[en] The ion species of H_n⁺ (n = 1-3) and their relative intensities in the cathode sheath of the pure H₂ abnormal glow discharges with various H₂ gas pressure and different discharge current are analysed with a mass spectrometer. The result shows that the H⁺ component of ions is dominated, no less than 90 percent, and the H₃⁺ smaller, the H₂⁺ hardly detectable in the H₂ pressure range of 133 Pa to 665 Pa. The ratio between H⁺ and H₃⁺ intensities increases with the gas pressure. The total intensity increases with the discharge current, and the ratio between H⁺ and H₃⁺ intensities is unchanged. The main physics-chemistry processes in the hydrogen plasma are discussed

[en] Laser-induced breakdown spectroscopy (LIBS) is one of useful analytical methods for hydrogen isotopes with depth profiles on materials. Advantages of LIBS as ex-situ measurements are (1) short time analysis, (2) elements can be detected including hydrogen isotopes and helium, and (3) intensities mappings at 2-D positions. For research topics of hydrogen removal experiments for ITER, analyses of hydrogen isotope depth profiles are important. In this study, a demonstration of hydrogen depth profile analyses on graphite target exposed to LHD divertor plasmas was done using LIBS. From a comparison with LIBS and other analyses, effective approaches are discussed. (J.P.N.)

[en] The graphite tiles, located at the lower divertor and the fixed limiter of HL-2A tokamak, were measured with ex-situ laser-induced breakdown spectroscopy (LIBS). The deposition behavior on HL-2A was presented by analyzing different elements including C, Fe, Si, H and D within the tiles and their distributions on depth in different tiles. Comparing with the results obtained with time of flight secondary ion mass spectrometry (TOF-SIMS), the depth profiles show well consistency. According to the effective number of deposition layer erased by the laser, the average laser ablation rate is estimated to be 270 nm/pulse. (authors)

[en] Benzene is a major industrial air pollutant and can cause serious human health disorders. In this paper an investigation on benzene destruction, in an atmospheric-pressure fast-flow pulsed DC-discharge by means of laser ionization combined with time-of-flight (TOF) mass spectrometry, is reported. Most by-products including transient reactive species from the benzene discharge were characterized by molecular beam sampling combined with TOF mass spectrometry. It is showed that, with a gas mixture of 0.5% C₆H₆ in Ar, benzene can be effectively destroyed by discharge plasma. The intermediate species consisted of small fragments of C_nH_m (n = 3∼5, m = 1∼11), cycle-chain species of C_nH_m (n = 6∼9, m = 7∼10) and polycyclic species C_nH_m (n ≥9, m = 8∼12). The alternation of mass peaks (intensity) with even/odd electrons was observed in the measured mass spectra. The results indicated that the alternation is mainly due to the different ionization potentials of the open shell and close shell species. Based on the examination of the features of the species' composition, the primary reaction pathways are proposed and discussed. (low temperature plasma)

[en] Atmospheric pressure surface micro-discharge (SMD) has recently received considerable interest as an efficient source for its potential applications in biomedical, agricultural and environmental fields. The inherent physical separation between the discharge layer and the downstream target causes a reduction in the flux of reactive chemical species, resulting in the limitation of application efficacy. To obtain the development of healthcare devices, this contribution focuses on the spatial-temporal transport behavior of OH radicals generated by SMD devices in helium using laser induced fluorescence and optical emission spectroscopy with hexagon mesh electrode of lattice distance varying between 3 mm and 11 mm. It was observed that the lattice distance has a significant impact on the distribution of OH radicals and the uniformity of the SMD. The increased lattice distance of the hexagon mesh both increased the density, delivery distance and propagation velocity of OH radicals. When the lattice distance varied from 3 to 11 mm, the generated OH was spread over twice the distance. The maximum delivery distance was found to be approximately 10.4 mm under lattice distance of 11 mm condition, and the peak propagation velocity was estimated to be about 3.8 mm ms⁻¹. For the main distribution region of OH, the optimum lattice distance is 9 mm. In addition, the distribution region rapidly expands between 7 and 9 mm. However, the findings indicated that researchers should make a compromise between the delivery of reactive chemical species and uniformity. The decreased lattice distance facilitated a high degree of uniformity for reactive species on a downstream target, which is a key requirement in healthcare related areas. (paper)

[en] Laser-induced breakdown spectroscopy has been recognized as a significant tool for element diagnostics in plasma–wall interaction. In this work, a one-dimensional numerical model is developed to simulate the laser ablation processes of a molybdenum (Mo) target in vacuum conditions. The thermal process of the interaction between the ns-pulse laser with wavelength of 1064 nm and the Mo target is described by the heat conduction equation. The plasma plume generation and expansion are described by Euler equations, in which the conservation of mass density, momentum and energy are included. Saha equations are used to describe the local thermal equilibrium of electrons, Mo atoms, ${\mathrm{M}\mathrm{o}}^{+}$ and ${\mathrm{M}\mathrm{o}}^{2+}.$ Plasma shielding and emission are all considered in this model. The mainly numerical results are divided into three parts, as listed below. Firstly, the rule of the plasma shielding effect varying with laser intensity is demonstrated quantitatively and fitted with the Nelder function. Secondly, the key parameters of plasma plume, such as the number density of species, the propagation velocity and the temperature, are all calculated in this model. The results indicate that the propagation velocity of the plume center increased with time in a general trend, however, one valley value appeared at about 20 ns due to the pressure gradient near the target surface leading to negative plasma velocity. Thirdly, the persistent lines of a Mo atom in the wavelength range from 300 nm to 600 nm are selected and the spectrum is calculated. Moreover, the temporal evolutions of Mo’s spectral lines at wavelength of 550.6494 nm, 553.3031 nm and 557.0444 nm are given and the results are compared with experimental data in this work. (paper)

