Gas Heating Mechanism Research Articles

Gas heating mechanisms in capacitively coupled plasmas

Capacitively coupled plasma (CCP) tools utilized for plasma etching of dielectric features utilize large amounts of power for processing. As a result, neutral gas heats up significantly during processing. The resulting gas density variations across the reactor can affect reaction rates, radical densities, plasma characteristics and uniformity within the reactor. In this paper, results from a two-dimensional computational investigation of an Ar/CF4 CCP discharge incorporating an energy equation solution for all ions and neutrals are discussed. The dominant neutral gas heating process is identified to be elastic collisions with ions while conduction is found to be the major mechanism of heat transport. Some species such as F and CF3 demonstrate higher temperatures than the feedstock gases owing to additional heating via charge-exchange reactions and/or Franck–Condon heating. Typical process parameters such as pressure, frequency of excitation, power and gas composition are varied to investigate their impact on gas temperature. At higher excitation frequency and/or pressure, increased elastic collisions with ions lead to greater heat generation. The heat generated per molecule of the radicals, however, decreases with increase in pressure leading to a decrease in gas temperature. The increase in neutral collision frequencies with pressure also results in the decrease in temperature difference between species in the plasma. As CF4 fraction increases, both the elastic collision cross-section and Franck–Condon heating sources increase, leading to higher gas temperatures.

Simulation studies of RF excited micro-cavity discharges for micro-propulsion applications

A detailed computational modelling study of the micro-cavity discharge (MCD) thruster is presented. The MCD thruster concept incorporates a microdischarge with dielectric covered electrodes operated using alternating current (ac) excitation. The thruster geometry comprises a constant area pipe section followed by a divergent micro-nozzle. Two ring electrodes are embedded in the wall of the pipe section with the downstream electrode close to the pipe-micronozzle intersection. A microdischarge plasma is generated in argon propellant gas flowing through the thruster. A detailed plasma dynamics model coupled with the compressible Navier–Stokes equations is used to study the flow and plasma phenomenon in the thruster. Results show a highly pulsed microdischarge with plasma densities of ∼1019 m−3 and current densities ∼700 mA cm−2 for an ac excitation in the radio frequency (RF) regime of 10 and 20 MHz. The dominant gas heating mechanism in these discharges is through ion Joule heating. Higher electron densities and spatially dominant thermal source terms are observed for the 20 MHz excitation compared with 10 MHz excitation. The addition of 20% nitrogen to the flow resulted in much better performance compared with the pure argon cases. A peak gas temperature rise ∼200 K is seen for a cycle-averaged power deposition of 76 mW. For the conditions explored in this study, the overall specific impulse of the thruster operating with the microdischarge plasma is found to be about 25% higher than a corresponding cold gas case.

The effects of X-ray and UV background radiation on the low-mass slope of the galaxy mass function

Even though the dark-matter power spectrum in the absence of biasing predicts a number density of halos n(M) ~ M^-2 (i.e. a Schechter alpha value of -2) at the low-mass end (M < 10^10 M_solar), hydrodynamic simulations have typically produced values for stellar systems in good agreement with the observed value alpha ~ -1. We explain this with a simple physical argument and show that an efficient external gas-heating mechanism (such as the UV background included in all hydro codes) will produce a critical halo mass below which halos cannot retain their gas and form stars. We test this conclusion with GADGET-2-based simulations using various UV backgrounds, and for the first time we also investigate the effect of an X-ray background. We show that at the present epoch alpha is depends primarily on the mean gas temperature at the star-formation epoch for low-mass systems (z <~ 3): with no background we find alpha ~ -1.5, with UV only alpha ~ -1.0, and with UV and X-rays alpha ~ -0.75. We find the critical final halo mass for star formation to be ~4x10^8 M_solar with a UV background and ~7x10^8 M_solar with UV and X-rays.

