On the elimination of numerical Cerenkov radiation in PIC simulations

  • Authors:

  • Andrew D. Greenwood;Keith L. Cartwright;John W. Luginsland;Ernest A. Baca

  • Affiliations:

  • Air Force Research Laboratory, Directed Energy Directorate, 3550 Aberdeen Ave SE, Kirtland, AFB NM;Air Force Research Laboratory, Directed Energy Directorate, 3550 Aberdeen Ave SE, Kirtland, AFB NM;NumerEx, Ithaca, NY and Air Force Research Laboratory, Directed Energy Directorate, 3550 Aberdeen Ave SE, Kirtland, AFB NM;Air Force Research Laboratory, Directed Energy Directorate, 3550 Aberdeen Ave SE, Kirtland, AFB NM

  • Venue:
  • Journal of Computational Physics
  • Year:
  • 2004

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Abstract

Particle-in-cell (PIC) simulations are a useful tool in modeling plasma in physical devices. The Yee finite difference time domain (FDTD) method is commonly used in PIC simulations to model the electromagnetic fields. However, in the Yee FDTD method, poorly resolved waves at frequencies near the cut off frequency of the grid travel slower than the physical speed of light. These slowly traveling, poorly resolved waves are not a problem in many simulations because the physics of interest are at much lower frequencies. However, when high energy particles are present, the particles may travel faster than the numerical speed of their own radiation, leading to non-physical, numerical Cerenkov radiation. Due to non-linear interaction between the particles and the fields, the numerical Cerenkov radiation couples into the frequency band of physical interest and corrupts the PIC simulation. There are two methods of mitigating the effects of the numerical Cerenkov radiation. The computational stencil used to approximate the curl operator can be altered to improve the high frequency physics, or a filtering scheme can be introduced to attenuate the waves that cause the numerical Cerenkov radiation. Altering the computational stencil is more physically accurate but is difficult to implement while maintaining charge conservation in the code. Thus, filtering is more commonly used. Two previously published filters by Godfrey and Friedman are analyzed and compared to ideally desired filter properties.