Kehne D M, Karasik M, Aglitsky Y, Smyth Z, Terrell S, Weaver J L, Chan Y, Lehmberg R H, Obenschain S P
Plasma Physics Division, Naval Research Laboratory, Washington, DC 20375, USA.
Rev Sci Instrum. 2013 Jan;84(1):013509. doi: 10.1063/1.4789313.
In direct drive inertial confinement laser fusion, a pellet containing D-T fuel is imploded by ablation arising from absorption of laser energy at its outer surface. For optimal coupling, the focal spot of the laser would continuously decrease to match the reduction in the pellet's diameter, thereby minimizing wasted energy. A krypton-fluoride laser (λ = 248 nm) that incorporates beam smoothing by induced spatial incoherence has the ability to produce a high quality focal profile whose diameter varies with time, a property known as focal zooming. A two-stage focal zoom has been demonstrated on the Nike laser at the Naval Research Laboratory. In the experiment, a 4.4 ns laser pulse was created in which the on-target focal spot diameter was 1.3 mm (full width at half maximum) for the first 2.4 ns and 0.28 mm for the final 2 ns. These two diameters appear in time-integrated focal plane equivalent images taken at several locations in the amplification chain. Eight of the zoomed output beams were overlapped on a 60 μm thick planar polystyrene target. Time resolved images of self-emission from the rear of the target show the separate shocks launched by the two corresponding laser focal diameters.
在直接驱动惯性约束激光核聚变中，含有氘 - 氚燃料的小球通过其外表面吸收激光能量产生的烧蚀作用而内爆。为实现最佳耦合，激光的焦斑会持续减小以匹配小球直径的减小，从而将浪费的能量降至最低。一种通过诱导空间非相干实现光束平滑的氟化氪激光（波长λ = 248 nm）能够产生高质量的焦斑轮廓，其直径随时间变化，这一特性称为焦斑变焦。海军研究实验室的耐克激光装置已演示了两级焦斑变焦。在该实验中，产生了一个4.4 ns的激光脉冲，在前2.4 ns内靶上焦斑直径为1.3 mm（半高全宽），在最后2 ns内为0.28 mm。这两个直径出现在放大链中多个位置拍摄的时间积分焦平面等效图像中。八束变焦输出光束在一个60μm厚的平面聚苯乙烯靶上重叠。靶背自发光的时间分辨图像显示了由两个相应激光焦斑直径引发的独立激波。
