Effect of air pressure on the spatial and emission characteristics of an aluminum laser torch under subthreshold conditions of ablation

The spatial characteristic of an aluminum laser-induced plasma are studied at a laser radiation intensity of (3.8–4.8) × 10⁸ W/cm² and an air residual pressure of 6.7–133.3 Pa. It is found that the duration of the aluminum plasma glow is 50 μs and decreases with decreasing laser power output. The glow intensity reaches a maximum at t = 1.4 μs and rises with laser energy. Typical sizes of the emitting area on the laser torch are determined.     

Effect of cathode surface state on the characteristics of pulsed hollow-cathode glow discharges

A study is made of the effect of pulse repetition rate (0.1−10³ s⁻¹) and average discharge current (0–1 A) on the breakdown delay time and burning voltage of low-pressure glow discharges (p<0.1 Pa) in an electrode system of the reverse magnetron type with a large cathode surface area (≈10³ cm²). It is shown that increasing the repetition rate leads to a many-fold reduction in the statistical spread in the delay time and in the discharge formation time, while the average discharge current has a significant effect on the burning voltage. The mechanism for the observed phenomena is interpreted qualitatively in terms of the presence of thin dielectric films on the cathode surface. Zh. Tekh. Fiz. 69, 20–24 (May 1999)     

The characteristics of high current amorphous silicon diodes

Amorphous siliconp−n junctions with various doping profiles have been prepared by the glow discharge technique and the effect of the barrier profile on electrical properties investigated. Current densities of up to 40 A cm⁻² with rectification ratios of 10⁴–10⁵ were obtained withn ⁺−ν−p ⁺ structures. The diode quality factor has also been investigated, both in the dark and under illumination.     

Excitation of metal spectra in combined glow discharge

Consideration is given to the special features of the excitation of spectra of metals (Ag, Au, Fe, Hg, Pt, and Sb) that manifest themselves in investigating spectral sources with combined glow discharge (glow discharge and capillary discharge). It is established that the proposed method for introducing a substance into a plasma makes it possible to realize two fundamentally different regimes: without evaporation and with complete evaporation and matrix atomization. It is shown that a spectral source of combined glow discharge with a nonequilibrium plasma has significant advantages for the most important analytical characteristics over quasi-equilibrium combined discharge at atmospheric pressure. The gold and platinum sensitivity in combined glow discharge is 10⁻⁷%, and the reproducibility of spectral determinations is 0.001–0.100, depending on the measurement range. A fractional separation of elements that makes it possible to analyze the phase composition of substances, particularly metals, with high sensitivity is revealed in glow discharge. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 2, pp. 143–147, March–April, 1997.     

Investigation of scaling laws as applied to the gas discharge in the case of a barrier-discharge-excited Kr/CCl<Subscript>4</Subscript> mixture

The electrical and luminescent characteristics of a barrier-discharge lamp filled with a Kr/CCl₄ (150: 1) mixture are experimentally studied versus the value of pd, which varies in the range (7.6–14) × 10³ Pa cm. When simulating the gas discharge using similarity parameters, the following relationships are fulfilled: for pd = const (p is the pressure, d is the interelectrode distance), the pulse duration and the mean current density are τ _j ∼ 1/p and 〈j〉 ∼ p; the surface charge density on the electrodes, σ ∼ const; the duration of the UV radiation pulse and the efficiency of UV radiation due to a KrCl* (222 nm) exciplex, τ_rad ∼ 1/p and η ∼ p ². The maximal radiation efficiency achieved in the experiments is about 13%. Deviations from the similarity laws for the gas discharge are related to the filamentary form of the observed discharge. Qualitative analysis indicates that similarity laws may be fulfilled for such a form of discharge as well but locally, within a single filament.     

