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In this article, the plasma parameters of a direct current magnetron discharge in argon at moderate pressure (10 to 40 Pa) using an aluminium cathode are explored. The density of argon excited states, the sputtered aluminium atom density and the electron temperature are estimated using a collisional–radiative model. The electron temperature obtained using optical emission spectroscopic data agrees well with measurements taken using a Langmuir probe. The influence of the discharge parameters, namely the background argon pressure and the discharge current, are discussed.

We report measurements of the time-averaged surface floating potential of magnetic and non-magnetic spherical probes (or large dust particles) immersed in a magnetized capacitively coupled discharge. In this study, the size of the spherical probes is taken greater than the Debye length. The surface potential of a spherical probe first increases, i.e. becomes more negative at low magnetic field ( $B < 0.05\ \textrm {T}$ ), attains a maximum value and decreases with further increase of the magnetic field strength ( $B > 0.05\ \textrm {T}$ ). The rate of change of the surface potential in the presence of a $B$ -field mainly depends on the background plasma and types of material of the objects. The results show that the surface potential of the magnetic sphere is higher (more negative) compared with the non-magnetic spherical probe. Hence, the smaller magnetic sphere collects more negative charges on its surface than a bigger non-magnetic sphere in a magnetized plasma. The different sized spherical probes have nearly the same surface potential above a threshold magnetic field ( $B > 0.03\ \textrm {T}$ ), implying a smaller role of size dependence on the surface potential of spherical objects. The variation of the surface potential of the spherical probes is understood on the basis of a modification of the collection currents to their surface due to charge confinement and cross-field diffusion in the presence of an external magnetic field.

The collective dynamics of annulus dusty plasma formed between a co-centric conducting (non-conducting) disk and ring configuration is studied in a strongly magnetized radio-frequency (rf) discharge. A superconducting electromagnet is used to introduce a homogeneous magnetic field to the dusty plasma medium. In absence of the magnetic field, dust grains exhibit thermal motion around their equilibrium position. The dust grains start to rotate in anticlockwise direction with increasing magnetic field (B $>$ 0.02 T), and the constant value of the angular frequency at various strengths of magnetic field confirms the rigid body rotation. The angular frequency of dust grains linearly increases up to a threshold magnetic field (B $>$ 0.6 T) and after that its value remains nearly constant in a certain range of magnetic field. Further increase in magnetic field (B $>$ 1 T) lowers the angular frequency. Low value of angular frequency is expected by reducing the width of annulus dusty plasma or the input rf power. The azimuthal ion drag force due to the magnetic field is assumed to be the energy source which drives the rotational motion. The resultant radial electric field in the presence of magnetic field determines the direction of rotation. The variation of floating (plasma) potential across the annular region at given magnetic field explains the rotational properties of the annulus dusty plasma in the presence of magnetic field., 10,9

Tunable diode laser induced fluorescence (TDLIF) measurements are discussed and quantitatively evaluated for nitrogen admixtures in argon plasma under the influence of a strong magnetic field. TDLIF measurements were used to evaluate light-transport properties in a strongly magnetized optically thick argon/nitrogen plasma under different pressure conditions. Therefore, a coupled system of rate balance equations was constructed to describe laser pumping of individual magnetic sub-levels of $\mathrm{2p_8}$ state through frequency-separated sub-transitions originating from $\mathrm{1s_4}$ magnetic sub-levels. The density distribution (alignment) of $\mathrm{2p_8}$ multiplet was described by balancing laser pumping with losses including radiative decay, transfer of excitation between the neighboring sub-levels in the $\mathrm{2p_8}$ multiplet driven by neutral collisions (argon and nitrogen) and quenching due to electron and neutral collisions. Resulting $\mathrm{2p_8}$ magnetic sub-level densities were then used to model polarization dependent fluorescence, considering self-absorption, which could be directly compared with polarization-resolved TDLIF measurements. This enables to estimate the disalignment rate constant for the $\mathrm{2p_8}$ state due to collisions by molecular nitrogen. A comparison to molecular theory description is given providing satisfactory agreement. The presented measurement method and model can help to describe optical emission of argon and argon-nitrogen admixtures in magnetized conditions and provides a basis for further description of optical emission spectra in magnetized plasmas., Comment: 15 pages , 8 figures

