Benchmarking of Electro-Optic Monitors for Femtosecond Electron Bunches

Abstract

The longitudinal profiles of ultrashort relativistic electron bunches at the soft x-ray free-electron laser FLASH have been investigated using two single-shot detection schemes: an electro-optic (EO) detector measuring the Coulomb field of the bunch and a radio-frequency structure transforming the charge distribution into a transverse streak. A comparison permits an absolute calibration of the EO technique. EO signals as short as 60 fs (rms) have been observed, which is a new record in the EO detection of single electron bunches and close to the limit given by the EO material properties. DOI: 10.1103/PhysRevLett.99.164801 PACS numbers: 41.75.Ht, 41.60.Cr, 41.85.Ew, 42.65.Re Intense relativistic electron bunches with a duration of 100 femtoseconds or less are essential for free-electron lasers (FELs) based on the principle of self-amplified spontaneous emission (SASE), such as the ultraviolet and soft x-ray Free-electron LASer at Hamburg (FLASH) [1], and future x-ray FELs like the Linac Coherent Light Source [2] at SLAC and the European XFEL [3]. Ultrashort relativistic electron bunches are also produced in plasma wakefield accelerators which have made impressive progress in the last years, see [4 -6] and the references quoted therein. Precise knowledge of the temporal profile of the electron bunches is essential for a detailed understanding of the physical processes in all these accelerators [1,4]. Two of the most important current techniques for the single-shot direct visualization of longitudinal electron bunch profiles are transverse-deflecting structures (TDS) and electro-optic (EO) detection.The principle of the TDS was demonstrated in 1964 [7]: the temporal profile of the electron bunch charge density is transferred to a transverse streak on a view screen by a rapidly varying electromagnetic field, analogous to the sawtooth voltage in a conventional oscilloscope tube. The time resolution of the TDS installed at FLASH can reach 15 fs (rms) if the beam optics is optimized to yield the smallest possible beam spot on the view screen [8]. With the optics tuned for FEL operation, the resolution is about 20 fs. Thus, the diagnostic itself is part of the accelerator optics design. The TDS must be several meters long to achieve sufficient streak length, preventing its use in a plasma wakefield accelerator. Furthermore, it is inherently destructive, prohibiting the characterization of bunches that are needed further downstream.The EO effect has been used extensively in terahertz time domain spectroscopy for over a decade [9], but its exploitation in electron bunch diagnostics is more recent. The need for an absolute temporal signal, rather than just a relative change, which is sufficient in spectroscopic applications, is a uniquely demanding requirement of EO bunch diagnostics. Single-shot EO measurements of picosecond electron bunches were first demonstrated in 2002 [10]. Several variants of EO bunch diagnostics have been applied [10 -12], all sharing the underlying principle of utilizing the field-induced bir...

Extending electro-optic detection to ultrashort electron beams

“…Schematics for three of the most widely used detection schemes are contained in Berden et al [7]. All these schemes rely on polarization changes due to the Pockels effect to extract the bunch profile.…”

“…The next advancement in EOS was the development of a single shot technique that relies on spectral decoding [3,7,8,16]. Here instead of an ultrashort pulse, a stretched pulse is used as a probe and only the polarization state corresponding to the 1 " 10 axis is examined.…”

“…The latest refinement to EOS is temporal decoding [6][7][8]. As with the previous technique, a stretched pulse is used to probe the nonlinear crystal.…”

“…Recently, this has been adapted to the longitudinal profiling of accelerated charged particle beams. EOS has been used to characterize beams either directly, by measuring their self-fields [2][3][4][5][6][7][8][9][10][11], or indirectly by measuring the THz fields generated from transition radiation diagnostics [12,13]. In the case of the former, the particle beam's longitudinal profile can be deduced from its self-fields due to the fact that for sufficiently relativistic beams the temporal structure of the selffields matches the temporal structure of the beam [14].…”

Electro-optical measurement of sub-ps structures in low charge electron bunches

“…It was shown that the Coulomb field of sub-ps electron bunch structures with some 100 pC charge can be measured with about 200 fs resolution by the spectral decoding technique [3,4]. Also, measuring the coherent transition radiation has proven to be a suitable method to analyze the temporal profiles of electron bunches [5]. A drawback of both methods is that they require a crystal or a metal foil close to or even in the path of the electrons and, thus, can either trigger wakefields or distort the electron bunches otherwise.…”