LANGMUIR FIELD STRUCTURES FAVORED IN WAVE COLLAPSE

Study of Langmuir collapse thresholds shows that they have little pola rization dependence and that moving packets have the lowest thresholds in the undamped case. However, incorporation of damping into the dens ity response inhibits collapse of packets moving at more than a small fraction of the sound speed. Investigation of energy transfer to packe ts localized in density wells-the nucleation process-shows that at mos t a few trapped states can exist and that energy transfer is most effe ctive when there is a single barely-trapped state. Coupled with an arg ument that closely packed wave packets have lower collapse thresholds, this argument yields an estimate of the number density of localized n ucleating states in a turbulent plasma. It also leads to a simple and direct semiquantitative estimate of the collapse threshold. All these results are in accord with previous numerical simulations incorporatin g ion-sound damping, which show a preponderance of slow-moving or stat ionary packets with little or no intrinsic polarization dependence of thresholds. Likewise, the number densities obtained are in good agreem ent with simulation values, and the simple estimate of the threshold i s semiquantitatively correct. The extent of the agreement supports the nucleation scenario with close-packed nucleation sites in the turbule nt state. (C) 1996 American Institute of Physics.