Dirk Uhrlandt

The Dark Side of Light: A Transdisciplinary Research Agenda for Light Pollution Policy

Although the invention and widespread use of artificial light is clearly one of the most important human technological advances, the transformation of nightscapes is increasingly recognized as having adverse effects. Night lighting may have serious physiological consequences for humans, ecological and evolutionary implications for animal and plant populations, and may reshape entire ecosystems. However, knowledge on the adverse effects of light pollution is vague. In response to climate change and energy shortages, many countries, regions, and communities are developing new lighting programs and concepts with a strong focus on energy efficiency and greenhouse gas emissions. Given the dramatic increase in artificial light at night (0 - 20% per year, depending on geographic region), we see an urgent need for light pollution policies that go beyond energy efficiency to include human well-being, the structure and functioning of ecosystems, and inter-related socioeconomic consequences. Such a policy shift will require a sound transdisciplinary understanding of the significance of the night, and its loss, for humans and the natural systems upon which we depend. Knowledge is also urgently needed on suitable lighting technologies and concepts which are ecologically, socially, and economically sustainable. Unless managing darkness becomes an integral part of future conservation and lighting policies, modern society may run into a global self-experiment with unpredictable outcomes.

Transport mechanisms of metastable and resonance atoms in a gas discharge plasma

Atoms in electronically excited states are of significant importance in a large number of different gas discharges. The spatio-temporal distribution particularly of the lower excited states, the metastable and resonance ones, influences the overall behavior of the plasma because of their role in the ionization and energy budget. This article is a review of the theoretical and experimental studies on the spatial formation and temporal evolution of metastable and resonance atoms in weakly ionized low-temperature plasmas. Therefore, the transport mechanisms due to collisional diffusion and resonance radiation are compared step by step. The differences in formation of spatio-temporal structures of metastable and resonance atoms in plasmas are attributed to these different transport mechanisms. The analysis is performed by obtaining solutions of the diffusion and radiation transport equations. Solutions of stationary and non-stationary problems by decomposition over the eigenfunctions of the corresponding operators showed that there is, on the one hand, an effective suppression of the highest diffusion modes and, on the other hand, a survival of the highest radiation modes. The role of the highest modes is illustrated by examples. In addition, the differences in the Green functions for the diffusion and radiation transport operators are discussed. Numerical methods for the simultaneous solution of the balance equations for metastable and resonance atoms are proposed. The radiation transport calculations consider large absorption coefficients according to the Lorentz contour of a spectral line. Measurements of the distributions of metastable and resonance atoms are reviewed for a larger number of discharge conditions, i.e. in the positive column plasma, afterglow plasma, constricted pulsed discharge, stratified discharge, magnetron discharge, and in a discharge with a cathode spot.

Spectroscopic investigation of the high-current phase of a pulsed GMAW process

While metal vapours have an important impact on the efficiency of the pulsed gas metal arc welding process, only a few papers are focused on this effect. In this paper, methods based on emission spectroscopy are performed to improve the understanding of the physical phenomena occurring during the high-current pulse. Boltzmann plots applied to iron lines, the Stark broadening of the 696.5 nm argon line and composition calculations assuming local thermodynamic equilibrium are used to determine characteristic parameters of the plasma. It is observed that the central part of the arc is composed mainly of iron. The percentage of iron increases quickly at the beginning of the high-current pulse, and slowly decreases when the central part broadens. During the high-current phase the temperature profile has a minimum value of around 8000 K at the axis of the arc while the argon envelope of the central part reaches temperatures of approximately 13.000 K. The high percentage of iron and the high radiation of the plasma at the centre can explain the measured shape of the temperature profile.

High-speed three-dimensional plasma temperature determination of axially symmetric free-burning arcs

In this paper we introduce an experimental technique that allows for high-speed, three-dimensional determination of electron density and temperature in axially symmetric free-burning arcs. Optical filters with narrow spectral bands of 487.5?488.5?nm and 689?699?nm are utilized to gain two-dimensional spectral information of a free-burning argon tungsten inert gas arc. A setup of mirrors allows one to image identical arc sections of the two spectral bands onto a single camera chip. Two-different Abel inversion algorithms have been developed to reconstruct the original radial distribution of emission coefficients detected with each spectral window and to confirm the results. With the assumption of local thermodynamic equilibrium we calculate emission coefficients as a function of temperature by application of the Saha equation, the ideal gas law, the quasineutral gas condition and the NIST compilation of spectral lines. Ratios of calculated emission coefficients are compared with measured ones yielding local plasma temperatures. In the case of axial symmetry the three-dimensional plasma temperature distributions have been determined at dc currents of 100, 125, 150 and 200?A yielding temperatures up to 20000?K in the hot cathode region. These measurements have been validated by four different techniques utilizing a high-resolution spectrometer at different positions in the plasma. Plasma temperatures show good agreement throughout the different methods. Additionally spatially resolved transient plasma temperatures have been measured of a dc pulsed process employing a high-speed frame rate of 33000 frames per second showing the modulation of the arc isothermals with time and providing information about the sensitivity of the experimental approach.

