subroutine byfield(dwtp) ! Lui
c     this subroutine describe the applied transverse bending field
c     vertically traversing the cathode cavity
c------------------------------------------------------------------------
c
      include 'constcom.h'
      include 'pcordcom.h'
      include 'tstepcom.h'
c
      common/back/bmax,byz0,byzs,pzowmin,pzowmax,iback 
c-----------------------------------------------------------------------
c*
      data itest/0/
      bdis(z)=1./(1.+((z-byz0)/byzs)**2)  
      if(bmax.eq.0.0) go to 20
      if(itest.eq.0) then
     	      byc0=bmax/brhof
	      itest=1
      endif
      dd=dcon*dwtp
      bz=bgz/gamma
      bx=bgx/gamma
      byc=byc0*bdis(z)
      dbgx=byc*bz*dd
      dbgz=-byc*bx*dd
      bgx=bgx+dbgx
      bgz=bgz+dbgz
20    continue
      return
      end
c++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++*