Electron Impact De-excitation Research Articles

Self-consistent calculation of the optical emission spectrum of an argon capacitively coupled plasma based on the coupling of particle simulation with a collisional-radiative model

We report the development of a computational framework for the calculation of the optical emission spectrum of a low-pressure argon capacitively coupled plasma (CCP), which is based on the coupling of a particle-in-cell/Monte Carlo collision simulation code with a diffusion-reaction-radiation code for Ar I excited levels. In this framework, the particle simulation provides the rates of the direct and stepwise electron-impact excitation and electron-impact de-excitation for 30 excited levels, as well as the rates of electron-impact direct and stepwise ionization. These rates are used in the solutions of the diffusion equations of the excited species in the second code, along with the radiative rates for a high number of Ar-I transitions. The calculations also consider pooling ionization, quenching reactions, and radial diffusion losses. The electron energy distribution function and the population densities of the 30 excited atomic levels are computed self-consistently. The calculations then provide the emission intensities that reproduce reasonably well the experimentally measured optical emission spectrum of a symmetric CCP source operated at 13.56 MHz with 300 V peak-to-peak voltage, in the 2–100 Pa pressure range. The accuracy of the approach appears to be limited by the one-dimensional nature of the model, the treatment of the radiation trapping through the use of escape factors, and the effects of radiative cascades from higher excited levels not taken into account in the model.

Spectroscopic diagnosis and simulation analysis of tungsten cathode erosion during the start-up process for a magnetoplasmadynamic thruster

Measurement of tungsten density during the start-up process plays a vital role in monitoring real-time cathode mass loss in the study of plasma discharge instrument structure optimization. The relative intensity of the W II 239.71 nm spectral line, collected at the exit by the monochromator, is used to estimate the W II density in combination with the collision radiative model. The model of a small amount of tungsten in high-density plasma takes into account the main kinetic processes including the electron impact excitation process, electron impact de-excitation process, and electron impact ionization process. Additionally, A two-dimensional axisymmetric quasi-neutral fluid model, coupled with a thermal model of the cathode into a self-consistent model, is also constructed to analyze the mass loss mechanism of tungsten cathode during the start-up process. In the initial stage, the cold-state cathode has a large potential drop in the sheath region to accelerate the ion bombardment of the cathode for the requirement of electron emission, and a large number of tungsten atoms removal from the cathode surface due to sputtering and evaporation. In the steady-state operating phase, the primary reason for the mass loss of tungsten cathodes is attributed to thermal evaporation. Both experimental and simulation results indicate that the mass loss rate during the start-up phase is higher than that during steady-state operation phase by 1 to 2 orders of magnitude.

Diagnostic study of capacitively coupled neon rf plasma with traces of O2/H2 at intermediate pressure

In this work, we have performed the non-invasive diagnostic study of capacitively coupled Ne–O2/H2 mixture plasma through the optical emission spectroscopy (OES) coupled with a suitable collisional radiative (CR) model. Capacitively coupled neon radio-frequency (rf) discharge (flowing downstream) with small admixture of O2/H2 have been generated in a vacuum chamber using 13.56 MHz rf signal and 120 W power supply. Keeping O2 and H2 flow rates fixed at 0.01 and 0.015 LPM respectively, the neon flow rate has been varied as 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8 LPM to obtain different mixture concentration of Ne–O2 and Ne–H2 discharge. The pressure in the chamber has been observed in intermediate range (∼500–25 000 Pa) for different mixture concentrations. OES measurements are recorded at various operating conditions in the wavelength range from 200 to 1200 nm. To extract the information of plasma parameters from the OES measurements, a comprehensive fine-structure resolved CR model has been developed. In the diagnostic process, five intense Ne–I emission lines at 594.48, 607.43, 633.44, 638.30, 703.24 nm are used. The CR model considers all the important processes i.e. electron impact excitation, electron impact de-excitation, radiative decay, ionization, two–three body recombination, and diffusion. The quenching process of 1s (1s5,1s4,1s3) levels of neon by the O2/H2 molecule has been included in the model. The radiative transitions from the upper levels to the ground state (1S0) as well as to the 1s levels of neon are corrected for self-absorption. The electron temperature (T e) and electron density (n e) of the plasma have been extracted for all the mixture concentrations of Ne–O2 and Ne–H2. Also, with the different mixture concentrations of O2/H2 in neon discharge, the variation of population of metastable levels (1s5, 1s3) of neon as well as intensities of 656.28 nm of hydrogen and 777.4 nm of oxygen lines have been reported and discussed.

Diagnostic of Ar-CO2 mixture plasma using a fine-structure resolved collisional radiative model

In the present work, we develop a collisional radiative (CR) model for the Ar-CO2 mixture plasma. The model is applied to the diagnostics of a recently reported low-pressure DC generated Ar-CO2 plasma of Rodríguez et al. [Phys. Plasmas 25, 053512 (2018)] utilizing their spectroscopic measurements who reported the plasma parameters at different concentrations of CO2 in Ar at various pressures and powers. Our CR model includes various population transfer mechanisms between the fine structure levels, viz., electron-impact excitation, radiative decay, and ionization, as well as their reverse processes, e.g., electron-impact de-excitation as well as two- and three-body recombination. The effects of radiation trapping and diffusion are also taken into account. The plasma parameters viz. electron density (ne) and electron temperature (Te) are obtained as a function of different pressures (0.2, 0.3, and 0.6 mbars) and discharge powers at 25 and 50% concentrations of CO2 in argon. These results are determined using measured intensities of seven intense emission lines out of 3p54p (2pi) → 3p54s (1si) fine-structure transitions. It is observed that both the electron density and electron temperature increase with the increase of CO2 concentration, which is in confirmation with experimental predictions. The populations obtained for 1si and 2pi levels from our CR model are also reported and compared respectively with the corresponding available values from the simple CR model and experiment of Rodríguez et al. [Phys. Plasmas 25, 053512 (2018)]. A significant difference in the populations is observed from the two models.

