## 1 Introduction  
An accelerator-based THz source prototype for pump-probe experiments at the European XFEL was recently constructed at the Photo Injector Test Facility at DESY in Zeuthen (PITZ), which was established for the commissioning and testing of electron sources with various diagnostic systems from measurement of beam charge, beam momentum to transverse and longitudinal phase space characterization [1]. At PITZ, a space charge-dominated electron beam from the radio-frequency (RF) gun is accelerated to a beam momentum of 15-20 MeV/c by the Cut Disk Structure (CDS) booster cavity. In 2022, new elements such as an undulator, THz test stations, and a bunch compressor were installed at the Photo Injector Test Facility at DESY in Zeuthen (PITZ) beamline as parts of the THz source prototype. To achieve a THz Self-Amplified Spontaneous Emission (SASE) Free Electron Laser (FEL) with pulse energy in the mJ-range, electron beams of 1.5-2.5 nC compressed by the bunch compressor to a peak bunch current near 200 A are considered. Figure 1 shows a brief PITZ layout for the THz SASE FEL experiments, including the RF gun, the booster, the bunch compressor, the coherent transition radiation (CTR) measurement station, and the undulator. Furthermore, the bunch compressor has been foreseen and assigned as a part of the prototype for various applications such as the SASE FEL, seeded FEL, superradiant radiation, etc. [2]. The bunch compressor using a magnetic chicane consists of four rectangular dipole magnets (with the pole gap of 8 cm and the pole size of 15.5 \(\times\) 30 cm) obsoleted from the HERA beamline [3]. The magnets are reused, as the pole shoes are added with four shims optimized by the program CST Studio Suite 2018 [4] to flatten the magnetic field profile of the magnet. Thus, the electron beam experiences a magnetic field that is \(<\)0.1% different from the maximum field throughout the chicane dipole magnet within the constant field region (5 \(\times\) 7 \(\times\) 20 cm in the horizontal, vertical, and longitudinal axis, respectively). Moreover, these chicane dipole magnets have identical strength and length and a vertical bending plane with a deflection angle of 19 degrees. Given the angle, the beam remains within this \"good field\" region during transport through the magnet. In addition, the angle is designed due to the limited installation space along the original beamline. As the effective length of each chicane dipole magnet of 0.327 m is estimated from the magnetic field simulation via the program CST, \(R_{56}\) of this chicane is 0.215 m. However, there is no tuning knob for varying the chicane height (or \(R_{56}\)), as \(R_{56}\) is fixed to one value. Note that in general, the vertical chicane structure has few benefits. First, the emittance growth from collective effects in the vertical axis could be beneficial to an undulator with a large vertical pipe size (not in our case). Secondly, there is no interference from the earth magnetic field in bending (vertical). At PITZ, the space charge-dominated electron beam is transported throughout the beamline via the use of periodic and dense focusing elements. However, in addition to the space charge-dominated beam dynamics, this type of bunch compressor design becomes a challenge for beam dynamics and beam transport due to the impact of high bunch charge, high bending angle, and low beam energy on coherent synchrotron radiation (CSR) effects, resulting in energy spread and projected emittance growth [5,6]. Furthermore, such collective effects--both space charge and CSR effects--introduce dispersion leakage due to the imbalance in the longitudinal dynamics of particles [7,8]. According to our previous study via the particle tracking simulation [2], the compression performance (compression ratio of bunch length) is dropped, and projected vertical emittance is increased drastically when increasing bunch charge.