theta_derivative.f90

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subroutine theta_derivative(arrin, darr, nchi)
!---------------------------------------------------------------------
! subroutine to calculate the theta derivate using fft.
! arrin : the input array
! darr  : the resulting theta derivative 
! nchi  : the number of poloidal points
! ias   : 0 for up/down symmetric equilibria, 1 for asymmetric equil.
!---------------------------------------------------------------------

  use itm_types
  use mod_parameter
  use mod_mesh

  implicit none

  integer(itm_i4), intent(in) :: nchi
  real(r8), dimension(nchi), intent(in) :: arrin
  real(r8), dimension(nchi), intent(out) :: darr

  integer(itm_i4) :: j, n
  integer(itm_i4), dimension(2 * nchi - 2) :: indices
  real(r8), allocatable :: ff(:), df(:)

  do j = 1, nchi
    indices(j) = j
  end do

  if (ias == 0) then
    do j = nchi + 1, 2 * nchi - 2
      indices(j) = 2 * nchi - j
    end do
  end if

  if (ias == 0) then
     n = 2 * (nchi - 1)
  else
     n = nchi
  end if

  allocate(ff(2 * n))

  ff = 0._r8

  do j = 1, n       
    ff(j) = arrin(indices(j))
  end do 

  call rft2(ff, n, 1)

  allocate(df(2 * n))

  df = 0._r8

  do j = 1, n / 2
    df(2 * j - 1) = -dble(j - 1) * ff(2 * j)   
    df(2 * j) = dble(j - 1) * ff(2 * j - 1) 
  end do

  call rfi2(df, n, 1)

  do j = 1, nchi
    darr(j) = df(j)
  end do

  deallocate(ff)
  deallocate(df)

  return

end subroutine theta_derivative