A design for an X-band rf photoinjector that was developed jointly by SLAC National Accelerator Laboratory (SLAC) and Lawrence Livermore National Laboratory (LLNL) is presented. The photoinjector is based around a 5.59 cell rf gun that has state-of-the-art features including: elliptical contoured irises; improved mode separation; an optimized initial half cell length; a racetrack input coupler; and coupling that balances pulsed heating with cavity fill time. Radio-frequency and beam dynamics modeling have been done using a combination of codes including PARMELA, HFSS, IMPACT-T, ASTRA, and the ACE3P suite of codes developed at SLAC. The impact of lower gradient operation, magnet misalignment, solenoid multipole errors, beam offset, mode beating, wakefields, and beam line symmetry have been analyzed and are described. Fabrication and testing plans at both LLNL and SLAC are discussed.
We describe the computer modeling of relativistic electron guns (0.5MeV) using pulsed field emitter arrays. The special challenge lies in the fact, that current distributions vary at a submicron scale, whereas structural dimensions are in the millimeter range. The general approach uses two steps. The first one is the computation of individual field-emitter tips including gate and focusing layers. Real world influences as, e.g., the effect of adsorbates on the emitted currents are taken into account by parameterizing the phase space of the tips. Together with a stochastic distribution of emitter properties, this leads to an equivalent current distribution on the cathode itself, which is used in the second step for the calculation of the electron dynamics in the gun itself. We present results for a source using a field-emitter array of 17 700 tips. For the current field-emitter geometry, we see a large high base line effect resulting from nonlinear focusing forces inside the emitter itself. Of special interest are the effects of spatial correlations in the stochastic distribution on the emittance, showing pronounced performance degradations in the case of large correlation lengths.
SciDAC1, with its support for the "Advanced Computing for 21 st Century Accelerator Science and Technology" (AST) project, witnessed dramatic advances in electromagnetic (EM) simulations for the design and optimization of important accelerators across the Office of Science. In SciDAC2, EM simulations continue to play an important role in the "Community Petascale Project for Accelerator Science and Simulation" (ComPASS), through close collaborations with SciDAC CETs/Institutes in computational science. Existing codes will be improved and new multi-physics tools will be developed to model large accelerator systems with unprecedented realism and high accuracy using computing resources at petascale. These tools aim at targeting the most challenging problems facing the ComPASS project. Supported by advances in computational science research, they have been successfully applied to the International Linear Collider (ILC) and the Large Hadron Collider (LHC) in High Energy Physics (HEP), the JLab 12-GeV Upgrade in Nuclear Physics (NP), as well as the Spallation Neutron Source (SNS) and the Linac Coherent Light Source (LCLS) in Basic Energy Sciences (BES).
Over the past years, SLAC's Advanced Computations Department (ACD) has developed the parallel finite element (FE) particle-in-cell code Pic3P (Pic2P) for simulations of beam-cavity interactions dominated by spacecharge effects. As opposed to standard space-charge dominated beam transport codes, which are based on the electrostatic approximation, Pic3P (Pic2P) includes space-charge, retardation and boundary effects as it self-consistently solves the complete set of Maxwell-Lorentz equations using higher-order FE methods on conformal meshes. Use of efficient, large-scale parallel processing allows for the modeling of photoinjectors with unprecedented accuracy, aiding the design and operation of the next-generation of accelerator facilities. Applications to the Linac Coherent Light Source (LCLS) RF gun are presented.
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