Table of contents

A floating hairpin resonance probe has been used for the first time to measure the spatial and time evolution of local electron density in a laser produced plasma expanding in vacuum. The measured variation in electron density agrees closely with the variation of ion charge density as measured with a time-of-flight planar Langmuir ion probe confirming the reliability of Langmuir probe in the laser produced plasma.

The aim of this paper is to improve our knowledge concerning the power balance at the electrode surfaces in the case of an electric arc of short duration with a small electrode gap burning in air at atmospheric pressure.

With this aim in view, we propose a simple method using the experimental results obtained through the observation of the tracks left by the arc and a numerical simulation of the thermal phenomena occurring in the electrode during the arc heating.

This method, based on the analysis of the compatibility between experimental results and modelling results, allows us to assess a zone of possible values for the main parameters of the arc root (power and surface power density brought by the arc to the electrodes). A simple usual volt equivalent approach of the power balance is proposed. Calculations and experiments have been conducted for several copper anodes and cathodes. The values for the volt equivalent at the anode are found in the range 9–12.6 V, for the cathode 5.4–9 V. The values for the surface power density are found to be near 6.5 × 10⁹ W m⁻² at the cathode and 6 × 10⁹ W m⁻² for the anode.

The operation of ferroelectric plasma sources (FPSs) under the application of driving pulses with different amplitudes (4–40 kV) was studied. It was found that the dense plasma formation during the fast fall (a few tens of nanoseconds) in the driving pulse is accompanied by the generation of a highly diverging (∼180°) neutral flow with a velocity of ∼7 × 10⁷ cm s⁻¹ in addition to the electron/ion and charged micro-particle flows which were studied in earlier research. It was shown that the velocity and intensity of the generated neutral flow remained the same for different parameters of the driving pulse. In addition a flux of x-rays with an energy of ∼20 keV was observed for ∼2 µs after the ending of the 40 kV driving pulse. A model of neutral emission based on micro-explosions of ferroelectric ceramics is suggested.

A two-dimensional plasma model that includes the full set of Maxwell equations is used to understand the physics of very high frequency capacitively coupled plasmas. The effect of radio frequency (RF) source power, inter-electrode gap and gas mixture (Ar, Ar/SF₆, Ar/CF₄) on the plasma characteristics is investigated. The computational results show that the plasma spatial profile is influenced by both electrostatic and electromagnetic effects. The electrostatic power deposition is stronger at the electrode edges due to electric field enhancement at corners. Therefore, when the electrostatic effects are dominant, the plasma density peaks off-axis. Due to a standing electromagnetic wave in the chamber, the electron density peak moves to the chamber center under conditions where electromagnetic effects become strong. Inductive heating due to the radial electromagnetic electric field can also influence the plasma spatial profile. The relative importance of electromagnetic and electrostatic effects is found to be a function of the RF source power, the inter-electrode gap and the plasma electronegativity. While the electron density peaks on-axis at a low source power, inductive power deposition at higher source powers shifts the electron density peak towards the electrode edge. Electrostatic power deposition makes the plasma more uniform at smaller inter-electrode gaps. Due to a lower electron density and a larger applied RF potential, electrostatic effects become more dominant in electronegative discharges.

Optical emission spectroscopy (OES) and Langmuir probe measurements are used to determine the electron density and the electron energy distribution function (EEDF) in an inductively coupled plasma (ICP) intended for sterilization of materials in medical applications. Measuring the EEDF with the Langmuir probe in the ICP discharge is limited to energies below 11 eV. Using OES the EEDF can be determined between 1.5 and 30 eV (in a He : N₂ mixture). These methods are reliably compared in an ICP in a He : N₂ mixture. Both diagnostics complement one another to determine the EEDF in a wide kinetic energy range.

