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The electro-optic (EO) effect is a powerful diagnostic tool for determining the time profile of ultrashort relativistic electron bunches. When a relativistic bunch passes within a few mm of an electro-optic crystal, its transient electric field is equivalent to a half-cycle THz pulse passing through the crystal. The induced birefringence can be detected with polarized femtosecond laser pulses. A simulation code has been written in order to understand the faithfulness and the limitations of electron bunch shape reconstruction by EO sampling. The THz pulse and the laser pulse are propagated as wave packets through the EO crystal. Alternatively, the response function method is applied. Using experimental data on the material properties of zinc telluride (ZnTe) and gallium phosphide (GaP), the effects of velocity mismatch, pulse shape distortion, and signal broadening are explicitly taken into account. The simulations show that the most severe limitation on the time resolution is given by the transverse-optical (TO) lattice oscillation in the EO crystal. The lowest TO frequency is 5.3 THz in ZnTe and 11 THz in GaP. Only the Fourier components below the TO resonance are usable for the bunch shape reconstruction. This implies that the shortest rms bunch length which can be resolved with moderate distortion amounts to 90 fs in ZnTe and 50 fs in GaP. The influence of the crystal thickness on the amplitude and width of the EO signal is studied. The optimum thickness is in the range from 100 to 300 m for ZnTe and from 50 to 100 m for GaP.

A systematic study is presented on the superconductivity (sc) parameters of the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz cavities for the linear collider project TESLA. Cylindrical Nb samples have been subjected to the same surface treatments that are applied to the TESLA cavities: buffered chemical polishing (BCP), electrolytic polishing (EP), low-temperature bakeout (LTB). The magnetization curves and the complex magnetic susceptibility have been measured over a wide range of temperatures and dc magnetic fields, and also for different frequencies of the applied ac magnetic field. The bulk superconductivity parameters such as the critical temperature T c = 9.26 K and the upper critical field B c2 (0) = 410 mT are found to be in good agreement with previous data. Evidence for surface superconductivity at fields above B c2 is found in all samples. The critical surface field exceeds the Ginzburg-Landau field B c3 = 1.695B c2 by about 10% in BCP-treated samples and increases even further if EP or LTB are applied. From the field dependence of the susceptibility and a power-law analysis of the complex ac conductivity and resistivity the existence of two different phases of surface superconductivity can be established which resemble the Meissner and Abrikosov phases in the bulk: (1) "coherent surface superconductivity", allowing sc shielding currents flowing around the entire cylindrical sample, for external fields B in the range B c2 < B < B coh c3 , and (2) "incoherent surface superconductivity" with disconnected sc domains for B coh c3 < B < B c3 . The "coherent" critical surface field separating the two phases is found to be B coh c3 = 0.81 B c3 for all samples. The exponents in the power law analysis are different for BCP and EP samples, pointing to different surface topologies.

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...

Precise measurements of the temporal profile of ultrashort electron bunches are of high interest for the optimization and operation of ultraviolet and x-ray free-electron lasers. The electro-optic (EO) technique has been applied for a single-shot direct visualization of the time profile of individual electron bunches at FLASH. This paper presents a thorough description of the experimental setup and the results. An absolute calibration of the EO technique has been performed utilizing simultaneous measurements with a transverse-deflecting radio-frequency structure that transforms the longitudinal bunch charge distribution into a transverse streak. EO signals as short as 60 fs (rms) have been observed using a gallium-phosphide (GaP) crystal, which is a new record in the EO detection of single electron bunches and close to the physical limit imposed by the EO material properties. The data are in quantitative agreement with a numerical simulation of the EO detection process.

At the 1 GeV electron linac of the DESY vacuum-ultraviolet free-electron laser an electro-optic (EO) sampling experiment has been installed permitting to measure the time structure of the bunches with high resolution. The transient electric field of the relativistic bunch corresponds to a sub-picosecond THz pulse which induces a birefringence in an electro-optic crystal. The sampling of the resulting polarization anisotropy by femtosecond laser pulses is studied in detailed numerical calculations. The THz and the laser pulses are treated as wave packets which propagate in the zinc telluride resp. gallium phosphide crystals. Using experimental data on the material properties of ZnTe and GaP the effects of signal broadening and distortion are explicitely taken into account. The most severe limitation on the time resolution is given by the transverse optical (TO) lattice oscillation in the EO crystal. The lowest TO frequency is 5.3 THz in ZnTe and 11 THz in GaP. The shortest bunch length which can be resolved with moderate distortion amounts to about 200 fs (FWHM) in ZnTe and 100 fs in GaP. The influence of the crystal thickness on the amplitude and width of the EO signal is studied. The optimum thickness is in the range from 100 to 300 µm. Multiple internal reflections can be suppressed by using a wedge-shaped EO crystal.

The SwissFEL Injector Test Facility operated at the Paul Scherrer Institute between 2010 and 2014, serving as a pilot plant and testbed for the development and realization of SwissFEL, the X-ray Free-Electron Laser facility under construction at the same institute. The test facility consisted of a laser-driven rf electron gun followed by an S-band booster linac, a magnetic bunch compression chicane and a diagnostic section including a transverse deflecting rf cavity. It delivered electron bunches of up to 200 pC charge and up to 250 MeV beam energy at a repetition rate of 10 Hz. The measurements performed at the test facility not only demonstrated the beam parameters required to drive the first stage of an FEL facility, but also led to significant advances in instrumentation technologies, beam characterization methods and the generation, transport and compression of ultra-low-emittance beams. We give a comprehensive overview of the commissioning experience of the principal subsystems and the beam physics measurements performed during the operation of the test facility, including the results of the test of an in-vacuum undulator prototype generating radiation in the vacuum ultraviolet and optical range.

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