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 $B_\phi(z,r,t)$ using an off-axis expansion in $r$ and assuming $E \sim \sin (\omega t + \phi)$ and $B \sim \cos (\omega t + \phi)$.

  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, $\phi$, 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., $\phi=270^\circ$ 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:

    0

    $E_z$ is constant in $r$, while $E_r$ and $B_\phi$ are zero.

    1

    $E_z$ is constant in $r$, while $E_r$ and $B_\phi$ vary linearly with $r$.

    2

    Adds a quadratic variation with $r$ to $E_z$ .

    3

    Adds a cubic variation with $r$ to $E_r$ and $B_\phi$.

  • 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.