Author: Li, R.K.
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MOP021 LCLS-II Injector Beamline Design and RF Coupler Correction 77
  • F. Zhou, D. Dowell, R.K. Li, T.O. Raubenheimer, J.F. Schmerge
    SLAC, Menlo Park, California, USA
  • C.E. Mitchell, C. F. Papadopoulos, F. Sannibale
    LBNL, Berkeley, California, USA
  • A. Vivoli
    Fermilab, Batavia, Illinois, USA
  Funding: U.S. DOE contract #DE-AC02-76SF00515.
LCLS-II CW injector beamline consists of a 186 MHz normal conducting (NC) RF gun for beam generation and acceleration to 750 keV, two solenoids for the beam focusing, two BPMs, 1.3 GHz NC RF buncher for bunch compression down to 3-4 ps rms, 1.3 GHz superconducting standard 8-cavity cryomodule to boost beam energy to about 98 MeV. The beamline is being optimized to accommodate all essential components and maximize beam quality. The beamline layouts and beam dynamics are presented and compared. The 3D RF field perturbation due to cavity couplers where the beam energy is very low (<1 MeV) causes significant emittance growth especially for a large-size beam. A theory of rotated fields predicted and simulations verified using a weak skew quadrupole located even a significant distance from the perturbation can completely eliminate the emittance growth. A layout for future upgrade is developed. The results are presented and analysed.
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MOD03 Alkali Cathode Testing for LCLS-II at APEX 280
  • H.J. Qian, J. Feng, D. Filippetto, J.R. Nasiatka, H.A. Padmore, F. Sannibale
    LBNL, Berkeley, California, USA
  • R.K. Li, J.F. Schmerge, F. Zhou
    SLAC, Menlo Park, California, USA
  Funding: Work supported by the Director of the Office of Science of the US Department of Energy under Contract no. DEAC02-05CH11231
Electron sources of high brightness and high bunch charge (~300 pC) with MHz repetition rate are one of the key technologies for next generation X-FEL facilities such as the LCLS-II at SLAC and the Euro XFEL at DESY. The Advanced Photoinjector EXperiment (APEX) at the Lawrence Berkeley National Laboratory (LBNL) is developing such an electron source based on high quantum efficiency (QE) alkali photocathodes and the VHF-Gun, a new scheme normal conducting RF gun developed at LBNL. The VHF-Gun already demonstrated stable CW operation with high gradient (~ 20 MV/m), high gun voltage (~ 750 kV) and low vacuum pressure (~ 3 E-10 torr) laying the foundation for the generation of high brightness electron beams. In this paper, we report the test and characterization of several different alkali cathodes in high average current (several hundreds of pC/bunch with MHz repetition rate) operation at APEX. Measurements include cathode life time, QE map evolution and thermal emittance characterization, to investigate the compatibility of such cathodes with the challenging requirements of LCLS-II.
slides icon Slides MOD03 [6.030 MB]  
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Ultra-Low Charge, Ultra-High Brightness Frontiers of Photoinjectors: Challenges and Perspectives  
  • R.K. Li
    SLAC, Menlo Park, California, USA
  Photoinjectors deliver high brightness electron beams that are essential to the success of FELs. Higher beam brightness can boost the performances of existing facilities and enable new capabilities of future ones. Ultralow charge (a few pC and lower) is the choice to reach higher brightness thanks to the favorable scaling, as well as longitudinally coherent, single-spike FELs. This parameter space is particularly relevant for future high rep-rate machines. Despite the ultralow total charge, the charge density is still high and collective effects need to be studied in-depth. The extreme emittance, bunch length, spot size, and charge also pose challenges on the diagnosis of these beams. In this talk, we will discuss recent progress on the generation, manipulation, and diagnosis of ultralow charge beams, including e.g. shaping the space charge dominated beams to maximize the phase space density, new techniques to characterize the nm-rad-level emittance, and new advances in photocathode physics and rf structures. Many of the above progress are motivated by the recent development of ultrafast electron diffraction and microscopy using photoinjectors. Understanding and controlling the high brightness beams in accelerators with the high stability and precision typical in the electron microscopy community, constitutes an R&D frontier on electron beams and sources for the coming decades.  
slides icon Slides TUC02 [2.858 MB]  
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