High brightness injector for a high power superconducting rf FEL

Abstract

We have developed a concept for a high average power FEL which incorporates a superconducting rf linac in an energy recovery configuration to increase the overall efficiency. Our design minimizes space charge effects at low beam energy by using: (1) higher than usual voltages on our grid modulated electrostatic electron gun and (2) pulse widths much longer than usual, from the gun. These pulses are compressed a minimum amount at the superconducting accelerator, which contains cavities at both fundamental and third-harmonic frequencies to flatten the voltage gain over these long pulses. After acceleration to the full beam energy, the pulses are compressed further to produce the peak currents required for the design FEL conversion efficiency. To validate the concept, we have designed an experiment to produce the required beam, accelerate it to 5 MeV and then recirculate the beam to demonstrate the anticipated high recovery efficiency. The injector gun for this experiment has now been operated at its design voltage and the current and the beam characteristics measured (design specifications: 1 nC, 0.5 ns). The beam emittance ( ϵ n) over 90% of the beam profile has been determined for various accelerating gradients and is 8π mm mrad. The energy slewing cavity required for bunching the beam a factor of 3 is in fabrication, as is the superconducting 500 MHz accelerator cavity. A description of our injector system design and the latest superconducting accelerator cavity tests will be reported.