论文标题
分析自由电子激光器中的超短束
Analysis of Ultra-Short Bunches in Free-Electron Lasers
论文作者
论文摘要
自由电子激光器（FELS）以毫米波到硬X射线的波长运行。在X射线波长下，Fels通常依赖于自发自发发射（SASE）。典型的SASE发射包含多个时间“尖峰”，这些时间“尖峰”限制了光学输出的纵向相干性。因此，对SASE发射的纵向相干性有所改善的替代方案引起了人们的关注。在本文中，我们同意电子束比Sase Spike分离短。在这种情况下，自发产生的辐射由一个单个光脉冲组成，其纵向相干性比典型的Sase Fels中的纵向相干性更好。为了调查此制度，我们使用两个FEL模拟代码。一个（密涅瓦）使用缓慢变化的包膜近似（SVEA），该近似值（SVEA）分解了极短的脉冲。第二个（puffin）是一个粒子中的模拟代码，被认为是基础物理学的更完整的模型，并且能够模拟非常短的脉冲。我们首先通过表明在ENEA Frascati的Sparc Sase Fel实验模拟中的代码之间存在实质性一致性来锚定这些代码。然后，我们使用比SASE切片分离短的电子束长度比较了两个代码进行模拟。短束模拟的两个代码之间的比较阐明了该制度中SVEA的局限性，但表明SVEA可以处理与合作长度相当的短束。
Free-electron lasers (FELs) operate at wavelengths from millimeter waves through hard x-rays. At x-ray wavelengths, FELs typically rely on self-amplified spontaneous emission (SASE). Typical SASE emission contains multiple temporal "spikes" which limit the longitudinal coherence of the optical output; hence, alternate schemes that improve on the longitudinal coherence of the SASE emission are of interest. In this paper, we consnider electron bunches that are shorter than the SASE spike separation. In such cases, the spontaneously generated radiation consists of a single optical pulse with better longitudinal coherence than is found in typical SASE FELs. To investigate this regime, we use two FEL simulation codes. One (MINERVA) uses the slowly-varying envelope approximation(SVEA) which breaks down for extremely short pulses. The second (PUFFIN) is a particle-in-cell (PiC) simulation code that is considered to be a more complete model of the underlying physics and which is able to simulate very short pulses. We first anchor these codes by showing that there is substantial agreement between the codes in simulation of the SPARC SASE FEL experiment at ENEA Frascati. We then compare the two codes for simulations using electron bunch lengths that are shorter than the SASE slice separation. The comparisons between the two codes for short bunch simulations elucidate the limitations of the SVEA in this regime but indicate that the SVEA can treat short bunches that are comparable to the cooperation length.
