LOCALIZED LANGMUIR FIELD STRUCTURES INDUCED BY SHARP DENSITY GRADIENTS

Experimental measurements performed in an unmagnetized plasma with a c old electron beam as the free energy source reveal the development of localized, large amplitude, electrostatic field structures at sharp, d ownward-going gradients in the plasma density. The relative intensity of the spiky field structures is found to correlate with the reciproca l of the density gradient scale length. The plasma expelled by the ear lier growth of Langmuir waves contributes to the density gradient whic h propagates back toward the injection point at the sound speed. A 1D particle-in-cell simulation shows reasonable correlation with the expe rimental results.