Ei Ekaterina Iordanova

A novel method to determine the electron temperature and density from the absolute intensity of line and continuum emission: application to atmospheric microwave induced Ar plasmas

An absolute intensity measurement (AIM) technique is presented that combines the absolute measurements of the line and the continuum emitted by strongly ionizing argon plasmas. AIM is an iterative combination of the absolute line intensity–collisional radiative model (ALI–CRM) and the absolute continuum intensity (ACI) method. The basis of ALI–CRM is that the excitation temperature T13 determined by the method of ALI is transformed into the electron temperature Te using a CRM. This gives Te as a weak function of electron density ne. The ACI method is based on the absolute value of the continuum radiation and determines the electron density in a way that depends on Te. The iterative combination gives ne and Te. As a case study the AIM method is applied to plasmas created by torche a injection axiale (TIA) at atmospheric pressure and fixed frequency at 2.45 GHz. The standard operating settings are a gas flow of 1 slm and a power of 800 W; the measurements have been performed at a position of 1 mm above the nozzle. With AIM we found an electron temperature of 1.2 eV and electron density values around 1021 m−3. There is not much dependence of these values on the plasma control parameters (power and gas flow). From the error analysis we can conclude that the determination of Te is within 7% and thus rather accurate but comparison with other studies shows strong deviations. The ne determination comes with an error of 40% but is in reasonable agreement with other experimental results.

Atmospheric microwave-induced plasmas in Ar/H2 mixtures studied with a combination of passive and active spectroscopic methods

Several active and passive diagnostic methods have been used to study atmospheric microwave-induced plasmas created by a surfatron operating at a frequency of 2.45 GHz and with power values between 57 and 88 W. By comparing the results with each other, insight is obtained into essential plasma quantities, their radial distributions and the reliability of the diagnostic methods. Two laser techniques have been used, namely Thomson scattering for the determination of the electron density, ne, and temperature, Te, and Rayleigh scattering for the determination of the heavy particle temperature, Tg. In combination, three passive spectroscopic techniques are applied, the line broadening of the Hβ line to determine ne, and two methods of absolute intensity measurements to obtain ne and Te. The active techniques provide spatial resolution in small plasmas with sizes in the order of 0.5 mm. The results of ne measured with three different methods show good agreement, independent of the plasma settings. The Te values obtained with two techniques are in good agreement for the condition of a pure argon plasma, but they show deviations when H2 is introduced. The introduction of a small amount (0.3%) of H2 into an argon plasma induces contraction, reduces ne, increases Te, enhances the departure from equilibrium and leads to conditions that are close to those found in cool atmospheric plasmas.

Experimental investigation of the electron energy distribution function (EEDF) by Thomson scattering and optical emission spectroscopy

The electron temperature of an argon surface wave discharge generated by a surfatron plasma at intermediate pressures is measured by optical emission spectroscopy (OES) and Thomson scattering (TS). The OES method, namely absolute line intensity (ALI) measurements gives an electron temperature which is found to be (more or less) constant along the plasma column. TS, on the other hand, shows a different behaviour; the electron temperature is not constant but rises in the direction of the wave propagation. In the pressure range of this study, it is theoretically known that deviations from Maxwell equilibrium are expected towards the end of the plasma column. In this paper, we propose a combination of methods to probe the electron energy distribution function (EEDF) in this relatively high-pressure regime. The ALI method combined with a collisional–radiative model allows one to measure the effective (Maxwellian) creation temperature of the plasma while TS measures the mean electron energy of the EEDF. The differences between the two temperature methods can be explained by the changes in the form of the EEDF along the plasma column. A strong correlation is found with decreasing ionization degree for different pressures. Numerical calculations of the EEDF with a Boltzmann solver are used to investigate the departure from a Maxwellian EEDF. The relatively higher electron temperature found by TS compared with the ALI measurements is finally quantitatively correlated with the departure from a Maxwellian EEDF with a depleted tail. (Some figures may appear in colour only in the online journal)

Absolute measurements of the continuum radiation to determine the electron density in a microwave-induced argon plasma

A method for the determination of the electron density (ne) using the continuum radiation is presented. The radiation is calibrated with a standard tungsten ribbon lamp and thus expressed in absolute units. This method is applied to a microwave-induced argon plasma, created by a surfatron (2.45?GHz), for which the standard settings are: wavelength region at 648?nm, power of 60?W, pressure of 15?mbar, gas flow of 70?sccm and axial distance from the launcher of 3?cm. Due to the low degree of ionization, the influence of electron?ion interactions can be neglected; the radiation is predominantly generated by free?free interactions between electrons and atoms. The method provides the electron density values in the order of 1019?m?3 for different plasma settings. It is observed that the measured ne follows the well-known trends?it decreases in the direction of the propagating surface wave and increases with power.

