I Koleva

Spatially resolved diagnostics of an atmospheric pressure direct current helium microplasma

Optical emission spectroscopy measurements were performed with added trace probe gases in an atmospheric pressure direct current helium microplasma. Spatially resolved measurements (resolution ~6 µm) were taken across a 200 µm slot-type discharge. Gas temperature profiles were determined from N2 emission rotational spectroscopy. Stark splitting of the hydrogen Balmer-β line was used to investigate the electric field distribution in the cathode sheath region. Electron densities were evaluated from the analysis of the spectral line broadening of Hβ emission. The gas temperature was between 350 and 550 K, peaking nearer the cathode and increasing with power. The electron density in the bulk plasma was in the range (4–7) × 1013 cm−3. The electric field peaked at the cathode (~60 kV cm−1) and decayed to small values over a distance of ~50 µm (sheath edge) from the cathode. These experimental data were generally in good agreement with a self-consistent one-dimensional model of the discharge.

Plasma diagnostics by optical emission spectroscopy on argon and comparison with Thomson scattering

A novel optical emission spectroscopy (OES) technique for the determination of electron temperatures and densities in low-pressure argon discharges is compared with Thomson scattering (TS). The emission spectroscopy technique is based on the measurement of certain line ratios in argon and a collisional‐radiative model (CRM) including metastable transport. The investigations are carried out in a planar inductively coupled neutral loop discharge (NLD) over a wide range of pressures, p = 0.05Pa‐5Pa. This discharge is a weakly magnetized novel radio-frequency (rf) plasma source, proposed for plasma etching. The NLD is operated in pure argon at a frequency of f = 13.56MHz and powers varied between P = 1kW and 2kW. Both diagnostics, OES and TS, are applied in parallel. The electron energy distribution functions obtained by TS are clearly Maxwellian at low pressures but exhibit a certain enhancement of the energetic tail at higher pressures. Electron densities and temperatures obtained by both diagnostic techniques are compared. Further, absolute numbers of the metastable densities derived from the measurement by the CRM are compared with earlier measurements under similar conditions. Excellent agreement is found throughout if depletion of the neutral gas density by increasing gas temperature and electron pressure is included in the CRM. Electron pressure is the dominant depletion mechanism at gas pressures p 0.1Pa and rf powers P> 1kW. There, the electron pressure exceeds more than 3 times the neutral pressure and the ionization degree approaches 7% while at pressures p> 1Pa the degree of ionization is relatively low (<10 −3 ) and neutral gas depletion is dominated by gas heating. (Some figures in this article are in colour only in the electronic version)

Small surface wave discharge at atmospheric pressure

A small surface wave driven source produces plasma at atmospheric pressure. Microwave power at frequency 2.45 GHz is coupled with the source and a discharge is ignited at power levels below 10 W. The coaxial exciter of the surface waves has a length of 10 mm because its dielectric is a high permittivity discharge tube. The plasma source operates as a plasma jet in the case of plasma columns longer than the tube length. The source maintains stable plasma columns over a wide range of neutral gas flow and applied power in continuous and pulse regimes. An additional advantage of this source is the discharge self-ignition. An electron temperature of Te ~ 1.9 eV and a density of ne ~ 3.9 × 1014 cm−3 are estimated by the probe diagnostics method. The emission spectra in the wavelength range 200–1000 nm under different experimental conditions are analysed and they prove the applicability of the source for analytical spectroscopy. The dependences of column length, reflected power and plasma parameters on the gas flow and the input power are discussed.

