The design of CEBAF allows the electron beam to be continuous rather than the pulsed beam typical of ring shaped accelerators. (There is some beam structure, but the pulses are very much shorter and closer together.) The electron beam is directed onto three potential targets (see below). One of the distinguishing features of Jefferson Lab is the continuous nature of the electron beam, with a bunch length of less than 1 picosecond. Another is Jefferson Lab's use of superconducting Radio Frequency (SRF) technology, which uses liquid helium to cool niobium to approximately 4 K (−452.5 °F), removing electrical resistance and allowing the most efficient transfer of energy to an electron. To achieve this, Jefferson Lab houses the world's largest liquid helium refrigerator, and it was one of the first large-scale implementations of SRF technology. The accelerator is built 8 meters below the Earth's surface, or approximately 25 feet, and the walls of the accelerator tunnels are 2 feet thick.