The effect of ion temperature together with the concentration of low temperature nonisothermal electrons and their temperature contributions to the excitation of nonlinear solitary waves is studied in an inhomogeneous plasma having negative ions under the influence of a magnetic field. Unlike usual negative-ion-containing plasmas, the present plasma with nonisothermal electrons does not support the rarefactive solitons and only the compressive solitons evolve for two types of modes called the fast mode and slow mode. The fast compressive solitons corresponding to the fast mode propagate with higher amplitude in comparison with the slow compressive solitons, which occur only under a limiting condition on the obliqueness of the wave propagation with the magnetic field. The effect of ion temperature is found to be more significant when fewer negative ions are present in the plasma. Both solitons are found to be sensitive to the concentration and thermal motions of the low temperature electron species. With regard to the dependence of soliton characteristics on the magnetic field, it is observed that both solitons evolve with larger size when the wave propagates at a larger angle to the magnetic field and the magnetic field effect is found to narrow the solitons.

Fireballs are discharge phenomena on positively biased small electrodes in plasmas. The discharge arises from electron energization at a double layer. Fireballs can collect relatively large electron currents from the ambient plasma. Fireballs can become unstable to relaxation oscillations. This paper addresses the space–time evolution of pulsed fireballs. Growth and collapse of fireballs produce large density and potential variations near the electrode which couple into the background plasma production. Unstable fireballs emit bursts of fast ions and ion acoustic waves. High-frequency emissions near the electron plasma frequency have been observed and associated with the sheath–plasma instability rather than electron beam–plasma interactions. New shapes of fireballs have been observed in dipole magnetic fields.

In this paper, we report the influence of the various stages of the preionized high power pulsed magnetron discharge on the saturated ion substrate holder current. Our system allows superposition of a preionization low current dc discharge with high voltage pulses applied directly on the magnetron cathode. This system is characterized by a very fast and perfectly reproducible discharge current rise. For a 33 mm copper target, Ar pressure of ∼1 Pa, voltage applied in a pulse of ∼1 kV, the maximum cathode current of ∼40 A is reached in 6 µs. The dependence of the saturated ion substrate holder current was analyzed for varying time duration of the high power pulse from 2 up to 8 µs by 0.5 µs steps. It allows the discrimination of the contribution of elemental temporal intervals to the overall saturated ion substrate holder current. This analysis led to the conclusion that the transport of ballistic ions during the current pulse and in the afterglow is independent of time. We concluded that space charge effects are negligible for both discharge and post-discharge conditions and that electrons act as a neutralizing background. Finally, on the basis of a phenomenological kinetic model for the electron transport, physical explanations of these results are proposed which involve the transverse diffusion of low energy electrons out of the magnetized glow region through electron–ion Coulomb collisions.

We developed a cryo-microplasma, which can continuously control gas temperature below room temperature and below the freezing point of water. To develop the cryo-microplasma, we first developed an atmospheric-pressure low-temperature microplasma that can suppress the increase in its gas temperature. Helium gas was employed, which was generated in open air. The average estimated electron density and temperature were 10⁸–10⁹ cm⁻³ and 4–5 eV, respectively, independent of the applied voltage. Then, helium gas, which was the working gas of the atmospheric-pressure low-temperature microplasma, was cooled by liquid nitrogen to generate an atmospheric-pressure cryo-microplasma in open air. We observed the generation of frost around the quartz tube in which the plasma was generated and an increase in atomic oxygen emission by optical emission spectroscopy. Finally, to avoid the generation of frost, a cryo-microplasma was generated in a reactor chamber separated from open air. Helium, nitrogen and oxygen were employed as working gases. Using thermocouples and by estimation from the nitrogen rotational temperature, we verified that the gas temperature of the cryo-microplasma was much lower (T_g ≈ 180–300 K) than that of the conventional atmospheric-pressure low-temperature plasma (above 300 K).

