A flow compression device and an electron injector convert the pulsed high voltage into an electron beam. A high-intensity electromagnetic field accelerates it in a long electron gun and ejects it to a collector accelerator through an electronic nozzle as a medium for forming and accelerating protons. What does "it" mean in the passage? ['Flow compression device.', 'Electron injector.', 'Pulse high voltage.', 'Electron beam.']