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 JLab is the continuous nature of the electron beam, with a bunch length of less than 1 picosecond. Another is JLab's use of superconducting RF (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, JLab uses the world's largest liquid helium refrigerator, and was one of the first large-scale implementators of SRF technology. The accelerator is built 8 meters, or approximately 25 feet, below the Earth's surface, and the walls of the accelerator tunnels are 2 feet thick. 