Damping of Geodesic Acoustic Mode by Trapped Electrons

We study the Landau resonance between geodesic acoustic mode (GAM) and trapped electrons as a GAM's collisionless damping. The assumption of (omega) over bar (de) << omega(be))be is adopted. The damping rate induced by trapped electrons is found to be an increasing function of q. In low q range, circulating-ion-induced damping rate is larger than that induced by trapped electrons. As q increases, the latter becomes larger than the former. The reason is that trapped electrons' resonant velocity is close to v(te) from the lower side, whiles circulating ions' resonant velocity gets bigger further from v(ti). So the number of resonant trapped electrons increases, whiles the number of resonant circulating ions decreases. The amplitude of damping rate induced by trapped electrons in the edge plasma can be comparable to that induced by circulating ions in the low q range. Another phenomenon we found is that in the chosen range of epsilon, the damping caused by trapped electrons has a maximum value at point epsilon(q) for different q. The reason is that as epsilon is close to epsilon(q), trapped electorns' resonant velocity is close to v(te).