We studied experimentally the plasma discharge process of Ar gases by an Electron beam injection parallel to the magnetic field. The plasma discharge via the gas break down was explained by the production of higher energy electron based on the cyclotron damping of unstable electron cyclotron harmonic waves in an electron beam-plasma system. On the condition of higher gas pressure and lower external magnetic field, we have observed that generated plasma diffuses perpendicular to the magnetic field and disrupts rapidly in the short time (≈100μs), regarless of existence of thebeam injection (beam pulse ≈460μs). The disruption of the plasma discharge is attributed to the occurrence of unstable waves with the low frequency by the beam-plasma interaction. We have investigated the dispersion relation of these unstable waves and compared with the numerical dispersion one of the lower hybrid waves. The observed wave number and frequency agree with numerical ones, therefore unstable waves with the low frequency identified as lower hybrid waves.
In oreder to improve the plasma discharge via the suppression of unstable lower hybrid waves, the high frequency (hf) field near the electron cyclotron frequency (ω/ω_c≈1.1) is external applied, Then the amplitude of unstable waves is reduced and it is observed that the plasma discharge is kept up between the time of the beam injection and the density of the generated plasma increases. Therefore, the anomalous diffusion due to unstable lower hybrid waves is improved by the suppression of unstable waves. We attempt the theoretical and numerical analysis of the unstable lower hybrid waves in the presence of the applied hf field. The results of analysis agree qualitatively with the experimental ones.