A spectroscopic method in studying the dissociation kinetics of hydrogen molecules

The actinometry method was used to study experimentally the kinetics of the formation of hydrogen atoms in a low-pressure glow discharge initiated in a hydrogen-argon mixture. Dependences of the steady-state concentration of hydrogen atoms on current and pressure have been obtained for the ranges i/R=1 to 16 mA/cm and pR=0.7 to 10 Torr·cm, respectively. It has been established that for discharge-activated hydrogen, when the temperature of the system varies only slightly, the recombination frequency parameter v_H=γ_HD_H/Λ² can be used. Ivanovo State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 13–17, May, 1999.     

Crystallization of a dusty plasma in the positive column of a glow discharge

The formation of macroscopic ordered structures in the standing striations of a stationary glow discharge in Ne is observed. A Coulomb quasicrystal is formed by spherical glass particles with diameters of 50–63 μm and charge Z _p~7·10⁵ e. The interparticle distance is approximately 300 μm. This corresponds to a nonideality parameter Γ~5·10⁴, which leads to crystallization in the Yukawa model. The factors leading to the formation of a quasicrystal in the striations are discussed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 2, 86–91 (25 July 1996)     

Kinetics of the Excited States of Helium in an Atmospheric-Pressure Glow Discharge in a Helium-Nitrogen Mixture

Using an atomic-absorption spectral analysis technique, we determined the concentrations of helium atoms in states 2¹S, 2¹P, 2³S, and 2³P in an atmospheric-pressure glow discharge in helium (99.98%) and in a mixture of helium with nitrogen (99.5%He+0.5%N₂). It is shown that the population of the lower excited levels of helium atoms (n = 2) in its mixture with nitrogen is almost an order of magnitude smaller than in the case of a discharge in helium. The maximum of the concentration of excited atoms in a discharge both in helium and in its mixture with nitrogen is in the cathode region at a distance of about 0.1 mm from the cathode. The reaction of quenching of excited helium atoms by nitrogen molecules is responsible for the sharp decrease in the concentration of He(n = 2) on addition of nitrogen into helium. Presented at the 5th Symposium on Physics and Diagnostics of Laboratory and Astrophysical Plasmas, Belarus, Serbia and Montenegro, Minsk, Belarus, September 20–23, 2004; 12th International Congress on Plasma Physics, October 25–29, 2004, Nice, France. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 530–537, July–August, 2005.     

Excitation of the 7s2P1/2, 3/2 levels of Xe+ due to charge exchange in the negative glow of a glow discharge in a neon-xenon mixture

It is shown that in the negative glow of a glow discharge in mixtures of neon with xenon (xenon contents of 5.5·10^–3 to 2% of the total pressure P of the mixture) with P=15 torr and discharge currents i=40 mA (cathode diameter 30 mm), the 7s²P_{1/2, 3/2} levels of the Xe⁺ ion are excited by the charge exchange reaction Xe ^m + Ne⁺ Xe^+* + Ne.The effective cross section Q_3/2 for excitation of the 7s²P_3/2 level is estimated to lie between 10^–14 and 10^–15 cm² and Q_3/26.5 × Q_1/2, where Q_1/2 is the effective cross section for excitation of the 7s²P_1/2 level.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 3, pp. 96–99, March, 1979.     

Emission characteristics and parameters of a glow discharge in an argon-xenon-chlorine mixture

We present the results of a study of the emission characteristics of a low-pressure longitudinal glow discharge in an Ar-Xe-Cl₂ mixture (p = 0.5–6.0 kPa) and numerical modeling of its plasma parameters as a function of the parameter E/P. We optimized the average emission power of the discharge in the spectral range 160–310 nm as a function of the discharge current, the pressure and composition of the gas mixture. The plasma parameters were calculated for the optimal mixture (according to experimental data) and included obtaining the dependences of the electron transport and energy characteristics, the power losses per unit pressure in the discharge going toward elementary electron processes in the plasma, and also the ionization and attachment coefficients as a function of the parameter E/P. The results of numerical modeling of the glow discharge parameters allow us to provide a qualitative basis for the emission distribution in the spectrum of a plasma based on an Ar-Xe-Cl₂ mixture. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 4, pp. 563–567, July–August, 2008.     

