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Order-of-magnitude laser imprint reduction using pre-expanded high-Z coatings on targets driven by a third harmonic Nd:glass laser
Physics of Plasmas ( IF 2.2 ) Pub Date : 2021-03-22 , DOI: 10.1063/5.0042454
Max Karasik 1 , Jaechul Oh 1 , S. P. Obenschain 1 , A. J. Schmitt 1 , Y. Aglitskiy 1 , C. Stoeckl 2
Affiliation  

A hybrid early time x-ray drive followed by conventional laser direct drive obtained by utilizing a thin high-Z overcoat on plastic ablator targets has proven to be very effective at reducing laser imprint in experiments driven by the Nike krypton-fluoride laser. An inherent low level laser prepulse from that laser system heated and expanded the coating prior to the main pulse. Here, we report on results using a frequency tripled Nd:glass laser system (Omega EP) where the inherent prepulse was several orders of magnitude smaller. By applying a separate prepulse, these experiments allowed us to test if the prepulse is important for mitigating imprint with the high-Z layer. The results show that the high-Z coating is much more effective at reducing laser imprint when it is pre-expanded, in this case, by an externally generated low level soft x-ray prepulse of 10 J/cm2. With the x-ray prepulse, laser imprint at spatial wavelengths below 100 μm was reduced by an order of magnitude, similar to that observed with the laser prepulse on Nike. Furthermore, the experiments here establish that high-Z coating is effective in reducing imprint even in the case of a fixed speckle pattern. Rayleigh–Taylor-amplified laser imprint and high-Z layer dynamics were measured using through-foil and side-on x-ray radiography. Pre-expansion times from 30 down to 6 ns were effective, potentially compatible with laser prepulse generation using existing NIF and OMEGA front ends; however, temporal beam smoothing appears to be necessary for the laser prepulse that directly illuminates the high-Z coating. The highest imprint reduction is observed for Pd and Au coatings of at least 400 Å thickness; thicknesses down to 200 Å show a reduction in imprint with an adjusted prepulse.

中文翻译:

在由三次谐波Nd:玻璃激光器驱动的目标上使用预膨胀的高Z涂层在数量级上减少激光压印

在耐克氟化-激光器驱动的实验中,混合的早期X射线驱动和随后的常规激光直接驱动通过在塑料消融靶上利用薄的高Z外涂层获得,已被证明在减少激光烙印方面非常有效。来自该激光系统的固有低电平激光预脉冲在主脉冲之前加热并扩展了涂层。在这里,我们报告使用三倍频Nd:玻璃激光系统(Omega EP)的结果,该系统的固有预脉冲要小几个数量级。通过应用单独的预脉冲,这些实验使我们能够测试预脉冲对于减轻高Z层的印记是否重要。结果表明,高Z涂层在预扩展时在减少激光压印方面更为有效,在这种情况下, 10焦耳/厘米2。与x射线预脉冲,激光刻印在低于100的空间波长 μm减小了一个数量级,类似于在Nike上使用激光预脉冲所观察到的。此外,此处的实验证实,即使在固定斑点图案的情况下,高Z涂层也可有效减少压印。瑞利-泰勒放大的激光压印和高Z层动力学是通过箔和侧面X射线射线照相术测量的。从30 ns到6 ns的预扩展时间是有效的,可能与使用现有NIF和OMEGA前端的激光预脉冲生成兼容;但是,对于直接照射高Z涂层的激光预脉冲来说,临时光束平滑似乎是必需的。对于厚度至少为400Å的Pd和Au涂层,观察到了最高的压印减少率;厚度低至200Å表示在调整了预脉冲的情况下减少了压印。
更新日期:2021-03-31
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