Low-pressure mercury-free plasma light sources: experimental and theoretical perspectives

The replacement of mercury in conventional fluorescent lamps by other components is highly desirable for environmental reasons. This paper gives a short review of new types of mercury-free plasma light sources operating at low pressure. In particular, the features of cylindrical glow discharges in rare-gas mixtures including xenon are discussed, focusing on the generation of the 147 nm resonance radiation of xenon and its transition into visible light by new phosphors with sufficient efficiency. Laser absorption and vacuum ultraviolet emission spectroscopy are applied for several rare-gas mixtures to reveal the contributions of the different gas components and their excited states to the power balance and radiation output. The experimental research is assisted by theoretical studies applying self-consistent hybrid models of the cylindrical column plasma. The good agreement between experimental and theoretical results obtained at selected discharge conditions makes it possible to predict optimum discharge parameters by means of extensive model calculations. It is demonstrated that about half of the efficacy of a mercury fluorescent lamp can be reached if the rare-gas discharge is operated at pressures below 100 Pa.

Strict calculation of electron energy distribution functions in inhomogeneous plasmas

The paper reports on strict calculations of the velocity distribution function and of related macroscopic quantities of the electrons in inhomogeneous plasmas. The kinetic studies are based on the numerical solution of the inhomogeneous, time-independent Boltzmann equation of the electrons including the action of a nonuniform electric field and of elastic and inelastic collisions of the electrons with the gas atoms in weakly ionized plasmas. In particular, the relaxation of plasma electrons in uniform electric fields, the response of the electrons to spatial disturbances of the electric field, the electron kinetic behaviour in the case of space charge field confinement by enclosing walls and the reaction of the electrons to strong fields as occurring in the region of moderate cathode falls are analysed. Especially, unexpectedly large spatial relaxation lengths and distinctly nonlocal properties of the electrons are found to be typical of the weakly ionized plasmas in the range of medium electric fields.

Novel non-equilibrium modelling of a DC electric arc in argon

A novel non-equilibrium model has been developed to describe the interplay of heat and mass transfer and electric and magnetic fields in a DC electric arc. A complete diffusion treatment of particle fluxes, a generalized form of Ohms law, and numerical matching of the arc plasma with the space-charge sheaths adjacent to the electrodes are applied to analyze in detail the plasma parameters and the phenomena occurring in the plasma column and the near-electrode regions of a DC arc generated in atmospheric pressure argon for current levels from 20 A up to 200 A. Results comprising electric field and potential, current density, heating of the electrodes, and effects of thermal and chemical non-equilibrium are presented and discussed. The current–voltage characteristic obtained is in fair agreement with known experimental data. It indicates a minimum for arc current of about 80 A. For all current levels, a field reversal in front of the anode accompanied by a voltage drop of (0.7–2.6) V is observed. Another field reversal is observed near the cathode for arc currents below 80 A.

Study of the welding gas influence on a controlled short-arc GMAW process by optical emission spectroscopy

The controlled short-arc processes, variants of the gas metal arc welding (GMAW) process, which have recently been developed, are used to reduce the heat input into the workpiece. Such a process with a wire feeding speed which varies periodically, using a steel wire and a steel workpiece to produce bead-on-plate welds has been investigated. As welding gases CO2 and a mixture of Ar and O2 have been used. Depending on the gas, the properties of the plasma change, and as a consequence the weldseams themselves also differ distinctly. Optical emission spectroscopy has been applied to analyse the plasma. The radial profiles of the emission coefficients of an iron line and an argon line or an atomic oxygen line, respectively, have been determined. These profiles indicate the establishment of a metal vapour arc core which has a broader profile under CO2 but is more focused in the centre for argon. The measured iron line emission was near to its norm maximum in the case of CO2. From this fact, temperatures around 8000 K and a metal vapour molar fraction above 75% in the arc centre could be roughly estimated for this case. Estimations of the electrical conductivity and the arc field indicate that the current path must include not only the metal vapour arc core but also outer hot regions dominated by welding gas properties in the case of argon.

Account of near-cathode sheath in numerical models of high-pressure arc discharges

Three approaches to describing the separation of charges in near-cathode regions of high-pressure arc discharges are compared. The first approach employs a single set of equations, including the Poisson equation, in the whole interelectrode gap. The second approach employs a fully non-equilibrium description of the quasi-neutral bulk plasma, complemented with a newly developed description of the space-charge sheaths. The third, and the simplest, approach exploits the fact that significant power is deposited by the arc power supply into the near-cathode plasma layer, which allows one to simulate the plasma–cathode interaction to the first approximation independently of processes in the bulk plasma. It is found that results given by the different models are generally in good agreement, and in some cases the agreement is even surprisingly good. It follows that the predicted integral characteristics of the plasma–cathode interaction are not strongly affected by details of the model provided that the basic physics is right.

Comparing two non-equilibrium approaches to modelling of a free-burning arc

Two models of high-pressure arc discharges are compared with each other and with experimental data for an atmospheric-pressure free-burning arc in argon for arc currents of 20–200 A. The models account for space-charge effects and thermal and ionization non-equilibrium in somewhat different ways. One model considers space-charge effects, thermal and ionization non-equilibrium in the near-cathode region and thermal non-equilibrium in the bulk plasma. The other model considers thermal and ionization non-equilibrium in the entire arc plasma and space-charge effects in the near-cathode region. Both models are capable of predicting the arc voltage in fair agreement with experimental data. Differences are observed in the arc attachment to the cathode, which do not strongly affect the near-cathode voltage drop and the total arc voltage for arc currents exceeding 75 A. For lower arc currents the difference is significant but the arc column structure is quite similar and the predicted bulk plasma characteristics are relatively close to each other. (Some figures may appear in colour only in the online journal)