An investigation of different design parameters of a cold-cathode Penning ion source is described. The Penning source had an iron cathode body, samarium cobalt permanent magnet, stainless steel anode and iron cathode faceplate. The faceplate extraction orifice diameter, anode shape and permanent magnet location were adjusted. Discharge current–voltage characteristics and current to a grounded target were measured for the different ion source configurations. Results indicate that for hydrogen and helium operation, the largest orifice, cylinder anode and baseline magnet location provides optimum source operation. Specifically, a continuous positive hydrogen ion target current of 1.0 ± 0.15 mA was attainable at only 1.6 × 10⁻⁴ Torr of pressure. At 1.0 mTorr of hydrogen pressure a target current of 2.0 ± 0.15 mA was possible. These results far surpass the design goal of 1 mA hydrogen ion beam at 1 mTorr of pressure. This cold-cathode Penning ion source design has potential application in a wide variety of laboratory research and development projects.

Traditional magnetic field design techniques for dc ion thrusters typically focus on closing a sufficiently high maximum closed magnetic contour, B_cc, inside the discharge chamber. In this study, detailed computational analysis of several modified NSTAR thruster 3-ring and 4-ring magnetic field geometries reveals that the magnetic field line shape as well as B_cc determines important aspects of dc ion thruster performance (i.e. propellant efficiency, beam flatness and double ion content). The DC-ION ion thruster model results show that the baseline NSTAR configuration traps the primary electrons on-axis, which leads to the high on-axis plasma density peak and high double ion content observed in experimental measurements. These problems are further exacerbated by simply increasing B_cc and not changing the field line shape. Changing the field line shape to prevent on-axis confinement (while maintaining the NSTAR baseline B_cc) improves thruster performance, improves plasma uniformity and lowers double ion content. For these favorable field line geometries, we observe further improvements to performance with increased B_cc, while maintaining plasma uniformity and low double ion content. These improvements derive from the fact that the field lines guide the high-energy primaries to regions where they are most efficiently used to create ions while a higher B_cc prevents the loss of ions to the anode walls. Therefore, it is recommended that the ion thruster designer first establish a divergent field line shape that ensures favorable beam flatness, low double ion content and reasonable performance; then the designer may adjust the B_cc to attain desirable performance and stability for the target discharge plasma conditions.

Efficient (∼1%) electron cyclotron radio emissions are produced in the X-mode from regions of locally depleted plasma in the Earth's polar magnetosphere. These emissions are commonly referred to as auroral kilometric radiation. Two populations of electrons exist with rotational kinetic energy to contribute to this effect, the downward propagating auroral electron flux which acquires transverse momentum due to conservation of the magnetic moment as it experiences an increasing magnetic field and the mirrored component of this flux. This paper demonstrates the production of an electron beam having a controlled velocity spread for use in an experiment to investigate the available free energy in the earthbound electron flux. The experiment was scaled to microwave frequencies and used an electron gun to inject an electron beam into a controlled region of increasing magnetic field produced by a set of solenoids reproducing the magnetospheric situation. Results are presented of the measurements of diode voltage, beam current as a function of magnetic mirror ratio and estimates of the line density versus electron pitch angle consistent with the formation of a horseshoe velocity distribution and demonstrating control of the electron distribution in velocity space.

An inductively coupled plasma torch, working at atmospheric pressure, is used to create a CO₂–N₂ Martian-like plasma (97% CO₂–3% N₂). The operating frequency and power are 64 MHz and 3 kW, respectively. Spectral measurements covering the [250–850] nm range have been carried inside the induction coil. Spectral profiles of emitting diatomic species have been measured, intensity calibrated, Abel inverted and fitted with a line-by-line code. This has allowed rebuilding the radial temperature profiles of the plasma. A simplified kinetic model has then been developed in order to confirm that chemical equilibrium is reached in this region of the plasma. Then, the plasma chemical composition has been determined utilizing a calculation code based on the Gibbs free energy minimization method. Overall, a complete characterization for the thermal, chemical and radiative properties of the plasma has been achieved in the induction coils region.

In the first part of this work, described in a previous paper, the thermodynamic conditions in an atmospheric pressure inductively coupled CO₂–N₂ plasma have been determined, and the radiation emission spectrum has been measured and calibrated in the [250–850 nm] spectral region. In the second part of this work, a synthetic radiation spectrum is obtained taking into account (a) the geometry of the plasma torch and (b) the local thermodynamic conditions of the plasma. This synthetic spectrum has then been compared against the measured spectrum. The good agreement between the two spectra allows validating the spectral database of the line-by-line code SPARTAN for the simulation of the radiative emission of CO₂–N₂ plasmas from the near-UV to the near-IR spectral region.

