function Gamma_phi = Gamma_phi( phi )
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%  set diffusion parameter Gamma as function of phi
%%
%%   Gamma(phi) = Gamma(phi_ref) + Gamma_grad * (phi - phi_ref)
%%
%%  or
%%   Gamma = K/phi
%%      K = Gamma_ref * phi_ref
%% ------------------------------------------------------

    global phi_ref Gamma_ref Gamma_grad
    
    
   %% Gamma_phi = Gamma_ref + Gamma_grad * (phi - phi_ref );
%
    Gamma_phi = ( Gamma_ref * phi_ref ) ./ phi;
%
%
%