Quasi-trapped particle orbits due to toroidal plasma modes

A combined numerical and analytical treatment of the particle orbits in a two-dimensional toroidal configuration is given by analyzing the resonant effects of a special class of electrostatic standing modes such as those that can result from plasma microinstabilities. It is shown that trapped particle orbits with bounce frequencies close to the considered mode frequency can be significantly amplified and even untrapped for reasonable mode amplitudes. The orbits of particles which in the absence of fluctuations are weakly trapped or barely circulating are replaced by a new class of orbits, with non-vanishing bounce frequencies, which are alternatively trapped and circulating ('quasi- circulating'). The change on the average displacement of a particle orbit from a given magnetic line over a typical orbit time is evaluated in view of its importance for the stability of (low frequency) interchange trapped particle modes.