C. Wilke

Laser absorption spectroscopy with a blue diode laser in an aluminum hollow cathode discharge

A GaN-based blue diode laser is employed for laser absorption spectroscopy in an aluminum hollow cathode glow discharge plasma. A Littrow grating stabilized external resonator is used for tuning the wavelength of laser emission. Scanning the wavelength of the laser probes the absorption profile of aluminum transitions yielding ground state aluminum densities. Hyperfine structure has to be taken into account in order to obtain correct temperatures. The diode laser is used to investigate both the temperature and the density of aluminum atoms sputtered from a cathode surface by ion bombardment from the cathode fall region. The blue diode laser allows quick and easy access to aluminum ground state atoms at low costs.

Spatio-temporal response of stochastic resonance in an excitable discharge plasma

Stochastic resonance is experimentally studied in a discharge plasma which is operated close to the threshold of ionization instability. It is shown that ionization wave-trains are excited by noise superimposed on the discharge current. There is a maximum signal-to-noise ratio at an optimum level of noise, indicating stochastic resonance. The corresponding spatio-temporal dynamics is investigated and it is shown that noise drives the stable discharge stochastically into the convectively unstable regime. This also reveals similarities to other physical system, for instance chemical waves.

Investigation of the disturbance of a Langmuir probe and its influence on measurement results

Measurements of plasma parameters by Langmuir probes are influenced by the probe system itself. This article considers the deviation of plasma parameters determined by probe measurements from those in the undisturbed plasma. For this reason the particle density of the excited atoms of a neon plasma in the very vicinity of the probe is compared with the particle density in the undisturbed case. The densities of the excited metastable 1s5 level were measured by laser absorption spectroscopy. By solving the plasma balance equations the deviation of the electron density and the deviation of the electric field could be deduced.

On the density of metastable and resonance atoms in a stratified positive column in neon

The densities of metastable and resonance states 33P2 (S5) and 31P1 (S2) are measured by a method of laser absorption in S and P striations in neon at low (1.4 Torr) and intermediate (12.8 Torr) pressures. The theoretical description of excited atoms in striations requires the solution of the nonlocal kinetic equation in a spatially periodic field to obtain the excitation rates, as well as the analysis of both the resonance radiation transport and diffusion equations for excited atoms. The solution of the radiation transport equation is obtained for the case of both the radial and axial (in particular, periodic) inhomogeneities. The densities of excited atoms are calculated in different striation phases taking into account the transport processes, direct intermixing by atomic and electron impact, and intermixing by excitation and radiation of the system 2p53p. Comparison of the theoretical and experimental results is performed for weakly nonlinear striations near a lower bound of existence by pressure and striations at low and intermediate pressures.

Observation of the spatiotemporal dynamics of ionization wave mode transitions

Experimental observation of transitions between discrete longitudinal wave modes in a dc neon glow discharge is reported. Experimental findings concerning the pattern selecting instability agree with observations of the Eckhaus instability in fluid systems. This is supported by theoretical considerations of a model equation (Complex Ginzburg–Landau equation) based on a fluid model.

Hysteresis of ionization waves

A quasi-logistic, nonlinear model for ionization wave modes is introduced. Modes are due to finite size of the discharge and current feedback. The model consists of competing coupled modes and it incorporates spatial wave amplitude saturation. The hysteresis of wave mode transitions under current variation is reproduced. Sidebands are predicted by the model and found in experimental data. The ad hoc model is equivalent to a general—so-called universal—approach from bifurcation theory.

Spatio-temporal dynamics of hidden wave modes in a dc glow discharge plasma

Abstract We report on experimental preparation and investigation of hidden wave modes, which represent stable patterns that cannot be attained by a monotonic variation of a control parameter starting from any other stable, non-hidden state. In terms of a stability diagram this situation may occur if the stability region of patterns is bent in such a way that secondary instabilities of neighboring, non-hidden states cannot lead to hidden patterns. Hidden patterns are prepared by driving the system at the frequency of the hidden state and by choosing an appropriate control parameter. Then these states may remain in the hidden state after disconnecting the driving force. The spatio-temporal dynamics of the hidden patterns is found to be more complicated than non-hidden, regular patterns. In particular, saturated long-wavelength modulation and non-linear wave-wave interaction can be observed.

Peculiarities of the resonant structure of the electron distribution function in S-, P- and R-striations

S-, P- and R-striations, which can be observed in an inert gas dc discharge at low pressures and small current densities, are considered. Mechanisms of the resonant formation of the electron distribution function (EDF) in S-, P-, Q- and R-striation-like fields are discussed on the basis of an accurate numerical solution of the Boltzmann kinetic equation and from the standpoint of the analytical theory. For S-, P- and Q-resonances, the EDF has the form of maxima of equal magnitudes. The maxima move in the plane (energy, coordinate) along the resonance trajectories. In the case of R-resonance, alternation of the magnitudes of these maxima takes place. The cause of such behavior of the EDF is elucidated. The experimental investigation of the EDF in R-striations in neon has also been carried out. The probe voltage?current characteristics were measured in each striation phase. The EDFs were determined by the second derivative of the probe characteristics. The ill-posed problem of the computation of the second derivative was solved by the Tikhonov regularization method. Comparison of the results of the measurements with the calculations shows the adequacy of the developed theoretical notions about the resonant character of the EDF formation for both integer S-, P- and Q-resonances and non-integer R-resonance.

Onset of Chaotic Wave Dynamics near the Critical Point in a Glow Discharge, Theory and Experiment

Conditions for the onset of irregular wave dynamics are derived from a theoretical description of ionization waves in a glow discharge by means of amplitude equations. It is shown that the Benjamin–Feir condition of the cubic Ginzburg–Landau equation is a necessary but not sufficient condition for loss of stability of plane waves in a higher order amplitude equation. The onset conditions are numerically evaluated in the case of a neon discharge. Experimental findings concerning the onset of chaotic wave dynamics near the critical point agree very well with the theoretical predictions. This example reveals physical relevance of higher order nonlinearities close to the critical point.

Scaling laws and nonlinear wave dynamics in neon glow discharges

Scaling laws that are obtained from a hydrodynamic model of the positive column in neon gas discharges are checked in experiments. In contrast to previous investigations, most experiments were performed in parameter regions where nonlinear phenomena are dominant. The results show that the predicted scaling laws are also fulfilled in these cases with satisfactory accuracy.