Research and development of an accelerator-based THz source prototype for pump-probe experiments at the European XFEL are ongoing at the Photo Injector Test Facility at DESY in Zeuthen (PITZ). Proof-of-principle experiments have been performed to generate a high-gain THz Free-electron Laser (FEL) based on the Self-Amplified Spontaneous Emission scheme. The FEL radiation pulses with a central wavelength of about \SI{100}{\micro\metre} (\SI{3}{\tera\hertz}) and single pulse energy of several tens~\SI{}{\micro\metre} can be generated. In this paper, we present and discuss the photon diagnostic setup for the THz FEL together with examples of diagnostic results, including pulse energy and an FEL gain curve. The upgraded photon diagnostic setup, capable of measuring pulse energy, transverse distribution, and spectral distribution, is expected to be operational in the spring of 2023.

A THz SASE FEL is currently under operation at the Photo Injector Test facility at DESY in Zeuthen (PITZ) as a prototype THz source for pump-probe experiments at the European XFEL.This prototype should provide tunable (3-5 THz) narrowband THz radiation with THz pulse energies up to several hundred μJ from 17-20MeV electron beams with a beam charge of several nC and a peak current up to 200 A to demonstrate the THz SASE FEL concept. In experiments it has been learned that strong space charge effects, steering effects from quadrupoles and possibly geometrical and conductive wall wakefields should be carefully treated during the beam transport from the photocathode to the undulator. These effects have been reduced by applying a smooth beam transport and improving the beam trajectory in the booster accelerator and the quadrupole magnets. Furthermore, the beam trajectory and matching into the undulator is critical for the THz output energy. This has been optimized by the Bayesian optimization algorithm. In this paper, experimental findings regarding the optimization of electron beams and THz radiations will be reported.

Development of an accelerator-based tunable THz source prototype for pump-probe experiments at the European XFEL is ongoing at the Photo Injector Test facility at DESY in Zeuthen (PITZ). The proof-of-principle experiments on the THz SASE FEL are performed utilizing the LCLS-I undulator installed in the PITZ beamline. The first lasing at a center wavelength of 100 µm was observed in the summer of 2022. The lasing of the narrowband THz source was achieved using an electron beam with an energy of ~17 MeV and a bunch charge up to several nC. Optimization of beam transport and matching resulted in the measurement of THz radiation with a pulse energy of tens of µJ, measured with pyroelectric detectors. The THz FEL gain curves were measured by means of specially designed short coils along the undulator. The results of the first characterization of the THz source at PITZ will be presented.