Experimental investigation of the propagation of an electron beam in a homogeneous non-magnetized plasma

We conduct an experiment on the interaction of a beam with a plasma. The plasma is non-magnetized; its density is uniform, except for a 0.2 m gradient in front of the cathode. The parameters of our beam explore the domain of validity of the quasi-linear and the non-linear theories. In the experiments the solitons and their evolution were investigated. The relative parameters of our beam are similar to those used in other experiments; the density of our plasma, however, is higher than that of the plasmas used in other experiments. The extension of the solitons is practically inobservable: six Debye lengths, i.e. 0.5 mm. The particularity of our method lies in the investigation of the macroscopic effects of the interaction rather than in that of the microscopic effects. These particular conditions give our beam an energy of several keV. The normally employed diagnoses have turned out to be incapable of measuring such distribution functions. We have developed a new rapid electrons analyzer capable of measuring our beam. Nine different experimental conditions have been created. The thermalization distances of our beams are beyond quasi-linear predictions. We have observed that the interaction deeply modifies the plasma. During the shots, the plasma's density presents a gradient extending over the entire length of the vacuum cell. Langmuir waves are excited at both the fundamental and the harmonic frequency of the plasma's beat frequency. Theoretically a distribution function having a plateau can be unstable in a multidimensional system. In an experiment, we have measured the reappearance of a positive slope in a distribution which already had a quasi-linear plateau. 62 refs., 26 figs., 3 tabs