High-pressure Discharge Research Articles

Ignition of High-Pressure Discharge Lamps Supported by Microdischarges

Auxiliary ignition wires are a well-known tool to support the ignition of high-pressure discharge lamps. The effect of these wires in cylindrical high-pressure discharge lamps made from polycrystalline alumina is visualized by applying high-voltage pulses of a spark tester to one of the electrodes. Microdischarges are observed, which are capable to support lamp ignition.

Experimental and Theoretical Wall Temperatures of Hg HID Lamps

Thermography is an established experimental method to measure wall temperatures of high-pressure discharge lamps resulting in 2-D images. However, theoretical models, including the heat balance of the tube wall, are usually based on a cylinder symmetric 2-D model, where the tube wall surface is represented by a 1-D line. In this paper, a comparison between experimental and theoretical results are presented, where a 2-D thermographic image is reconstructed from the model calculations. This allows visualizing and discussing of some capabilities and limitations of the theoretical model.

Radio frequency emission from high-pressure xenon arcs: A systematic experimental analysis of the underlying near-anode plasma instability

High-pressure Xe discharge lamps at DC operation can show unwanted strong RF (radio-frequency) emission to beyond 1 GHz, correlated to a sharp periodic lamp-voltage instability in the near-anode plasma with a pulse repetition rate ε of 1–10 MHz. The physical origin of the instability is unclear. Here, its existence and pulse rate have been measured as a function of arc current I = 0.2–1.2 A and anode temperature Ta = 1700–3400 K independently, in experimental lamps with pure-tungsten electrodes and a Xe operating pressure around p = 10 MPa. Surprisingly, the instability is not affected by I or current density j but exists if Ta is lower than a threshold value around 2800–2900 K. The pulse rate ε is simply a rising linear function of the inverse anode temperature 1/Ta, with only a small I-dependent correction. The average anode heat load is slightly lower in the unstable regime and possibly depends on ε. The results allow a consistent re-interpretation of earlier and present experimental observations and should be both a valuable help in practical lamp engineering and a tight constraint for future theories of this effect.

Optical emission characteristics of medium- to high-pressure N2 dielectric barrier discharge plasmas during surface modification of polymers

The authors measured the band spectra (first and second positive systems) of the nitrogen molecule by optical emission spectroscopy with an aim to understand the mechanism of surface processing by medium- to high-pressure dielectric barrier discharge (DBD) plasmas. The experimentally measured and calculated spectra were compared to determine the vibrational and rotational temperatures of the N2 (C3Πu) state in the generated plasmas. The authors generated the N2 DBD plasmas at a driving frequency of 1–7 kHz and a discharge pressure of 20–105 Pa for the surface modification of a polyethylene terephthalate (PET) sample. It was found that the vibrational temperature was greatly affected by the N2 pressure while the rotational temperature remained constant in the N2 pressure range of 20–105 Pa. The emission intensity of N2 first positive system (B3Π → A3Σ) rapidly decreased at an increasing N2 pressure due to the collisional relaxation process of the B3Π state with N2 molecules. The N2+(B2Σu+→X2Σg+) radiative transition was observed in the low-pressure DBD plasmas, which was attributed to the direct electron impact ionization of N2 molecules. The surface characterizations of treated PET samples by contact angle measurement and atomic force microscopy indicate that the low-pressure N2 DBD plasma is an effective method for the surface modification of polymers. Analysis indicates the plasma characteristics such as electron temperature and ion energy are mainly dependent on the N2 pressure, which turn to determine the surface properties of treated PET samples.

Positive streamer formation in cathode region of pulsed high-pressure discharges for transversely excited atmosphere laser applications

A two-dimensional model of a high-pressure cathode region with nonuniform secondary electron emission has been used for conditions close to those used in pulsed discharge XeCl lasers. To simulate processes leading to a cathode spot formation, an inhomogeneity in the cathode electron emission was introduced by varying the electron emission in a 100 μm-radius cathode surface region. The results obtained indicate that the inhomogeneous cathode emission can induce an instability in the cathode-faced surface of the bulk discharge plasma leading to the development of a cathode-directed streamer inside of the cathode region.

