Study of the plasma parameters in a high-current pulsed magnetron sputtering system

Results are presented from experimental studies of the current-voltage characteristics and spatial and temporal parameters of the plasma in a high-current pulsed magnetron sputtering system with a 10-cm-diameter plane disk cathode. It is shown that the plasma density in such a system is three orders of magnitude higher than that in conventional dc magnetron discharges and reaches 10¹³ cm⁻³ at a distance of 250 mm from the cathode at a peak discharge current of 500 A. The plasma propagates from the cathode region at a velocity of 1 cm/μs in the axial direction and 0.25 cm/μs in the radial direction. Optical emission spectroscopy shows that the degree of plasma ionization increases severalfold with increasing discharge current, mainly at the expense of the sputtered material.     

Epitaxial synthesis of diamond layers on a monocrystalline diamond substrate in a torch microwave plasmatron

The epitaxial growth of a diamond single-crystal film in a torch microwave discharge excited by a magnetron of a domestic microwave oven with the power of ≤1 kW in an argon-hydrogen-methane mixture with a high concentration of methane (up to 25% with respect to hydrogen) at atmospheric pressure on a sub-strate of a synthetic diamond single crystal (HPHP) with the orientation (100) and 4 × 4 mm in size is obtained. A discharge with the torch diameter of ∼2 mm and the concentration of the microwave power absorbed in the torch volume of >10³ W/cm³ is shown to be effective for epitaxial enlargement of a single crystal of synthetic diamond. The structure of the deposited film with the thickness up to 10 μm with high-quality morphology is investigated with an optical microscope as well as using the methods of the Raman scattering and scanning electron microscopy.     

Dispersal of Septoria nodorum Pycnidiospores by Simulated Raindrops in Still Air

Simulated raindrops, diameter c. 3 or 4 mm, fell 13 m down a raintower onto suspensions of Septoria nodorum pycnidiospores, depth 0.5 mm, or infected straw pieces. Splash droplets were collected on pieces of fixed photographic film. It was estimated that one drop generated c. 300 spore carrying splash droplets, containing c. 6000 spores, from a concentrated spore suspension (6.5 × 10⁵ spores/ml) and c. 25 spore-carrying droplets, containing c. 30 spores, from infected straw pieces (11 × 10⁶ spores/g dry wt). When the target was a spore suspension in water without surfactant, most spore-carrying droplets were in the 200—400 μm size category and most spores were carried in droplets with diameter >1000 μm. When surfactant was added to spore suspensions, most spore-carrying droplets were in the 0–200 μm category and most spores were carried in droplets with diameter 200–400 μm and none in droplets >1000 μm. Regression analyses showed a significant (p < 0.001) relationship between square root (number of spores per droplet) and droplet diameter; the slope of the regression line was greatest when surfactant was added to the spore suspensions. The distribution of splash droplets with distance travelled from the target was better fitted by an exponential model than by power law or Gaussian models. The distributions of spore-carrying droplets and spores with distance were fitted better by an exponential model than by a power law model. Thus regressions of log, (number collected) against distance were all significant (p < 0.01); the slopes of the regression lines were steepest when surfactant was added to the spore suspension. At a distance of 10 cm from target spore suspensions most splash droplets and spore-carrying droplets were collected at height 10–20 cm, with none above 40 cm; at a distance of 20 cm there were most at heights 0–10 cm and 40–50 cm.     

Spectroscopic and probe measurements of the electron temperature in the plasma of a pulse-periodic microwave discharge in argon

A pulse-periodic 2.45-GHz electron-cyclotron resonance plasma source on the basis of a permanent- magnet mirror trap has been constructed and tested. Variations in the discharge parameters and the electron temperature of argon plasma have been investigated in the argon pressure range of 1 × 10^–4 to 4 × 10^–3 Torr at a net pulsed input microwave power of up to 600 W. The plasma electron temperature in the above ranges of gas pressures and input powers has been measured by a Langmuir probe and determined using optical emission spectroscopy (OES) from the intensity ratios of spectral lines. The OES results agree qualitatively and quantitatively with the data obtained using the double probe.     