Subnormal glow discharge in a Xe/Cl2 mixture in a narrow discharge tube

Electrical and optical characteristics of a subnormal glow discharge in a short (L=10 cm) discharge tube with an inner diameter of 5 mm are investigated. The dependences of the discharge current-voltage characteristic, the energy deposition in the discharge, the plasma spectral characteristics in the 130-to 350-nm wavelength range, the emission intensities of the XeCl(D-X) 236-nm and XeCl(B-X) 308-nm bands, and the total emission intensity in the range 180–340 nm on the pressure and composition of the Xe/Cl₂ mixture are studied. Two modes of glow discharge are shown to exist: the low-current mode at a discharge current of I _ch ≤2 mA and the high-current mode at I _ch >2 mA. The transition from one mode to another occurs in a stepwise manner. The increase in the chlorine content causes the discharge voltage and the energy deposition in the plasma to increase. At low pressures of the Xe/Cl₂ mixture (P≤0.7 kPa), stationary strata form in the cathode region. The lower the discharge current, the greater the volume occupied by the strata. This longitudinal discharge acts as a powerful source of continuous broadband emission in the range 180–340 nm, which forms due to overlapping the XeCl(D, B-X) and Cl ₂ ^* bands with edges at λ=236, 308, and 258 nm. The intensity of the 236-nm band is at most 20% of the total intensity of UV radiation. The maximum power of UV radiation (3 W at an efficiency of 8%) is attained at a xenon partial pressure of 250–320 Pa and a total pressure of the mixture of 2 kPa.     

Effect of gas pressure on amplitude and duration of electron beam current in a gas-filled diode

The parameters of an electron beam generated in helium in the pressure range p = 10⁻⁴−12 atm are studied. Nanosecond high-voltage pulses are applied to a gap between a tubular cathode and planar anode, which is made of 45-μm-thick AlBe foil. Behind the anode, an electron beam is detected at a helium pressure of 12 atm. The pressure dependence of the beam current amplitude shows three peaks at p ≈ 0.01, ≈ 0.07, and ≈ 3 atm. The beam-induced glow of a luminescent film placed behind the foil and the discharge glow at different helium pressures in the gas-filled diode are photographed.     

Model of pulsed electric discharge expansion in dense gas taking into account electron and radiative thermal conductivities. II. Energy balance and equation

Using an ionization sensor, it was found that weakly ionized plasma with an ionization degree larger than 10⁻⁶ is formed under exposure to UV radiation of a high-current pulsed electric discharge in gas (air, nitrogen, xenon, and krypton) at atmospheric pressure at a distance of ∼1.2–2.5 cm from the discharge boundary. It was shown that the structure of such discharge includes, in addition to the discharge channel, a dense shell and a shock wave, also a region of weakly ionized and excited gas before the shock wave front. The mechanism of discharge expansion in dense gas is ionization and heating of gas involved in the discharge due to absorption of the UV energy flux from the discharge channel and the flux of the thermal energy transferred from the discharge channel to the discharge shell due to electron thermal conductivity.     

Energy efficiency of femtosecond laser ablation of refractory metals

We present the results of an experimental study of the ablation energy thresholds and ablated mass for a number of refractory metals (Ti, Zr, Nb, Mo) by femtosecond (τ _0.5 = 45–70 fs) exposed to laser pulses in the ultraviolet — near infrared range (λ = 266, 400, 800 nm) under atmospheric conditions and under vacuum (p ~ 10^–2 Pa). We have analyzed the ablation efficiency (mass yield per unit energy of the acting coherent radiation) and ablation energy thresholds vs. the laser pulse duration and photon energy.     

