Electromagnetic field and VUV light tracking simulations for the DARWIN field cage

The goal of the DARWIN experiment is to design and construct the ultimate dark matter detector, using a multi-ton target of liquid xenon for the direct detection of particle dark matter in a sensitive time projection chamber (TPC). Together with achieving very low radioactivity conditions and the required xenon purity, one of the most challenging aspects is the ability to have a very homogeneous drift field inside the TPC with ideally no radial electrical field. It is therefor of particular importance to gain a detailed understanding of the electrical field with high precision simulation algorithms. In this work we present detailed calculations of the large-scale drift field of the DARWIN TPC with the aim to maximize the field homogeneity, using the KEMField and COMSOL codes. We also investigate the impact of surface charge on the PTFE-LXe interface and the local field of small Cu-pins, which are proposed as countermeasure against surface charges. We show that the impact of pins on the local drift field is well under control. In addition, we present simulations of the effect of the Cu-pins on the propagation of VUV light.