High brightness, high current injector design for the Cornell ERL prototype

Cornell University, in collaboration with Jefferson Laboratory, has proposed the construction of a 100 MeV, 100 mA CW Energy Recovery Linac (ERL) prototype, to study and resolve the many accelerator physics and technology issues of this type of machine. The long term goal of this work is the construction of a state-of-the-art 5 to 7 GeV ERL-based synchrotron light source. A key element of this machine is a high brightness injector with every bunch filled (77 pC/bunch at 1300 MHz). We report the design of a versatile injector for the prototype ERL which also meets the requirements for a full energy light source. The injector uses a very high voltage DC photoemission electron gun with a GaAs photocathode and a conventional bunching cavity. A cryomodule with five two-cell superconducting RF cavities allows energies between 5 and 15 MeV to be delivered, with an average beam power of 500 kW limited by the installed RF power. Thorough simulations, using realistic particle distributions at the photocathode, indicate this injector should provide a normalized rms transverse emittance approaching 1 mm-mrad, and an rms longitudinal emittance of 10 keV-degrees. Operation at reduced bunch charge will provide a smaller transverse emittance, until aberrations and the time dependence of the RF fields impose limitations.