Stix Award: The ponderomotive effect beyond the ponderomotive force

The classical ponderomotive effect (PE) is typically understood as the nonlinear time-average force produced by a rapidly oscillating electromagnetic field on a nonresonant particle. It is instructive to contrast this understanding with the common quantum interpretation of the PE as the ac Stark shift, i.e., phase modulation, or a Kerr effect experienced by the wave function. Then the PE is naturally extended from particles to waves and can be calculated efficiently in general settings [1], including for strongly nonlinear interactions and resonant dynamics. In particular, photons (plasmons, etc.) are hence seen to have polarizability and contribute to the linear dielectric tensor exactly like ``true'' particles such as electrons and ions. The talk will briefly review the underlying variational theory [1-4] and some nonintuitive PE-based techniques of wave and particle manipulation that the theory predicts. It will also be shown that the PE can be understood as \textit{the} cause for the basic properties of both linear and nonlinear waves in plasma, including their dispersion, energy-momentum transport, and various modulational instabilities. Linear collisionless dissipation (both on particles and classical waves, treated on the same footing [2]) also appears merely as a special case of the modulational dynamics.\\[4pt] [1] I. Y. Dodin and N. J. Fisch, Phys. Rev. Lett. {\bf 112}, 205002 (2014).\\[0pt] [2] I. Y. Dodin, Phys. Lett. A {\bf 378}, 1598 (2014).\\[0pt] [3] I. Y. Dodin, Fusion Sci. Tech. {\bf 65}, 54 (2014).\\[0pt] [4] I. Y. Dodin and N. J. Fisch, Phys. Rev. A {\bf 86}, 053834 (2012).