Local transit-time damping of electrostatic wave packets

The theory of local transit-time damping is generalized to coherent electro static wave packets with nonzero mean wave number in an unmagnetized plasma . A general analytic formula for the phase-averaged power dissipated locall y within an arbitrary three-dimensional wave packet is derived to second or der in the fields. This expression is evaluated explicitly for a representa tive one-dimensional field structure. The result agrees with independent nu merical test-particle calculations to within numerical rounding errors for small to moderate field amplitudes, which justify the perturbation expansio ns. The resulting damping involves both Landau (resonant) and non-Landau (n onresonant) terms, the latter having been omitted in previous works. It is found that the dissipated power depends sensitively on the ratio of the par ticle velocity to the phase velocity of the packet, the ratio of the wavele ngth to the size of the packet, and the form of the particle distribution. In general, particles remove energy from some parts of the packet and depos it it in others, thus reshaping it. (C) 1999 American Institute of Physics. [S1070-664X(99)01404-4].