A plasma under the influence of an external magnetic field changes the optical properties due to the Zeeman splitting of the energy levels. This splitting degenerates an initial single spectral line into a system of spectral lines with different transition frequencies defined by the electronic structure of the energy levels. Newly created magnetic sub-levels redefine the spectral profile of the line emission and therefore radiation transport mechanism in optically thick plasma. Self-absorption which defines the excited state-densities is an important mechanism and can be used with other methods to describe the state densities for an optically thick plasma. This method is an established tool to retrieve state-density and plasma parameters. To measure each magnetic sub-level density of argon 1s4 and 1s5 (in Paschen's notation) a tunable diode laser absorption spectroscopy (TDLAS) was used. Based on reconstructed excited state densities, the self-absorption coefficient was calculated for individual magnetic sub-levels. A decrease in self-absorption with an external magnetic field was noticed indicating a higher transparency of the plasma. Furthermore a polarization dependent self-absorption was found. The presented results can help to model optical properties and interpret the absorption of a low-pressure optically thick magnetized plasma., Comment: 16 pages, 10 figures

This paper reports experiments on self$-$excited dust acoustic waves (DAWs) and its propagation characteristics in a magnetized rf discharge plasma. The DAWs are spontaneously excited in dusty plasma after adding more particles in the confining potential well and found to propagate in the direction of streaming ions. The spontaneous excitation of such low-frequency modes is possible due to the instabilities associated with streaming ions through the dust grain medium. The background E-field and neutral pressure determine the stability of excited DAWs. The characteristics of DAWs strongly depend on the strength of external magnetic field. The magnetic field of strength B $$ 0.05 T and get completely damped at higher magnetic field B $\sim$ 0.13 T. After lowering the power and pressure to 3 W and 23 Pa respectively, the excited DAWs in the absence of B are slightly unstable. In this case, the magnetic field only stabilizes and modifies the propagation characteristics of DAWs while the strength of B is increased up to 0.1 T or even higher. The modification of the sheath electric field where particles are confined in the presence of the external magnetic field is the main cause of the modification and damping of the DAWs in a magnetized rf discharge plasma., Comment: 9,6

A new quantitative evaluation of tunable diode laser induced fluorescence (TDLIF) measurements in magnetized plasma is presented in this article, taking into account Zeeman splitting of energetic levels as well as inter- and intra-multiplet mixing, defining the density distribution (alignment) of the excited 2 p 8 multiplet of argon. TDLIF measurements were used to evaluate light-transport properties in a strongly magnetized optically thick argon plasma under different pressure conditions. Therefore, a coupled system of rate balance equations was constructed to describe laser pumping of individual magnetic sub-levels of the 2 p 8 state through frequency-separated sub-transitions originating from 1 s 4 magnetic sub-levels. The density distribution of the 2 p 8 multiplet was described by balancing laser pumping with losses, including radiative decay, transfer of excitation between the neighboring levels within the 2 p 8 multiplet driven by neutral collisions, and quenching due to electron and neutral collisions. Resulting 2 p 8 magnetic sub-level densities were then used to model polarization dependent fluorescence, considering self-absorption, which could be directly compared with measured polarization-resolved TDLIF measurements. The achieved results enable to obtain unique solutions for the 1 s 4 and 1 s 5 magnetic sub-level densities which were found to be in good agreement with the densities obtained by laser absorption measurements. It is shown that polarization resolved TDLIF measurements in magnetized plasma conditions have strong pressure dependence. The effective disalignment rate constant which redistributes the 2 p 8 sub-levels among each other has to be considered for a correct description of the TDLIF. This rate is dependent on the neutral gas density and a specific rate coefficient. With the presented method, 1s state densities involved in the TDLIF can be determined without any absolute intensity calibration in an optically thick plasma. Additionally, the presented measurement method and model can help to further understand and improve the description of optical emission of argon based on individual sub-transition descriptions under magnetized conditions.

