5th generation light sources based on underdense photocathode plasma wakefield acceleration

由自由电子激光器产生的高亮度、单色、极短的辐射脉冲（具有fs-duration）的核心重要性，使人们能够深入了解物质的结构，这使得自由电子激光器（FEL）的发展具有极大的吸引力。科学、信息技术、工业和医疗都受益于像FEL这样的设备，并在各自的工作中向前发展。上述辐射源需要高质量的电子束，能量在几百MeV到几十GeV之间，不相关的能量扩散在0.1%的水平，低发射度（发射度< 0.1mm - mrad）和大电流（kA范围）。目前，超导射频（SRF）腔中的加速梯度约为100MV/m，这意味着电子束需要一米长的SRF腔才能达到100兆电子伏的束流能量。因此，基于SRF技术的XFEL设备变得极其笨重和昂贵。对于第五代光源，未来的加速器将需要新颖的技术，以更紧凑的设计和适中的成本。等离子体腔可以实现更高的加速梯度。等离子体腔内的加速度梯度可达100GV/m或更高。注入电子并将其捕获在等离子体腔中，可以将电子加速到厘米级的高能量。这大大减少了尺寸和成本，这可能会导致一个大学实验室规模的fel设施。将电子注入等离子体腔是PWFA领域的一个重大挑战。一种有前途的方法是等离子体低密度光电阴极（也称为“特洛伊木马”）技术，该技术直接将电子注入到中等强度激光脉冲引起的井喷中。这种方法有可能产生发射率低至纳米级的高质量电子束。利用“特洛伊木马”技术产生的电子可能适用于大学规模的紧凑型自由电子激光器等设备，并在世界范围内扩大XFEL设备的使用范围。为了满足FEL对电子束的要求，基于等离子体加速器的电子束质量需要进一步提高，主要是在能量扩散方面。此外，第五代光源的一个不可忽视的方面是等离子体加速器中产生的电子束的提取和传输到目标应用。特别是势输运光学必须避免电子束退化。

The central importance of high-brightness, monochromatic, extremely short radiation pulses (with fs-duration) generated by a free-electron laser, which gives an in-depth insight into the structure of matter makes the development of free- electron laser (FEL) immensely attractive. Science, information technology, industry and medical benefit from such devices like FEL and moved forward in the past the respective discipline in their work. The above-mentioned radiation sources need high quality electron-beams with energy in the range of few hundred MeV up to tens of GeV, uncorrelated energy spread at the level of 0.1%, low emittance (emittance < 0.1mm - mrad) and high current (kA range). Currently the accelerating gradients in superconducting radiofrequency (SRF) cavities are in order of 100MV/m, meaning that an electron beam would require one meters of such SRF cavities to reach 100 MeV beam energy. Therefore, XFEL facility based on SRF technique becomes extremely bulky and expensive. Towards 5th generation light sources, future accelerators will require novel techniques for more compact designs at moderate costs. Much higher accelerating gradients can be realized by plasmas cavities. The acceleration gradients in thus plasmas cavities can be up to 100GV/m or more. Injecting electrons and trapping them in the plasmas cavities enable the potential to accelerate electrons to high energy in cm scale. This reduces significantly in size and cost, which may leads to a university laboratory scale FEL-facility. Injecting electrons into this plasmas cavity is a significant challenge in the PWFA community. One proposed, promising method is the plasma underdense photocathode (also called the "Trojan Horse") technique for directly injecting electrons in to the blowout due to a moderately intense laser-pulse. This method can potentially produce high quality electron beams with emittances down to nm level. With the "Trojan Horse"-technique generated electrons may be suitable for devices such as university-scale compact FEL and enlarged worldwide the access to XFEL facilities. In order to meet the requirement on electron beams for FEL the quality plasma accelerator based electron beams need to be improved further, i.e mainly in term of energy spread. In addition, a not negligible aspect of a 5th generation light sources is extraction and transport of electron beams generated in plasma accelerator to the aimed application. In particular potential transport optics has to avoid electron beam degradation.