On the two modes of operation of planar-coil-driven inductive discharges in hydrogen

Langmuir probe and laser photodetachment technique diagnostics as well as optical emission spectroscopy and phase-resolved optical emission spectroscopy are used to study a planar-coil-driven inductive discharge in hydrogen sustained in the first chamber of a two-chamber plasma source. The discharge is operated in the power range 50?400?W at 27?MHz. The gas-pressure range studied is p?=?20?60?mTorr. The results obtained over the first half of the discharge length, that is starting from the position of the coil, outline discharge maintenance in the two modes of the inductive discharge. The inductive mode sustained at a high rf power appears with rf power deposition by ring-shaped rf electric field intensity close to the coil and high electron density there, followed by its strong drop in the remote plasma region. The capacitive mode sustaining low-density plasmas also appears with two regions specified by different mechanisms of rf power deposition: plasma heating by an electron beam acceleration in the wall sheath during its expansion and Joule heating in the plasma bulk. The similarity?at high and low rf power?in the axial structure of the discharge over the second half of its length, that is touching the transition between the two chambers of the source, and the appearance of a second maximum of the dc potential there are related to the configuration of the source. The obtained axial variation of the negative ion density obeys that of the potential of the dc electric field in the discharge.

An experimental study of the axial structure of a gas discharge sustained by a surface electromagnetic wave in the presence of a uniform external magnetic field

Examines experimentally the axial profiles of the plasma parameters (electron temperature Te and the electron number density n) of a helium low-pressure gas discharge column sustained by a travelling electromagnetic wave in the presence of a strong magnetic field for two values of magnetic induction corresponding to the magnitudes of omega c/ omega =2.5 and 5 respectively ( omega c is the electron cyclotron frequency and omega is the wave angular frequency). It is found that the electron temperature is diminished and the plasma density is increased (accompanied by a decrease of its axial gradient) in comparison with the magnitudes of the same parameters and characteristics of an isotropic plasma column. The measured axial plasma density gradients are compared with theoretically calculated ones and the agreement between theory and experiment is good.

Axial variation of line emission from surface wave sustained discharges

The intensities of ArI emission lines 750.4 and 811.5 nm and of H (hydrogen appearing as a small admixture to the discharge in Ar) were measured along the column of a stationary, surface wave sustained plasma. Simultaneously, electron densities along the discharge were determined, probing the spatial phase characteristics of test surface waves. Using test waves of different frequencies, the evaluation of electron densities was based on the dispersion relation, which connects densities with axial wavenumbers of the m = 0 mode. The emitted line intensities plotted against the electron density, particularly those of H and the 750.4 nm line, reveal that the discharge economizes on dissipated RF power in fulfilling the local energy and particle balance. A theoretical model of the diffusion dominated plasma confirms the conclusion drawn from the spectroscopically observed trends. According to this model, the lowering of the RF field intensities is correlated with the transition from ground-state excitation/ionization (prevailing at low electron densities) to additional, less power consumptive, stepwise excitation/ionization, which is enabled due to enlarged densities of metastables.

Waveguide Stationary and Nonstationary Discharges: Modelling and Experiments

Discharges produced in electromagnetic (EM) wave fields are the last ones entered the rich collection of the gas discharges. With their status of the most recent type of discharges and with the extended research on them, the wave sustained discharges support the development of modern trends in the field of the gas discharge physics. Based on the mechanism of the ionization nonlinearity, the gas discharges are nonlinear dissipative systems composed by two interrelated components: electric fields and plasmas. Creation of discharges and their evolution are processes of dynamics [1] of nonequilibrium dissipative systems.

Microwave discharge in a finite length vessel

A microwave surface-wave discharge at low pressure in a finite length vessel is experimentally investigated. Argon plasma is created in a dielectric capillary with length of 15 mm, the tube being an extension of an open-ended coaxial structure. Microwave power at frequency 2.45 GHz is coupled into the source applicator at power levels 6-15 W. The plasma column increases with the power applied and standing wave mode is realized when its length is equal to the capillary length. The electron temperature and plasma density are obtained simultaneously by passive optical emission spectroscopy using the line-ratio method. The surface waves phase-diagram shows that their wavelengths are much lower than the free-space wavelength. The input impedance of the plasma column - modelled as a lossy transmission line - has resonance behaviour when its length is equal to λg/2 or λg (λg being the surface waves wavelength). The results obtained for the plasma parameters, the surface waves wavelength and the vessel length show correlation to the power reflected, absorbed and radiated from the plasma column. The radiation pattern of the column shows that the main lobe is nearly perpendicular to the capillary axis.