SLAC, Menlo Park, California, USA
LCLS is presently operating with a third copper photocathode in the original rf gun, with a quantum efficiency (QE) of ~1x10-4 and projected emittance eNx,y=0.45 μm at 250 pC bunch charge. The spare LCLS gun is installed in the SLAC Accelerator Structure Test Area (ASTA), processed to the design rf gradient of >120 MV/m. As part of a wider photocathode R&D program, a UV laser system and additional gun diagnostics are being installed at ASTA to measure QE, QE lifetime, and electron beam emittance under a variety of operating conditions. The near-term goals are to test and verify the spare photocathode production/installation sequence, including transfer from the final holding chamber to the rf gun. Mid- and longer-term goals include development of a rigorous understanding of plasma and laser-assisted surface conditioning and investigation of new, high-QE photocathode materials. In parallel, an x-ray photoemission spectroscopy station is nearing completion, to analyze Cu photocathode surface chemistry. In this paper we review the status and anticipated operating parameters of ASTA and the spectroscopy test chamber.
The University of Melbourne, Melbourne, Australia
ASCo, Clayton, Victoria, Australia
SLAC, Menlo Park, California, USA
SLAC, Menlo Park, California, USA
The electron gun for the SPEAR3 injector operates with a warm thermionic dispenser cathode immersed in a 1.5-cell RF structure. At each injection cycle the gun accelerates several thousand electron bunches up to ~3 MeV during a 2.5us rf pulse. The individual bunches are then compressed by an alpha magnet and a traveling-wave chopper selects 3-5 bunches so they don’t cause beam loading to the linac, where the accelerated bunches reach 120 MeV for subsequent capture in a single booster synchrotron bucket. Tests are underway to operate the dispenser cathode as a cold electron photo-emitter driven by an external laser system. Eventually, without the copper, this will enable multi-bunch injections to the Booster and SPEAR3. In parallel, tests are underway to evaluate quantum efficiency and beam emittance for a beam emitted from a CsBr photocathode with ns- and ps-pulses of UV laser light. In this paper we report on both the cold cathode electron gun operation studies for SPEAR3 and the CsBr research aimed at developing advanced cathode materials for future applications.
SLAC, Menlo Park, California, USA
The SPEAR2 accelerator system originally had 3 RF stations (2 for storage ring, 1 Booster) operating at 358.5 MHz. SPEAR3 now operates at 476.3 MHz with PEP-II type RF system, while the Booster RF frequency remains unchanged. For top-off operation, the Booster injects single 3.0 GeV electron bunches into SPEAR3 at 10 Hz every 5 minutes to replenish lost charge. Due to the frequency mismatch between SPEAR3 and the Booster, only one SPEAR3 bucket can injected per shot limiting injection rate and overall system flexibility. The aging high-power RF subsystems of the Booster pose a reliability issue as well. In order to remove these constraints, studies are underway to replace the Booster RF system using the PEP-II type RF system as a baseline. The new Booster RF system will be tuned to 475.036 MHz, and phase-locked to the SPEAR3 RF system. The project calls for ramping the Booster cavity gap voltage to 0.80 MV at 10 Hz, each with a 40 ms acceleration interval. With very low beam loading and low average RF power, there are many subsystems that can be operationally simplified. In this paper we present the results of analysis leading to a new Booster RF system.
UMD, College Park, Maryland, USA
SLAC, Menlo Park, California, USA
At SPEAR3, the light source operates in top-up injection mode with 273nC charge circulating in the storage ring (350mA). Each individual injection pulse contains only 40pC, or a contrast ration of 1:6800. In order to monitor injected beam dynamics during User operations, it is desirable to optically image the injected charge distribution on a turn-by-turn basis in the presence of the bright stored beam. The measurement is made by re-imaging visible synchrotron radiation onto a 1024x768 pixel Digital-Micro-Array mirror device (DMD) which is used to 'mask' light from the central stored beam while observing the weak injected beam signal on an intensified, fast-gated CCD camera. Complex beam dynamics are observed after only a few 10's of turns around the synchrotron. In this paper we report on the DMD optical configuration, masking considerations, measurement timing and initial tests imaging the injected beam in the presence of stored beam.