[en] One of the great difficulties in understanding nitrogen plasma elementary processes is the lack of an available database of the cross-sections of electron-impact excitations and radiations. Ab initio calculations of vibrational excitation cross sections for electron collisions with nitrogen molecules in low-lying states using similarity function approach, such as a-a', a-w, B-B' and B-W transition systems, are reported here for the first time. In the meantime, the average excitation energies of neighboring levels of these systems have been calculated. In order to obtain the cross sections, accurate spectroscopic constants and transition dipole moments have been investigated. Potential energy curves and other electronic transition dipole moments for the low-lying states of N₂ have been re-evaluated using complete active space self-consistent field (CASSCF) approach with aug-cc-pVqZ basis set. The calculated cross-sections could provide a database for studying the elementary processes and the properties in N₂ plasma. (low temperature plasma)

[en] This work presents the transport behaviour of OH radicals in an atmospheric pressure pulse-modulated surface micro-discharge in helium. Laser-induced fluorescence is employed to measure the time development of OH radicals density distribution during the pulse duration and the inter-pulse period. It is shown that convection caused by electrohydrodynamic force enhances and dominates the transport of OH radicals from the surface plasma layer to the afterglow region with a distance up to 8 mm away from the dielectric surface. Interestingly, after the plasma switches off, OH density decreases quickly near the electrode but the downstream distribution region keeps constant, showing that the impact of convection is still present during the post-discharge. Additionally, the propagation velocity of OH declines monotonously with time. The maximum value is estimated as 1.86 m s⁻¹ during the first 1 ms when the plasma is on. The influence of power delivered to plasma is investigated, and the results indicate that the OH density everywhere in the detection zone is directly proportional to the applied power. However, the input power has no significant influence on the propagation velocity and delivery distance, suggesting that it is unrealistic to increase the delivery distance of reactive species by increasing the power delivered to plasma. (paper)

[en] The dynamic evolution of plasma optical emission from an array of surface micro-discharges has been investigated by optical emission imaging. The array was operated in helium at atmospheric pressure and driven at 2.0 W at a frequency of 30 kHz. The findings indicate that surface charges and external voltage have a significant contribution to the splitting of the plasma streamer, with luminous fronts moving at velocities of 8.3–22.4 km s⁻¹. The split plasmas induce new discharge events within a single hexagonal cell. Furthemore, we present the case of two co- and counter-propagating streamers generated within one hexagon mesh element. Experimental evidence reveals that the co-propagating streamers merge and produce a new streamer front with enhanced intensity under the combined effects of electrostatic repulsion, gas dynamic interaction and a photolytic process. As the spacing between the counter-propagating streamers decreases, the streamers interact electrically, resulting in a modification of the shape of these streamers as well as a decrease in their velocities and emission intensities. The emergence of secondary streamers is also observed. This behavior is related to surface charges accumulated during a previous half cycle and their redistribution due to the turbulence fluctuations dominated by electrohydrodynamic force. From the propagation of an individual streamer, it is shown that surface charges accumulated in a previous negative half cycle can determine the plasma path to some extent. The ionization wave propagates over the rim electrode with a velocity of about 20 km s⁻¹, resulting in a distinct discharge channel and a strong interaction between neighboring hexagonal units in an array. The ionization wave leads to the propagation of plasma across the dielectric surface of the array. (paper)

[en] The purpose of this study is to design a diagnostic system for reactive plasma environment by combining molecular-beam time-of-flight (TOF) mass spectroscopy with laser spectroscopy technique. The combination of TOF mass spectrometers and pulsed lasers is favorable in the diagnosis of intermediate species distribution since they allow the simultaneous but separate recording of the spectra of different species. In the plasma system, the intermediate species in electronic ground state or low lying excited state is pumped to higher energy level with resonant laser excitation, and then, the ionization with a second laser system is possible which can readily be detected by the TOF analyzer. The ionization itself is only used as a detection mechanism for the observation of the excitation of these states. In this manner, the population distribution of intermediate species can be determined with state-selective and mass-selective feature. Also, in this article, a flexible data acquisition and automatic control system based on LABVIEW was designed to integrate all the stand-alone measurement instruments including a TOF spectrometer, a laser system, a high performance oscilloscope, and a digital delay generator into a single personal computer-based control unit. Moreover, a virtual Boxcar integrator with hundreds of channels has been developed to enhance the signal while filtering out the random noises. Finally, the many potentials of this technique in the application of plasma diagnosis will be discussed.