Analysis of Discharge Parameters in Xenon-Filled Coaxial DBD Tube

In this paper, a xenon-filled coaxial dielectric barrier discharge (DBD) has been studied to understand the high-pressure nonequilibrium nonthermal plasma discharge. A quartz coaxial DBD tube (ID: 6 mm, OD: 12 mm) at 400-mbar xenon-filled pressure has been used in the experiment. A unipolar pulselike voltage up to a-6-kV peak working at 30 kHz has been applied to the discharge electrodes for the generation of microdischarges. A single discharge is observed per applied voltage pulse. Visual images of the discharge and electrical waveform confirm the diffused-type discharges. The knowledge obtained by dynamic processes of DBDs in the discharge gap explains quantitatively the mechanism that is obtained in the ignition, development, and extinction of DBDs. The behavior of different discharge parameters has also been analyzed. From the experimental results and equivalent electrical circuit, the dynamic nature of equivalent capacitance has been reported. The relative intensity analysis of the Xe peak in the optical emission spectra (172 nm) has also been carried out for different supplied powers, and it is found that the radiation power has increased with supplied power.

Low-temperature synthesis of microcrystalline 3C-SiC film by high-pressure hydrogen-plasma-enhanced chemical transport

The synthesis of microcrystalline 3C-SiC films on glass substrates at relatively low temperatures (⩽600 °C) by the plasma-enhanced chemical transport method was carried out using a high-pressure pure hydrogen glow discharge. This method used the chemical erosion products of graphite and silicon in the hydrogen plasma as the deposition source. The temperature dependence of the etching rate of graphite and the generated volatile C-species were investigated by exposing sintered graphite to a pure hydrogen plasma at 100 Torr. Infrared absorption gas analysis indicated that the C-related film precursor generated from the graphite was mainly CH4. The etch rate of graphite reached a maximum at a sample temperature of 200 °C. The deposition rate of the SiC film prepared at a hydrogen pressure of 200 Torr increased monotonically from 14 to 29 nm min−1 with an increase in substrate temperature (Tsub) from 100 to 600 °C. The Si/C composition ratio in the films was independent of Tsub and all the film compositions were nearly stoichiometric. The prepared SiC films were microcrystalline 3C-SiC. Raman spectroscopy indicated that the film quality significantly depends on Tsub. The electrical conductivity of the films at room temperature was found to increase exponentially from 3 × 10−4 to 1.7 S cm−1 with Tsub.

Mercury-free high pressure discharge lamps dominated by molecular radiation

High intensity discharge (HID) lamps dominated by molecular radiation offer a very promising alternative for use in future light sources. They are able to deliver competitive efficacies of about 110 lm W−1 and higher, excellent colour rendering index above 90 and a correlated colour temperature in the 3000–4000 K region at the operating point near the Planckian locus. Moreover, these lamps are opening up the possibility of dimming. Due to the fact that they are able to omit mercury they are environmentally friendly.The emission spectra generated by these HID lamps differ significantly from those of conventional lamps. The reason for this is the dominance of molecular radiation processes. In comparison with conventional HID lamps atomic contributions are usually rather small. In the present case they amount to less than about 10% of the total intensity in the visible range.

An Alternative Optical Method for Acoustic Resonance Detection in HID Lamps

Acoustic resonances are observed in high-pressure discharge lamps operated with ac input modulated power frequencies in the kilohertz range. This paper describes an optical resonance detection method for high-intensity discharge lamps using computer-controlled cameras and image processing software. Experimental results showing acoustic resonances in high-pressure sodium lamps are presented.

Dynamic model of streamer coupling for the homogeneity of glowlike dielectric barrier discharges at near-atmospheric pressure

A streamer coupling theory is developed to describe the formation of homogenous emission and the high propagation speed of emission patterns in near-atmospheric pressure discharges. By considering the effects of both electron diffusion and electronic drift in the streamer head, the minimum required preionization level n(min) for the formation of streamer coupling is found to be dependent on electric field strength, gas pressure, and electron temperature. The final stage of discharge is a microdischarge, when the preionization level n(0) is smaller than n(min). However, when n(0) is larger than n(min), streamers can couple to each other and form a glowlike discharge, and the homogeneity and propagation speed of the emission pattern in the streamer coupling head increases with the preionization level. The streamer coupling model can also be possibly used to explain many phenomenon in near-atmospheric pressure discharges, such as the bulletlike luminous discharge when atmospheric pressure plasma jets eject into ambient air.

