P. Kudrna

An experimental study of plasma density determination by a cylindrical Langmuir probe at different pressures and magnetic fields in a cylindrical magnetron discharge in heavy rare gases

This article presents an experimental study that contributes to the problem of interpretation of cylindrical Langmuir probe data obtained in a non-isothermal low-temperature plasma in magnetic field. A discussion on the influence of positive ion - neutral collisions on the charged particle density estimation is also given and the effect is demonstrated on the experimental data. A Maxwellian electron energy distribution is assumed throughout the present study. The Langmuir probe data are obtained in a cylindrical magnetron discharge in argon at pressures from 1.5 to 6 Pa and magnetic fields between 100 and 500 G. The radially movable Langmuir probe was made of either m or m diameter tungsten wire in order to investigate the effect of the probe dimensions on the estimated plasma density. The electron density is calculated from the electron current at the space potential (used as a reference) and from the OML collisionless theory. The ion density is calculated by using ABR - Chen theory without and with the correction due to the collisions of positive ions in the probe sheath. Also, the recent collisional positive-ion-collection-theory is used for comparison. The resulting numerical values of plasma density are compared over more than one order of magnitude change in the plasma density given by its radial dependence in the cylindrical magnetron discharge. Optical measurements were made in order to quantitatively assess the neutral gas temperature in the discharge and the density of particles in excited states that could induce secondary electron emission from the probe surface and thus apparently enhance the positive-ion density estimated from the probe positive-ion current. The effect of secondary electron emission from the probe surface on the probe data interpretation has been found small compared to the experimental error limits and consequently not substantial for our experimental conditions. In the range of our experimental conditions the ABR - Chen theory with the collisional correction gives the best agreement of the estimated numerical values of ion and electron densities in the whole range of its investigated change. Also from our results it follows that the effect of the magnetic field on the thinner-probe-electron-current at the space potential and hence on the reference-electron-density-estimation is negligible within the experimental uncertainties up to a magnetic field strength of 500 G which was the maximum used in our experimental study.

The recombination of H3+ ions with electrons: dependence on partial pressure of H2

Abstract We observed an increase in the effective recombination coefficient ( α eff ) of the recombination of H 3 + ions with electrons with a number density of H 2 , n( H 2 ) . With increasing n (H 2 ) from 1×10 11 to 2×10 12 cm −3 the α eff increased from 1.3×10 −8 to 1.5×10 −7 cm 3 s −1 . The dependence of α eff on n (H 2 ) indicates that the recombination of H 3 + ions in an afterglow plasma is a complex process in which collisions with H 2 take place. It is stressed that at conditions corresponding to plasma in interstellar space the dissociative recombination of H 3 + ions is very slow with a rate coefficient α⩽1.3×10 −8 cm 3 s −1 .

The recombination of and ions with electrons in hydrogen plasma: dependence on temperature and on pressure of H2

The recombination of H + 3 and H + 5 ions with electrons has been studied in a low-temperature, high-pressure flowing afterglow in a mixture of He-Ar-H 2 . At high H 2 number densities and lower temperatures, H + 5 ions are formed and the electron decay is controlled by their recombination with electrons (rate coefficient α(H + 5 ). At lower H 2 number densities, H + 3 ions dominate the plasma and the decay is controlled by these ions (rate coefficient α(H + 3)). In the intermediate pressure regime the decay of the afterglow plasma depends on the ratio R = [H + 5]/[H + 3] = [H 2 ]K C (T) and both rate coefficients. In the experiment the overall effective recombination rate coefficient, α eff , as a function of the H 2 number density was measured. Recombination coefficients α(H + 3 ) and α(H + 5 ) and the equilibrium constant, K C (T), were determined at several temperatures. The observed pressure and temperature dependences are in good agreement with thermodynamical data.

Radial behaviour of the electron energy distribution function in the cylindrical magnetron discharge in argon

The cylindrical magnetron consists of a coaxial inner cathode and an outer anode. The magnetic field is applied in the axial direction and is almost homogeneous in the whole magnetron volume. The electric field has radial direction and therefore the charged particles in the cylindrical magnetron discharge move under the influence of the × field. Due to its comparatively simple geometry, the cylindrical magnetron represents a suitable experimental tool that can be used to confirm theoretical results of modelling and theoretical studies of magnetrons in general. We studied experimentally the radial behaviour of the electron velocity distribution function (EVDF) in a cylindrical magnetron discharge in argon. We checked experimentally the anisotropy of the EVDF due to the influence of the magnetic field. For the assessment of the anisotropy of the EVDF we used a planar probe, whose collecting surface was adjustable at different angles to the direction of the magnetic field in the plane perpendicular to the electric field, as well as being movable in the radial direction. We found that in the measurable range of electron energies (energies greater than approximately 2 eV) and at magnetic fields up to 40 mT the anisotropy of the EVDF is not detectable within the experimental error limits. Therefore, for the study of the radial behaviour of the EVDF we used the thin (42 µm in diameter) tungsten cylindrical probe that was movable in the radial direction by a precise screw. For the theoretical determination of the EVDF in the cylindrical magnetron discharge we solved numerically the Boltzmann equation in a crossed × field, assuming the usual simplifications. The results of the calculation and the experiment in argon are compared and discussed.