TheHerschelview of GAS in Protoplanetary Systems (GASPS)

The Herschel GASPS Key Program is a survey of the gas phase of protoplanetary discs, targeting 240 objects which cover a large range of ages, spectral types, and disc properties. To interpret this large quantity of data and initiate self-consistent analyses of the gas and dust properties of protoplanetary discs, we have combined the capabilities of the radiative transfer code MCFOST with the gas thermal balance and chemistry code ProDiMo to compute a grid of 300 000 disc models (DENT). We present a comparison of the first Herschel/GASPS line and continuum data with the predictions from the DENT grid of models. Our objective is to test some of the main trends already identified in the DENT grid, as well as to define better empirical diagnostics to estimate the total gas mass of protoplanetary discs. Photospheric UV radiation appears to be the dominant gas-heating mechanism for Herbig stars, whereas UV excess and/or X-rays emission dominates for T Tauri stars. The DENT grid reveals the complexity in the analysis of far-IR lines and the difficulty to invert these observations into physical quantities. The combination of Herschel line observations with continuum data and/or with rotational lines in the (sub-)millimetre regime, in particular CO lines, is required for a detailed characterisation of the physical and chemical properties of circumstellar discs.

One-Dimensional Fluid Simulation of Atmospheric-Pressure Helium DC Glow Discharges

Atmospheric-pressure glow discharges (APGDs) have been widely used for material processing such as etching, deposition and surface modification and biomedical applications because of the material processing without thermal damage, discharge uniformity over the large area, and so on. In order to suitably utilize APGDs for these applications, further understanding of fundamental properties in APGDs is strongly required. In this paper, the APGDs in helium produced by DC power supply have been simulated using a spatially one-dimensional fluid model. The structure of discharges can be distinguished into five specific regions: a cathode fall, a negative glow, a Faraday dark space, a positive column, and an anode fall. The structure is similar to that of typical low-pressure dc glow discharges. Reasonably good agreement between measured and calculated current-voltage characteristics can be obtained in the present current range (∼10 mA). The discharge-current dependence of the spatial structure, and the mechanism of gas-heating and power-consumption are also discussed.

Massive molecular outflows associated with UCHII/HII regions

Received date ; Accepted date Abstract. Aims: We searched for the molecular outflows from fifteen mole cular clouds associated with ultra-compact and compact HII (UCHII/HII) regions and discussed possible gas heating mechanism. Methods: Mapping observations of CO J = 3− 2 and J = 2− 1 lines were carried out with the KOSMA 3m-Telescope towards the 15 HII regions/molecular cloud complexes. Results: Ten molecular outflows were identified o ut of the fifteen HII region /molecular cloud complexes. The higher outflow detection rate (67%) suggested that such outflows are as common in high mass star forming regions as those in low mass star forming regions, which is consistent with the results o f other authors. The observations also showed that the outflo w might occur in the HII region. The integrated CO line intensity ratios (RICO(3−2)/ICO(2−1) ) were determined from the core component of the spectra as well as from both the blue and red wings. Maximum line intensity ratios from the wings and core components appeared to be related to the mid-infrared sources imaged by Midcourse Space Experiment (MSX). The relationship between the maximum line intensity ratios and MSX sources indicates that the molecular gas could be heated by the emission of dust associated with massive stars. Based on maser observations reported in the literature, we found that H 2O masers were only detected in seven regions. The H2O masers in these regions are located near the MSX sources and within the maximum line intensity ratio regions, suggesting that H 2O masers occur in relatively warm environments.

Modeling of direct current micro-plasma discharges in atmospheric pressure hydrogen

Numerical simulations and experimental studies were conducted to characterize direct current (dc) hydrogen discharge for a pin plate electrode configuration having an inter-electrode separation distance of 400 µm. A self-consistent two-dimensional hybrid model was developed to simulate the atmospheric pressure dc hydrogen micro-discharges. The discharge simulation model considered consists of momentum and energy conservation equations for a multi-component gas mixture, conservation equations for each component of the mixture (electrons, ions, excited species and neutrals) and state relations. The model uses a drift–diffusion approximation for the electron and the ion fluxes. The species considered include H, H2, H+, , , , H2 v=1 and the electrons. The electric field is obtained from the solution of Poisson's equation. Numerical simulations and experimental measurements indicated some of the key features of a normal glow discharge: flat voltage–current characteristics and constant cathode current density. Basic plasma properties such as electron number density, gas temperature, electric field and electron temperature were studied. The model predicted a constant current density of ∼22 A cm−2 in the normal glow regime. The normal current density was found to be a temperature scaled value of a low pressure normal current density. The ion Joule heating and Frank–Condon heating were found to be the dominant gas heating mechanisms. The peak gas temperature of ∼500 K indicated the discharge to be a non-thermal non-equilibrium discharge. Predictions from the model compares favorably well with the experimental measurements.

