subroutine cathode(dwtp,iend,ibeg)
c----------------------------------------------------------------------
c
      include 'param_sz.h'
      include 'bfieldcom.h'
      include 'constcom.h'
      include 'pcordcom.h'
      include 'syscom.h'
      include 'ucom.h'
c
      common/cathcur/rcur
c--------------------------------------------------------------------------
c*
      ne=rne
      bz=bgz/gamma
      dz=bz*dcon*dwtp
c----particle will be moved into next element during this step.
c----the next element should be a cell of length greater then
c----the distance this element moves the particles.
c----the next element in particular must not have zero length!
c----the location of this element should be at z=0. a zero length
c----drift could preceed this element.
      if(dz.ge.0.)iend=1
c-----allow particle to go backwards
      if(ne.ge.1.and.dz.lt.0.and.z+dz.le.zloc(ne-1))then
c----particle will cross into previous element during this step
      ibeg=1
      endif
      r=sqrt(x*x+y*y)
      if(iend.eq.1.and.rcur.gt.r)then
      theata=asin(r/rcur)
      dz=rcur*(1.-cos(theata))
      z=z+dz
      bgz=bgz*cos(theata)
      bgr=-bgz*sin(theata)
      if(r.ne.0.)then
      bgx=x/r*bgr
      bgy=y/r*bgr
      endif
      endif
      if(ibeg.eq.1)z=zloc(ne-1)
      dwtp=0.
      return
      end
c++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++*