The relaxation due to Coulomb collision of the plateau of the electron distribution function in the post-r.f. phase, after the lower hybrid r.f. power has been removed, is investigated numerically. The authors show that in the process of this collisional relaxation, a positive slope is formed to the distribution function. This offers a mechanism by which plasma waves can be excited during the relaxation process, via the Cerenkov resonance interaction in the velocity region with the positive slope.