Parametric investigation of nonlinear fluctuations in a dc glow discharge plasma

Glow discharge plasmas exhibit various types of self excited oscillations for different initial conditions like discharge voltages and filling pressures. The behavior of such oscillations associated with the anode glow have been investigated using nonlinear techniques like correlation dimension, largest Lyapunov exponent etc. It is seen that these oscillations go to an ordered state from a chaotic state with increase in input energy i.e. with discharge voltages implying occurrence of inverse bifurcations. These results are different from the other observations wherein the fluctuations have been observed to go from ordered to chaotic state.Comment: Submitted to Chao

Ion laser plasmas

The typical noble gas ion laser plasma consists of a high-current-density glow discharge in a noble gas, in the presence of a magnetic field. Typical CW plasma conditions are current densities of 100 to 2000 A/cm^2, tube diameters of 1 to 10 mm, filling pressures of 0.1 to 1.0 torr, and an axial magnetic field of the order of 1000 G. Under these conditions the typical fractional ionization is about 2 percent and the electron temperature between 2 and 4 eV. Pulsed ion lasers typically use higher current densities and lower operating pressures. This paper discusses the properties of ion laser plasmas, in terms of both their external discharge parameters and their internal ion and excited state densities. The effect these properties have on laser operation is explained. Many interesting plasma effects, which are important in ion lasers, are given attention. Among these are discharge nonuniformity near tube constrictions, extremely high ion radial drift velocities, wall losses intermediate between ambipolar diffusion and free fall, gas pumping effects, and radiation trapping. The current status of ion laser technology is briefly reviewed

The prevention of electrical breakdown in spacecraft

Prevention of electrical breakdown in spacecraf

Electron and ion thermal forces in complex (dusty) plasmas

Expressions for the ion and electron thermal forces acting on a charged grain, suspended in a weakly ionized plasma subject to temperature gradients, are derived. The main emphasize is on the conditions pertinent to the investigations of complex (dusty) plasmas in gas discharges. Estimates show that for the electron temperature gradients $\sim {\mathcal O}$(eV/cm) typically encountered in laboratory gas discharges, the electron thermal force can become an important player among other forces acting on micron-size grains.Comment: 6 pages, 1 figur

Nonthermal Plasma Technology as a Versatile Strategy for Polymeric Biomaterials Surface Modification: A Review

In modern technology, there is a constant need to solve very complex problems and to fine-tune existing solutions. This is definitely the case in modern medicine with emerging fields such as regenerative medicine and tissue engineering. The problems, which are studied in these fields, set very high demands on the applied materials. In most cases, it is impossible to find a single material that meets all demands such as biocompatibility, mechanical strength, biodegradability (if required), and promotion of cell-adhesion, proliferation, and differentiation. A common strategy to circumvent this problem is the application of composite materials, which combine the properties of the different constituents. Another possible strategy is to selectively modify the surface of a material using different modification techniques. In the past decade, the use of nonthermal plasmas for selective surface modification has been a rapidly growing research field. This will be the highlight of this review. In a first part of this paper, a general introduction in the field of surface engineering will be given. Thereafter, we will focus on plasma-based strategies for surface modification. The purpose of the present review is twofold. First, we wish to provide a tutorial-type review that allows a fast introduction for researchers into the field. Second, we aim to give a comprehensive overview of recent work on surface modification of polymeric biomaterials, with a focus on plasma-based strategies. Some recent trends will be exemplified. On the basis of this literature study, we will conclude with some future trends for research

Probing photo-ionization: Experiments on positive streamers in pure gasses and mixtures

Positive streamers are thought to propagate by photo-ionization whose parameters depend on the nitrogen:oxygen ratio. Therefore we study streamers in nitrogen with 20%, 0.2% and 0.01% oxygen and in pure nitrogen, as well as in pure oxygen and argon. Our new experimental set-up guarantees contamination of the pure gases to be well below 1 ppm. Streamers in oxygen are difficult to measure as they emit considerably less light in the sensitivity range of our fast ICCD camera than the other gasses. Streamers in pure nitrogen and in all nitrogen/oxygen mixtures look generally similar, but become somewhat thinner and branch more with decreasing oxygen content. In pure nitrogen the streamers can branch so much that they resemble feathers. This feature is even more pronounced in pure argon, with approximately 10^2 hair tips/cm^3 in the feathers at 200 mbar; this density could be interpreted as the free electron density creating avalanches towards the streamer stem. It is remarkable that the streamer velocity is essentially the same for similar voltage and pressure in all nitrogen/oxygen mixtures as well as in pure nitrogen, while the oxygen concentration and therefore the photo-ionization lengths vary by more than five orders of magnitude. Streamers in argon have essentially the same velocity as well. The physical similarity of streamers at different pressures is confirmed in all gases; the minimal diameters are smaller than in earlier measurements.Comment: 28 pages, 14 figures. Major differences with v1: - appendix and spectra removed - subsection regarding effects of repetition frequency added - many more smaller change