We describe levitation of diamond fine particles in an H₂ rf plasma chamber equipped with an adaptive rf electrode. Since suppression of ion bombardment is essential for crystalline diamond growth, we use an adaptive rf electrode system in a parallel-plate capacitively coupled rf plasma in order to levitate particles in a quasineutral 'spot plasma' region. Here the ions' energy corresponds only to the floating potential of the particles without the additional energy of the streaming ions as in the sheath. One can expect ion bombardment with considerably reduced ion energy when this technique is applied to diamond deposition on levitated particles.

Numerical simulations of radio frequency atmospheric pressure argon glow discharges were performed using a one-dimensional hybrid model. The discharge simulations were carried out for a parallel plate electrode configuration with an inter-electrode gap of 1.0 mm together with an external matching circuit. The external matching circuit parameters were found to have significant effect on the discharge characteristics. The results indicate that the discharge can operate at either the α or γ mode depending on the matching circuit parameters. The two modes of operation were found to be distinctly different. The predicted Ar^* density was considered to provide qualitatively the visual appearance of the α or γ mode discharge. The α mode was found to have a luminous region in the center of the discharge. On the other hand, the γ mode had luminous regions very close to the electrodes which were followed by alternating dark and bright regions. The appearance of the simulated γ mode was found to resemble that of an atmospheric pressure direct current glow discharge. The predicted gas temperature indicated the γ mode to have higher gas temperature compared with the α mode.

This paper deals with properties of air thermal plasmas containing vapours of iron, silver or copper. The plasma is supposed to be in local thermodynamic equilibrium, for temperatures ranging from 2000 to 30 000 K. First, the equilibrium composition and thermodynamic properties are presented. Then, the radiative properties are calculated using the method of the net emission coefficient. Finally, the viscosity, electrical and thermal conductivities are calculated using the method of Chapman–Enskog. For all mixtures, mole fractions have been used. The results are computed for various values of pressure, plasma size and proportions of vapours. The influence of metallic vapour is important on the electrical conductivity and on the radiation, even at low concentration. All the metallic vapours present a similar behaviour except iron, which has a stronger radiation emission than the other components.

This study presents a two-dimensional fluid-plasma model developed for describing the cw regime operation of a tandem plasma source, consisting of a driver and an expansion plasma volume of different sizes. The moderate pressure range considered (tens to hundreds of milliTorr) allows a description within the drift–diffusion approximation, as employed in the model. Argon discharges maintained in a metal gas-discharge vessel are treated. The discussions stress charged-particle and electron-energy fluxes as well as the spatial distribution of their components. The main conclusions are for (i) different electron and ion fluxes resulting in a net current in the discharge; (ii) a radial ion flux prevailing over the axial one and an axial electron flux prevailing over the radial one; (iii) ion motion determined by the dc electric field and drift–diffusion electron motion influenced by thermal diffusion; (iv) plasma maintenance in the expansion plasma chamber due to charged-particle and electron-energy fluxes from the driver; (v) importance of the convective flux in the electron-energy balance; (vi) electron-energy losses for sustaining the dc electric field in the expansion plasma volume strongly predominating over the losses through collisions and (vii) electron cooling accompanied by a strong drop in the plasma density and in the potential of the dc electric field, due to the plasma expansion in a bigger volume. In general, the results show that the gas pressure range usually considered to be governed by ambipolar diffusion shows up in a different regime: a regime with a dc current, when the discharge is in a metal chamber with different dimensions in the transverse and longitudinal directions.

