X-ray free-electron lasers based on superconducting accelerator technology deliver ultrashort photon pulses with unprecedented peak brilliance at high repetition rates. Continuous and noninvasive monitoring of the current profile of the electron bunches is essential for the operation and control of the accelerator. Longitudinal diagnostics based on coherent radiation have already shown their potential at various free-electron laser facilities, and the multi-GeV electron beams of x-ray free-electron lasers are powerful sources for the generation of broadband coherent diffraction radiation. We present noninvasive current profile measurements with a few femtoseconds resolution based on spectroscopy of coherent diffraction radiation in the frequency range 0.7–58 THz. The current profiles, reconstructed from the spectroscopic data with an advanced phase retrieval method, are compared with measurement results obtained with a transverse deflecting structure. For the first time, bunch-resolved current profiles have been recorded simultaneously to user operation at European XFEL for all bunches in the bunch train at MHz repetition rates.

中文翻译：

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无创THz光谱以MHz重复频率重建束电流分布

基于超导加速器技术的X射线自由电子激光器以高重复频率提供超短光子脉冲，具有前所未有的峰值亮度。对电子束的电流分布进行连续且无创的监控对于加速器的操作和控制至关重要。基于相干辐射的纵向诊断已显示出其在各种自由电子激光设备中的潜力，x射线自由电子激光器的多GeV电子束是产生宽带相干衍射辐射的有力来源。我们基于在0.7–58 THz频率范围内的相干衍射辐射的光谱学，提出了具有几飞秒分辨率的无创电流剖面测量结果。当前的个人资料，使用先进的相位检索方法从光谱数据中重建数据，并与横向偏转结构获得的测量结果进行比较。第一次，在欧洲XFEL上以用户群重复记录了束解析的电流分布图，以供用户操作，以MHz重复频率进行。