Novel accelerator concepts such as all-optical terahertz (THz) based compact accelerators demand high-power THz sources that are robust in order to enable reliable testing. THz sources based on the tilted-pulse front scheme have become the method of choice for table-top, high-energy, single-cycle (SC) THz generation due to both their versatility and scalability. However, due to the noncollinear interaction geometry, fine-tuning of the performance and tailoring of the THz beam properties requires a detailed understanding of the dependences on the setup parameters. Here, we present on the use of multi-dimensional parameter scans to systematically map out sensitivities of such THz sources on the primary interaction parameters and show experimental characterization of a robust, high-energy, single-cycle THz source designed and constructed based on these findings. This setup delivers pulses centered at 300 GHz with pulse energies exceeding 400 µJ at 52 Hz repetition rate and a shot-to-shot rms stability < 3.8%. Such robust, high-energy THz sources are crucial for the development of next generation THz-driven particle accelerators and manipulators.

Recent demonstrations of terahertz (THz) powered accelerators and beam manipulators have opened a pathway towards miniaturized accelerators that promise to enable new science due to unique features such as reduced timing-jitter and reduced space-charge broadening of the electron bunches. Here, we present on the development of a matchbox sized multi-layered accelerator structure powered by a single few-cycle terahertz pulse and designed to boost the output of a 55 keV DC electron gun to energies up to ~ 400 keV. An integrated actuated mirror is used to interfere the transversely injected THz pulse with itself, creating a transient standing wave optimized for efficient acceleration of the electrons. In contrast to a double-side-pumped approach this reduces the complexity of the optical setup by using the available THz energy more efficiently. We demonstrate first acceleration and map out the booster performance by varying the injection timing of the electrons and fine-tuning of the transient THz standing wave. Such a table-top source is promising for ultrafast electron diffraction experiments as well as precursor for subsequent acceleration to MeV energy by THz-driven LINACs.

Ultrafast electron diffraction (UED) is a powerful tool for the direct visualization of structural dynamic process-es in matter on atomic length and time scales. Observa-tions on a femtosecond time scale with atomic resolution spatially have long been a goal in science and are current-ly achieved with large photo injectors developed for FEL frontends. Here we demonstrate a compact 180 keV photocathode S-band electron gun, which employs field-enhancement at a pin-shaped cathode to produce an extraction field strength of 102 MV/m driven by a rack-mountable solid state 10 kW peak power supply. Simula-tions predict that high-brightness electron bunches with RMS duration of 10 fs, a radius of 135 μm, and spatial emittance of 0.1 mm-mrad are possible for a bunch charge of 10 fC. The impact of laser spot size and dura-tion, as well as their spatial distribution, on the temporal bunch length of electrons on the specimen was investigat-ed. Following the successful completion of the condition-ing phase of the RF gun and multipacting suppression, photo-triggered electrons using a UV laser on the photo-cathode were observed.

A promising approach for compact linear accelerators in the THz frequency range is based on dielectric-loaded waveguides (DLWs). Higher breakdown fields expected at THz frequencies should enable higher acceleration gradients. However, the accelerating mode of a cylindrical DLW (TM₀₁) is not the fundamental and only mode inside the waveguide at operating frequency. Therefore, a method is required to ensure excitation of the proper mode only. Here we present a coupler design to convert the guided electromagnetic TE₁₀ mode in a rectangular waveguide to the TM₀₁ mode of a cylindrical DLW. The symmetry of the structure and its feeding waveguides allow us to suppress all undesired modes and consequently increase the coupling efficiency to the desired mode. Moreover, this configuration shows an extremely wide bandwidth and low quality factor suggesting the coupler is also suitable for short THz pulses.