Impact of stochastic gas motions on galaxy cluster abundance profiles

The impact of stochastic gas motions on the metal distribution in cluster core is evaluated. Peaked abundance profiles are a characteristic feature of clusters with cool cores and abundance peaks are likely associated with the brightest cluster galaxies (BCGs) which dwell in cluster cores. The width of the abundance peaks is however significantly broader than the BCG light distribution, suggesting that some gas motions are transporting metals originating from within the BCG. Assuming that this process can be treated as diffusive and using the brightest X-ray cluster A426 (Perseus) as an example, we estimate that a diffusion coefficient of the order of $2 10^{29} {\rm cm^2 s^{-1}}$ is needed to explain the width of the observed abundance profiles. Much lower (higher) diffusion coefficients would result in too peaked (too shallow) profiles. Such diffusion could be produced by stochastic gas motions and our analysis provides constraints on the product of their characteristic velocity and their spatial coherence scale. We speculate that the activity of the supermassive black hole of the BCG is driving the stochastic gas motions in cluster cores. When combined with the assumption that the dissipation of the same motions is a key gas heating mechanism, one can estimate both the velocity and the spatial scale of such a diffusive processes.

C II] emission and star formation in late-type galaxies

We study the relationship between gas cooling via the [C II] (A = 158 μm) line emission and dust cooling via the far-IR continuum emission on the global scale of a galaxy in normal (i.e. non-AGN dominated and non-starburst) late-type systems. It is known that the luminosity ratio of total gas and dust cooling, L C I I /L F I R , shows a non-linear behaviour with the equivalent width of the Hα (A = 6563 A) line emission, the ratio decreasing in galaxies of lower massive star-formation activity. This result holds despite the fact that known individual Galactic and extragalactic sources of the [C II] line emission show different [C II] line-to-far-IR continuum emission ratios. This non-linear behaviour is reproduced by a simple quantitative theoretical model of gas and dust heating from different stellar populations, assuming that the photoelectric effect on dust, induced by far-UV photons, is the dominant mechanism of gas heating in the general diffuse interstellar medium of the galaxies under investigation. According to the model, the global L C I I /L F I R provides a direct measure of the fractional amount of non-ionizing UV light in the interstellar radiation field and not of the efficiency of the photoelectric heating. The theory also defines a method to constrain the stellar initial mass function from measurements of L C I I and L F I R . A sample of 20 Virgo cluster galaxies observed in the [C II] line with the Long Wavelength Spectrometer on board the Infrared Space Observatory is used to illustrate the model. The limited statistics and the necessary assumptions behind the determination of the global [C II] luminosities from the spatially limited data do not allow us to establish definitive conclusions but data-sets available in the future will allow tests of both the reliability of the assumptions behind our model and the statistical significance of our results.

Spatial distributions of translational and vibrational temperatures of molecules in a waveguide CO2laser plasma

Methods developed for diagnostics of the active media of waveguide carbon dioxide lasers at elevated pressures and input energies in a capillary gas discharge were used to study the mechanism of gas heating and pumping of vibrational levels in a CO2–N2–He (1:1:8) mixture. As a result of spectroscopic experiments with an actual spatial resolution of ~0.2 mm, radial distributions of the translational and vibrational temperatures of the molecules were reconstructed as a function of changes in the discharge conditions. The results could be used to establish the mechanism of gas heating in a capillary discharge of a waveguide CO2 laser.