Argon Plasma Column Research Articles

Populations of excited atomic states along argon surface-wave plasma columns at low and intermediate pressures

The axial distributions of the electrons and 3p54s and 3p54p excited atoms in argon plasma columns sustained by traveling electromagnetic waves have been studied both experimentally and theoretically in the gas pressure range of 0.2–2.8 Torr. Various diagnostic methods (surface-wave interferometry, emission and absorption spectroscopy) have been used in data gathering. The theoretical model includes a self-consistent solution to the electron Boltzmann equation, electron energy balance equation, a set of balance equations for excited atoms and charged particles, the gas thermal balance equation, the wave dispersion relation, and the wave energy balance equation. The agreement between experimental data and theoretical results is very good.

Self-consistent axial modeling of surface-wave-produced discharges at low and intermediate pressures.

A model for description of the axial structure of a surface-wave-produced and -sustained plasma based on numerical calculation of a complete set of electrodynamic and kinetic equations is presented. The model includes a self-consistent solution to the electron Boltzmann equation, a set of particle balance equations for electrons, excited atoms, atomic and molecular ions, as well as Maxwell's equations with appropriate boundary conditions. A gas thermal balance equation is used to predict the neutral gas temperature self-consistently. Precise calculations of discharge characteristics of an argon plasma column sustained by an azimuthally symmetric surface wave at low and intermediate gas pressures have been performed. A comparison with available experimental data is done in order to test the validity of the model.

Modeling of an axially inhomogeneous microwave argon plasma column at a moderate pressure

In this article, we present theoretical results for axial distributions of charged particles and excited atoms in an axially inhomogeneous microwave sustained argon plasma column at a pressure of 1.8 Torr. The model includes two self-consistently linked parts—electrodynamic and kinetic ones. It is found that the electron temperature is not a constant along the column length. The dependence of the mean power required for sustaining an electron–ion pair in the discharge on the electron number density is obtained taking into account different elementary processes. The theoretical results are in agreement with the available experimental data.

Operation and output pulse characteristics of an extremely compact capillary-discharge tabletop soft-x-ray laser

We report on the operating parameters and laser output pulse characteristics of an extremely compact discharge-pumped 46.9-nm laser of a size comparable with that of many widely utilized visible and ultraviolet gas lasers. This capillary-discharge laser generated subnanosecond laser pulses with energies of as much as 25microJ by single-pass amplification in an 18.6-cm-long argon-plasma column. Measurements of the laser output energy, pulse width, far-field beam profile, and beam divergence are reported.

Axial distributions of metastable atoms and charged particles in an ultrahigh frequency argon plasma column at moderate pressures

The axial distributions of electrons, atomic and molecular ions as well as metastable atoms in an argon ultrahigh frequency (UHF) discharge at a moderate pressure sustained by a traveling electromagnetic wave have been theoretically obtained. The dependence between the absorbed wave power and the electron density necessary for plasma modeling has been numerically derived by taking into account both direct and step-wise ionization processes. The inclusion of step-wise ionization process leads to a lower electron temperature comparing to that calculated under the assumption that only the direct ionization takes place. The results have been compared with the experimental data for an UHF argon plasma at 1.8 Torr. A better agreement is achieved when the ionization by metastable–metastable collisions is taken into account together with the step-wise ionization.

Excitation Kinetic in an Argon Plasma Column Produced by a Surface Wave at Atmospheric Pressure

The excitation kinetic of a surface wave plasma produced by microwave is not well-known. This work presents an experimental study of this kinetic in Ar plasma. The 4p, 5p and higher levels of the Ar atomic system have been studied to determine their equilibrium separation. 5p and higher levels were found that followed an equilibrium distribution but the 4p levels were underpopulated in respect to this ditribution. Then the plasma is a recombining plasma which is in partial local thermodynamic equilibrium (pLTE). Using the phase diagram that is divided in different areas by the Green and Byron criterium we have established that the 4p levels are in saturation zone and the 5p levels and higher ones are in pLTE. For our discharge positions, those nearest to the excitator are the nearest to equilibrium population, because higher electron temperature and density imply that the plasma is nearer to thermodynamic equilibrium.

Far-forward and forward CW. CO2 laser scattering measurements of ion-acoustic waves in a quiescent plasma

Far-forward and forward collective light scattering measurements of externally launched ion-acoustic waves in a low density argon plasma column have been performed with a c.w. CO2 laser. The scattering processes taking place are described theoretically by the same general formalism. Wave dispersive and damping properties are studied consistently from 2 to 200 cm-1. Plasma parameters are deduced and compared to electrostatic probe and antenna measurements. It is found that high frequency wave properties are erroneously measured with a grid antenna.

Large volume, high density rf inductively coupled plasma

Magnetized argon plasma columns with peak densities of greater than 1018 m−3, lengths of 2 m, and full width half-maximum diameters of 0.3 m have been created at pressures of 40 mPa (3×10−4 Torr) with input powers of 1.5 kW of 7 MHz rf.

Hysteresis in a low-pressure argon discharge

Hysteresis in the discharge characteristics of a low pressure (p≂10−4 Torr), magnetized argon plasma column is reported. The hysteresis is associated with sudden jumps in plasma density and discharge current as either the discharge voltage or magnetic field strength is varied. A substantial change in the plasma density profile and the appearance of coherent plasma oscillations are also observed.

Radial variation of excited atom densities in an argon plasma column produced by a microwave surface wave

Radial variation of excited atom densities in an argon plasma column produced by a microwave surface wave

Radial variation of excited atom densities in an argon plasma column produced by a microwave surface wave

The radial variations of radiative and metastable atom densities in an argon plasma column produced by a microwave surface wave are obtained. A large variety of radial profiles is observed as a function of wave frequency (300–1000 MHz), gas pressure (50–200 mTorr), tube diameter (17.5–34 mm) and axial magnetic field. The results differ significantly from those reported for the dc positive column, where the radial distributions keep approximately the same J0 Bessel-like profile.

