FERMI is a seeded free electron laser (FEL) based on the high gain harmonic generation (HGHG) scheme and has been generating intense and fully coherent extreme ultra-violet and soft x-ray pulses for several years. The high-degree performance of the FEL leans on a high brightness electron beam, with small transverse emittance ($\sim 1\mu \mathrm{m}$) and a peak current of about 700-800 A. The main constraint on lasing at high harmonics of the seed is low electron time-slice energy spread. The optimization of the photoinjector and of some linac parameters has allowed a reduction of the relative slice energy spread to the level of few times of ${10}^{-5}$. With these new conditions, the FEL can be operated without the need of a laser heater to suppress micro bunching instabilities and this “cold” beam has allowed generation of extreme UV pulses with pulse energy exceeding a mJ, and with peak power of about 10 GW. We describe the electron beam characterization and the FEL performance improvement, including the extension of the range of harmonics of the seed which can be amplified, up to the twenty-fifth harmonic, i.e., 10 nm.

中文翻译：

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“冷”电子束增强了种子自由电子激光性能

FERMI是一种基于高增益谐波产生（HGHG）方案的种子自由电子激光器（FEL），并且已经产生强且完全相干的极端紫外线和软X射线脉冲已有几年了。FEL的高度性能依赖于高亮度电子束，而横向发射较小（$〜\mathrm{1个}\mu \mathrm{米}$）和约700-800 A的峰值电流。在高种子高次谐波激射的主要限制因素是低电子时间片能量散布。通过优化光注入器和一些直线加速器参数，可以将相对的切片能量散布减小到几倍于${10}^{-5}$。在这些新条件下，可以在不需要激光加热器的情况下操作FEL，从而抑制微束不稳定性，并且这种“冷”光束允许产生脉冲能量超过mJ，峰值功率约为10 GW的极端UV脉冲。 。我们描述了电子束的表征和FEL性能的改进，包括可以被放大的种子的谐波范围的扩展，直至第二十五个谐波，即10 nm。