A Subthreshold High-Pressure Discharge Excited by a Microwave Beam: Physical Basics and Applications

A new form of discharge excited by a microwave beam in a high-pressure (up to atmospheric and higher) gas in free space and in a closed chamber is discussed. For the first time, the discharge was implemented by means of a gyrotron with a pulse power of 200 ≤ P ≤ 600 kW, a pulse duration of 0.5 ≤ τ ≤ 20 ms, and a wavelength of λ = 0.4 cm. Under deeply subthreshold conditions in atmospheric-pressure air, a plasma column with a length of L = 50 cm was generated by a microwave beam formed with the help of a quasi-optical transmission line. With the use of the MIG-3 gyrotron complex with the above parameters, generation of a plasma column with a length of several meters is possible in principle. The parameters and structure of the formation of the plasma investigated make it possible to class it as a self-non-self-sustained (SNSS) discharge, discovered and described for the first time at the Prokhorov General Physics Institute, Russian Academy of Sciences. One of the important applications of this type of discharge is plasmachemical cleaning of the urban air environment of hazardous contaminants.     

Effects of water conductivity on electrocommunication in the weak-electric fish Brienomyrus niger (Mormyriformes)

A characteristic electric organ discharge display in social encounters between mormyrid fish is a temporary discharge cessation. Using this response, we have investigated the useful range of electrocommunication under different water conductivity conditions in the mormyrid Brienomyrus niger. An individual fish was confined to a porous ceramic shelter tube and moved from a starting distance of 380 cm toward a similarly confined conspecific until discharge, cessation occurred. The moved fish was subsequently returned to its original, position. Water conductivity affects the peak-to-peak source voltage of the electric organ and the sensitivity of the fish's electroreceptors. Within a range of 10 to 36 000 μS/cm, the peak-to-peak amplitude of the electric organ discharge declined as a power function. At 120 μS/cm, the amplitude was 50%, and at 300μS/cm, 30% of the 10 μS/cm value. The interfish distance at which discharge cessation occurred and the associated electric field gradients were dependent on water conductivity and upon the spatial orientation of the two fish (end-to-end or parallel orientations of their shelter tubes). The respective ranges were from 135 cm and 0.02 mV/cm at 52 μS/cm (parallel orientation) to 22 cm and 0.36 mV/cm at 678 μS/cm (end-to-end orientation). When the data for both tube orientations were combined, the relationship between water conductivity (x) and the distance at which discharge cessation occurred (y) could be expressed by a power function, y=K·x^a (with K=10^2.97 and a=?0.56). When an electrically ‘silent’ fish was moved away from its conspecific, a discharge resumption in the form of a high-frequency rebound occasionally effected changes in the other fish's discharge activity at distances up to 157 cm (with an associated electric, field gradient of 0.01 mV/cm under the lowest conductivity condition).     

Ultrahigh charging of dust grains by the beam−plasma method for creating a compact neutron source

Generation of high-voltage high-current electron beams in a low-pressure (P = 0.1–1 Torr) gas discharge is studied experimentally as a function of the discharge voltage and the sort and pressure of the plasma-forming gas. The density of the plasma formed by a high-current electron beam is measured. Experiments on ultrahigh charging of targets exposed to a pulsed electron beam with an energy of up to 25 keV, an electron current density of higher than 1 A/cm², a pulse duration of up to 1 μs, and a repetition rate of up to 1 kHz are described. A numerical model of ultrahigh charging of dust grains exposed to a high-energy electron beam is developed. The formation of high-energy positive ions in the field of negatively charged plane and spherical targets is calculated. The calculations performed for a pulse-periodic mode demonstrate the possibility of achieving neutron yields of higher than 10⁶ s^–1 cm^–2 in the case of a plane target and about 10⁹ s^–1 in the case of 10³ spherical targets, each with a radius of 250 μm.     

Hydrodynamic model of a dielectric-barrier discharge in pure chlorine

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 Cl ₂ ⁺ 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.     

Plasma parameters in the channel of a long leader in air

The time evolution of the electric field in the leader channel and other characteristics of the leader plasma in long air gaps are simulated. Calculations are performed in the one-dimensional time-dependent model with allowance for the time-varying energy deposition in the channel, the channel expansion, and the nonequilibrium ionization kinetics in the leader plasma. The calculations show that, at a gas temperature of 4500–6000 K, associative ionization becomes a dominant ionization mechanism in the leader channel; as a result, the electric field decreases to 100–200 V/cm in 10^?4–10^?3 s under the conditions typical of the leader discharge. The calculated electric field agrees well with the data from the experimental modeling of long leaders by a spark discharge in short gaps.     

Microwave-induced pressure waves in a model of muscle tissue

Microwave-induced mechanical stress waves were studied in simulated muscle tissue. Pulsed microwave energy at 5.655 GH_z induced pressure waves that were recorded with a hydrophone transducer. Each pulse produced a peak power density greater than 1.5 kW/cm². Microwave absorption measurements within the model showed energy deposition to be mostly confined to a region within 2 cm of the irradiated surface. The average specific absorption rate (SAR) at the surface of the sample was about 100 W/kg. The microwave-induced stress wave propagated at a velocity of 1,600 m/sec with peak pressures of approximately 300 pascals and was detectable after having traveled a total distance of 0.61 m on a path that included two reflections at model-container interfaces.     

