Far infrared (FIR) radiation of high-power pulsed laser incident normal to the surface of GaAs/metal tunnel structures with a self-consistent Schottky barrier gives rise to a change in the tunnel conductance. It has been shown that the observed photoresistive effects are caused by ponderomotive forces of the radiation field on the free electron plasma in the junctions. The change of tunnel conductance rises linearly with increasing intensity at low power levels and proceeds into a strongly superlinear dependence at high intensities. It is shown that this superlinearity is a result of an enhancement of the local radiation field in the near zone of diffraction by inhomogeneities at the metal-semiconductor interface and depends strongly on the roughness of the metal electrode. Experimental results are compared to a nonlinear extension of the theory of electron redistribution due to the radiation pressure.