subroutine field
c---define background field  (e.g., helmholz coil field)
c---parameters for field coil are position,radius,current,(z0,zmax on 
c---first only)
c----------------------------------------------------------------------
c
      include 'param_sz.h'
      include 'bfieldcom.h'
      include 'flagcom.h'
      include 'misccom.h'
      include 'syscom.h'
c
      real*8 rz3ov2,rz5ov2,rz7ov2,rz9ov2,rz11ov2,rz13ov2
c----------------------------------------------------------------------
      div=nptcbu
cbm      if(ifld.eq.0)then
cbm         write(nnout,*) 
cbm     * 'Number of used values for magnetic field ',vv(6)
cbm         div=vv(6)
cbm         nptcbu=div
cbm      endif
      if(ifld.eq.0)then
cbm28/09/09         do 1 i=1,1002
         do 1 i=1,nptcbu+2
         do 1 j=0,6
         bf(i,j)=0.
1        continue  
         zmin=vv(4)
         zmax=vv(5)
         order=6.
cbm28/09/09         dzz=(zmax-zmin)/1000.
         dzz=(zmax-zmin)/div
         ifld=1
      endif
cbm      if(nn.eq.7) order=vv(7)
      if(nn.eq.6) order=vv(6)
      cur=vv(3)
      pos=vv(1)
      radus2=vv(2)**2
      a=.6283*cur*radus2
cbm28/09/09      do 10 i=1,1001
      do 10 i=1,nptcbu+1
      z=((i-1)*dzz+zmin)-pos
      zsq=z**2
      rsqzsq=radus2+zsq
      rz3ov2=rsqzsq**1.5
      rz5ov2=rz3ov2*rsqzsq
      rz7ov2=rz5ov2*rsqzsq
      rz9ov2=rz7ov2*rsqzsq
      rz11ov2=rz9ov2*rsqzsq
      rz13ov2=rz11ov2*rsqzsq
      bf(i,0)=bf(i,0)+a/rz3ov2
      bf(i,1)=bf(i,1)-a*3.*z/rz5ov2
      if(order.ne.1)then
      bf(i,2)=bf(i,2)-a*(3.*rsqzsq-15.*zsq)/rz7ov2
      bf(i,3)=bf(i,3)+a*(45.*z*rsqzsq-105.*z*zsq)/rz9ov2
      bf(i,4)=bf(i,4)+a*(45.*rsqzsq**2-630.*zsq*rsqzsq+945.*zsq**2)/
     *                   rz11ov2
      bf(i,5)=bf(1,5)-a*(1575.*z*rsqzsq**2-9450.*z*zsq*rsqzsq+
     *                   10395.*z**5)/rz13ov2
      bf(i,6)=bf(i,6)-a*(1575.*rsqzsq**3-42525.*zsq*rsqzsq**2+
     * 155925.*zsq**2*rsqzsq-.135135*z**6)/(rz13ov2*rsqzsq)
      endif
   10 continue
      return
      end
c++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++*