During the last twenty years, there has been a world wide effort to develop the physics and technology of linear colliders. Present goals at SLAC, KEK, and DESY are to bring the R&D efforts to the point where proposals for 500/1000 GeV cms electron-positron colliders can be officially submitted in the years 2002/2003. The CLIC study at CERN aims at a second generation very high energy electron-positron collider, to be considered after completion of the LHC. The main areas of hardware R&D include efficient accelerating waveguides without harmful higher order mode (h.o.m.) effects, high peak power klystrons, klystron modulators, and rf-power compression. Test facilities have been put in place for the testing of h.o.m. behavior of new waveguide designs (ASSET), focusing of low emittance beams to spot sizes in the nanometer range (FFTB), and damping particle oscillations in a special damping ring (ATF) to prepare low emittance bunch trains of electrons for injection into linear colliders. The TESLA collaboration is making a major effort to develop the required technology for a superconducting linear collider. Test accelerator sections, which employ all the necessary new accelerator components, have been built and are currently being tested at SLAC and DESY.
A low emittance, flat-beam electron source for linear colliders