On the Formation of the Inverse EDF and the Absolute Negative Conductivity of Electrons in a Gas-Discharge Plasma

A plasma with an inverse electron distribution function (EDF) can have an absolute negative conductivity (ANC) and amplify electromagnetic waves. Unfortunately, this important problem remains unresolved due to the significant simplification of the problem and the reduction of the Boltzmann equation to the homogeneous case. Finding the inverse EDF is reduced to searching for reactions that "eat away" the low-energy region on the EDF. However, such processes inevitably contribute to an increase in the electron transport frequency and prevent the formation of ANC. Since a real plasma is spatially inhomogeneous, one should solve the complete kinetic equation, which depends on energy and on spatial variables. In this case, an increase in the number of degrees of freedom of the system leads to the emergence of fundamentally new scenarios for the formation of the EDF. Thus, the divergence of the spatial flow can lead to an additional sink of electrons and thus provide the inversion of the EDF. The analysis shows that the EDF inversion conditions should be sought near the points of field reversal and/or a non-monotonic change in the plasma density [1, 2]. The simulation results showed that these conditions are fulfilled after the second field reversal point in the transition region from the Faraday dark space to the positive column of the glow discharge.