X-ray characterisation in high-intensity laser-solid interactions

当高强度激光脉冲与固体靶碰撞时，存在的电场足够强以电离固体原子，在靶表面上产生离子和电子的等离子体。这些等离子体电子在激光场中的加速通过非线性康普顿散射（NCS）过程产生X射线。虽然预计NCS可以在下一代激光设施中提供高效的X射线源，并且还可以作为激光器内电子运动的诊断，但我们尚未在现代激光器达到的较低强度下在激光-固体相互作用中观察到这一过程。这是由于等离子体电子在固体靶核的电场中散射时产生的韧致辐射的背景。本文试图研究激光-固体相互作用中产生的NCS和韧致辐射X射线，并寻找NCS的特征信号，以便对这一过程进行实验验证。这将通过将模拟韧致辐射的能力添加到粒子在细胞中的代码EPOCH来实现。通过大尺度激光-固体相互作用模拟，研究了不同激光/固体参数下的X射线分布，以确定两种辐射机制之间的角度/光谱差异。建立了单粒子模型，解析描述了电子片在固定离子通道中的运动。这个简单的框架将测试对EPOCH模拟，以确定它的适用性，并可用于描述的NCS发射的激光-固体相互作用的预期形式。如果成功的话，这个代码的进一步应用将有助于开发阿秒时间尺度上的X射线脉冲源

When a high intensity laser pulse collides with a solid target, the electric fields present are strong enough to ionise the solid atoms, creating a plasma of ions and electrons on the target surface. The acceleration of these plasma electrons in the laser fields produce X-rays via the non-linear Compton scatter (NCS) process. While it is expected that NCS may provide a highly efficient source of X-rays in next generation laser facilities, and may also act as a diagnostic for the electron motion within the laser, we have not yet observed this process in laser-solid interactions at the lower intensities reached by modern lasers. This is due to a background of bremsstrahlung radiation, produced as plasma electrons scatter in the electric fields of the solid target nuclei. This work attempts to characterise the NCS and bremsstrahlung X-rays produced in laser-solid interactions, and to find NCS signatures which would allow for experimental identification of this process. This will be accomplished by adding the capability to simulate bremsstrahlung radiation to the particle-in-cell code EPOCH. Large scale laser-solid interaction simulations will be run to study the expected X-ray distributions over a variety of laser/solid parameters to identify angular/spectral differences between the two radiation mechanisms.A single particle model has been developed to analytically describe electron sheets moving through an immobile ion channel. This simple framework will be tested against EPOCH simulations to determine it's applicability, and may be used to describe the expected form of the NCS emission in laser-solid interactions. If successful, further applications of this code could help develop a source of X-ray pulses on the attosecond timescale