极端光场驱动超短相干强X射线源产生

Compared to the X-ray free electron lasers, relativistic laser plasmas may provide coherent X-ray sources with shorter pulse duration in a table-top size, which is beneficial for broad applications. However, the general problems presenting in current schemes based upon laser plasmas is the low conversion efficiency, in particular in the keV energy spectrum range. In this proposal, we will mainly explore two schemes to improve the conversion efficiency for the purpose of producing intense coherent X-ray sources. One of them is based upon the interaction of petawatt laser pulses with gas targets. Attention is paid mainly on the control of the electron injection into the laser wakefields by controlling the gas density profiles and/or the laser pulse injection process. The key point is to produce an ultra-thin dense relativistic electron sheet by the realization of synchronized injection of electrons. Via nonlinear Thomson scattering of another laser pulse with the relativistic electron sheet, bright coherent X-ray emission at the keV range may be produced with a reasonable conversion efficiency. Another scheme is based upon the interaction of petawatt laser pulses with thin solid targets with certain structures (such as grating, etc). It is possible to break the well-known power-law limit of high harmonic emission with a plane solid target given by n^(-8/3), thereby the X-ray conversion efficiency in the keV range can be significantly improved.

基于相对论激光等离子体产生的相干X射线源，与X射线自由电子激光相比具有产生的X射线脉宽更短、小型化等特点，有助于满足广泛的应用需求。但现有激光等离子体机制和方案中普遍存在的问题是转换效率偏低，特别是要产生在keV能段的相干辐射。本申请项目将主要研究两种方案来探索提高其转换效率，产生相干强X射线。其中之一是研究拍瓦（PW）级强激光与气体靶作用，通过气体的密度分布和激光脉冲控制等方式，控制激光尾波场中电子的注入。最终实现电子的同步注入，以形成超薄的高密度相对论电子层。利用该电子层来实现非线性汤姆逊散射，产生keV波段的高亮度相干辐射。另一种方案是通过拍瓦级强激光与具有一定结构（譬如光栅等）的固体薄膜靶作用，突破现有机制中高次谐波的效率按n^(-8/3)的幂指数下降的局限，显著提高keV量级的X射线相干辐射产生效率。

围绕本项目的研究计划和目标，主要开展了在激光尾场加速高品质电子束产生、高亮度X射线产生、激光与固体靶相互作用的加速和辐射，以及利用等离子体对强激光进行操控等4方面工作。有关电子加速方面提出了在激光尾场加速中利用激光离化过程控制电子注入的自截止注入机制和双色激光控制注入机制、激光尾场结构振荡导致电子注入机制，并利用激光模式组合实现非线性尾波场中的正电子加速和聚焦；在高亮度X射线产生方面，提出利用激光尾波场中阿秒电子束实现超强阿秒光脉冲以及相干Thomson背散射方案，提出基于等离子体通道的线性等离子体波荡器和螺旋等离子体波荡器概念，提出利用激光离化注入提高激光尾场加速中的betatron辐射亮度，实验和理论研究基于强激光与固体靶（包括光栅靶）作用的高次谐波辐射，揭示了光栅靶对提高高次谐波效率、操控谐波频率的效应。在等离子体对强激光操控方面提出10PW级超强激光驱动gamma射线源与稠密正负电子对产生方案，强磁场对10PW强激光偏振态的控制等。项目已经完全达到预期目标。