A ponderomotive potential is an effective potential seen by a particle in ac field on average over the fast oscillations. It is not a true potential though, and hence can be used for particle manipulations more advanced compared to those via static potentials. If the field scale is small enough, the particle motion in a ponderomotive barrier is essentially phase-dependent and resembles the dynamics of a quantum object in a conservative field. Probabilistic transmission is possible in this case [1, 2] and can produce attosecond electron bunches when a uniform relativistic electron beam is scattered off an intense laser wave in vacuum.

For particles exhibiting natural oscillations (e.g., Larmor rotation or internal vibrations), nonadiabatic yet phase-independent ponderomotive manipulations by resonant ac fields are also available [3-5]. An approximate integral of particle motion is found for resonant nonlinear interactions, and a new ponderomotive potential is introduced accordingly [6]. Because of the irreversibility of the effective potential, a resonant barrier can also produce a ratchet effect by asymmetrically transmitting thermal particles in a preferential direction [3, 4, 7]. Possible applications of this and related effects comprise current drive [3, 4], selective separation, and cooling of plasma species [6].

[1] I. Y. Dodin and N. J. Fisch, Phys. Rev. Lett. 95, 115001 (2005).
[2] I. Y. Dodin and N. J. Fisch, submitted to Phys. Rev. E.
[3] N. J. Fisch, J. M. Rax, and I. Y. Dodin, Phys. Rev. Lett. 91, 205004 (2003).
[4] I. Y. Dodin, N. J. Fisch, and J. M. Rax, Phys. Plasmas 11, 5046 (2004).
[5] I. Y. Dodin and N. J. Fisch, J. Plasma Phys. 71, 289 (2005).
[6] I. Y. Dodin and N. J. Fisch, Phys. Lett. A 349, 356 (2006).
[7] I. Y. Dodin and N. J. Fisch, Phys. Rev. E 72, 046602 (2005).

1 In collaboration with N. J. Fisch