Generation of uniform low-temperature plasma in a pulsed non-self-sustained glow discharge with a large-area hollow cathode

Generation of plasma in a pulsed non-self-sustained glow discharge with a hollow cathode with an area of ≥2 m² at gas pressures of 0.4–1 Pa was studied experimentally. At an auxiliary arc-discharge current of 100 A and a main discharge voltage of 240 V, a pulse-periodic glow discharge with a current amplitude of 370 A, pulse duration of 340 μs, and repetition rate of 1 kHz was obtained. The possibility of creating a uniform gas-discharge plasma with a density of up to 10¹² cm^?3 and an electron temperature of 1 eV in a volume of >0.2 m³ was demonstrated. Such plasma can be efficiently used to treat material surfaces and generate pulsed ion beams with a current density of up to 15 mA/cm².     

Influence of the plasma density and heating power on the intensity of electron cyclotron emission in the L-2M stellarator

Results are presented from experiments on studying the plasma behavior in the L-2M stellarator in regimes with a high power deposition in electrons during electron cyclotron heating at the second harmonic of the electron gyrofrequency (X mode) at heating powers of P _in=120–400 kW and average plasma densities from n _e≤3×10¹⁹ to 0.3×10¹⁹ m^?3. It is shown that, as the plasma density decreases and the heating power increases, the electron cyclotron emission spectrum is modified; this may be attributed to a deviation of the electron energy distribution from a Maxwellian and the generation of suprathermal electrons. At low plasma densities, the emission intensity at the second harmonic of the electron gyrofrequency increases, whereas the plasma energy measured by diamagnetic diagnostics does not increase. This poses the question of the correctness of determining the plasma electron temperature by electron cyclotron emission diagnostics under these conditions.     

Nonsputtering impulse magnetron discharge

Experiments with quasi-steady high-current discharges in crossed E × B fields in various gases (Ar, N₂, H₂, and SF₆) and gas mixtures (Ar/SF₆ and Ar/O₂) at pressures from 10⁻³ to 5 Torr in discharge systems with different configurations of electric and magnetic fields revealed a specific type of stable low-voltage discharge that does not transform into an arc. This type of discharge came to be known as a high-current diffuse discharge and, later, a nonsputtering impulse magnetron discharge. This paper presents results from experimental studies of the plasma parameters (the electron temperature, the plasma density, and the temperature of ions and atoms of the plasma-forming gas) of a high-current low-pressure diffuse discharge in crossed E × B fields.     

Effect of hydrogen ratio on plasma parameters of N2-H2 gas mixture glow discharge

A dc plane glow discharge in a nitrogen-hydrogen (N₂-H₂) gas mixture has been operated at discharge currents of 10 and 20 mA. The electron energy distribution function (EEDF) at different hydrogen concentrations is measured. A Maxwellian EEDF is found in the positive column region, while in both cathode fall and negative glow regions, a non-Maxwellian one is observed. Langmuir electric probes are used at different axial positions, gas pressures, and hydrogen concentrations to measure the electron temperature and plasma density. The electron temperature is found to increase with increasing H₂ concentration and decrease with increasing both the axial distance from the cathode and the mixture pressure. At first, with increasing distance from the cathode, the ion density decreases, while the electron density increases; then, as the anode is further approached, they remain nearly constant. At different H₂ concentrations, the electron and ion densities decrease with increasing the mixture pressure. Both the electron and ion densities slightly decrease with increasing H₂ concentration.     

A biresonant plasma source based on a gapped linear microwave vibrator

The operating principle of a novel microwave plasma source—a linear microwave vibrator with a gap—is discussed. The source is placed on a microwave-transparent window of a chamber filled with a plasma-forming gas (argon or methane). The device operation is based on the combination of two resonances—geometric and plasma ones. The results of experimental tests of the source are presented. For a microwave frequency of 2.45 GHz, microwave power of ≤1 kW, and plasma-forming gas pressure in the range 5 × 10⁻²–10⁻¹ Torr, the source is capable of filling the reactor volume with a plasma having an electron density of about 10¹² cm⁻³ and electron temperature of a few electronvolts.     