The modelling of a dc plasma source operating at low power and at low mass flow rate is presented. The physical description of the electric arc in volume with chemical and thermal non-equilibrium conditions is given: two-temperature model, ionization–neutralization process and arc–gas interactions are discussed. The modelling assumptions are justified by the local non-dimensional numbers characterizing the system (Damkhöler, Reynolds and Knudsen numbers). Special attention is focused on the influence of the arc power on the thermal non-equilibrium between electrons and heavy particles and the resulting ionization rate. The kinetic thermal non-equilibrium is shown as a decreasing function of the arc discharge current and it is established that the electron density never reaches equilibrium conditions, even at the throat exit. Calculations have been performed for a stationary arc confined to the throat. The gas is argon and the flow is axisymmetric and stationary.

A Nd : Yag laser was employed to irradiate thick silver targets in vacuum. The ion emission from the Ag plasma was detected in situ using a ring ion collector and an electrostatic ion energy analyzer which permitted measurement of the ion kinetic energy-to-charge state ratio. The total ion yield, the ion threshold fluence, the ion energy distributions and the mean temperature for the different ion charge states of the non-equilibrium plasma were investigated. The visible radiation emission spectrum, the light emission threshold, the electronic temperature and density were also investigated by using optical spectroscopy. Ex situ surface profile measurements, performed on the generated craters after the ablation process, permitted the evaluation of the ablation threshold fluence and performance of a comparison with a semi-empirical model.

Tunable diode laser absorption spectroscopy is used as a non-invasive method for the diagnosis of a helium atmospheric pressure dielectric barrier discharge, containing oxygen as contaminant. The time evolution of the absorption coefficient corresponding to the oxygen metastable atoms on the 3⁵S₂ level is recorded as a function of the laser absorbing path, in the axial symmetrical electrode geometry. The bi-dimensional Abel transform is used to obtain local information on the space distribution of the metastable atoms in the discharge. The results show that the oxygen metastable atoms have distinct time–space dynamics, in relation to plasma kinetics involving helium high-energy species and to the marked role of the dielectric barrier.

In this study, the effect on thin film growth due to an anomalous electron transport, found in high power impulse magnetron sputtering (HiPIMS), has been investigated for the case of a planar circular magnetron. An important consequence of this type of transport is that it affects the way ions are being transported in the plasma. It was found that a significant fraction of ions are transported radially outwards in the vicinity of the cathode, across the magnetic field lines, leading to increased deposition rates directly at the side of the cathode (perpendicular to the target surface). Furthermore, this mass transport parallel to the target surface leads to that the fraction of sputtered material reaching a substrate placed directly in front of the target is substantially lower in HiPIMS compared with conventional direct current magnetron sputtering (dcMS). This would help to explain the lower deposition rates generally observed for HiPIMS compared with dcMS. Moreover, time-averaged mass spectrometry measurements of the energy distribution of the cross-field transported ions were carried out. The measured distributions show a direction-dependent high-energy tail, in agreement with predictions of the anomalous transport mechanism.

In order to study the low frequency oscillation (20–40 kHz) in a plume of Hall thrusters, the plasma parameter oscillations are measured with a Langmuir probe in axial and circumferential directions, and oscillations of different energy ions are measured with a multi-grid probe by varying the potential to repel selected ions. Experimental results indicate that the oscillation of plasma parameters in the plume exhibits the same frequency as a low frequency discharge current oscillation and is synchronous in the circumferential direction at the time scale of the low frequency discharge current oscillation. The time delay of the oscillation in different axial positions is related to the propagation of plasma density along the axial direction. The oscillation of Xe⁺ has the same frequency as the low frequency discharge current oscillation, but the oscillation of Xe²⁺ or a high-valence ion is more complex and scattered due to their instability in the ionization process. It is therefore concluded that the low frequency oscillation of plasma parameters in the plume is strongly related to the low frequency discharge current oscillation and the ionization process of a neutral atom.

