Theoretical studies of Raman Scattering and Chirped Pulse Amplification in Plasma

该提案旨在探索利用完全电离的气体或等离子体作为高效短脉冲放大器的潜力。用等离子体中的两个碰撞脉冲（种子脉冲和泵脉冲）激发等离子体波，可以有效地放大短种子脉冲。利用啁啾泵浦脉冲可以提高等离子体放大介质的带宽。在线性状态下，在放大过程饱和之前，长啁啾泵浦激光脉冲提供了分布式放大，其中种子的不同光谱成分在等离子体中的不同纵向位置被放大，通过创建啁啾等离子体密度梯队，很像衍射光栅。这就像一个长啁啾反射镜，同时对啁啾泵浦脉冲进行反向散射和压缩，有效地将增益带宽扩大到泵浦的增益带宽。放大器的增益和带宽取决于等离子体的固有振荡频率（等离子体频率）和啁啾率（频率沿泵浦脉冲变化的速率）和泵浦的频谱带宽。这与传统的啁啾脉冲放大器（CPAs）和光学参数啁啾脉冲放大器（OPCPAs）形成对比，其中探针是啁啾的，而泵通常是单色的（非啁啾）。啁啾脉冲拉曼放大器既可以作为高保真超短脉冲大功率线性放大器，也可以作为传统CPA放大器的高能啁啾脉冲的压缩器。它还避免了在巨大的真空室中对极其庞大和昂贵的光学元件和压缩机的需求。此外，由于啁啾脉冲拉曼放大是一个三波参数相互作用，它提供了一种消除预脉冲和基础的手段，这通常限制了传统固态CPA放大器的实用性。这项研究计划将研究拉曼放大的线性和非线性阶段，以期开发具有开辟物理学新领域潜力的极高功率激光器，例如使用激光从真空中产生粒子或在实验室中创造天体物理条件。

The proposal is to explore the potential of using a fully ionised gas or plasma as an efficient short pulse amplifier. By exciting a plasma wave by two colliding (seed and pump respectively) pulses in plasma, it is possible to amplify the short seed pulse efficiently. The bandwidth of the plasma amplifier medium is enhanced when a chirped pump pulse is utilised. In the linear regime, before saturation of the amplifying process takes over, the long chirped pump laser pulse provides distributed amplification where different spectral components of the seed are amplified at different longitudinal positions in the plasma through the creation of a chirped plasma density echelon, much like a diffraction grating. This behaves as a long chirped mirror which simultaneously backscatters and compresses the chirped pump pulse and effectively broadens the gain bandwidth to that of the pump. The gain and the bandwidth of the amplifier depend on the natural oscillation frequency of the plasma (the plasma frequency) and the chirp rate (the rate at which the frequency changes along the pump pulse) and spectral bandwidth of the pump. This contrasts with conventional chirped pulse amplifiers (CPAs) and optical parametric chirped pulse amplifiers (OPCPAs) where the probe is chirped while the pump is usually monochromatic (un-chirped). The chirped pulse Raman amplifier has potential use either as a high fidelity ultra-short pulse high power linear amplifier or as a compressor of high energy chirped pulses from a conventional CPA amplifier. It also avoids the requirement for extremely large and expensive optical elements and compressors in vast vacuum chambers. Furthermore, because chirped pulse Raman amplification is a three wave parametric interaction it provides a means of eliminating pre-pulses and pedestals which usually limit the usefulness of conventional solid state CPA amplifiers. This research proposal will investigate the linear and non-linear stages of Raman amplification with a view to develop extremely high power lasers which have the potential of opening up new frontiers of physics such as using lasers to create particles from vacuum or create astrophysical conditions in the laboratory.