Origin of surface microwave discharges

The high-pressure gas discharge initiated at the focus of pulsed powerful microwave radiation is a branched network of thin plasma channels completely filling the volume of the focal zone. A detailed experimental investigation of such discharges is carried out with a setup operating at a wavelength of 8.9 cm and at pressures ranging from 0.05 to 1.00 atm. When the initiator is placed on a dielectric plate, the discharge channels are found to propagate solely over the surface of the dielectric. It is shown that electrodynamic interaction between the gas discharge and dielectric is the main reason for the surface propagation of the discharge.

The influence of the discharge parameters on the plasma spatial structuring in argon DBDs

The plasma parameters, discharge plasma uniformity and filamentation processes in high pressure (near atmospheric pressure) dielectric barrier discharges (DBD) in argon are studied using the developed two-dimensional 2D(r, z) model. The applied voltage frequency, the voltage shape, the dielectric layers material and its thickness are varied and the effects of such variations on plasma uniformity, discharge structure and operation are studied. The DBD discharges with different dielectric layers thickness, dielectric constants and secondary electron emission coefficients are simulated. It was shown that the dielectric layer thickness is an important parameter for producing high pressure discharges uniform over the radius. The possibility of the radially uniform discharges at atmospheric pressure was shown in the present study.

Comparison of visible and infrared spectrum of light sources

We compared the visible and the infrared spectrum of cesium and the sodium high pressure discharge light sources of 70 W power, run at different voltages from 180 to 240 V. Although the cesium discharge lamp exhibits remarkable white light in the visible spectrum, it suffers from the large infrared radiation intensity. The study of the causes of large infrared losses and their possible reduction was the main motivation for the present investigation. Sodium discharge plasma appears to be more efficient light source than the cesium discharge plasma due to the smaller infrared emission, although the latter has much better colour rendering index.

Experimental and theoretical investigations on the warm-up of a high-pressure mercury discharge lamp

Modern high-pressure discharge lamps are forced to provide instant light and hot relight capabilities—if possible at lower power units. A detailed understanding of the warm-up of high-pressure discharge lamps is therefore required. Complex fluid model codes were developed for the past years including more and more processes like two-dimensional treatment of convection trying to provide a more comprehensive and consistent description of high-pressure discharge lamps. However, there is a lack of experimental data to examine the performance of these models. This work provides a very complete set of geometrical, electrical, spectroscopic, and thermographic data according to the warm-up of a high-pressure mercury discharge lamp that is compared to the results of a state of the art fluid code. Quantitative agreement is achieved for single parameters like wall temperatures. But the paper also reveals the need for further investigations and improvements of the code.

Model of High-Pressure Ablative Capillary Discharge for Plasma Thrusters

A zero-dimensional time-dependent high-pressure slab capillary discharge model with capillary wall thermal conduction and radiation absorption is presented. The model includes a resistor-inductor-capacitor circuit and a heat transfer radiation model based on a radiation database constructed using a commercially available radiation software, to calculate the radiation heat flux output from a uniform plasma slab. The model also includes a model of the thin transition boundary layer between the uniform plasma core and the ablative capillary walls. This transition boundary-layer model is used to obtain the boundary conditions connecting the plasma core parameters with the parameters at the ablative surface to calculate the thermal and radiation heat fluxes at the capillary walls. The radiation wall absorption coefficient is assumed to be a constant independent of wavelength and wall temperature andisaninputparameterofthemodel.Thewallthermalconductivityisalsoassumedtobeaconstantindependentof the wall temperature. Thus, the model self-consistently calculates plasma parameters of the capillary discharge and distribution of wall temperature vs time. The model is used to model high-pressure ablative capillary discharge for plasma thrusters.

