New contributions to transit-time damping in multidimensional systems

The existence of two previously unrecognized contributions to transit-time damping in systems of more than one dimension is demonstrated and discussed. It is shown that these contributions cannot be treated by one-dimensional analyses unless it is assumed that the gradient of the field perpendicular to itself always vanishes. Such an assumption is unjustified in general and the new contributions can dominate damping by fast particles in more general situations. Analytic expressions obtained using a Born approximation are found to be in excellent agreement with numerical test-particle calculations of transit-time damping for a variety of field configurations. These configurations include those of a resonance layer and of a spherical wave packet, which approximates a collapsing wave packet in a strongly turbulent plasma. It is found that the fractional power absorption can be strongly enhanced in non-slablike field configurations.