A spectroscopic method to determine the electron temperature of an argon surface wave sustained plasmas using a collision radiative model

A method is presented to determine the electron temperature in a low pressure argon plasma using emission spectroscopic measurements and a collisional radiative (CR) model. Absolute line intensity measurements are made in order to construct the atomic state distribution function. In addition to the excited states, the ground state density is also taken into account. Because of this, the excitation temperature can be determined with high precision. A CR-model has been used to determine the degree of equilibrium departure and to obtain the relationship between the excitation temperature and the electron temperature. This method is applied to a microwave plasma which has been generated inside a quartz tube using a surfatron device. The densities of argon levels close to the continuum are used to get an estimated value of the electron density. These values are used as input data for the CR-model. For an argon pressure of 6 mbar, the 4p level densities vary between 8 × 1014 and 6 × 1015 m−3. Using the estimated values for the electron density, between 2 × 1019 and 3 × 1019 m−3, the electron temperature was found to range between 1.15 and 1.20 eV. An extensive error analysis showed that the relative error in the electron temperature is less than 6%.

Revision of the criterion to avoid electron heating during laser aided plasma diagnostics (LAPD)

A criterion is given for the laser fluency (in J/m2) such that, when satisfied, disturbance of the plasma by the laser is avoided. This criterion accounts for laser heating of the electron gas intermediated by electron-ion (ei) and electron-atom (ea) interactions. The first heating mechanism is well known and was extensively dealt with in the past. The second is often overlooked but of importance for plasmas of low degree of ionization. It is especially important for cold atmospheric plasmas, plasmas that nowadays stand in the focus of attention. The new criterion, based on the concerted action of both ei and ea interactions is validated by Thomson scattering experiments performed on four different plasmas.

Thomson scattering measurements on a low pressure surface wave sustained plasma in argon

Thomson scattering (TS) experiments have been made on a low pressure surfatron induced plasma. TS is an active diagnostic method and the experimental results are directly related to important plasma properties such as the electron density, ne, and the electron temperature, Te. Therefore, the TS results for ne and Te can be used to calibrate passive diagnostic methods which are often based on plasma models. However, to apply TS on a surfatron induced plasma inside a quartz tube is experimentally demanding because of the large amount of stray light and a low intensity of the TS signal. To achieve low detection limits and high stray light rejection, a triple grating spectrograph was used in the detection branch and an iCCD was used to record the TS spectrum. For a typical plasma condition with an argon pressure of 10 mbar and an absorbed power of 50 W, the measured electron density was found to be equal to ne ≈ 4 × 1019 m−3 and the electron temperature Te ≈ 1.2 eV. In addition, frame-averaged results for 6, 10, 15 and 20 mbar argon plasmas for absorbed microwave powers in between 25 ≤ Pab ≤ 60 W are presented. The trends found in the dependence of the pressure and power density are according to theory.

Polydiagnostic calibration performed on a low pressure surface wave sustained argon plasma

The electron density and electron temperature of a low pressure surface wave sustained argon plasma have been determined using passive and active (laser) spectroscopic methods simultaneously. In this way the validity of the various techniques is established while the plasma properties are determined more precisely. The electron density, ne, is determined with Thomson scattering (TS), absolute continuum measurements, Stark broadening and an extrapolation of the atomic state distribution function (ASDF). The electron temperature, Te, is obtained using TS and absolute line intensity (ALI) measurements combined with a collisional–radiative (CR) model for argon. At an argon pressure of 15 mbar, the ne values obtained with TS and Stark broadening agree with each other within the error bars and are equal to (4 ± 0.5) × 1019 m−3, whereas the ne value (2 ± 0.5) × 1019 m−3 obtained from the continuum is about 30% lower. This suggests that the used formula and cross-section values for the continuum method have to be reconsidered. The electron density determined by means of extrapolation of the ASDF to the continuum is too high (~1020 m−3). This is most probably related to the fact that the plasma is strongly ionizing so that the extrapolation method is not justified. At 15 mbar, the Te values obtained with TS are equal to 13 400 ± 1100 K while the ALI/CR-model yields an electron temperature that is about 10% lower. It can be concluded that the passive results are in good or fair agreement with the active results. Therefore, the calibrated passive methods can be applied to other plasmas in a similar regime for which active diagnostic techniques cannot be used.

Thomson Scattering Imaging of the Very End of Surfatron Plasmas

The direct imaging capacity of a triple-grating spectrometer designed for Thomson scattering is used for a cartographic study of the end of the column created by intermediate-pressure surfatron plasmas in argon. It is shown that the electron density decreases abruptly at the very end of the column. This effect is more pronounced for higher pressures and accompanied by a large rise in electron temperature.

Towards poly-diagnostics on cool atmospheric plasmas

A route toward the experimental characterization of Cool Atmospheric Plasmas (CAPs) is described. It is a step-by-step approach, in which, for each step different experimental techniques are compared with each others. These can be divided in passive and active spectroscopic methods. It is seen that especially the passive methods for the electron temperature determination are very sensitive to the degree of equilibrium departure suggesting that active spectroscopy is preferable. However, one should realize that lasers can easily heat cool plasmas. This is due to the fact that the ionization degree of CAPs is smalls.