Ultrafast High-Field THz beamline at X-ray FEL

THz sources at FLASH utilize spent electron beam from an soft X-ray FEL to generate very intense (up to $150 {\mu } \mathrm {J})$, tunable frequency (1–300THz) and ultrafast narrowband $( \sim 10$%) THz pulses, which are naturally synchronized to soft X-ray pulses. This unique combination allows for wide range of element specific pump-probe experiments in physics, material science and biology. Here we discuss the unique features of the FLASH THz pulses and the accelerator source that bring along a set of challenges in the diagnostics of their key parameters: pulse energy, spectral, temporal and spatial profiles.