First Experimental Observation of Electron Acoustic Wave Propagation in Laboratory Plasma*

The electron acoustic wave (EAW) is one of the basic electrostatic waves in an unmagnetized collisionless plasma. EAW is an acoustic-like mode that requires a non-Maxwellian electron distribution. It is also known to be heavily Landau damped and never seen in a laboratory device directly. Our theoretical analysis in the fluid description, predicts that in presence of a drifting, cold electron component, this mode can be destabilized. Incidentally, our Magnetize Plasma Linear Experimental (MaPLE) device provided such favorable platform and we have succeeded for the first time in observing the EAW propagation. Detailed experimental studies verified a wave indeed was excited and it propagated axially with a phase velocity of∼ 1.8 times the electron thermal velocity. Analytic treatment predicted the cold drifting electrons in the plasma can ease the stringent condition of hot-to-cold electron temperature ratio, T_eh >> T_ec and destabilizes EAW, which is also established experimentally. The observed dispersion relation matches well with the analytical outcome. A critical drift velocity, dependent on density and temperature ratios, is observed above which the mode ceases to exist. Experimental observations also support this aspect.