Self-Organization in Cathode Boundary Layer Discharges

Cathode boundary layer (CBL) discharge, which has been developed as a UV light source, operates in a direct current between a planar cathode and a ring-shape anode that are separated by a dielectric with an opening of the same diameter as the anode. The nonthermal CBL discharges operate in a medium pressure range down to 30 Torr, emitting excimer radiation when operated with noble gases. The radiant excimer emittance at 172 nm in xenon reaches 1.7 W/cm2, and a maximum excimer efficiency of 6 % has been obtained. The high excimer radiant emittance, in addition to low cost and simple geometry compared to other UV sources, makes CBL discharges an excellent choice for deep UV lamps and a candidate for integrated flat UV panels (Moselhy et al. 2004). It has been found that CBL discharges spontaneously give rise to regularly arranged filaments, i.e., self-organization, at a low current, e.g., less than 0.2 mA at 75 Torr (Schoenbach et al. 2004). In this thesis, the self-organization of direct current xenon discharges in the CBL configuration and parallel-plate geometry have been studied for a pressure range from 30 to 140 Torr and currents from 20 μA to 1 mA. Comprehensive examinations have been performed to investigate the behavior of those filaments by the use of optical, electrical, and spectral measurements. Side-on and end-on observations of the discharges have provided information on axial structure and distance of the filaments from the cathode fall. The electrical measurement has recorded a discrete I-V characteristic associated with the change of the numbers of the filaments. The spectral measurement provides scaling information on the relative population of high-lying states (1s4, 1s5, and 2p6) of excited xenon atoms. Moreover, temperature measurement has revealed that the thermal electron emission from the cathode surface is negligible for the formation of filaments. The reactor geometry with parallel-plate electrodes analogously gives self-organization. The gas species, the cathode material, and the reactor geometry are varied to facilitate the understanding of the CBL xenon discharges and the self organization. When krypton is used instead of xenon, rather homogeneous plasma far from organized pattern formation is observed with decreasing current. Of the tested aluminum, copper, and tungsten cathodes, the aluminum cathode achieved higher excimer intensity at 250 Torr than that of the molybdenum cathode by a factor of two. The diameter of the plasma reactor was reduced to 300 μm, and it gave rise to a single filament, illuminating with an enhanced excimer power density of 500 mW/cm2 at 62 Torr. Three mechanisms of these self-organizations are given and discussed in this thesis. The first mechanism explains that the axial electric field can initiate instability. This instability is caused by N-shaped negative differential conductivity (NNDC) in the vicinity of negative glow, which is attributed to electron-electron collisions. Positive feedback of the current density and the electric field due to the NNDC causes fluctuation to develop. Another positive feedback effect of the gas temperature is that the first Townsend coefficient can become dominant as the current density increases. The filaments are assumed to undergo Coulomb force from the positively charged cathode fall channels and positive space charges on the surface of the surrounding dielectric spacer. The calculations, based on these assumptions on the Coulombic interactions, showed good agreement with experimental data. The second mechanism for the pattern formation is presented as to the development of Turing instability. The discussion is based on the idea that the regular arrangement of filaments is merely a result of general phenomena such as diffusion, ionization, or drift. A reaction-diffusion process with respect to the variation of local current density j and voltage v is the physical effect in the discharge. The numerical calculation was performed and obtained was a 2-D patterned structure exhibiting resemblance to the observed pattern. The transition time to self-organization was calculated to be 6.2 ms. A discussion of the dynamics of the fluctuations and the qualitative explanation for the pattern formation is presented. Benilov’s argument (2007) is introduced as the third mechanism. The standpoint of this discussion shows an analogy to that of the second mechanism. The potential distribution behaves as the standing wave described by the Helmholtz equation in the discharge area. In conclusion, applicability of these three mechanisms to self-organization is discussed and compared