Experimental and computational study of the passage of an electron beam through the curvilinear element of the Drakon stellarator system in a tenuous plasma

Results are presented from the first stage of studies on the passage of an electron beam with energy 100–500 eV in a magnetic field of 300–700 Oe through the curvilinear solenoid of the KRéL unit, the latter being a prototype of the closing segment of the Drakon stellarator system, in the plasma-beam discharge regime. The ion density at the end of the curvilinear part of the chamber, n _i ≈8×10⁸–10¹⁰ cm⁻³, the electron temperature T _e ≈4–15 eV, and the positions at which the beam hits the target for different distances from it to the electron source are determined experimentally. The motion of the electron beam is computationally modeled with allowance for the space charge created by the beam and the secondary plasma. From a comparison of the experimentally measured trajectories and trajectories calculated for different values of the space charge, we have obtained an estimate for the unneutralized ion density of the order of 5×10⁷ cm⁻³. Zh. Tekh. Fiz. 69, 22–26 (February 1999)     

Emission characteristics of a gas-discharge plasma of water vapor in the vacuum UV region of the spectrum

We present the results of a study of emission from a low-density water vapor plasma in the vacuum ultraviolet (VUV) region of the spectrum. The plasma was formed in a longitudinal glow discharge. We have studied the spectral characteristics of the plasma and also the dependences of the relative energy characteristics of the hydroxyl (OH) emission band on the discharge current and the helium partial pressure in a He-H₂O mixture in the spectral range 130–190 nm. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 831–833, November–December, 2006.     

Glow discharge in low-pressure excilamps

This paper presents a review of the principal publications devoted to the study of the spontaneous radiation sources operating in the uv region and discusses the properties of a low-pressure glow discharge in binary mixtures of inert gases with halides. It has been demonstrated that for variation of the excitation parameters over a wide range more than 10% emission efficiencies are realized for exciplex molecules and thatXeCl ^* andKrCl ^* molecules are formed in the main by the harpoon reaction. The current-voltage characteristics of glow discharges in cylindrical, coaxial, and planar excilamps and the parameters of the radiation ofXeCl ^*,KrCl ^*, andXeF ^* molecules generated on excitation of inert-gas-halide mixtures by normal and high-voltage glow discharges are presented. It is pointed out that low-pressure glow-discharge excilamps enable one to use simple power supplies. Institute of High Current Electronics. Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 50–66, June, 1999.     

Emission properties of a plasma cathode based on a glow discharge for generating nanosecond electron beams

The emission properties of a plasma cathode based on a nanosecond pulsed glow discharge with currents of up to 200A at a pressure of 5×10⁻² Pa are studied experimentally. Stable ignition and burning of the discharge are ensured if the current in the auxiliary pulsed discharge is 25–30% of that in the main discharge and its pulse duration exceeds that of the main discharge by more than an order of magnitude. Emission current pulses from the cathode with amplitudes of up to 140A fully reproduce the discharge current and are determined by the transparency of the grid anode. Zh. Tekh. Fiz. 69, 62–65 (November 1999)     

High-power discharge with a plasma cathode in dense gases

An attempt is made to create an electric-discharge source for pumping argon, krypton, and xenon dimer lasers. The device is based on a method proposed previously by the authors, wherein confinement of the discharge is achieved by removing the cathode spot from the main discharge region and closing the discharge to the spot along a narrow extended auxiliary plasma channel. The conditions for the formation of such a discharge are investigated. The high stability of the sparkless stage of the discharge permits the first-ever attainment of energy depositions at the level of 100 J/cm² at pressures ∼10 atm, a level several orders of magnitude higher than is attainable by conventional methods. A discharge cell and power supply system are designed for a multisectional discharge with an active length of 200 mm, and the reliability of the entire apparatus is demonstrated in long-term use. Zh. Tekh. Fiz. 67, 10–14 (November 1997)     

Non-self-sustained hollow-cathode glow discharge for large-aperture ion sources

A discharge system is proposed in which an auxiliary gas discharge is used to inject electrons into the cathode cavity of a hollow-cathode glow discharge. A study is made of the region of stable existence of a non-self-sustaining hollow-cathode discharge. It is shown that the injection of electrons permits a reduction to <10⁻² Pa in the minimum pressure at which a discharge can exist. It is shown experimentally that this discharge can be used to generate wide-aperture ion beams. Zh. Tekh. Fiz. 67, 27–31 (June 1997)