Noninvasive THz spectroscopy for bunch current profile reconstructions at MHz repetition rates

Nils Maris Lockmann,Bernhard Schmidt,Christopher Gerth,Stephan Wesch

doi:10.1103/physrevaccelbeams.23.112801

Nils Maris Lockmann, Bernhard Schmidt + Show 2 more

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https://doi.org/10.1103/physrevaccelbeams.23.112801

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Abstract

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.

Highlights

Single-pass x-ray free-electron lasers (XFELs) [1,2,3,4] require high-brightness electron beams with peak currents in the kiloampere range to start the self-amplification of spontaneous emission (SASE) process [5] in the undulators
Such high peak currents and the constant quest to produce ever shorter photon pulses push longitudinal bunch compression into a regime in which the formation of the current profile can be strongly affected by collective and nonlinear effects such as coherent synchrotron radiation or longitudinal space charge [6,7,8]. It is crucial for the preparation of the electron beam for XFEL user experiments as well as stable accelerator operation to monitor the current profile with femtosecond resolution
We present current profile measurements with a few femtoseconds resolution obtained at the European XFEL (EuXFEL) by combining coherent diffraction radiation (CDR) and a spectrometer capable of measuring at MHz repetition rates

Summary

Introduction

Single-pass x-ray free-electron lasers (XFELs) [1,2,3,4] require high-brightness electron beams with peak currents in the kiloampere range to start the self-amplification of spontaneous emission (SASE) process [5] in the undulators Such high peak currents and the constant quest to produce ever shorter photon pulses push longitudinal bunch compression into a regime in which the formation of the current profile can be strongly affected by collective and nonlinear effects such as coherent synchrotron radiation or longitudinal space charge [6,7,8].

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