J. Krištof

Spectroscopic diagnostics and modelling of a N2–Ar mixture discharge created by an RF helical coupling device: I. Kinetics of N2(B 3Πg) and N2(C 3Πu) states

Optical emission spectroscopy, ranging from visible to near infrared, is used to determine densities and rotational temperatures of N2(B?3?g) and N2(C?3?u) states in a nitrogen?argon (0?95% Ar) discharge, under moderate pressures (200?400?Pa). The plasma is sustained by a helical cavity with an excitation frequency of 27?MHz and power fixed to 28?W. Firstly, in the case of a pure N2 discharge, the two states turn out to have a similar rotational temperature, which approximates the gas temperature reasonably well. With a gradual increase in the Ar concentration up to 95%, the rotational temperature of N2(C?3?u) roughly doubles while that of N2(B?3?g) stays unchanged at 430???50?K regardless of the gas composition. Secondly, as observed, the densities of the N2(B?3?g) and N2(C?3?u) states increase with increasing Ar percentage in the gas mixture. The increase in the emission intensity values is less marked for positions corresponding to both ends of the cavity. In fact, the difference in the emission level between the power input and helix middle positions is reduced, revealing that the total discharge is more uniform along the cavity for large argon concentrations. The experimental results show a strong dependence of temperatures and densities on the Ar amount in the gas mixture. A kinetic model is developed to explain this phenomenon, which is then used in modelling density evolutions versus relative abundance of Ar and versus the position along the cavity axis. The model indicates the importance of the role of electron and metastable species in the above-described discharge.

Vacuum UV and UV spectroscopy of a N2–Ar mixture discharge created by an RF helical coupling device

Optical emission spectroscopy in vacuum ultraviolet and UV spectral ranges is applied to study densities, and vibrational and rotational temperatures of the N2 molecule in a nitrogen–argon (0–95% Ar) plasma sustained at a pressure of 400 Pa by a helical cavity supplied with a power of 28 W and an excitation frequency of 27 MHz. The spatial investigation of all emission systems from UV to NIR shows a minimum situated in the middle of the helical structure and two maxima located at the positions where the RF power is transmitted to the gas and at the end of the helix. The minimum was deepest for emission of the first positive (1+) nitrogen system. This hollow shaped density profile due to the presence of a non-linear phenomenon in the discharge is constant whatever the gas composition. The emissions related to Lyman–Birge–Hopfield and the second positive (2+) systems of molecular nitrogen, and N(2P) atoms, are analyzed versus the Ar percentage. Additionally, the NO(A 2Σ+ → X 2Π) and OH(A 2Σ+ → X 2Π) emission systems coming from impurities are investigated. All the densities of the considered species increase with Ar amount. The rotational and vibrational temperatures of the emitter species are determined through the comparison between experimental and simulated spectra. In the case of a N2 discharge, all the rotational temperatures deduced through the nitrogen emission systems are equal and can be assimilated to the gas temperature. With the increase in the Ar amount, only the rotational temperature obtained from the 1+ system is close to the gas temperature. The rotational and vibrational temperatures related to the NO(A 2Σ+) species are constant whatever the gas composition. The vibrational distribution function of N2(a 1Πg) state presents a Boltzmann law with a vibrational temperature in the range 5600–8000 K (±1000 K) for the N2–x% Ar mixture with x < 75%. When the Ar percentage increases above this limit, we observe strong deviations from the Boltzmann law and no temperature can be deduced. Some kinetic considerations, where the nitrogen and argon metastables play an important role, are discussed to explain the strong dependence of the temperatures and density species toward the Ar amount in the gas mixture.

Use of the near vacuum UV spectral range for the analysis of W-based materials for fusion applications using LIBS

The vacuum UV (VUV)-near Infrared (NIR) laser induced breakdown spectroscopy (LIBS) technique was applied to investigate the composition of W-based samples with a protective carbon layer. The sample was analyzed under pressures from 5 to 105 Pa and atmosphere (air, He). The spectra were recorded with three spectrometers at delays from 200 ns to 10 μs at atmospheric pressures and from 100 to 500 ns at low pressures. The electron density was determined from the measured spectra using Stark broadening and the electron temperature from the W I–W III Saha–plot in the VUV–NIR spectral range. The better precision was achieved due to usage W III spectral lines of tungsten. The achieved results are more reliable than results obtained without W III spectral lines. The calibration free LIBS method was then applied to determine the W and C contents of the analyzed sample.

Diagnostics of low-pressure hydrogen discharge created in a 13.56 MHz RF plasma reactor

A 13.56 MHz RF discharge in hydrogen was studied within the pressure range of 1-10 Pa, and at a power range of 400-1000 W. The electron energy distribution function and electron density were measured by a Langmuir probe. The gas temperature was determined by the Fulcher-alpha system in pure H-2, and by the second positive system of nitrogen using N-2 as the probing gas. The gas temperature was constant and equal to 450 +/- 50 K in the capacitively coupled plasma (CCP) mode, and it increased with pressure and power in the inductively coupled plasma (ICP) mode. Also, the vibrational temperature of the ground state of hydrogen molecules was determined to be around 3100 and 2000 +/- 500 K in the ICP and CCP mode, respectively. The concentration of atomic hydrogen was determined by means of actinometry, either by using Ar (5%) as the probing gas, or by using H-2 as the actinometer in pure hydrogen (Q1 rotational line of Fulcher-alpha system). The concentration of hydrogen density increased with pressure in both modes, but with a dissociation degree slightly higher in the ICP mode (a factor 2).