Time and space resolved Langmuir probe measurements of a pulsed vacuum arc plasma

The time and space evolution of pulsed vacuum arc plasma parameters have been measured using a single cylindrical Langmuir probe in a free expansion cup. Electron density n_e, effective electron temperature T_eff and electron energy distribution function (EEDF) are derived from the I-V curves using Druyvesteyn method. Results show that during the discharge time, the electron density n_e is between 0.27 and 1.82 × 10¹⁸ m⁻³ and the effective electron temperature T_eff is between 6.14 and 14.72 eV, both of which decrease as a function of the discharge time. The electron energy distribution function (EEDF) is no-Maxwellian since the high-energy electrons depart from the Maxwellian distribution. Due to the plasma expansion, the electron density n_e decreases as increase of the expansion distance, but the effective electron temperature T_eff is weakly dependent on the distance in the free expansion cup.     

Plasma measurements in pulse discharge with resistively heated emissive probe

A resistively heated emissive probe has been developed to work in low-pressure air plasma produced by 100 Hz pulsed DC source. The evolution of the discharge and consequent rapid changes in plasma potential and electron temperature are characterized for different fill pressures at constant input voltage of 300 V. The floating point method in the strong emission regime is applied to determine the plasma potential. Emissive probe responds to rapid changes in the discharge current during different stages of the pulse cycle. The electron temperature is determined from the potential difference of hot probe in the strong emission regime and the cold one incorporating the space charge effects of the hot probe. Temporal measurements of V _p and T _e describe the development and characteristics of the emissive probe technique for fast measurements in pulsating discharges.     

The features of vacancy formation at low temperatures

Expressions for the calculation of the thermodynamic potential, enthalpy, entropy s _v, and volume of vacancy formation in a simple crystal at temperatures close to absolute zero are derived. It is established that the vacancy concentration Φ as a function of the temperature T exhibits a minimum at a certain temperature T ₀. At T<T ₀, the Φ(T) function increases with decreasing temperature; the s _v(T) value changes sign at T=T ₀ and is negative at T<T ₀. It is shown that the existence of the “zero-point vacancies” does not violate the third law of thermodynamics. Three new effects are predicted, and the prospects for their experimental observation are considered.     

A two-dimensional mathematical model of a short vacuum arc in external magnetic field: results of numerical calculations

The paper deals with the investigation of the impact made by two-dimensional effects on the process of passage of current in a short vacuum arc in an axial magnetic field. A two-fluid mathematical model is used, which is based on hydrodynamic and electrodynamic equations. The axial magnetic field B _z affects significantly the magnitude of two-dimensional effects: the two-dimensional effects increase with decreasing B _z . The simulation results demonstrate that the contraction of plasma density exceeds that of current density. The distribution of anode drop of potential on the anode surface is nonuniform; in the case of certain (critical) values of current, the anode drop goes to zero on the external boundary of plasma. The dependence of the critical current on B _z is determined. The distribution of current density on the starting plane is nonuniform with a maximum on the axis, and the ion trajectories are inclined to the discharge axis. The possibility is discussed of matching the solution in the plasma region of vacuum arc with that for cathode flames.     

Langmuir probe diagnostics of microwave electron cyclotron resonance (ECR) plasma

A Langmuir probe diagnostics is done on the microwave ECR generated plasma in a 2.45 GHz, 1.5 kW facility set up in our laboratory (for thin film deposition) by inserting a probe in the plasma close to substrate location (640 mm away from main ECR zone). A program using Graphical User Interface (GUI) was used for data analysis of I-V probe characteristics to obtain the radial electron energy distribution function (EEDF) in plasma. Plasma parameters such as charged particle density (n_e and n_i), electron temperature (T_e), plasma potential (V_pl) and floating potential (V_float) were estimated at substrate location for two incident microwave power levels at a fixed operating pressure. These parameters were estimated by different methods like orbital motion limited (OML) theory, electron energy distribution function (EEDF) and conventional method. The results obtained by the different methods are compared and observed differences are explained. The results indicate that even though the diffusion of plasma at the substrate location is mainly forced by particle collisions that lead to radial plasma uniformity, it still shows a non-Maxwellian behavior for the electrons with two groups having different energies.     

