High Electron Temperature Te Research Articles

Particle-in-cell and global simulations of α to γ transition in atmospheric pressure Penning-dominated capacitive discharges

Atmospheric pressure radio-frequency (rf) capacitive micro-discharges are of interest due to emerging applications, especially in the bio-medical field. A previous global model did not consider high-power phenomena such as sheath multiplication, thus limiting its applicability to the lower power range. To overcome this, we use one-dimensional particle-in-cell (PIC) simulations of atmospheric He/0.1% N2 capacitive discharges over a wide range of currents and frequencies to guide the development of a more general global model which is also valid at higher powers. The new model includes sheath multiplication and two classes of electrons: the higher temperature ‘hot’ electrons associated with the sheaths, and the cooler ‘warm’ electrons associated with the bulk. The electric field and the electron power balance are solved analytically to determine the time-varying hot and warm temperatures and the effective rate coefficients. The particle balance equations are integrated numerically to determine the species densities. The model and PIC results are compared, showing reasonable agreement over the range of currents and frequencies studied. They indicate a transition from an α mode at low power characterized by relatively high electron temperature Te with a near uniform profile to a γ mode at high power with a Te profile strongly depressed in the bulk plasma. The transition is accompanied by an increase in density and a decrease in sheath widths. The current and frequency scalings of the model are confirmed by the PIC simulations.

Heating of blue compact dwarf galaxies: gas distribution and photoionization by stars in I Zw 18

Photoionization models so far are unable to account for the high electron temperature Te([O III]) implied by the line ratio [O III]4363A/[O III]5007A in low-metallicity blue compact dwarf galaxies, casting doubts on the assumption of photoionization by hot stars as the dominant source of heating of the gas in these objects. Combinations of runs of the 1-D photoionization code NEBU are used to explore alternative models for the giant H II region shell I Zw 18 NW. Acceptable models are obtained, which represent schematically an incomplete shell comprising radiation-bounded condensations embedded in a low-density matter-bounded diffuse medium. The thermal pressure contrast between gas components is about a factor 7. The diffuse phase can be in pressure balance with the hot superbubble fed by mechanical energy from the inner massive star cluster. The failure of previous modellings is ascribed to (1) the adoption of an inadequate small-scale gas density distribution, which proves critical when the collisional excitation of hydrogen contributes significantly to the cooling of the gas, and possibly (2) a too restrictive implementation of Wolf-Rayet stars in synthetic stellar cluster spectral energy distributions. A neutral gas component heated by soft X-rays, whose power is less than 1% of the star cluster luminosity and consistent with CHANDRA data, can explain the low-ionization fine-structure lines detected by SPITZER. [O/Fe] is slightly smaller in I Zw 18 NW than in Galactic Halo stars of similar metallicity and [C/O] is correlatively large. Extra heating by, e.g., dissipation of mechanical energy is not required to explain Te([O III]) in I Zw 18. Important astrophysical developments are at stakes in the 5% uncertainty attached to [O III] collision strengths.

Electron energy distribution functions and the influence onfluorocarbon plasma chemistry

Two different modes of electron heating are found inmicrowavedischarges: the bulk heating mode characterized with low electrondensityne and high electron temperature Te (~10 eV), andthesurface heating mode with high ne and low Te (~3 eV). Thecorrelation between the heating mode and the electron energydistributionfunction (EEDF) is qualitatively interpreted in terms of non-localkinetictheory, taking account of the ambipolar potential well. A biasedoptical probediagnostics of a surface wave plasma (SWP) reveals that the surfaceheatingmode gives a bi-Maxwellian type EEDF, that is, a sum of two Maxwelliandistributions of bulk temperature Tb and tail temperatureTt>Tb. On theother hand, the EEDF of inductively coupled plasma (ICP) is close to asingle-Maxwellian distribution with electron temperature higher thanthe bulktemperature Tb of the SWP. Such differences in the EEDFs make thecomposition of the reactive species of the two plasmas different;namely, ionand radical measurements at the same electron density show that theICPcontains more F radicals and less CF3 and CF2radicals incomparison with the SWP. In addition, a simplified model based on thebi-Maxwellian EEDF shows how the EEDF determines the ion and radicalcompositions, supporting the major experimental results. Theseobservationsand calculations suggest that plasma chemistry is controllable bytailoringthe EEDF with proper adjustment of bulk heating and/or surface heatingofelectrons.

CO2-laser gas discharges in narrow gaps

We have studied RF discharges as excitation mechanisms for distributed feedback (DFB) CO2 lasers. For CO2 laser plasmas the reduced electric fieldE/N has to be in a well-defined range. The reduced electric fieldsE/N of gas discharges in the narrow gaps with widths of the order of 100 μm required for DFB are considerably above this range. In order to study the feasibility of these RF-excited discharges for DFB CO2 lasers we have measured the electron temperatureT e in their plasmas. From helium-line-intensity ratios we have deduced a lower limit of the electron temperatureT e of 4eV. The observed high intensities of bands of singly ionized nitrogen indicate an even higher electron temperature, but an efficient pumping of the upper laser level is not possible with an electron temperature above 2.5 eV. We have estimated the electron densityn e and the current densityj e from ratios of the intesities of forbidden and allowed helium lines. The high current densityj e is in the range of abnormal glow discharges. In the gas discharges between narrow gaps the electron oscillation amplitudex e is large than the electrode separationd. In order to replace the resulting high electron losses a high electron temperatureT e is necessary to sustain the gas discharge. Because of this high electron temperatureT e an efficient pumping of the upper laser level is not possible.