Localized Langmuir Field Structures Induced by Sharp Density Gradients

Abstract

Experimental measurements performed in an unmagnetized plasma with a cold electron beam as the free energy source reveal the development of localized, large amplitude, electrostatic field structures at sharp, downward-going gradients in the plasma density. The relative intensity of the spiky field structures is found to correlate with the reciprocal of the density gradient scale length. The plasma expelled by the earlier growth of Langmuir waves contributes to the density gradient which propagates back toward the injection point at the sound speed. A 1D particle-in-cell simulation shows reasonable correlation with the experimental results.