The X-Ray Emission Effectiveness of Plasma Mirrors: Reexamining Power-Law Scaling for Relativistic High-Order Harmonic Generation
The X-Ray Emission Effectiveness of Plasma Mirrors: Reexamining Power-Law Scaling for Relativistic High-Order Harmonic Generation
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DOI：
10.1038/s41598-020-61255-0
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发表时间：
2020-03-20
影响因子：
4.6
通讯作者：
Mikhailova, Julia M.
中科院分区：
文献类型：
--
作者：
Edwards, Matthew R.;Mikhailova, Julia M.
超短脉冲激光提供了对物理、化学和生物系统超快动力学的独特详细访问，但只有少数波长是由固体激光直接产生的，这就需要高效的高功率频率转换。相对论等离子体镜会产生宽带功率谱，这可能会跨越拍瓦级红外激光设备和x射线自由电子激光之间的差距；尽管进行了大量的理论工作，但这种相对论高次谐波产生的最终效率仍不清楚。结果表明，等离子体反射镜发出的相干辐射遵循能量随频率的指数分布，即使在极端相对论极限下，它也强烈地依赖于激光强度与等离子体密度的比值，并且在很大范围内超过了经常引用的-8/3的值。当对发射电子束的有限宽度进行适当修正时，相干同步辐射模型不仅适用于p偏振光和薄箔目标，而且通常描述相对论谐波产生，包括在垂直入射和有限梯度等离子体中。我们的数值结果支持在大多数情况下，同步辐射模型的-4/3标度是该过程的极限效率。通过这种定标可以产生的最高频率通常受到发射电子束的宽度的限制，而不是最快电子的洛伦兹因子。例如，本文建立的理论标度关系表明，对于20PW的800 nm驱动激光器，1keV的光子可以产生1TW/谐波。
Ultrashort pulsed lasers provide uniquely detailed access to the ultrafast dynamics of physical, chemical, and biological systems, but only a handful of wavelengths are directly produced by solid-state lasers, necessitating efficient high-power frequency conversion. Relativistic plasma mirrors generate broadband power-law spectra, that may span the gap between petawatt-class infrared laser facilities and x-ray free-electron lasers; despite substantial theoretical work the ultimate efficiency of this relativistic high-order-harmonic generation remains unclear. We show that the coherent radiation emitted by plasma mirrors follows a power-law distribution of energy over frequency with an exponent that, even in the ultrarelativistic limit, strongly depends on the ratio of laser intensity to plasma density and exceeds the frequently quoted value of -8/3 over a wide range of parameters. The coherent synchrotron emission model, when adequately corrected for the finite width of emitting electron bunches, is not just valid for p-polarized light and thin foil targets, but generally describes relativistic harmonic generation, including at normal incidence and with finite-gradient plasmas. Our numerical results support the.-4/3 scaling of the synchrotron emission model as a limiting efficiency of the process under most conditions. The highest frequencies that can be generated with this scaling are usually restricted by the width of the emitting electron bunch rather than the Lorentz factor of the fastest electrons. The theoretical scaling relations developed here suggest, for example, that with a 20-PW 800-nm driving laser, 1 TW/harmonic can be produced for 1-keV photons.