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Applied physics letters 120, 104102 (2022). doi:10.1063/5.0078927, Electron photoinjectors provide high-brightness electron beams to numerous research applications in physics, chemistry, material, and life sciences. Semiconductor photocathodes are widely used here, as they enable the production of low-emittance beams with variable charge at high repetition rates. One of the key figures of merit of photocathodes is the minimum achievable bunch length. In semiconductor cathodes, this is dominated by scattering effects and varying penetration depths of the extracting photons, which leads to a characteristic electron emission function. We present a method to determine this cathode time response with resolution on the tens of femtoseconds level, breaking the resolution barrier encountered in previous studies. The method is demonstrated with cesium-telluride (Cs$_2$Te) and gold cathodes, revealing response times of (184 �� 41) fs up to (253 �� 58) fs for the semiconductor and an upper limit of (93 �� 17) fs for the metal. Monte Carlo simulations of Cs$_2$Te emission benchmarked to these results give detailed information about the cathode material., Published by American Inst. of Physics, Melville, NY

10th International Particle Accelerator Conference, IPAC'19, Melbourne, Australia, 19 May 2019 - 24 May 2019; Journal of physics / Conference series Conference series [...] 1350, 012036 - (2019). doi:10.1088/1742-6596/1350/1/012036, A free-electron laser based THz source is undergoing design studies at the Photo Injector Test facility at DESY in Zeuthen (PITZ). It is considered as a prototype for pump-probe experiments at the European XFEL, benefiting from the fact that the electron beam from the PITZ facility has an identical pulse train structure as the XFEL pulses. In the proposed proof-of-principle experiment, the electron beam (up to 4 nC bunch charge and 200 A peak current) will be accelerated to 16-22 MeV/c to generate SASE radiations in an LCLS-I undulator in the THz range between 60 and 100 pm with an expected energy of up to ~1 mJ/pulse. In this paper, we report our simulations on the optimization of the photo-injector and the design of the transport and matching beamline. Experimental investigations on the generation, characterization and matching of the high charge beam in the existing 22-m-long beamline will also be presented., Published by IOP Publ.20382, Bristol

10th International Particle Accelerator Conference, IPAC’19, Melbourne, Australia, 2 May 2019 - 24 May 2019; Journal of physics / Conference series Conference series [...] 1350, 012046 - (2019). doi:10.1088/1742-6596/1350/1/012046, Laser pulse shaping is one of the key elements to generate low emittance electron beams with RF photoinjectors. Ultimately high performance can be achieved with ellipsoidal laser pulses, but 3-dimensional shaping is challenging. High beam quality can also be reached by simple transverse pulse shaping, which has demonstrated improved beam emittance compared to a transversely uniform laser in the 'pancake' photoemission regime. In this contribution we present the truncation of a Gaussian laser at a radius of approximately one sigma in the intermediate (electron bunch length directly after emission about the same as radius) photoemission regime with high acceleration gradients (up to 60 MV/m). This type of electron bunch is used e.g. at the European XFEL and FLASH free electron lasers at DESY, Hamburg site and is being investigated in detail at the Photoinjector Test facility at DESY in Zeuthen (PITZ). Here we present ray-tracing simulations and experimental data of a laser beamline upgrade enabling variable transverse truncation. Initial projected emittance measurements taken with help of this setup are shown, as well as supporting beam dynamics simulations. Additional simulations show the potential for substantial reduction of slice emittance at PITZ., Published by IOP Publ.20382, Bristol

44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) : [Proceedings] - IEEE, 2019. - ISBN 978-1-5386-8285-2 - doi:10.1109/IRMMW-THz.2019.8874425 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2019, Paris, France, 1 Sep 2019 - 6 Sep 2019; IEEE 1 pp. (2019). doi:10.1109/IRMMW-THz.2019.8874425, There is a high demand for intense THz sources since “many excitation mechanisms of matter resonate in the terahertz regime” especially for condensed matters. “Accelerator-based THz sources provide the wide tunability together with high intensity and repetition rates beyond 100 kHz, that will enable broad application at the European XFEL to the most interesting scientific problems in the field” [1]. Supported by European XFEL a proof of principle study is started at the Photo-Injector Test Facility located at DESY in Zeuthen site (PITZ). Since PITZ and European XFEL electron sources are identical the X-ray and THz radiation can be produced with identical bunch train structure so that for every X-ray pulse a corresponding THz pulse can be provided for the pump-and- probe experiments., Published by IEEE

Plasma physics and controlled fusion 61(4), 045012 (2019). doi:10.1088/1361-6587/ab04b9, We present a new method to determine the electron density of a plasma by measuring the periodicity of modulations introduced to the longitudinal phase space of a relativistic particle bunch by the interaction with the plasma via the self-modulation instability. As the modulation is solely depending on the plasma density and the beam parameters, this method allows to determine the time-resolved density of a plasma at the position of beam passage, which is confirmed in particle-in-cell simulations. Densities in the range of 3.6 × 10$^{12}$ cm$^{−3}$ – 7.2 × 10$^{15}$ cm$^{−3}$ have been measured and the measurement accuracy is confirmed by comparison to spectroscopic plasma density measurements., Published by IOP Publ., Bristol

Physical review letters 121(6), 064801 (2018). doi:10.1103/PhysRevLett.121.064801, Particle-beam-driven plasma wakefield acceleration (PWFA) enables various novel high-gradient techniques for powering future compact light-source and high-energy physics applications. Here, a driving particle bunch excites a wakefield response in a plasma medium, which may rapidly accelerate a trailing witness beam. In this Letter, we present the measurement of ratios of acceleration of the witness bunch to deceleration of the driver bunch, the so-called transformer ratio, significantly exceeding the fundamental theoretical and thus far experimental limit of 2 in a PWFA. An electron bunch with ramped current profile was utilized to accelerate a witness bunch with a transformer ratio of $4.6^{+2.2}_{−0.7}$ in a plasma with length $∼10 cm$, also demonstrating stable transport of driver bunches with lengths on the order of the plasma wavelength., Published by APS, College Park, Md.