Investigations on the stability of the low pressure positive column in oxygen

Abstract

The stability of the low pressure positive column in oxygen was investigated in a pressure range from 0.5 to 0.9 Torr within a discharge current interval from 0.5 to 90 mA. The transition between the well-known T-and H-modes has been studied. The H-to T-mode transition showed a marked hysteresis in the E(I) characteristic which is affected by wall processes. For the first time temporally resolved electric field measurements were realized. At small discharge current the electric field showed a significant modulation, characterized by incoherent fluctuations with a broadband Fourier spectrum. With increasing current the discharge operates in the T-mode, where a mode selection with high modulation degree occurred, resulting in a periodic oscillation of the electric field at a discrete frequency spectrum. The dynamic state in the T-mode is expressed by T-waves moving from the cathode to the anode. It seems that they were excited by oscillations in the cathode region. The waves were damped in the direction of the anode and show no dispersion. The discharge stability was studied using a hydrodynamic model considering electrons, positive and negative ions as well as metastable O 2 (a 1 g) molecules. Here the negative O −-ions play a crucial role. In good agreement with the experiments the transition between the Hand T-modes was explained as a linear unstable equilibrium state where the energy dependence of the corresponding rate coefficients is the driving mechanism (attachment-induced instability).

HOPF‐Bifurcations, Global Coupling and Hysteresis in dc‐Driven Oxygen Discharges

“…Actually,the same behavior is observed experimentally at the transition from the H-mode to the T-mode [8], [14] at the upper critical current. Furthermore, there appears apronounced hysteresis right at this transition.…”

“…Here we are only interested in the space-independent case, i.e. we choose ∂A/∂ξ =0.T hen (4.13) will reduce to the normal form of aHOPF-bifurcation 14) and the HOPF constant (c + a) contains twodifferent contributions. c is directly connected to the pure plasma properties of the column, whereas a describes the effects of the external circuit on the stability of the system.…”

“…The normal form equations are discussed for this case for example in [32]. We have not analyzed this situation in detail yet, butitispossible that the dynamics near this point can be used to understand the recently observed dynamical states (the so-called H*-mode, [14]) at small currents.…”

“…In particular the axial eletric field E z is of major importance here. Furthermore we have to specify the temperature of the walls and along the axial direction, to rotational temperature profile measured by experimentalists [14]. This defines the equilibrium state for the particle densities and the mean electron energy and the axial electric field.…”

“…[7] contains as tudy of dynamical effects, which can be compared to theoretical results presented in this paper.B ut little is known about the theoretical description of nonlinear dynamical states in such plasmas. On the other hand, there is al ot of experimental data for instabilities, oscillations and wavesi nd c-discharges, as well as in rf-plasmas [8][9][10][11][12][13][14]. The onset of such waves, i.e.…”