自吸收，且减少了光线反射次数，因此，能够获得最大耦合效率且泵浦均匀。 3、放大器增益均匀性将影响激光光束质量，非成像系统的主要目的是在目标面辐照均匀。因此，采用漫反射理论设计非成像反射腔，针对成像和非成像两种泵浦方式，研究泵浦方式对棒状放大器增益特性的影响规律。设计了成像椭圆反射腔，六边形、十二边形和圆形非成像反射腔，利用几何光学对四种腔结构的能量转换效率进行理论分析计算，数值模拟了钕玻璃棒表面的泵浦光照度，测试了钕玻璃棒横截面任意直径上的小信号增益系数和增益倍数。结果表明：成像椭圆反射腔的小信号增益系数方差最小，增益均匀性最好；非成像泵浦腔结构差异对增益特性影响较大，其中六边形的增益和小信号增益系数最大。漫反射设计理论，没有考虑光线在钕玻璃棒内的传输过程，从而影响了其增益和增益均匀性。 4、基于边缘光线理论设计非成像反射腔，其结构是以氙灯外圆为基圆，通过切线绕氙灯转动形成渐开线反射腔。类渐开线的几何结构能够100%传输氙灯的几何外形，即没有光线反射至氙灯本身。单支氙灯辐照区域固定，相互之间没有影响，避免了因相互吸收光线而导致氙灯负载增加。相比较漫反射理论设计的反射腔，非成像泵浦均匀性高。钕玻璃棒横截面内储能密度的数值模拟和实验结果吻合，相比较成像椭圆反射腔，非成像反射腔小信号增益系数在同等泵浦条件下增大。因此，非成像反射腔能够提高棒状放大器的转换效率。Laser inertial confinement fusion (ICF) relies on either X-ray (indirect drive) or electron (direct drive) to drive the thermonuclear fuel in a chamber. To reach ignition conditions, a laser driver for ICF should have output energy on the order of mega joules at the appropriate wavelength over a few nanoseconds pulse-length. The ultimate goal of research in small-signal gain coefficient is to enhance the electro-optical conversion, and decrease the waste heat, thus develop laser driver as a high efficiency, economic and safe device. The pump reflector as a coupler between the flashlamp and Nd: glass, which greatly affect amplifier’s small-signal gain coefficient. Therefore, according to the related technology issues of amplifiers, we researched on how to enhance the reflector’s transfer efficiency, and then increase the working efficiency of the driver. The main works and achievements are as follows: 1. When we designed the reflector, flashlamp were simplified as a point source or Lambertian source. Basing on the first radiation model of the flashlamp, which formed the imaging ellipse reflector with flashlamp and Nd: glass at the foci. Here, most rays could be transferred to the surface of the Nd: glass through the geometric feature of ellipse. The second radiation model treated the xenon plasma as a grey-body radiator, which could emit as well as absorbed pump light. The radiating xenon plasma was optically thick, meaning that pump light emanated from the surface of the plasma and not from within its bulk. We believed that xenon plasma absorption coefficients change with current density, lamp diameter, and xenon pressure. So, the absorption of emitted radiation by xenon plasma itself was relatively important. In order to determine the model in amplifier, the absolute spectral-intensity measurements at one point on the surface of a lamp tube under different current densities and lamp diameter were tested, and the good agreement between experimental data and numerical simulation demonstrated that if the spacing between the pump source and object is short enough, we can sure that it is a Lambertian source. 2. As we knew that flashlamp was a Lambertian source in an amplifier system. We wanted to design a non-imaging reflector structure with the theoretical maximum coupling efficiency. The multi-lamp non-imaging pump cavities were designed by the edge-ray principle and constant string length method. Two kinds of calculation model were designed by the system structural difference. The Nd: glass surfaces illumination in the non-imaging and elliptical imaging pump cavity were simulated under the same pump energy. The results show that there have lamp self-absorption and mutual-absorption in the elliptical imaging pump cavity, and then affect its gain property, in the non-imaging pump cavity, it avoid the light reflecting back to the lamp and reduce the number of reflections. Thus, the maximum coupling efficiency and pump uniformity are obtained. 3. Laser beam quality sort of being decided by the gain uniformity of amplifier, and the main goal of the non-imaging system was pumping uniformity on the object. Therefore, we designed the non-imaging reflector basing on the theory of diffuse reflection. According to the two pump styles of imaging and non-imaging, the effect rules of the pump style to the rod amplifier gain uniformity was researched. Elliptical imaging pump cavity, non-imaging pump cavities of hexagonal, dodecagonal and circular were designed. Pump energy transfer coefficient of the four kind of cavity structures were analyzed and calculated by geometrical optics. The illumination at the surface of Nd: glass was simulated, small-signal gain coefficient and energy gain at arbitrary diameter of the Nd: glass cross section were tested. The results show that elliptical imaging pump cavity has the minimum variance of small-signal gain coefficient and the best gain uniformity, the structure difference in non-imaging pump cavities have a great effect on the gain properties, and the hexagonal pump cavity has the biggest small-signal gain and gain coefficient. 4. Using the edge-ray principle, the basic shape of the non-imaging cavity was of an involute to the flashlamp’s outer circumference. This profile produced a 100% geometrical transfer efficiency which meant it can reflect all light rays from flashlamp toward Nd: glass, with no light rays travelling back to the flashlamp itself, that would increased amplifier efficiency. Each flashlamp pumped the fixed area on the surface of Nd: glass, either of them had a connect which isolating the load increase, Compared with an imaging cavity, a non-imaging cavity increased the small-signal gain coefficient for the same xenon flashlamp electrical energy and dimensions of the rod amplifier. Numerical results for the stored energy density were compared with measured data and a good agreement was found for the small-signal gain coefficient. Therefore, a non-imaging cavity with several optical features can enhance the pumping efficiency and may increase repetition frequency in the future