First high peak and average power THz SASE FEL in operation

Advanced experiments using THz pump and X-ray probe pulses at modern free-electron lasers (FELs) like the European X-ray FEL require a frequency-tunable (from 0.1 THz to 30 THz), high-power (> 10 microjoule), narrow-band (~ 1–2%) THz source maintaining the repetition rate and pulse structure of the X-ray pulses. This paper reports the first results from a THz source, that is based on a self-amplified spontaneous emission (SASE) FEL operating with a central wavelength of 100 micrometers. The THz SASE FEL prototype is currently under development at the Photo Injector Test facility at DESY in Zeuthen (PITZ) and uses the same type of electron source as the European XFEL photo injector. The first proof-of-principle experiments were done at PITZ using an LCLS-I undulator to generate high-power, high-repetition-rate THz SASE FEL radiation. Electron bunches with a beam energy of ~ 17 MeV and a bunch charge of up to several nC are used to generate THz pulses with a pulse energy of several tens of microjoules. For example, for an electron beam with a charge of ~ 2.4 nC, 65 microjoules were measured at a central wavelength of 100 micrometers. These proof-of-principle experiments pave the way for a tunable, high-repetition-rate THz source providing pulses with energies in the millijoule range.