function Gamma_phi = Gamma_phi_bad( phi )
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%%  set diffusion parameter Gamma as function of phi
%%
%%   Gamma(phi) = Gamma(phi_ref) + Gamma_grad * (phi - phi_ref)
%%
%%  This linearization can produce negative Gamma - beware!
%% ------------------------------------------------------

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