Description of the input parameters for xbfield
                   ===============================================

PERIOD: magnet period in cm.

N:      number of periods

NPTS: Number of points per period (for internal use in calculations)

Undulator magnet:  Magnet composition  (Nd-Fe-B of Sm-Co)

Undulator type: Plane (convensional) or tapered undulator

K: Deflection parameter K

For tapared undulators define:
  GAP: initial gap in cm.
  GAPTAP: Gap taper in %

FILE: The output file name. 


It uses a linearly modulated gap model of the form (see BFIELD.F):

B=0.95*3.44*exp(-r*(5.08-1.54*r))

where B is the magnetic field
      r  = lgap/period
      lgap is the "local" gap, i.e., lgap=GAP*(1.0+z*(GAPTAP/100)/PERIOD)
      z is the spatial position along the undulator axis