Laboratory Observation of a Very Low Frequency Instability in an Argon Plasma

Very low frequency (50 to 500 Hz) self-excited electrostatic waves are detected in a cylindrical argon plasma column with a weak axial magnetic field (10 to 100 G). The waves propagate azimuthally with a phase velocity in the electron diamagnetic drift direction, but with a speed at least an order of magnitude less than the ion acoustic speed. A number of known plasma instabilities are considered as possible explanations, though most of them do not seem to account for the observed characteristics of the waves.

Non-linear wave interaction in a magnetoplasma column 2. Experiment

Experiments on non-linear mode conversion in a cylindrical magnetoplasma column are described. An argon plasma column was produced by a diffusion source, and modulated simultaneously by a strong pump wave and a weaker higher frequency signal, to produce an idler wave at the difference frequency by non-linear interaction. The results are compared with theoretical predictions made in Part 1 of the paper for a cold, homogeneous, cylindrical column in an infinite, axial, static magnetic field. Good qualitative agreement is demonstrated, but quantitative comparisons are complicated by the discrepancies from precise synchronism observed for experimental conditions giving the strongest interactions.

Microwave plasma at atmospheric pressure and measurement of its density

At atmospheric pressure, a dense argon plasma column ( approximately 1014 cm-3) is created in capillary quartz tubes by a plasma surface wave. The electron density ne is measured through the reflectivity of the TE01 mode of an X-band waveguide crossed by the plasma. Discussion shows that ne is determinated with an error less than 20% if the plasma diameter is known accurately.

Electrostatic waves near the electron cyclotron harmonics

Electron cyclotron harmonic waves are investigated in an argon plasma column with axial magnetic field. Two coaxialT-shaped probes are used to launch and to detect the waves. By amplitude and phase radial measurements of the r.f. signal in the plasma we determine the wavelengths, the direction of the phase velocity and the region where the waves are generated. The results are limited to backward waves. Electron-neutral collision frequency effects on the amplitude of these waves have been also pointed out.

Transient flow and expansion of a pinch discharge plasma in self-induced magnetic fields

The formation of a dense argon plasma column in a linear Z-pinch and the ejection of the plasma through an axial orifice electrode are investigated photographically, and with electric, magnetic and piezoelectric pressure probes. The plasma-magnetic field interaction and gas dynamic flow transients in the exhaust are also studied. Pronounced orifice electrode polarity effects on the plasma acceleration and ejection mechanisms are observed. It is found that the distribution and dynamics of the ejected plasma cannot be established from luminosity profiles. The development of plane blast waves in the far field flow is identified.

Ion temperature and steady-state characteristics of a low density magnetoplasma column

Theory and experimental results are presented for the ion temperature and the steady-state characteristics of a low density magnetoplasma column. An Argon plasma column is generated using a Lisitano structure with 10-50 W of 2.8 GHz power. This method of plasma production results in radially varying end losses and radial transport related to ion perpendicular mobility and ion viscosity. Ion temperature is derived from an approximate transport theory and experimentally verified by measuring the Doppler broadening of Argon II 4610 AA radiation.

Dual Magnetic Probe System for Phase Measurements in Thermal Induction Plasmas

Previously reported measurements of rf magnetic field distributions in thermal induction plasmas by a water-cooled coil probe have been extended to phase measurements through the use of a second identical probe placed at a fixed position near the plasma boundary. From measurements of the vector sum and/or difference in addition to the individual signals from both probes, data for the variation of the phase angle with radius were obtained. The reliability of the method was checked with an electrolyte of known conductivity. Tests conducted at 2.6 MHz in a 14-cm-wide argon-plasma column at ambient pressure yielded a change in phase angle of 285 deg over the radius. The variations in phase and magnitude of the magnetic field were used to derive those of the induced electric field, the current density, the electrical conductivity, and the real part of the Poynting vector. From the latter a specific power input of about 1500 W/cm was obtained. A correlation of the conductivity data with temperatures derived from absolute line and continuum intensities is in good agreement with predictions from an equilibrium theory.

Axial Current Density Measurements in a Wide Unstable Plasma Column

Axial current density measurements are made in an argon plasma column by means of a multiring composite anode. The existence of an instability in a column which has an L/R of six is confirmed. The critical magnetic field Bc is determined and found to depend on the neutral gas pressure pg in proportion to pg0.45. In all cases, when B ≤ 1.36Bc, a reversion is observed to begin, back to a configuration similar to that which exists when B < Bc. When B < Bc, the axial current density on the tube axis is observed to increase with increasing B, indicating that a “modified Schottky” theory must apply.

Ion Temperature, Charge Exchange, and Coulomb Collisions in an Argon Plasma Column

Ion temperatures in a highly ionized argon plasma column have been measured with a Fabry-Perot interferometer. Plasma-column conditions were altered over a range of parameters from 10% to 70% ionized, electron temperature Te=2–10 eV, and magnetic field 1400–2800 G. In general, Ti is about one-tenth Te, determined by the balance between electron heating and charge-exchange cooling. The measured values of Ti agree fairly well with those calculated, using accepted values of Coulomb collision and charge-exchange cross sections. Alternately, the present data are processed to derive approximate values of the charge-exchange cross section to neutral argon as a function of Ar+ ion energy in the range 0.1–1.0 eV. It can also be concluded that the theoretical rate for electron-ion energy relaxation is not seriously in error.