Ultrabroadband terahertz source and beamline based on coherent transition radiation

Abstract

Coherent transition radiation (CTR) in the THz regime is an important diagnostic tool for analyzing the temporal structure of the ultrashort electron bunches needed in ultraviolet and x-ray free-electron lasers. It is also a powerful source of such radiation, covering an exceptionally broad frequency range from about 200 GHz to 100 THz. At the soft x-ray free-electron laser FLASH we have installed a beam transport channel for transition radiation (TR) with the intention to guide a large fraction of the radiation to a laboratory outside the accelerator tunnel. The radiation is produced on a screen inside the ultrahigh vacuum beam pipe of the linac, coupled out through a diamond window and transported to the laboratory through an evacuated tube equipped with five focusing and four plane mirrors. The design of the beamline has been based on a thorough analysis of the generation of TR on metallic screens of limited size. The optical propagation of the radiation has been computed taking into account the effects of near-field (Fresnel) diffraction. The theoretical description of the TR source is presented in the first part of the paper, while the design principles and the technical layout of the beamline are described in the second part. First experimental results demonstrate that the CTR beamline covers the specified frequency range and preserves the narrow time structure of CTR pulses emitted by short electron bunches.

Intense terahertz pulses from SLAC electron beams using coherent transition radiation

“…Several accelerator-based sources of THz radiation using these approaches have been constructed. [1][2][3][4][5][6][7][8][9][10][11] In CTR from a thin metallic foil, surface currents that match boundary conditions on the foil produce the fields of a collapsing dipole as the electrons approach, and an expanding dipole as they leave, generating an impulse of dipole radiation on both sides. 12 The temporal shape has a long-wavelength cutoff due to the finite size of the foil and apertures.…”

Coherent single-cycle pulses with MV/cm field strengths from a relativistic transition radiation light source

“…The transport tube has a minimum diameter of 200 mm to maintain low frequencies. A detailed description of the beam transport theory of the CTR generation at FLASH can be found in [10].In order to enable direct characterization of the THz field in the time domain, it is vital to use a synchronized laser system with a jitter on the order of 100 fs or less relative to the electron bunch. For our experiments a Ti:sapphire femtosecond oscillator is synchronized to the master oscillator of the FLASH linear accelerator by high frequency to baseband mixing (Fig.…”

MV/cm THz pulses from a coherent transition radiation source

“…Coherent transition radiation (CTR) is a source of single cycle electro-magnetic radiation with a broad spectrum spanning the terahertz through infrared region. At the soft x-ray free-electron laser FLASH [2] transition radiation is generated from collision of electrons bunches with energies up to 1.2 GeV with a metal screen and then guided through an evacuated beam line [3] with 20 m length into a user accessible lab. While the main purpose of this beam line has been high-resolution diagnostics of the longitudinal electron bunch characteristics, it can also serve as a powerful and broadband source for experiments with subpicosecond single cycle pulses.…”