This study reports on the optimization of the radio frequency capture phase during the operational cycle of the SIS-18 synchrotron at Gesellschaft für Schwerionenforschung, Darmstadt, Germany. The ion species studied were 238U+28 and 238U73+ at an injection energy of 11.4 MeV/u. The longitudinal relative momentum spread derived from Schottky spectra of the coasting beam at injection provides a value of |Δp/p0|full-width ∼ 5 × 10−3. Simulation results from the synchrotron tracking code ESME (FermiLab) were compared with beam-current profile measurements obtained from a pickup. To gain further insight, the Tomography program (European Organization for Nuclear Research) has been used to derive the longitudinal phase space development from waterfall plots of the measured beam current profile, which may then be compared against simulation. Possible causes of this nonadiabaticity are discussed and solutions are proposed.