4.6 add_on_axis_fields

• description:

  Adds on-axis field data to the simulation. The user must provide an SDDS file giving E_z(z,r = 0). This data is used to compute E_z(z,r,t), E_r(z,r,t), and B_ϕ(z,r,t) using an off-axis expansion in r and assuming E ~ sin(ωt + ϕ) and B ~ cos(ωt + ϕ).

  Any number of add_on_axis_fields commands may be given.

• example:
  &add_on_axis_fields  
    filename = fieldProfile.sdds,  
    z_name = z,  
    Ez_name = Ez,  
    Ez_peak = 30e6,  
    phase = 180,  
    expansion_order = 2  
    fields_used = fieldUsed.sdds,  
  &end

  This command loads on-axis field data from columns z and Ez in fieldProfile.sdds, and scales it so that the peak field is 30 MV/m. The phase, ϕ, is set to 180 degrees. Because spiffe simulates electrons, if E(z,r) is positive, the phase factor must be negative to provide acceleration. I.e., ϕ = 270^∘ is the accelerating phase.

• synopsis and defaults:
  #namelist add_on_axis_fields  
          STRING filename = NULL;  
          STRING z_name = NULL;  
          STRING Ez_name = NULL;  
          double Ez_peak = 0;  
          double frequency = 0;  
          double z_offset = 0;  
          long expansion_order = 3;  
          STRING fields_used = NULL;  
  #end

• details:
  • filename: Name of the SDDS file containing the data.
  • z_name: Name of the column containing z values, which must be monotonically increasing and equispaced.
  • Ez_name: Name of the column containing E_z values.
  • Ez_peak: Absolute value, in V/m, of the maximum on-axis electric field due to this field profile. The E_z values are scaled to obtain this maximum, but the signs are unchanged.
  • frequency: Frequency of the fields, in Hz.
  • z_offset: Offset, in meters, to be added to the z values.
  • expansion_order: Order of the off-axis expansion:
    • E_z is constant in r, while E_r and B_ϕ are zero.
    • E_z is constant in r, while E_r and B_ϕ vary linearly with r.
    • Adds a quadratic variation with r to E_z .
    • Adds a cubic variation with r to E_r and B_ϕ.
  • fields_used: Name of an SDDS file to which field profile data will be written for all on-axis fields specified up to and including this command.