Effect of ambient density fluctuations on Langmuir wave collapse and strong turbulence

Abstract

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.

Nonlinear spatiotemporal stability of solitons against external perturbations

“…It is worthwhile to note that average calculations suggest that the type of external drive analyzed here is shown to be ineffective in altering several solitary properties, like the multi-dimensional collapse in the dynamics of plasma waves when a s /W 2 ( 1 [9]. Our work nevertheless suggests that soliton existence can be firmly affected even at these small perturbing amplitudes provided we take fully into account the nonintegrability properties of the driver.…”

“…(4), which does not involve radiation, proves convenient and accurate; radiative effects, when present, are small and can be examined with perturbation theories [20]. In addition ion-acoustic and soliton scale lengths are comparable under this same condition, which enhances the interaction [9]. Note that the system length satisfies kL % 10p and the soliton length L s , kL s $ 1.…”

“…This interesting result should be tested with full nonlinear dynamics machinery to examine its effectiveness [8,14]. In the case of Langmuir turbulence, average techniques lead to an estimate on conditions for soliton stability against collapse, but nonintegrable effects have not been included which can set additional limits [9].…”

“…Chaos may not only affect the dynamics of solitons, but can also prevent their formation and survival. This feature is of course of critical relevance in ambients where NLSE solitons coexist with high level integrability-breaking external perturbations, 0960 and has been the subject of recent investigation in a number of settings of which we cite two: (i) NLSE solitons or cavitons of Langmuir turbulence subjected to the action of surrounding ion-acoustic waves [9,10]; (ii) Bose-Einstein condensates under time varying fields [11][12][13][14]. In both cases thresholds for soliton existence have been determined based on average, or integrable approximations of the driven system, but full account of chaotic motion has not been taken, which can alter the average estimates.…”

Ion heating, burnout of the high-frequency field, and ion sound generation under the development of a modulation instability of an intense Langmuir wave in a plasma

“…The modulation instability of intense Langmuir waves in non-isothermal plasmas also leads to collective ion perturbations, in particular, to the generation of ion-sound waves [32][33][34][35] .…”