Diagnostics of pulsed vacuum arc discharges by optical emission spectroscopy and electrostatic double-probe measurements

In this work the study of plasma characteristics by means of two of the most common techniques employed by the scientific community dedicated to the experimental plasma research is presented. The plasma was generated in a vacuum reaction chamber which was filled with hydrogen gas. Inside the chamber, two opposite electrodes were placed: the cathode, which was formed by a target of highly oriented pirolitic carbon and the anode. The electron temperature T_e and the electron density n_e were measured by using optical emission spectroscopy and electrostatic double probe, obtaining very close values for each case. T_e was calculated as approximately 0.7 eV and n_e of the order of 10¹³ cm⁻³. The optical emission measurement allows one to identify the substances that are in the plasma like C I, C II, C III, H I and H₂ and some possible reactions. The double-probe technique showed the plasma potential of about 24 V. The characteristic curve of the double probe exhibited oscillational plasma instabilities which could be attributed to the charge density variation or other factors, such as the employed AC signal and the geometric probe.     

Low-voltage beam discharge in light inert gases to solve problems of voltage stabilization

By means of the flat one-sided probe, the electron velocity distribution function is investigated along with plasma parameters distribution and electrokinetic characteristics in short (without positive column) low-voltage beam discharge in helium, neon, and argon. With the pressure increase, the discharge voltage obtains the value of the excitation potential of the metastable level of the filling gas, U _m. The results of the probe measurements of the electron velocity distribution function make it possible to calculate the generation functions of the inelastic process and to make the conclusion that, at attaining the critical pressure, P _cr, when the processes of atom stepwise ionization begin to dominate over the direct ionization, the discharge burning voltage drops below the ionization potential, U _ion. Based on the performed studies, a technique and a device are proposed to stabilize the voltage within the range of 9–50 V by filling the interelectrode gap with inert gases of different excitation and ionization potentials.     

The parameters of nonequilibrium microwave discharge in nitrogen in a tube in a rectangular waveguide

Computer codes are developed for the processing of emission spectra of nonequilibrium plasma in nitrogen for the purpose of obtaining information about the translational T _g and rotational T _rot temperatures, the populations of vibrational levels in the ground electron and electron-excited states, the electron energy distribution function, the electron concentration N _e , and the electric field intensity E. The computer codes are used to determine the parameters of microwave-discharge plasma in nitrogen in discharge systems of two types, namely, in a discharge tube (with a radius of 1 cm), which crosses a rectangular waveguide (plasmatron on the H ₁₀ wavelength, at a pressure of 1.7 torr and absorbed power density of 1.5 W/cm³), and in a discharge section of similar structure on the basis of prismatic resonator (at a pressure of 1.0 torr and absorbed power density of 0.4 W/cm³). The mechanisms of population of the N₂(C ³Π_u) state are treated.     

Plasma-enhanced chemical vapor deposition of nanocrystalline diamond

Nanocrystalline diamond films have attracted considerable attention because they have a low coefficient of friction and a low electron emission threshold voltage. In this paper, the author reviews the plasma-enhanced chemical vapor deposition (PE-CVD) of nanocrystalline diamond and mainly focuses on the growth of nanocrystalline diamond by low-pressure PE-CVD. Nanocrystalline diamond particles of 200–700 nm diameter have been prepared in a 13.56 MHz low-pressure inductively coupled CH₄/CO/H₂ plasma. The bonding state of carbon atoms was investigated by ultraviolet-excited Raman spectroscopy. Electron energy loss spectroscopy identified sp²-bonded carbons around the 20–50 nm subgrains of nanocrystalline diamond particles. Plasma diagnostics using a Langmuir probe and the comparison with plasma simulation are also reviewed. The electron energy distribution functions are discussed by considering different inelastic interaction channels between electrons and heavy particles in a molecular CH₄/H₂ plasma.     

Optimization of the heating of a metal by an electric arc

We investigate and generalize the results of experimental measurements of heat fluxes in a metal anode from a plasma jet of a plasmatron with an external arc (PEA) whose operating conditions vary over a wide range.Notation efficiency of heating of the metal - Q_el heat flux into the calorimeter - I current of the electric arc - U voltage drop across the electric arc - V linear velocity of rotation of the sensor - r_o thermal radius of arc spot - _o time until surface of calorimeter begins to melt - t_p melting point of copper - t_o initial temperature of calorimeter surface - q specific heat flux in the heating spot - a thermal diffusivity - D diameter of rotating calorimeter - t₁ temperature of surrounding medium - t₂ temperature of the surface of the rotating calorimeter - G argon flow rate - h_s specific enthalpy of argon in the region near the anode - h_a specific enthalpy of argon at the surface of the calorimeter - k Boltzmann constant - T_e electron temperature in the anode region - e charge of the electron - U_a potential drop at the anode - _a work function of electrons leaving copper - L length of open part of the arc—distance from the cut in the nozzle of the PEA to the sensor - h depth of cathode insertion into the nozzle—distance from the cut in the PEA nozzle to the cathode - d diameter of the PEA nozzle - U _a ^* effective voltage equivalent Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 4, pp. 687–691, October, 1980.     

