Measurement of the inhomogeneity of the KATRIN tritium source electric potential by high-resolution spectroscopy of conversion electrons from 83mKr. - KATRIN Collaboration; Acharya, H.; Aker, M.; Batzler, D.; Beglarian, A.; Beisenkötter, J.; Biassoni, M.; Bieringer, B.; Biondi, Y.; Block, F.; Bornschein, B.; Bornschein, L.; et al. (European Physical Journal C -- Particles & Fields, 2025)