The Linac III linear accelerator at DESY (Deutsche Elektronen Synchrotron), Hamburg. Linac III is used to accelerate negatively-charged hydrogen atoms to an energy of 50 million electron volts (50 MeV). The hydrogen atoms are then injected into the DESY III synchrotron ring accelerator, where two electrons are stripped from each atom. This leaves the central proton from each atom in the ring to be accelerated further, up to 9 billion electron volts (GeV). The protons then proceed into the PETRA II ring for acceleration to 40 GeV, thence into the 6.3km HERA ring, where they are collided with high-energy electrons.