[en] The former 5-cell capture cavity of the S-DALINAC injector was insufficiently adapted to the beam energies provided by the electron sources. This led to an inefficient energy gain and a deterioration of the beam quality in this acceleration structure. In order to optimize the performance of the injector, especially with regard to an accelerator operation in the energy-recovery mode, it was therefore necessary to replace the 5-cell cavity with a dedicated β-adapted capture structure. This project was implemented within the scope of this dissertation. A new β-reduced 6-cell structure was manufactured, tested and installed at the accelerator. Tuner frame components had to be adapted to the novel geometry and mechanical features of the cavity. To achieve an exchange of the cavities within a regular maintenance shutdown, full compatibility with the existing cryostat was prioritized during the design of the modified cryomodule components. In addition, a beam diagnostic setup was conceptualized, installed and commissioned upstream of the capture cavity. This setup was used to characterize momentum spreads of the beam along the normal-conducting injector section with a relative uncertainty of ±2 %. Furthermore, parameters of the beam entering the superconducting injector section, which have a significant influence on the achievable injector beam quality, could be optimized. After the cool-down of the installed capture cavity, full functionality of the upgraded cryomodule was shown. First beam parameter studies were conducted at the optimized injector. The results were benchmarked against the former injector setup. Besides a reduction of the momentum spread by a factor of around four, a substantially improved energy gain of up to 1.4 MeV was achieved in the new capture structure for an incident beam energy of 250 keV. Furthermore, a decreased bunch length was observed at the injector beam, which had already proven beneficial for the overall S-DALINAC performance during ERL experiments following the commissioning. The optimized injector is fully operational and is now available for further beam operation at the S-DALINAC.