Session UO1 - Reconnection, Nonlinear Phenomena, and Turbulence.
ORAL session, Thursday morning, October 26
Room 301AB, Qu\'{e}bec City Convention Centre
Lenard \textitet al.(A. Lenard \textitet al.), Phys. Rev. \textbf112, 1456 (1958). studied this using an approximate collision operator. Observation of plasma echoes led Su \textitet al.(C. H. Su \textitet al.), Phys. Rev. Lett. \textbf20, 427 (1968). to study the effect of weak collisions on the propagation of f_1 from a localized source. They used boundary layer theory to obtain the v-dependence of f_1 and found that the spatial echo is attenuated as exp(-\betax^3). Recently, this problem has been revisited by Ng \textitet al.(C. S. Ng \textitet al.), Phys. Rev. Lett. \textbf83, 1974 (1999). They use a f.t. in x and t and obtain eigenvalues of the velocity equation by a numerical procedure and state that the results in Ref. 2 are in error. We show that this is not the case: they recover instead the collective oscillations (identical to those in Ref. 1) but not the dominant f_1 behavior. In addition, it is incorrect to state that Ref. 2 assumed that the Van Kampen spectrum is preserved in the presence of collisions. We use the b.l. method with complex ømega. Our results are in essential agreement with those of Ref. 2 (and Landau). This work was supported by the U.S. DOE Office of ICF under Coop. Agreem. DE-FC03-92SF19460.