Abstract
The Advanced Photoinjector Experiment (APEX) at the Lawrence Berkeley National Laboratory is dedicated to the development of a high-brightness high-repetition rate (MHz-class) electron injector for x-ray free-electron laser (FEL) and other applications where high repetition rates and high brightness are simultaneously required. The injector is based on a new concept rf gun utilizing a normal-conducting (NC) cavity resonating in the VHF band at 186 MHz, and operating in continuous wave (cw) mode in conjunction with high quantum efficiency photocathodes capable of delivering the required charge at MHz repetition rates with available laser technology. The APEX activities are staged in three phases. In phase 0, the NC cw gun is built and tested to demonstrate the major milestones to validate the gun design and performance. Also, starting in phase 0 and continuing in phase I, different photocathodes are tested at the gun energy and at full repetition rate for validating candidate materials to operate in a high-repetition rate FEL. In phase II, a room-temperature pulsed linac is added for accelerating the beam at several tens of MeV to reduce space charge effects and allow the measurement of the brightness of the beam from the gun when integrated in an injector scheme. The installation of the phase 0 beam line and the commissioning of the VHF gun are completed, phase I components are under fabrication, and initial design and specification of components and layout for phase II are under way. This paper presents the phase 0 commissioning results with emphasis on the experimental milestones that have successfully demonstrated the APEX gun capability of operating at the required performance.
Highlights
The advent of free-electron laser (FEL) based x-ray light sources [1,2,3,4] represented a revolution in terms of capability and opportunities that such facilities offer to the user community
More details on the gun can be found elsewhere [17,18,19]; here we want just to remark that the two major goals targeted by the gun design are the cw operation, and the low-vacuum performance (10À11–10À9 Torr) necessary to operate with acceptable lifetime high quantum efficiency (QE) semiconductor photocathodes sensitive to ion back bombardment and contamination
The commissioning of the Advanced Photoinjector Experiment (APEX) photogun has been completed
Summary
The advent of free-electron laser (FEL) based x-ray light sources [1,2,3,4] represented a revolution in terms of capability and opportunities that such facilities offer to the user community. More details on the gun can be found elsewhere [17,18,19]; here we want just to remark that the two major goals targeted by the gun design are the cw operation, and the low-vacuum performance (10À11–10À9 Torr) necessary to operate with acceptable lifetime high quantum efficiency (QE) semiconductor photocathodes sensitive to ion back bombardment and contamination Such cathodes are required to generate the desired charge per bunch at high repetition rate with the power available by present laser technology. A pulsed L-band linac system is chosen to allow realistic characterization of beam dynamics with components similar in rf fields and wakefields to superconducting cavities, while minimizing costs and remaining within the radiation safety envelope of the test area (the existing Beam Test Facility at the Advanced Light Source) This arrangement allows demonstration of the brightness performance of an injector based on the VHF gun, and brightness being a single bunch property, the limited repetition rate does not affect the scope of this phase. The FPGA-based low-level rf system (LLRF) is developed at LBNL and allows control of the rf system, the synchronization with the laser, and the tuning of the rf source frequency to follow the cavity frequency [36]
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