In the present analysis, we study the dynamics of charged particles submitted to the action of slowly modulated electromagnetic carrier waves. While the velocity of the particles remains smaller than the carrier's phase-velocity, their dynamics is well described by a refined ponderomotive approach. The ponderomotive approach has its own validity limits well established, beyond which particles are resonantly trapped by the carrier waves. We show that under adequate conditions, the trapping mechanism places particles at an optimal relative phase with respect to the carrier for maximum acceleration. In addition to the analytical approach involved in the ponderomotive description, we use numerical simulations to validate the corresponding dynamics as well as to explore various features of the resonant trapping and acceleration. Published by AIP Publishing.

In the present paper we investigate the process of energy transfer in the Zakharov equations. Energy is initially injected into modes with small wave vectors. When the modulational instability threshold is exceeded, some additional modes with small wave vectors are excited and solitons are formed if one lies in a quasiintegrable regime and if the number of excited modes is large enough. These solitons are formed as a direct result of the modulational instability and in fact saturate the instability. However, use of a low-dimensional formalism based on collective variables shows that if the largest length scale of the linearly excited modes is much longer than the most unstable, these solitons may be greatly influenced as they interact with ion-acoustic waves. In those cases, full simulation of the space-time problem indicates that energy is progressively transferred to modes with very small length scales. Since we work with one spatial dimension, collapse is absent and energy transfer is due to the stochastic dynamics. ͓S1063-651X͑98͒02303-4͔ PACS number͑s͒: 05.45.ϩb, 52.35.Ra

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We study soliton stability under the action of strong external perturbations. Limits on the weak perturbation approach are established with the help of average Lagrangian methods and full simulations. We found that for the same relative perturbation, larger amplitude solitons develop instability earlier than weaker amplitude solitons.

The effect of ambient density fluctuations on Langmuir wave collapse and strong Langmuir turbulence is investigated. Hamiltonian analysis of the collapse threshold implies that fluctuations with scales near those of nucleating wave packets can disrupt them before they can accumulate enough energy to collapse, provided the ambient fluctuation level is greater than that generated ponderomotively by the Langmuir waves. If packet disruption is effective, Langmuir energy cannot be dissipated via wave collapse and burnout, but must be scattered off density fluctuations directly to high wave numbers, as predicted by previous analyses. Numerical simulations of strong Langmuir turbulence confirm these predictions, with sudden transitions occurring from a strong-turbulence regime to one dominated by scattering or one with relatively rare wave collapses as a result of disruption of nascent wave packets. A corresponding sudden drop in Langmuir energy density is observed. Simulations of individual wave packets near the threshold for collapse show that such packets are easily disrupted by fluctuations with wavelengths near their linear scale, and confirm previous analytic disruption criteria.

In the present analysis we study the dynamics of charged particles under the action of slowly modulated electromagnetic carrier waves. With the use of a high-frequency laser mode along with a modulated static magnetic wiggler, we show that the ensuing total field effectively acts as a slowly modulated high-frequency beat-wave field typical of inverse free-electron laser schemes. This effective resulting field is capable of accelerating particles in much the same way as space-charge wake fields do in plasma accelerators, with the advantage of being more stable than plasma related methods. Acceleration occurs as particles transition from ponderomotive to resonant regimes, so we develop the ponderomotive formalism needed to examine this problem. The ponderomotive formalism includes terms that, although not discussed in the usual applications of the approximation, are nevertheless of crucial importance in the vicinity of resonant capture. The role of these terms is also briefly discussed in the context of generic laser-plasma interactions.

In this paper we identify phase-locked states among the solutions of the Zakharov equations. Locked states appear as resonant island chains in the appropriate Poincaré plots, with the relevant surface of section obtained by projecting out the full dynamical set on a subspace defined in terms of a pair of center-manifold variables. This pair allows an accurate canonical description of the system immediately after an inverse pitchfork bifurcation destabilizes an initial homogeneous steady state. If one is very close to the bifurcation point, nonlinear saturation of the initial instability is provided by quasistatic integrable ion-acoustic fluctuations, but as one proceeds away from that point, resonant nonintegrable ion-acoustic fluctuations become gradually more important; we show that the phase-locked states result from those resonant fluctuations. If one is not too far from the pitchfork bifurcation, locking is the stable asymptotic state of the interaction. As one moves farther away, locking exists only over long but finite amounts of time. In addition, the resonance separatrix appears to bring the first chaotic activity into the system. ͓S1063-651X͑96͒09109-X͔ PACS number͑s͒: 05.45.ϩb

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