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DeFord, J.F.

Paper Title Page
WPAT077 Finite-Element 2D & 3D PIC Modeling of RF Devices with Applications to Multipacting
 
  • J.F. DeFord
    LLNL, Livermore, California
  • E.M. Nelson
    LANL, Los Alamos, New Mexico
  • J.J. Petillo
    SAIC, Burlington, Massachusetts
 
  Multipacting currently limits the performance of many high power radio-frequency (RF) devices, particularly couplers and windows. Models have helped researchers understand and mitigate this problem in 2D structures, but useful multipacting models for complicated 3D structures are still a challenge. A combination of three recent technologies that have been developed in the Analyst and MICHELLE codes begin to address this challenge: high-order adaptive finite-element RF field calculations, advanced particle tracking on unstructured grids, and comprehensive secondary emission models. Analyst employs high-order adaptive finite-element methods to accurately compute driven RF fields and eigenmodes in complex geometries, particularly near edges, corners, and curved surfaces. To perform a multipacting analysis, we use the mesh and fields from Analyst in a modified version of the self-consistent, finite-element gun code MICHELLE. MICHELLE has both a fast, accurate, and reliable particle tracker for unstructured grids and a comprehensive secondary emission model. We will demonstrate this capability on an RF coupler.  
TPAT043 The MICHELLE 2D/3D ES PIC Code: Advances and Applications
 
  • J.J. Petillo, N.J. Dionne, K. Eppley, J. N. P. Panagos, X. Z. Zhai
    SAIC, Burlington, Massachusetts
  • L. C. Chernyakova, J.F. DeFord, B. H. Held
    STAR, Inc., Mequon, Wisconsin
  • B. Levush
    NRL, Washington, DC
  • E.M. Nelson
    LANL, Los Alamos, New Mexico
 
  Funding: Office of Naval Research, Naval Research Laboratory.

MICHELLE is a new 2D/3D steady-state and time-domain particle-in-cell (PIC) code* that employs electrostatic and now magnetostatic finite-element field solvers. The code has been used to design and analyze a wide variety of devices that includes multistage depressed collectors, gridded guns, multibeam guns, annular-beam guns, sheet-beam guns, beam-transport sections, and ion thrusters. Latest additions to the MICHELLE/Voyager tool are as follows: 1) a prototype 3D self magnetic field solver using the curl-curl finite-element formulation for the magnetic vector potential, employing edge basis functions and accumulating current with MICHELLE's new unstructured grid particle tracker, 2) the electrostatic field solver now accommodates dielectric media, 3) periodic boundary conditions are now functional on all grids, not just structured grids, 4) the addition of a global optimization module to the user interface where both electrical parameters (such as electrode voltages)can be optimized, and 5) adaptive mesh refinement improvements. Applications illustrating these latest additions will be presented, including a relativistic sheet beam gun, a relativistic MIG gun, and a depressed collector optimization example.

*John Petillo, et al., IEEE Trans. Plasma Sci., vol. 30, no. 3, June 2002, pp. 1238-1264.

 
WPAT080 Calculation of Beam-Loaded Q in High-Power Klystrons 4060
 
  • J.F. DeFord, B. H. Held
    STAR, Inc., Mequon, Wisconsin
  • V. Ivanov, K. Ko
    SLAC, Menlo Park, California
 
  Funding: Work supported by DOE SBIR Grant DE-FG02-03ER83776.

Instabilities in the gun region of a high-power klystron can occur when there is positive feedback between a mode and an induced current on the quasi-steady state beam emitted by the gun cathode.* This instability is dependent on the gun voltage, is predicted on the basis of a negative beam-loaded Q. The established method for computing the beam-loaded Q of a cavity involves using a time-dependent electromagnetic particle-in-cell (PIC) code to track beam particles through the quasi-static gun fields perturbed by the electromagnetic fields of a cavity eigenmode.** The energy imparted to the beam by the mode is obtained by integrating the Lorentz force along the particle tracks, and this quantity is simply related to the beam-loaded Q. We have developed an alternative approach that yields comparable accuracy but is computationally much simpler. The new method is based on a much simpler time-independent electrostatic PIC calculation, resulting in much faster solutions without loss of accuracy. We will present the theory and implementation of the new method, as well as benchmarks and results from analysis of the XP-4 klystron that show a potential instability near 3 GHz.

*B. Krietenstein, et al., "Spurious oscillations in high-power klystrons," PAC95, 1995. **U. Becker, et al., "Simulation of oscillations in high-power klystrons," EPAC, 1996.