Radial structure of low pressure rf capacitive discharges

This paper studies the glow intensity distribution of the discharge plasma against the tube radius and reports the radial profiles of electron temperature and plasma concentration in the rf capacitive discharge registered with a Langmuir probe. An abrupt increase of electron temperature and glow intensity near the tube wall in the weak-current α-mode of the rf capacitive discharge is revealed, the radial distribution of plasma concentration and ion flow to the electrodes possessing a maximum near the radial sheath boundary. In the γ-mode of the rf capacitive discharge the electron temperature decrease in the total plasma volume leads to an electric field weakening and the peak of the glow intensity near the tube wall vanishes. The radial sheath thickness in the α-mode of the rf capacitive discharge obtained with 2D simulation experiences pulsations during the rf field period, the changing radial electric field heating electrons and increasing the plasma concentration near the boundary of the radial sheath.     

Plasma-enhanced chemical vapor deposition of nanocrystalline diamond

Nanocrystalline diamond films have attracted considerable attention because they have a low coefficient of friction and a low electron emission threshold voltage. In this paper, the author reviews the plasma-enhanced chemical vapor deposition (PE-CVD) of nanocrystalline diamond and mainly focuses on the growth of nanocrystalline diamond by low-pressure PE-CVD. Nanocrystalline diamond particles of 200–700 nm diameter have been prepared in a 13.56 MHz low-pressure inductively coupled CH₄/CO/H₂ plasma. The bonding state of carbon atoms was investigated by ultraviolet-excited Raman spectroscopy. Electron energy loss spectroscopy identified sp²-bonded carbons around the 20–50 nm subgrains of nanocrystalline diamond particles. Plasma diagnostics using a Langmuir probe and the comparison with plasma simulation are also reviewed. The electron energy distribution functions are discussed by considering different inelastic interaction channels between electrons and heavy particles in a molecular CH₄/H₂ plasma.     

Work function variations of incandescent filaments during self-cooling in vacuum

A novel method of dynamic measurement of work function (WF) variations of hot metal filaments is described. It is essential in this method that electron emission current (I_e) is recorded during filament self-cooling when no heating power is supplied, thereby I_e is not disturbed by the potential gradient along the filament. WF shift due to the presence of a low-pressure gas, where the main active compounds are O₂ and H₂O, is calculated from an equation derived on the basis of the Richardson formula. The relative increase of WF found by this method was 5 times larger for tungsten than that for tantalum over the entire temperature range from 900 to 1800 K. Our method may be used in research studies of adsorption-related phenomena on metallic surfaces at high temperatures.     

Double layer onset inside the near-electrode sheath of a RF capacitive discharge in oxygen

This paper reports the axial profiles of the electron temperature T_e in the RF capacitive discharge in oxygen recorded with a probe technique. We observed the T_e peaks near the boundaries of the sheaths as well as inside the near-electrode sheath. The T_e peak inside the sheath is, probably, due to the formation of the double layer at the anode phase of this near-electrode sheath. The assumption of a double layer formation is also supported by the photos of the sheath glow making evident the bright region inside the sheath. The results of our measurements agree with our fluid simulation satisfactorily.     

On the theory of recombination of ultracold plasma

Results are given of the solution of an algebroid set of kinetic differential equations of populations of states of excited atoms in recombining plasma. Homogeneous nonequilibrium two-temperature hydrogen plasma with initial electron temperatures T _e0 = 1, 6, 13.33 K and initial density n _e0 = (1–2.7) × 10⁹ cm^?3 is considered. Comparison is made with theoretical and experimental data. The question is considered of the validity of the formulas for the probabilities of quenching an excited atom by electron impact in ultracold plasma (T _e0 < 100 K). The calculation is performed for weakly nonideal (γ _e0 < 1) and nonideal (γ _e0 ≥ 1) plasma. Dependences are obtained of the concentration of excited atoms and free electrons, the temperature of free electrons, and the nonideality parameter on the time and on the distribution of excited atoms with respect to the main quantum number.     