Study of photometric properties of high-pressure mercury discharge with thallium iodide additives (HgTℓI) using the ray-tracing method

The photometric properties of HgTℓI high intensity discharge (HID) lamps have been calculated by solving the radiation transport equations in the discharge, using the ray-tracing method. The partial pressures of the various chemical species in the plasma are computed assuming the plasma is in local thermodynamic equilibrium (LTE), using a curve fit to an experimentally measured temperature profile. The plasma is also assumed to be fully mixed, implying a constant mercury/thallium ratio throughout the discharge tube. The spectral line broadening includes neutral Van der Waals and charged particle Stark broadening. The potential influence of ‘de-mixing’ on the spectra is also considered.

The effect of He plasma treatment on properties of organosilicate glass low-k films

The effect of low-pressure He plasma on properties of nanoporous organosilicate glasses low-k films with 24% and 33% open porosity is studied. The influence of ions, VUV radiation, and metastable atoms are extracted separately using a special experimental system designed for this purpose. The low-k films treated in He plasma were exposed to O or H atoms in the downstream of high-pressure O2 or H2 rf discharge. The changes in chemical composition and structure occurring in low-k films were measured before and after all treatments. The loss probabilities of oxygen and hydrogen atoms on the low-k film surface were measured for both treated and pristine films. It is shown that the film pretreatment in He plasma leads to the noticeable densification of the top surface layer up to complete sealing all the films studied. The sealing layer prevents O atoms from deep penetration to the film bulk and carbon extraction. The sealing mechanism related to the joint impact of low-energy ions and VUV photons with metastable atoms in He plasma is discussed in detail.

Study of the effect of a silver nanoparticle seeding layer on the crystallisation temperature, photoinduced hydrophylic and catalytic properties of TiO2 thin films deposited on glass by magnetron sputtering

The optimization of the photocatalytic activity and hydrophilic conversion of TiO2 is an active research area in the field of self-cleaning materials and energy storage/conversion. One major focus is the crystalline phase of TiO2, known to be the most efficient of the anatase structures. Another issue is the decoration of TiO2 with noble metals, which act as charge carrier traps for electrons. The latter hinders or reduces the electron–hole recombination rate and often leads to a more efficient photocatalytic activity.In this paper, we describe how an interlayer consisting of 3–4nm silver nanoparticles (Ag-NPs) promotes TiO2 anatase crystallisation and has a positive effect on the photoinduced catalytic and hydrophylic properties of TiO2 thin films. Ag-NPs and TiO2 were deposited by magnetron sputtering in the same reactor in a two-step process: a) condensation of Ag-NPs produced in the gas phase thanks to a high-pressure discharge, and b) conventional TiO2 magnetron deposition in oxide mode. Four temperatures from RT to 288°C were investigated and film thickness was 80nm.Particle size and film structure were determined by TEM, HRTEM and XRD. Photocatalytic activities of the samples were tracked by the evaluation of the surface hydrophilicity after UV illumination and 24hours post-illumination, and by UV-induced palmitic acid degradation.

Transition to the obstructed discharge and a sharp change in the voltage-current characteristic as a result of gas heating in a short (positive-column-free) high-pressure glow discharge

The formation of a high-pressure glow discharge is studied under conditions when the discharge evolves from the normal glow to the abnormal glow. It is shown that the transition to the voltage-current characteristic of the obstructed discharge may take place as a result of heating the gas and a decrease in its density. The obstructed discharge follows the left-hand branch of the Paschen curve and features a sharp voltage rise and current density limitation.

Diagnostics of High‐Pressure Arc Plasmas by Hydrogen Balmer Spectra

AbstractWe re‐examine several Balmer spectra obtained from high‐pressure arc discharges (Radtke and Günther, Contrib. Plasma Phys. 26, 143 (1986); Radtke, Günther, and Spanke, ibid. 26, 151 (1986)). By matching synthetic spectra to the experimental line profiles, we infer plasma parameters such as density, temperature, and composition. Spectral line modeling is based on a microscopic quantum statistical approach. Corrections accounting to the merging lines near the threshold are describe following a model of D'yachkov. Self‐absorption is clearly visible in the spectra and accounted for in one‐dimensional model assuming thermal equilibrium. A consistency check is performed by comparing to the broadening of silicon impurity lines, where present in some of the spectra. Furthermore, a comparison is made with predictions from a fluid variational model for the composition and the equation of state of the plasma (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)