Numerical Studies on the Electro-Optic Sampling of Relativistic Electron Bunches

Abstract

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.

Characterization of the THz radiation source at the Frascati linear accelerator

“…As a consequence, at small observation angles, typical for radiation from highly relativistic electrons, and/or for finely collimated beams, the bunch transverse form factor has a weak influence on the coherent radiation spectrum. 50 In the following, we refer to F(ω), defined by…”

Phonon‐ and electron‐related far‐infrared absorption in CdTe and ZnTe crystals

“…Gallot et al [19] deduced from the low-frequency scope of the resonance frequency ν TO = 5.32 THz, and the damping factor γ TO /ν TO ≈ 4.7 × 10 -3 . The latter turned out to be significantly lower than that reported in early works (γ TO /ν TO ≈ 1.7 × 10 -2 ) [15,25] and in the handbook [26], and referred in recent works [27,33]. Experimental reflection data from early works used in recent analysis [33] showed irregularities in the reststrahl band that were again ascribed entirely to broadening TO mode resonance.…”

“…The latter turned out to be significantly lower than that reported in early works (γ TO /ν TO ≈ 1.7 × 10 -2 ) [15,25] and in the handbook [26], and referred in recent works [27,33]. Experimental reflection data from early works used in recent analysis [33] showed irregularities in the reststrahl band that were again ascribed entirely to broadening TO mode resonance. There is no experimental evidence on phonon difference transitions at ≈6 THz in ZnTe although the phonon band population inversion obtained in Monte Carlo modelling encourages for a deeper look on what mechanisms do control the infrared absorption above the TO mode resonance in the reststrahl band.…”

Coherent radiation diagnostics for short bunches

“…Transition radiation is significantly more intensive and can be reasonably well calculated for realistic geometries [10]. It is, however, destructive (even for a thin radiator, the emittance is degraded strongly).…”