critical_current_density_nbti Function

  function critical_current_density_nbti(c_pt,T,B,St,Tc0,Bc) result(critical_current_density) bind(C)
      type(c_ptr), value :: c_pt
      real(dp), value :: T,B,St,Tc0,Bc
      real(dp) :: critical_current_density
      real(dp) :: Blim,Tlim,tt,bb,h,fp,Blow,Tlow

      type(nbti_cfg_t), pointer :: nbti_ptr
      call c_f_pointer(c_pt,nbti_ptr)

      ! M.A. Green, Calculating the Jc, B, T Surface for Niobium Titanium
      ! Using a Reduced State Model, IEEE Trans. Mag., 25, 2, (1989).
  
      ! G. Morgan, A Comparison of Two Analytic Forms for the Jc(B,T)
      ! surface, SSC Magnet Division Notes, 310-1 (SSC-MD-218), (1989)
  
      ! L. Bottura, B. Bordini, Jc(B,T,e) Parameterization for the ITER
      ! Nb3Sn Production, IEEE Trans. Appl. Sup., 19(2), 1477-1480, 2009
              
      Blow=1.0e-3_dp
      Tlow=0.0_dp
      Blim=max(abs(B),Blow)
      Tlim=max(T,Tlow)
      tt = Tlim/nbti_ptr%Tc0_p

      if(tt>1.0_dp) then
         critical_current_density=0.0_dp
         return
      endif

      bb = Blim/Bc
      if(bb>1.0_dp) then
         critical_current_density=0.0_dp
         return
      endif

      h = hNbTi(tt,nbti_ptr%nu)
      fp = fpNbTi(bb,nbti_ptr%p,nbti_ptr%q)
      critical_current_density = nbti_ptr%CC0/Blim * h**nbti_ptr%n * fp
  end function critical_current_density_nbti