subroutine rdiffset
c      
c This routine calculates the diffusion associated with particle noise
c in GK PIC simulations.  The code evaluates an analytical derivation 
c by Greg Hammett.  The parameters ifilter, N_p, n_zpic, and bmax are 
c described in the input file (itg.in).  
c      
      implicit none
      include 'itg.par'
      include 'itg.cmn'
      
      integer m, n
      real tmp, fa_par, fac

      fa_par=2*pi*qsf/epsn/float(n_zpic)   ! a_par/L_n
      
      do n=1,nd
         do m=1,md
            tmp=0.5*sqrt(pi/2)*eta(1)**2/float(n_p)*fa_par*chi_int
     .           *bmax/(2+bmax)
c     first time step hack
            if(rdiff(m,n).eq.0.) then
               rdiff(m,n)=tmp
            else
               fac=1+sqrt(2/pi)/fa_par
     .              /(rdiff(m,n)-wpfx(ne,1)/eta(1)/1.5)
     .              *2/bmax*(2+bmax)/(1+bmax)
               rdiff(m,n)=tmp*alog(fac)
            endif
         enddo
      enddo
           
      return
      end