Dissociative recombination and electron-impact de-excitation in CH photon emission under ITER divertor-relevant plasma conditions

For understanding carbon erosion and redeposition in nuclear fusion devices, it is important to understand the transport and chemical break-up of hydrocarbon molecules in edge plasmas, often diagnosed by emission of the CH A 2Δ–X 2Π Gerö band around 430 nm. The CH A-level can be excited either by electron-impact (EI) or by dissociative recombination (DR) of hydrocarbon ions. These processes were included in the 3D Monte Carlo impurity transport code ERO. A series of methane injection experiments was performed in the high-density, low-temperature linear plasma generator Pilot-PSI, and simulated emission intensity profiles were benchmarked against these experiments. It was confirmed that excitation by DR dominates at Te < 1.5 eV. The results indicate that the fraction of DR events that lead to a CH radical in the A-level and consequent photon emission is at least 10%. Additionally, quenching of the excited CH radicals by EI de-excitation was included in the modeling. This quenching is shown to be significant: depending on the electron density, it reduces the effective CH emission by a factor of 1.4 at ne = 1.3 × 1020 m−3, to 2.8 at ne = 9.3 × 1020 m−3. Its inclusion significantly improved agreement between experiment and modeling.

Argon metastables in HiPIMS: time-resolved tunable diode-laser diagnostics

Time-resolved tunable diode-laser absorption spectroscopy measurements were performed on the argon metastable (Arm) level 3s23p54s excited at 801.478 nm, in the dense plasma region in front of the magnetron target in a high power impulse magnetron sputtering (HiPIMS) discharge. From the Doppler profile the evolution of the temperature and density was derived during the pulse as well as during the plasma decay, i.e. in the afterglow. It is shown that the Arm density sharply increases at the beginning of the discharge pulse, followed by a severe Arm depletion along with increasing gas temperature around the peak of the HiPIMS discharge current. The strong dynamics of these parameters involve many elementary processes such as electron-impact excitation, electron-impact de-excitation and ionization of Arm, gas rarefaction, electron temperature increase at the end of the pulse and gas diffusion. These phenomena are discussed with respect to several parameters: distance from the target, peak discharge current during the pulse, pulse length, and gas pressure.

Superelastic collisions between slow electrons and excited Kr and Xe atoms. Possible reaction mechanism for rare-gas atoms

Measurements of the electron energy distribution in krypton and xenon afterglow plasmas are used to determine the rate coefficients for superelastic collisions between excited np5(n+1)s Kr or Xe atoms and thermal electrons. The rate coefficient for electron impact de-excitation of the state Kr 3P2 is beta e2=(1.5+or-0.2)*10-10 cm3 s-1, for Kr 3P1 it is beta e1=5.2*10-10 cm3 s-1, for Kr 3P0 it is beta e0=9.8*10-10 cm3 s-1, for Xe 3P2 it is beta e2=(3.2+or-0.5)*10-10 cm3 s-1 and for Xe 3P1 it is beta e1<1*10-9 cm3 s-1. It is also shown that if the energy of the incident electron is in the range 0-0.2 eV, a binding of this electron with the excited atom in an autodetachment state of the negative ion, followed by its decay, is a quite possible channel of the reaction.

Effect of dielectronic recombination excitation in the solar chromosphere-corona transition region

The intensities of lines emitted from the solar chromosphere-corona transition region and observed on the earth are calculated by considering excitation of electrons to the upper level caused by electron-impact excitation and dielectronic recombination excitation processes. For the lines under investigation, the electron-impact deexcitation rates are negligibly small in comparison with spontaneous transition probabilities. Hence, electrons from the upper level deexcite through spontaneous transition only, emitting photons of wavelength corresponding to the energy difference between two levels. To determine the contribution of dielectronic recombination excitation (DRE) to the excitation mechanism, the calculations are repeated with exclusion of DRE. It is found that DRE contributes significantly and that its contribution may be much larger than that of electron-impact excitation.

Intensity of some astrophysical lines

Intensities of lines emitted from the solar atmosphere and observed on earth are calculated. The process for excitation of the electron to the upper level is taken as electron impact excitation because the other excitation processes, for instance photo excitation and dielectronic recombination excitation, do not contribute significantly. The electron de-excites from the upper level through photo emission. The electron impact de-excitation probability is negligible in comparison to the spontaneous transition probability. Results are reported in tabular form.

Investigation of some elementary processes in a vacuum-UV hydrogen laser discharge

A novel type of pulse generator is utilized for travelling wave excitation of hydrogen in a multiple electrode laser tube. With this arrangement a laser peak power in the Lyman bands of about 45 kW and a pulse width of 1 ns are achieved. The effect of electron impact deexcitation of upper laser levels on the pulse width was investigated. Some irregularities in the intensity distribution of laser lines could be explained by considering collisions of molecules in the upper laser levels with molecules in the electronic ground state.