Penetration and failure of lead and borosilicate glass against rod impact

This paper presents the experimental design and results for gold rod impact on DEDF (5.19 g/cm³) and Borofloat (2.2 g/cm³) glass by visualizing simultaneously failure propagation in the glass with a high-speed camera and rod penetration with flash radiography. At a given impact velocity, the velocity of the failure front is significantly higher during early penetration than during steady-state penetration of the rod. For equal pressures but different stress states, the failure front velocities determined from Taylor tests or planar-impact tests are greater than those observed during steady-state rod penetration. The ratio of average failure front velocity to rod penetration velocity decreases with increasing impact velocity (v_p) in the range of v_p=0.4–2.8 km/s. As a consequence, the distance between the rod tip and the failure front is reduced with increasing _vp$v_{\mathrm{p}}$. The Tate term R_T increases with impact velocity.     

The Vibrational Temperature in High-Enthalpy Jets of Atmospheric-Pressure Nitrogen Plasma

The paper deals with the spectroscopic determination of the temperature T _v of population of low-lying vibrational levels of radiating electron terms of an N₂ molecule and N ₂ ⁺ molecular ion in different cross sections of a high-enthalpy jet of atmospheric-pressure nitrogen plasma. Use is made of the method of optimal fitting of the measured and simulated rovibronic spectra in some selected spectral range of the (0–1) and (1–2) bands of the first negative system of N ₂ ⁺ ion and of the method of finding T _v by the integral intensities of molecular bands (sequence Δv = −1 of the second positive system of N₂). The obtained values of vibrational temperature are compared to the previously found values of rotational temperature T _r and electron temperature T _e . The pattern of variation of temperatures along the plasma jet in the zone of relaxation is discussed.__________Translated from Teplofizika Vysokikh Temperatur, Vol. 43, No. 4, 2005, pp. 501–509.Original Russian Text Copyright © 2005 by A. A. Belevtsev, E. Kh. Isakaev, A. V. Fedorov, and V. F. Chinnov.     

High-current electron beam in a hollow-cathode gas discharge at elevated pressures (∼100 Torr)

A discharge with plasma filling a flat-bottom cavity of depth δ in the cathode, partly closed by a dielectric plate with a hole (determining the aperture of the discharge between the cavity bottom and the anode), has been studied. In a discharge cell of type 1 with δ = 0.5 mm and a hole diameter of 22 mm, a pulsed electron beam was obtained with a duration of t _EB = 700 ns and a beam current j _EB approximately 10 times greater than that (j _AD) of the equivalent anomalous discharge (at fixed discharge voltage U and gas pressure p _He = 3.5 Torr). An electric field with the direction opposite to the field of applied voltage appeared at the cathode that was related to a space charge formed at the cathode plasma boundary, which could not follow a rapid drop of voltage across the discharge gap. In a discharge cell of type 2 with δ = 0.5 mm and a narrow slit (S = 0.1 × 5 cm²) in the dielectric plate, a pulsed electron beam was obtained with a duration of t _EB = 2 ns and a beam current of j _EB = 0.7 kA/cm² (j _EB/j _AD = 1.5) at U = 4.2 kV and p _He = 50 Torr.     

The Rotational Temperature in High-Enthalpy Jets of Atmospheric- Pressure Nitrogen Plasma

The method of comparison of the measured and model spectra of radiation of the (0-0) and (0-1) bands of the first negative system of N ₂ ⁺ ion and zero sequent bands (v = 0) of the "violet" system of CN is used to find the rotational temperature T _r in different cross sections of a high-enthalpy jet of atmospheric-pressure nitrogen plasma. Also determined in the far zone of jet relaxation is the temperature T _v of population of low-lying vibrational levels of the B ²⁺ state of a CN molecule. The translational temperature of heavy particles T _m is estimated. The obtained values of the temperature T _r are compared to the axial values of the electron temperature T _e .     

SO2 and NOx removal by electron beam and electrical discharge induced non-thermal plasmas

An installation containing a DC negative corona discharge reactor, a pulse corona discharge reactor and a combined electron beam and microwave induced plasma reactor is presented. SO₂ is removed up to 42% through spontaneous reaction with ammonia without electron beam or microwave irradiation at the temperature below 70 °C. For the same removal efficiency of 98% for SO₂ and 80% for NO_x at separate EB irradiation of 40 kGy, the required absorbed dose is about two times smaller for simultaneous electron beam and microwave irradiation. The SO₂ removal efficiency of simultaneous DC or positive discharge and microwave discharge is higher than separate DC, pulse and MW discharge. Also, the applied voltage level at which the removal efficiency reaches the maximum value is less than for the separate application of DC or pulse discharge. The NO_x removal efficiency of DC or pulse discharge suffers little change by additional use of the microwave energy.