We measure H⁻ negative ions by means of a mass spectrometer in a helicon plasma reactor. The H₂ plasma operates at a low injected RF power (50–300 W), in a capacitive regime, under low pressure conditions (between 0.4 and 1 Pa). A highly oriented pyrolytic graphite (HOPG) graphite sample centred in the expanding chamber and facing the mass spectrometer nozzle placed 40 mm away is negatively biased. Negative ions formed on the graphite surface upon positive ion bombardment are detected according to their energy by the mass spectrometer. We obtain the H⁻ ion distribution function (IDF) showing two main features: first, a high energy tail attributed to negative ions created via two-electron capture following and impact on the HOPG sample and, second, a main peak which can be attributed to negative ions created on the surface by the sputtering of adsorbed hydrogen and/or two-electron capture. Finally, we show that negative ion production is proportional to the positive ion flux and strongly depends on the positive ion energy.

Plasmas generated at atmospheric pressure are used to functionalize the surfaces of polymers by creating new surface-resident chemical groups. The polymers used in textiles and biomedical applications often have non-planar surfaces whose functionalization requires penetration of plasma generated species into sometimes complex surface features. In this regard, the atmospheric pressure plasma treatment of a rough polypropylene surface was computationally investigated using a two-dimensional plasma hydrodynamics model integrated with a surface kinetics model. Repetitively pulsed discharges produced in a dielectric barrier–corona configuration in humid air were considered to affix O. Macroscopic non-uniformities in treatment result from the spatial variations in radical densities which depend on the polarity of the discharge. Microscopic non-uniformities arise due to the higher reactivity of plasma produced species, such as OH radicals, which are consumed before they can diffuse deeper into surface features. The consequences of applied voltage magnitude and polarity, and the relative humidity on discharge dynamics and radical generation leading to surface functionalization, are discussed.

Plasmas are increasingly being used to functionalize the surface of polymers having complex shapes for biomedical applications such as tissue scaffolds and drug delivering micro-beads. The functionalization often requires affixation of amine (NH₂) or O-containing groups. In this paper, results are discussed from a two-dimensional computational investigation of the atmospheric pressure plasma functionalization of non-planar and porous surfaces of polypropylene with NH_x and O-containing groups. For the former, the discharge is sustained in He/NH₃/H₂O mixtures in a dielectric barrier–corona configuration. Significant microscopic non-uniformities arise due to competing pathways for reactive gas phase radicals such as OH and NH₂, and on the surface by the availability of OH to initiate amine attachment. The treatment of inside surfaces of porous polymer micro-beads placed on an electrode is particularly sensitive to view angles to the discharge and pore size, and is ultimately controlled by the relative rates of radical transport and surface reactions deep into the pores. The functionalization of micro-beads suspended in He/O₂/H₂O discharges is rapid with comparable treatment of the outer and interior surfaces, but varies with the location of the micro-bead in the discharge volume.

A comparison is made between plasma parameters measured with a retarding field energy analyzer (RFEA), mounted at a grounded electrode in an inductive discharge, and a Langmuir probe located in bulk plasma close to the analyzer. Good agreement between measured plasma parameters is obtained for argon gas pressure in the range 2–10 mTorr. Parameters compared include time averaged plasma potential, the tail of the electron energy distribution function (EEDF), the electron temperature and the ion flux. This highlights the versatility of the RFEA for determining plasma parameters adjacent to the surface where probe measurements are not easily made. Combination of the probe and energy analyzer has enabled the measurement of the EEDF to a higher energy than otherwise possible.

A microwave plasma system based on microstripline technology is created and implemented for the experimental investigation of miniature size plasmas. Plasmas are generated across a wide range of input parameters, including pressure variation from below 1 Torr to 1 atm, microwave power at 2.45 GHz from 1 to 30 W, and a variety of discharge sizes and shapes. Data are measured for the discharge power density as a function of discharge pressure, input power and size. The power densities for discharges of diameters in the range 2–0.45 mm vary from 10 to 1000 W cm⁻³. Basic characteristics of the plasmas such as the electron temperature, plasma density and gas temperature are measured using probe diagnostics and optical emission spectroscopy (OES). In the range of one to 10 Torr, the electron temperatures are 1.9–2.3 eV, gas temperatures range from 600–1200 K and the plasma densities are in the range of 10¹²–10¹⁵ cm⁻³.

