Linear accelerators, or Linacs, are generally used for fixed target experiments or as injectors to circular accelerators. There are examples of linear colliders but to achieve comparable energies to those reached in synchrotrons the beam pipes (the evacuated pipes through which the particles travel) would have to be extremely long and contain an impractical number R-F cavities. They are, however, much the cheaper and easier accelerators to build because they do not require the large bending magnets used in their circular counterparts.