A study of the electron energy distribution function in the cylindrical magnetron discharge in argon and xenon

We studied the behaviour of the cylindrical magnetron discharge in argon and xenon. We concentrated ourselves mainly on description of the transport of charge carriers in the region from the negative glow to the anode. We attempted to describe this transport using Monte-Carlo simulations as well as by standard transport parameters, mobility and diffusion coefficients. We also experimentally determined the radial shape of the electron energy distribution function (EEDF) including its detectable anisotropy caused by the presence of magnetic field. For the EEDF determination we used the planar probe whose collecting surface was adjustable at different angles to the direction of the magnetic field as well as movable in radial direction. The results of modelling and experiment are discussed.

Time-resolved measurement of plasma parameters in the far-field plume of a low-power Hall effect thruster

Time-resolved measurements using electrostatic probes are performed in the far-field plume of a low-power permanent magnet Hall effect thruster. These measurements are necessary in order to account for the non-stationary behavior of the discharge. The plasma potential is measured by means of a cylindrical Langmuir and a sufficiently heated emissive probe, the electron temperature and density are measured with a cylindrical Langmuir probe. The thruster is maintained in a periodic quasi-harmonic oscillation regime by applying a sinusoidal modulation to a floating electrode in the vicinity of the cathode in order to guarantee repeatable conditions for all measurements. The modulation depth of the discharge current does not exceed approximately 10%. In order to achieve synchronism, the frequency of the modulation has to be close to the natural frequency of the observed phenomena. It is different depending on whether the discharge current or the plasma potential is selected as a reference. The measurements show that the fluctuations of the electron density follow the discharge current fluctuations. The time evolution of the plasma potential and the electron temperature is similar. The time-averaged properties of the discharge remain almost uninfluenced by the modulation. Measurements of the plasma potential with the two different probes are in good agreement. The observed phenomena are similar for Xe and Kr used as propellant gases.

Measurement of plasma parameters in the far-field plume of a Hall effect thruster

The far-field plume of a 1.5 kW Hall effect thruster is mapped with a Langmuir probe and an emissive probe. Time-averaged measurements of the plasma potential, the electron temperature and the electron number density are performed for different operating conditions of the thruster. The influence of the discharge voltage, the cathode mass flow rate as well as the magnetic field strength is investigated. The plasma potential decreases from 30 V at 300 mm on the thruster axis to 5 V at 660 mm and at 60 ◦ , the electron temperature decreases from 5 to 1.5 eV. The electron number density drops from 3.5 × 10 16 to 1 × 10 15 m −3 in the far-field plume. The values of the plasma potential and electron temperature measured with the Langmuir probe and the emissive probe are in good agreement. (Some figures may appear in colour only in the online journal)

Electron kinetics in cylindrical discharges of magnetron configurations

Cylindrical dc magnetron discharges in argon in post-cathode (direct) and hollow-cathode (inverted) configurations are studied experimentally and theoretically. The electron component is modelled based on the non-hydrodynamic solution of the Boltzmann kinetic equation in the radially inhomogeneous electric and axially directed uniform magnetic fields. Two different representations of the ionization operator involving discrete and continuous spectra of secondary electrons are studied. The form of the ionization operator is shown to affect the electron distribution functions and macroscopic properties. The absolute values and radial profiles of the electron density, excitation and ionization rates and charged particle fluxes are obtained and compared for the direct and inverted magnetron configurations.

TiO2 nanoparticle detection by means of laser beam scattering in a hollow cathode plasma jet

The distribution of TiO2 nanoparticles in a plasma plume of a hollow cathode plasma jet is being studied in this work. The method is based on the detection of light from a laser beam scattered on nanoparticles by three detectors: a photodiode, an optical spectrometer and a digital photo camera. The Rayleigh–Mie theory was used as a basis for the description of the results. Two lasers with wavelengths of 446 and 661 nm were used as light sources. 2D maps of the scattered light intensity were assembled from photo camera pictures at different discharge parameters. Dependencies of the signal intensity on the O2 flow rate and wavelength are discussed. Direct measurements of particle size on the substrate were carried out by means of a scanning electron microscope in order to verify the agreement of results with the theory. Aside from nanoparticles we detected TiO2 columnar structures in the deposited samples. Crystalline structure of the samples was investigated using x-ray diffraction analysis.

Langmuir probe measurement of plasma splitting during pulsed laser deposition

The properties of plasma formed during pulsed laser deposition from a Bi–Sr–Ca–Cu–O target were measured using a Langmuir probe in the electron accelerating region “Plasma splitting” was observed at pressures in the range of 1–5 Pa and distances of the probe from the target of 3–14 mm.