Plasma sputtering is one of the most promising methods for reducing the amount of platinum catalyst in porous electrodes for low temperature fuel cells. Here, a simulation of the platinum deposition by radio frequency plasma sputtering has been developed and compared with experimental results to allow optimization of the deposition process. In the simulation, the transport of sputtered atoms through the argon plasma is obtained using a 3D Monte Carlo model called SPaTinG (Sputtered Particles Transport in Gas). The Yamamura formula provides the Pt sputtering yield on the target, and the initial energy distribution of sputtered atoms is given by the Thompson distribution. A 1D hybrid model is used to estimate the mean energy of argon ions impinging onto the platinum target. Experimentally, platinum is deposited on silicon in two plasma sputtering chambers with different geometries. The deposition rate is measured by Rutherford backscattering spectroscopy. The angular distribution of the Pt atoms ejected from the target surface and the condensation coefficient of the Pt atoms on silicon are calculated by adjusting the simulated and experimental deposition rates at 0.5 Pa. A good agreement between the simulation and the experiment is observed as a function of the target–substrate distance for the two system geometries at low pressure (0.5 Pa).

Very low frequency (VLF) oscillations in the DC self-bias voltage and the optical emission spectroscopy signatures of diluted SiH₄, GeH₄ and SiF₄ plasmas are documented. The oscillations occur under conditions where nanoparticles are generated in the plasma, and close to the transition between amorphous and nanocrystalline material growth. The frequency, intensity and waveforms of the oscillations are shown to have a dependence on gas temperature, RF-power, total pressure and flow rate. The observation of VLF oscillations is a positive indication of the formation of a nanocrystalline film, but the converse inference is not valid. A macroscopic, zero-dimensional model for the plasma dynamics is proposed incorporating a feedback mechanism involving nanoparticles to generate the VLF oscillations. The requirements for the model to reproduce the experimental results are (i) long particle residence times (>10 s), (ii) slow particle growth rates (0.4 nm s⁻¹) and (iii) the rapid onset of nucleation suppression by large particles. These conditions allow us to reproduce the undamped oscillations and oscillation frequencies observed experimentally. The long particle residence times may explain the complete crystallization of the nanoparticles in the plasma.

A long-probe technique was utilized to record the expansion and retreat of the dynamic sheath around a spherical substrate immersed in pulsed cathode arc metal plasma. Positively biased, long cylindrical probes were placed on the side and downstream of a negatively pulsed biased stainless steel sphere of 1 in. (25.4 mm) diameter. The amplitude and width of the negative high voltage pulses (HVPs) were 2 kV, 5 kV, 10 kV, and 2 µs, 4 µs, 10 µs, respectively. The variation of the probe (electron) current during the HVP is a direct measure for the sheath expansion and retreat. Maximum sheath sizes were determined for the different parameters of the HVP. The expected rarefaction zone behind the biased sphere (wake) due to the fast plasma flow was clearly established and quantified.

The effects leading to the falling behavior of the current–voltage characteristic of a Townsend discharge from 'zero' current are considered. The conditions investigated correspond to the breakdown of argon at pd ∼ 1 Torr cm. Using a Monte Carlo method for modeling electron multiplication the probability of ion-induced electron emission is extracted from the experimental breakdown data and the initial negative slopes of the current–voltage characteristic are found. This paper shows that two mechanisms provide the existence of the negative differential resistance R_1D. One of them is connected to the decrease in the probability of an electron to return to the cathode due to backscattering when increasing the discharge current; this mechanism was discussed before. The other mechanism is a result of non-equilibrium electron motion in a discharge gap. The higher the current and consequently the electric field at the cathode, the earlier an emitted electron acquires the energy necessary for ionization. This leads to an increase in the electron multiplication. Although this non-equilibrium effect is known, it was not considered previously as the possible cause of the appearance of the negative differential resistance. This paper shows that the role of the second mechanism in the formation of the negative differential resistance turns out to be even higher than that of the first one. The calculated R_1D values are in good agreement with the experiment.