Tungsten etching in pulsed plasmas has been investigated in a helicon plasma source reactor. The time dependence of the fluorine atom concentration has been measured using the time-resolved actinometry technique and related to the etch rate. According to our observations, it appears that fluorine adsorption on the tungsten substrate surface continues in the post-discharge period until the surface saturates. Moreover, it appears that two etching regimes exist. For short discharge off periods, the etching is limited by the fluorine adsorption ability of the surface, whereas for long periods, the etching is limited by the desorption rate of the etch products. Experiments were performed at different substrate temperatures and plasma gas pressures. An empirical model has been developed, in good agreement with the experimental data.