Description
Langmuir waves, electrostatic plasma waves characterized by a frequency near the electron plasma frequency, are routinely observed to be excited in the solar wind arising, for instance, from beam-driven instabilities or from magnetic structures, such as magnetic holes. A variety of in-situ observations from spacecrafts missions, such as Parker Solar Probe (PSP), are reported; however, there are still open questions related to the dynamics of this phenomenon. Laboratory systems in this respect offer the possibility of reproducible experimental data, with tunable parameters, and high diagnostic capabilities. We therefore realized a small-scale laboratory experiment for the investigation of Langmuir waves excitation in a weakly ionized plasma. A beam-plasma instability is induced through the acceleration of a supra-thermal electron population (electron beam), generated by thermionic emission from a tungsten filament, in a background thermal plasma.
A retarding field analyzer allows to resolve the electron velocity distribution function, characterized by a Maxwellian population with a super-imposed bump-on-tail, and to extrapolate the properties of the generated electron beam. Langmuir Probes are adopted to measure the background plasma parameters. A system of two closely spaced antennas connected to a multichannel high-frequency data acquisition tool allows the observation of the onset and non-linear evolution of Langmuir waves, their high frequency spectrum, and the characterization of the dispersion relation in mm wavelength regimes.
Our experimental measurements are compared with PSP observations and the results of dedicated simulations with a non-linear 1D-1V Vlasov-Poisson and a linear Vlasov-Maxwell numerical models. The experimental frequency and $\omega$ - $\kappa$ spectra are well reproduced by the numerical codes and their dispersion relations align with theoretical Bohm–Gross prediction at the Landau-Cherenkov resonance condition. Above a threshold on the electron beam energy, Langmuir wavepackets show a clear non-linear behaviour, which also characterizes PSP electrostatic waveforms.
Our work is an example of how experimental plasma systems can serve as effective analogues for understanding wave-particle interactions in astrophysical environments, despite the different physical scale.