  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, high-power, narrow-band 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 single-pass high-gain THz
FEL operating with a central wavelength of 100 micrometers. The THz FEL
prototype is currently in operation 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. A self-amplified spontaneous emission (SASE) FEL was
envisioned as the main mechanism for generating the THz pulses. Although the
THz FEL at PITZ is supposed to use the same mechanism as at X-ray facilities,
it cannot be considered as a simple scaling of the radiation wavelength because
there is a large difference in the number of electrons per radiation
wavelength, which is five orders of magnitude higher for the THz case. The
bunching factor arising from the electron beam current profile contributes
strongly to the initial spontaneous emission starting the FEL process.
Proof-of-principle experiments were done at PITZ using an LCLS-I undulator to
generate the first high-power, high-repetition-rate single-pass THz 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, more than 100 microjoules were generated at a central wavelength of 100
micrometers. The narrowband spectrum was also demonstrated by spectral
measurements. These proof-of-principle experiments pave the way for a tunable,
high-repetition-rate THz source providing pulses with energies in the
millijoule range.
