A high brightness photoinjector.

摘要

Linear colliders, future electron acceleration schemes, and short pulse, ultrawideband millimeter-wave sources require very bright electron beams. Conventional electron injectors including thermionic cathodes and RF bunchers or DC guns have intrinsic limitations which preclude their usage for many of these applications. RF photoinjectors have shown their ability to produce relativistic electron beams with low emittance and energy spread. However, previously developed RF photoinjectors are also subject to significant limitations. These include extreme sensitivity to timing between the RF in the accelerator structure and the drive laser, low efficiency with respect to the number and charge of the electron bunches produced by the injector, and high cost associated with both the RF drive and laser systems. The presently described system has addressed these issues by combining state-of-the-art capabilities in the laser and RF systems, photocathode materials, and new concepts for synchronization. Phase jitter generated by sources including Klystron modulator voltage fluctuation has been measured in detail, and schemes for alleviating this problem have undergone initial proof-of-principle testing. New concepts for the drive laser system have been tested which will lead to further improvements in performance, simplicity, cost-effectiveness, and compactness. The analytical and experimental work associated with the development of a high brightness, high gradient electron accelerator is presented. The presentation emphasizes the systematic progress toward the original design goals of the project, as well as the state-of-the-art innovations characterizing the system. The linear electron accelerator system is based on a 1 1/2 cell side-wall coupled, {dollar}pi{dollar}-mode standing wave accelerator structure, driven by a 20 MW SLAC Klystron operating at 8.548 GHz, a Ti:Sapphire laser oscillator, and an 8-pass, chirped pulse Ti:Sapphire laser amplifier. Simulations show an rms transverse invariant emittance at the gun exit of less than 1.0 {dollar}pi{dollar} mm-mrad for a 0.1 nC bunch. A cold test cavity demonstrated a Q{dollar}sb0{dollar} value of about 3400 under critically coupled conditions. A high power cavity was constructed and operated at the MW level. High quantum efficiency photocathodes were also tested.