Adaptive FEM and CIPFEM for a nonlinear Helmholtz equation with high wave number

Abstract A nonlinear Helmholtz (NLH) equation with high frequencies and corner singularities is discretized by the linear finite element method (FEM) and the continuous interior penalty finite element method (CIPFEM). After deriving some wave-number-explicit stability estimates and the singularity decomposition for the NLH problem, a priori stability and error estimates are established for the (CIP)FEM on shape regular meshes including the case of locally refined meshes. Then, a posteriori upper and lower bounds using a new residual-type error estimator, which is equivalent to the standard one, are derived for the (CIP)FE solutions to the NLH problem. These a posteriori estimates have confirmed a significant fact that is also valid for the NLH problem, namely the residual-type estimator seriously underestimates the error of the FE solution in the preasymptotic regime, which was first observed by Babuška et al. [Int J Numer Methods Eng 40 (1997)] for a one-dimensional linear problem. Based on the new a posteriori error estimator, both the convergence and the quasi-optimality of the resulting adaptive finite element algorithm are proved for the first time for the NLH problem, when the initial mesh size lies in the preasymptotic regime. Finally, numerical examples are presented to validate the theoretical findings and demonstrate that applying the CIP technique with appropriate penalty parameters can reduce the pollution errors efficiently. In particular, the nonlinear phenomenon of optical bistability with Gaussian incident waves is successfully simulated by the adaptive CIPFEM.