Vol 43, No 8 (2017)

This work is devoted to excimer lamp efficiency optimization by using a homogenous discharge model of a dielectric barrier discharge in a Ne−Xe mixture. The model includes the plasma chemistry, electrical circuit, and Boltzmann equation. In this paper, we are particularly interested in the electrical and kinetic properties and light output generated by the DBD. Xenon is chosen for its high luminescence in the range of vacuum UV radiation around 173 nm. Our study is motivated by interest in this type of discharge in many industrial applications, including the achievement of high light output lamps. In this work, we used an applied sinusoidal voltage, frequency, gas pressure, and concentration in the ranges of 2–8 kV, 10–200 kHz, 100–800 Torr, and 10–50%, respectively. The analyzed results concern the voltage V_p across the gap, the dielectric voltage V_d, the discharge current I, and the particles densities. We also investigated the effect of the electric parameters and xenon concentration on the lamp efficiency. This investigation will allow one to find out the appropriate parameters for Ne/Xe DBD excilamps to improve their efficiency.

A one-dimensional hydrodynamic model of a dielectric-barrier discharge (DBD) in pure chlorine is developed, and the properties of the discharge are modeled. The discharge is excited in an 8-mm-long discharge gap between 2-mm-thick dielectric quartz layers covering metal electrodes. The DBD spatiotemporal characteristics at gas pressures of 15–100 Torr are modeled for the case in which a 100-kHz harmonic voltage with an amplitude of 8 kV is applied to the electrodes. The average power density deposited in the discharge over one voltage period is 2.5–5.8 W/cm³. It is shown that ions and electrons absorb about 95 and 5% of the discharge power, respectively. In this case, from 67 to 97% of the power absorbed by electrons is spent on the dissociation and ionization of Cl₂ molecules. Two phases can be distinguished in the discharge dynamics: the active (multispike) phase, which follows the breakdown of the discharge gap, and the passive phase. The active phase is characterized by the presence of multiple current spikes, a relatively high current, small surface charge density on the dielectrics, and large voltage drop across the discharge gap. The passive phase (with no current spikes) is characterized by a low current, large surface charge density on the dielectrics, and small voltage drop across the discharge gap. The peak current density in the spikes at all pressures is about 4 mA/cm². In the multispike phase, there are distinct space charge sheaths with thicknesses of 1.5–1.8 mm and a mean electron energy of 4.3–7 eV and the central region of quasineutral plasma with a weak electric field and a mean electron energy of 0.8–3 eV. The degree of ionization of chlorine molecules in the discharge is ~0.02% at a pressure of 15 Torr and ~0.01% at 100 Torr. The DBD plasma is electronegative due to the fast attachment of electrons to chlorine atoms: e + Cl₂ → Cl + Cl^–. The most abundant charged particles are Cl₂⁺ and Cl⁻ ions, and the degree of ionization during current spikes in the active phase is (4.1–5.5) × 10^–7. The mechanism of discharge sustainment is analyzed. The appearance of a series of current spikes in the active phase of the discharge is explained.

Specific features of the energy distributions of fast ions during neutral beam injection at the Globus-М tokamak are considered. Different loss mechanisms that can lead to the formation of a specific shape of the energy spectrum of superthermal ions are analyzed. The effect of sawtooth oscillations on the loss of fast ions is discussed.

Impurity injection into plasma caused by the sputtering of the wall coating in the L-2M stellarator during auxiliary electron cyclotron resonance heating leads to a change in the level of plasma density fluctuations with frequencies above 0.25 MHz: suppression of long-wavelength (k_⊥ = 2 cm^–1) density fluctuations in the edge plasma, intensification of short-wavelength (k_⊥ = 30 cm^–1) and long-wavelength (k_⊥ = 1 cm^–1) fluctuations at the midradius of the plasma column, and intensification of short-wavelength fluctuations (k_⊥ = 20 cm^–1) in the plasma center (including the gyroresonance region). At the same time, the level of fluctuations with frequencies below 0.25 MHz remains unchanged. In the edge plasma, a decrease in the plasma potential and suppression of its fluctuations is observed during impurity injection, which also causes an increase in MHD activity.

