4.1 define_geometry

• description: Define the simulation region and the geometry of a cavity. This includes defining the grid sizes, the boundary conditions, which types of fields to include (TE or TM), and how to interpolate on the grid. This is done with the define_geometry command, which requires existence of a boundary definition file in a POISSON-like format. This file specifies not only the location of the metal surfaces in the problem, but optionally the potential of each.
• example:
  &define_geometry  
      nz = 165, zmin = 0.0, zmax = 0.0959148653920,  
      nr = 165, rmax=0.04289777777,  
      boundary = "mg6mm-3.geo",  
      boundary_output = "mg6mm-3.bnd",  
      interior_points = "mg6mm-3.pts"  
  &end

  This command defines a problem with an extent of about 9.6cm in the longitudinal direction and about 4.3cm in the radial direction. The number of grid lines in each dimension is 165. Boundary input is taken from file mg6mm-3.geo. In addition, output of the boundary coordinates is to be placed in file mg6mm-3.bnd while output of the coordinates of all interior grid points is to be placed in mg6mm-3.pts.

• synopsis and defaults:
  &define_geometry  
      long nz = 0;  
      long nr = 0;  
      double zmin = 0;  
      double zmax = 0;  
      double rmax = 0;  
      double zr_factor = 1;  
      STRING rootname = NULL;  
      STRING boundary = NULL;  
      STRING boundary_output = NULL;  
      STRING urmel_boundary_output = NULL;  
      STRING discrete_boundary_output = NULL;  
      STRING interior_points = NULL;  
      STRING lower = "Dirichlet";  
      STRING upper = "Neumann";  
      STRING right = "Neumann";  
      STRING left  = "Neumann";  
      long include_TE_fields = 0;  
      long exclude_TM_fields = 0;  
      long turn_off_Er = 0;  
      long turn_off_Ez = 0;  
      long turn_off_Ephi = 0;  
      long turn_off_Br = 0;  
      long turn_off_Bz = 0;  
      long turn_off_Bphi = 0;  
      long print_grids = 0;  
      long radial_interpolation = 1;  
      long longitudinal_interpolation = 1;  
      long radial_smearing = 0;  
      long longitudinal_smearing = 0;  
  &end

• details:
  • nz, nr: number of grid lines in z and r dimensions, respectively.
  • zmin, zmax: starting and ending longitudinal coordinate, respectively.
  • rmax: maximum radial coordinate.
  • zr_factor: a factor by which to multiply the z and r values in the boundary input file to convert the values to meters. For example, zr_factor=0.01 would be used if the boundary input file values were in centimeters.
  • rootname: rootname for construction of output filenames. Defaults to the rootname of the input file.
  • boundary: name of input file containing POISSON-like specification of the cavity boundary and surface potentials.
  • boundary_output: (incomplete) name of output file to which SDDS-protocol data will be sent containing the coordinates of points on the ideal boundary, i.e., the boundary you would get if your grid spacing was zero. Recommended value: “
  • discrete_boundary_output: (incomplete) name of output file to which SDDS-protocol data will be sent containing the coordinates of points on the actual boundary used in the simulation. This differs from the ideal boundary because every point on the actual boundary must be a grid point. Recommended value: “
  • interior_points: (incomplete) name of output file to which SDDS-protocol data will be sent containing the coordinates of all interior points of the cavity. May be used together with boundary output files and sddsplot to manually confirm the interpretation of the cavity specification by spiffe.
  • lower, upper, right, left: boundary conditions for the edges of the simulation region. The defaults are listed above. “Dirichlet” boundary conditions means that electric field lines are parallel to the boundary. “Neumann” boundary conditions means that electric field lines are perpendicular to the boundary.
  • include_TE_fields: flag indicating whether to include transverse-electric fields, i.e., fields having no longitudinal electric field components. If you include space-charge and the beam is rotating, you should set this to 1.
  • exclude_TM_fields: flag indicating whether to exclude transverse-magnetic fields, i.e., fields having no longitudinal magnetic field components.
  • turn_off_...: flags indicating that the specified fields should be “turned off,” which means that the don’t affect particles. Used for testing purposes.
  • print_grids: flag requesting a text-based picture of the simulation grids.
  • radial_interpolation, longitudinal_interpolation: flags requesting that field components be interpolated in the radial and longitudinal direction when fields are applied to particles. If 0, then fields will change abruptly as particles move from one grid square to the next.
  • radial_smearing, longitudinal_smearing: flags requesting that charge and current from simulation macro particles be smeared over the grid points surrounding each particle. If 0, charge and current are assigned to the nearest grid point.