Influence of Magnetic Field on Spectra of Electrostatic Oscillations in the Plasma of the E × B Discharge

Abstract

Simultaneous measurements of the amplitude–frequency characteristics (AFCs) of oscillations of the derivative of the discharge current and the ion current at the frequencies of 20 kHz–30 MHz in the plasma of a self-sustained E × B discharge in an accelerator with an anodic layer under conditions of a strong inhomogeneous magnetic field (the radial component is up to B_rC = 4200 G at the cathode; up to B_rA = 1010 G at the anode) have revealed both identical and different properties of oscillations of the derivative of discharge and ion currents. Common features are the discrete spectrum and, mainly, the cluster character of oscillations. The threshold magnetic field values of B_rA = 660–720 G, at which there is a rapid increase in the frequency of oscillations having a maximum amplitude up to f_max ∼ 4.5 MHz, have been discovered. At the same time, there are jumps of the selected peaks of the amplitude–frequency characteristic from tens of kilohertz to hundreds of kilohertz in the frequency range of not higher than 1 MHz. The differences in the amplitude–frequency characteristics of the discharge current and ion current oscillations are the frequencies of discharge current oscillations with the maximum amplitude, which are ∼5 times lower than those of the ion current at 205 ≤ B_rA ≤ 660 G, sharp decay of f_max for AFC of the discharge current, but sharp increase in f_max for AFC of the ion current, when B_rA becomes larger than 820 G. The results of the measurement of characteristics are analyzed together with the plasma emission spectra in the wavelength range of 200–1100 nm and ion energy distributions in the range of 50–1200 eV measured in the same discharge modes. The possible origin of the generation of discharge and ion current oscillations during excitation of the modified two-stream and electron–cyclotron drift instabilities in the plasma of the E × B discharge for frequencies f ≤ 1 MHz are discussed. At higher frequencies, the influence of the axial instability of the flow of non-magnetized ions on the energy ion distribution is analyzed.