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 . 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 .
 I. Y. Dodin and N. J. Fisch, Phys. Rev. Lett. 95, 115001 (2005).
 I. Y. Dodin and N. J. Fisch, submitted to Phys. Rev. E.
 N. J. Fisch, J. M. Rax, and I. Y. Dodin, Phys. Rev. Lett. 91, 205004 (2003).
 I. Y. Dodin, N. J. Fisch, and J. M. Rax, Phys. Plasmas 11, 5046 (2004).
 I. Y. Dodin and N. J. Fisch, J. Plasma Phys. 71, 289 (2005).
 I. Y. Dodin and N. J. Fisch, Phys. Lett. A 349, 356 (2006).
 I. Y. Dodin and N. J. Fisch, Phys. Rev. E 72, 046602 (2005).
1 In collaboration with N. J. Fisch
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