The components of the first 50 MeV accelerator in the chain of injectors to the 820 GeV HERA proton ring are described including the high energy transport line. An H− beam originates in the cesium loaded magnetron source, is preaccelerated by a radio frequency quadrupole to 750 keV, raised to 50 MeV energy in three rf resonators, and is analyzed in the transport line. The H− ions are converted to protons by stripping, when the beam is injected over several turns into the synchrotron DESY3.