The implosion dynamics of a condensed Z-pinch at load currents of up to 3.5 MA and a current rise time of 100 ns was studied experimentally at the Angara-5-1 facility. To increase the energy density, 1- to 3-mm-diameter cylinders made of a deuterated polyethylene−agar-agar mixture or microporous deuterated polyethylene with a mass density of 0.03–0.5 g/cm³ were installed in the central region of the loads. The plasma spatiotemporal characteristics were studied using the diagnostic complex of the Angara-5-1 facility, including electron-optical streak and frame imaging, time-integrated X-ray imaging, soft X-ray (SXR) measurements, and vacuum UV spectroscopy. Most information on the plasma dynamics was obtained using a ten-frame X-ray camera (Е > 100 eV) with an exposure of 4 ns. SXR pulses were recorded using photoemissive vacuum X-ray detectors. The energy characteristics of neutron emission were measured using the time-offlight method with the help of scintillation detectors arranged along and across the pinch axis. The neutron yield was measured by activation detectors. The experimental results indicate that the plasma dynamics depends weakly on the load density. As a rule, two stages of plasma implosion were observed. The formation of hot plasma spots in the initial stage of plasma expansion from the pinch axis was accompanied by short pulses of SXR and neutron emission. The neutron yield reached (0.4–3) × 10¹⁰ neutrons/shot and was almost independent of the load density due to specific features of Z-pinch dynamics.

The most important parameter responsible for the quality of high-aspect structures produced by plasmachemical etching is the angular distribution of ions near the processed surface. In this work, the effect of collisions and gas pressure on the angular distributions of ions and chemically active radicals in the chamber of a high-pressure plasmachemical reactor with a remote plasma source is analyzed theoretically.

Two-dimensional numerical simulations of a dc discharge in a CH₄/H₂/N₂ mixture in the regime of deposition of nanostructured carbon films are carried out with account of the cathode electron beam effects. The distributions of the gas temperature and species number densities are calculated, and the main plasmachemical kinetic processes governing the distribution of methyl radicals above the substrate are analyzed. It is shown that the number density of methyl radicals above the substrate is several orders of magnitude higher than the number densities of other hydrocarbon radicals, which indicates that the former play a dominant role in the growth of nanostructured carbon films. The model is verified by comparing the measured optical emission profiles of the H(n ≡ 3), C₂^*, CH*, and CN* species and the calculated number densities of excited species, as well as the measured and calculated values of the discharge voltage and heat fluxes onto the electrodes and reactor walls. The key role of ion–electron recombination and dissociative excitation of H₂, C₂H₂, CH₄, and HCN molecules in the generation of emitting species (first of all, in the cold regions adjacent to the electrodes) is revealed.

The plasma parameter studies of the Nd:YAG (neodymium-doped yttrium aluminum garnet, Nd:Y₃Al₁₅O₁₂) crystal by using the fundamental (1064 nm) and second (532 nm) harmonics of Nd:YAG laser are reported. The electron temperature (T_e) and electron number density (N_e) were determined using the Boltzmann plot method and the Stark-broadened line profile, respectively. An increase in the plasma parameters have been observed with an increase in the laser irradiance for both laser modes. The electron temperatures were calculated in the range of 0.53–0.66 eV for 1064 nm and 0.47–0.60 eV for 532 nm, and the electron number densities were determined in the range of 7.43 × 10¹⁵–3.27 × 10¹⁶ cm⁻³ for 1064 nm and 1.35 × 10¹⁶–3.97 × 10¹⁶ cm⁻³ for 532 nm in the studied irradiance range of 1.19–12.5 GW/cm². However, the spatial evolution of the plasma parameters investigated up to 2.75 mm away from the target surface at a fixed laser irradiance of 6.51 GW/cm2 showed a decreasing trend. In addition, the estimated values of the inverse bremsstrahlung (IB) absorption coefficients at both laser wavelengths showed that the IB process is dominant for the 1064-nm laser.

Supersoliton (SS) can be mainly featured in two ways, namely, by focusing on subsidiary maxima on its electric field or by meeting the requirement that the appropriate Sagdeev pseudopotential (SP) has three local extrema between the equilibrium conditions and its amplitude. In this paper, by using the SP method, double layers and ion-acoustic SSs are studied in a plasma with Maxwellian cold electrons, nonthermal hot electrons, and fluid ions. The existence of the SS regime in parameter space is obtained in a methodical fashion. The existence domains for positive solitary waves are also presented. It is found that there is no SSs at the acoustic speed.