Electrons leaving the injector have to have about 45 million volts, but the electron source can only give electrons about 100,000 volts. How do you get the rest? One way to do it is to send the electrons through some acceleration cavities. The main portion of the injector contains 18 cavities and each cavity can give an electron 2.5 million volts.
The cavities are kept in large tanks called cryomodules. Most of the cryomodules have names, like Atlantic. The cryomodules are filled with liquid Helium to keep the cavities cold.