Memory effect in the ignition of a low-pressure glow discharge in nitrogen in a long discharge tube

Results are presented from experimental studies of breakdown and the initial stage of a discharge in a long tube (with an interelectrode distance of 40 cm and a diameter of 2.8 cm) at a pressure of ～1 Torr and pulse discharge current of ～10 mA. Breakdown was produced by positive voltage pulses with a linearly growing leading edge with a steepness of dU/dt ～ 10⁶–10⁸ V/s. The time interval between pulses was varied from τ = 0.5 ms to 1 s, the pulse duration being 10 ms. The work was aimed at studying the memory effect of the discharge gap, namely, the influence of the previous pulse on the breakdown characteristics of the next one. In the experiments, the breakdown voltage was measured at different values of dU/dt and τ. It was found that the memory effect was absent at τ ～ 1 s. At the same time, an increase in the breakdown voltage with increasing dU/dt was observed. In the range of τ ≈ 50–200 ms, the breakdown voltage also did not depend on τ, but the memory effect took place. The memory effect in this case consisted in that the breakdown voltage decreased with increasing dU/dt, so that, at dU/dt ～ 10⁷ V/s, the breakdown voltage was two times lower than in the case of τ ～ 1 s. For τ ～ 1–10 ms, the memory effect manifested itself in that the breakdown voltage depended on τ: it could either decrease (the “normal” effect) or increase (the “anomalous” effect) with increasing τ. Breakdown of the discharge gap was preceded by the propagation of an ionization wave, except for the case of small τ values in the domain of existence of the anomalous effect. Estimates allowing one to qualitatively explain the experimental results are made.     

Parameters of radiation processes in the microwave resonant plasma

This work presents results of experimental studies of the spectral and photometric characteristics of optical radiation generated by a pulse-periodic microwave discharge close to ECR (2.45 GHz, average power of up to 200 W, argon pressure of 10^–4–10^–1 Torr). Under these conditions, dense (n _e = 10¹⁰–4 × 10¹¹ cm^–3) low-temperature (T _e = 3–5 eV) plasma is produced in the working volume at an ionization rate of 10^–3–5 × 10^–5. It is shown that the increase in the electron density near the upper boundary of the pressure range at a constant level of the input power leads to a drastic change in the type and spectral composition of plasma radiation and a jumplike increase in the light flux. The results of probe and optical measurements made it possible to determine the range of the operating parameters defining the character and parameters of the radiation processes under study.     

Plasma discharge and time-dependence of its effect to bacteria

Several types of plasma discharge have been proven to have a capacity for sterilization. Our goal is to introduce new nonthermal plasma pencil. We used it to sterilize different microbial populations with differing ages. We used a plasma discharge of the following characteristics: radio frequency barrier discharger at atmospheric pressure with a working frequency of 13.56 MHz, and the working gas used was argon. We performed 110 tests with the following microbial populations: Pseudomonas aeruginosa, Staphylococcus aureus, Proteus species, and Klebsiella pneumoniae. All populations were inoculated on the previous day and also on the day of our experiment. We made our evaluations the following day and also after 5 days, with all our microbial populations. Eradication of microbial populations is dependent on the plasma discharge exposure time in all cases. With regard to freshly inoculated microbes, we were able to sterilize agar with intensive exposure lasting for 10 s of colonies Pseudomonas, Proteus, and Klebsiella. The most resistant microbe seems to be S. aureus, which survives 5 s of coherent exposure in half of the cases. Using the lightest plasma discharge exposure, we achieved a maximum of 10⁴–10⁵ CFU/mL (colony-forming unit — CFU). Regarding older microbial populations inoculated the day before the experiment, we can only decrease population growth to 10⁵ CFU/mL approximately, but never completely sterilize. The plasma discharge with our characteristics could be used for the sterilization of the aforementioned superficially growing microbes, but does not sufficiently affect deeper layers and thus seems to be a limitation for eradication of the already erupted colonies.     

Development of Discharge in a Saline Solution at Near-Threshold Voltages

The development of a discharge in a point?plane gap filled with a saline solution with a salt content of 3% was studied experimentally. The duration of the voltage pulse applied to the gap was about 2 ms. Data are presented on the formation dynamics of gas microcavities at near-threshold voltages at which gas-discharge plasma appears in some microcavities. The cavities are conglomerates of microbubbles with a typical size of ≈100 μm. At the threshold voltage (≈750 V), the active electrode is covered with a gas layer and the gap voltage is in fact applied to this layer, which leads to the development of discharges in individual microbubbles. In this case, the discharge operates in the form of short current pulses. The number of microcavities filled with plasma increases as the voltage grows above the threshold value. At the plasma boundary, new microbubbles are formed, in which discharges are ignited. As a result, the plasma front propagates from the active electrode into the gap with a characteristic velocity of 10³ cm/s.