BBFIELD

  Define the parameters for the calculation of beam-beam field.  BBFIELD WX=WX=([,]), [WXMAX=WXMAX=(,),] R=r, [NX=,] [NY=,] [PSIZE=,] [NMOM=,] ;

,

Horizontal width of the mesh in meters for right and left-going beams. If is not specified, is adopted. No default for .

,

If WXMAX is given, the with of the mesh region can vary in the range when the beam fraction outside the range defined by WX and R is significant. Note .

r

Aspect ratio ()/() of the horizontal to vertical mesh size. This is common to right and left-going beams. No default.

,

Number of horizontal and vertical bins. Present version uses Fast Fourier Transformation so that a power of 2 is the best choice. Other numbers are also allowed but those of the form or or are recommended. Default=32.

Macro-particle size in units of the bin size. Macro-particles are treated as a rectangular of uniform distribution. Must be . Default=1.

For (x,y) points outside the mesh region, a harmonic expansion using the elliptic coordinate is used. The parameter specifies the truncation of harmonics. takes only the total charge term and ignores the field outside. Default=10.
Note that the particles outside mesh region receive the beam-beam kick unless , but the field created by them is not taken into account. See Sec.5.6 for more detail.

Note that the longitudinal mesh size,  which is common to beam-beam field and luminosity calculations, has to be defined by the parameter Smesh  by the SET command. Its value at the time when PUSH started is used thoughout the PUSH loop.