This paper reports on the dynamics of a 3-dimensional dusty plasma in a strong magnetic field. An electrostatic potential well created by a conducting or non-conducting ring in the rf discharge confines the charged dust particles. In the absence of the magnetic field, dust grains exhibit a thermal motion about their equilibrium position. As the magnetic field crosses a threshold value (B > 0.02 T), the edge particles start to rotate and form a vortex in the vertical plane. At the same time, the central region particles either exhibit thermal motion or E → × B → motion in the horizontal plane. At B > 0.15 T, the central region dust grains start to rotate in the opposite direction resulting in a pair of counter-rotating vortices in the vertical plane. The characteristics of the vortex pair change with increasing the strength of the magnetic field (B ∼ 0.8 T). At B > 0.8 T, the dust grains exhibit very complex motion in the rotating torus. The angular frequency variation of rotating particles indicates a differential or sheared dust rotation in a vortex. The angular frequency increases with increasing the magnetic field from 0.05 T to 0.8 T. The ion drag force and dust charge gradient along with the E-field are considered as possible energy sources for driving the edge vortex flow and central region vortex motion, respectively. The directions of rotation also confirm the different energy sources responsible for the vortex motion.

Potential in biomedical-related applications by atmospheric pressure plasma-treated water gradually increased recently. In order to enhance the generation of reactive species in atmospheric pressure plasma in regards to liquid treatment, this study aims to investigate the effect of external axial magnetic field on a helium atmospheric pressure plasma jet (APPJ) and plasma-irradiated water. Magnetic field strength up to 2.0 T was generated by an electromagnet in Helmholtz configuration. The dielectric barrier discharge-based APPJ is driven by 4.4 slm helium and sinusoidal 30 kHz, 10 k V p p signals. Plasma was mainly characterized by optical emission where vibrational and rotational temperature was estimated by nitrogen second positive system, and electron temperature and density by emission intensity line ratio method. Representative reactive species of nitrite, nitrate, and hydrogen peroxide concentration in the plasma-treated water were also quantified. Results of vibrational temperature and electron density showed a first drop then rise trend which partially corresponds to the reactive species in the plasma-treated water. No significant changes were found in rotational temperature, while electron temperature monotonically increased with the magnetic field.

Three-dimensional plasma crystals are often described as Yukawa systems for which a phase transition between the crystal structures fcc and bcc has been predicted. However, experimental investigations of this transition are missing. We use a fast scanning video camera to record the crystallization process of 70 000 microparticles and investigate the existence of the fcc-bcc phase transition at neutral gas pressures of 30, 40, and 50 Pa. To analyze the crystal, robust phase diagrams with the help of a machine learning algorithm are calculated. This work shows that the phase transition can be investigated experimentally and makes a comparison with numerical results of Yukawa systems. The phase transition is analyzed in dependence on the screening parameter and structural order. We suggest that the transition is an effect of gravitational compression of the plasma crystal. Experimental investigations of the fcc-bcc phase transition will provide an opportunity to estimate the coupling strength $\mathrm{\ensuremath{\Gamma}}$ by comparison with numerical results of Yukawa systems.

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Nonstoichiometric NiO x thin films were grown on single crystal substrates of c-plane ( 0001 ) sapphires by ion-beam sputter-deposition (IBSD) of a Ni metal target in a mixed argon and oxygen atmosphere. Structural characterization was carried out by X-ray diffraction and scanning electron microscopy. All samples grew ( 111 )-oriented out-of-plane and with a defined in-plane orientation relationship relative to the crystalline substrate. The chemical bonding information of the films was examined by X-ray photoelectron spectroscopy showing that the composition x could be varied by adjusting the oxygen-to-argon ratio in the IBSD process. However, a growth anomaly was detected for a certain range of synthesis parameters, standing out due to an enhanced growth rate, nickel excess, and unusually elongated surface structures. With joint solid-state and plasma diagnostic tools, the underlying processes on the atomic scale were studied. An increased proportion of atomic oxygen species in the intermediate range of the oxygen-to-argon gas flux ratio was identified to be responsible for an enhanced generation of NiO x species. Optical emission spectroscopy was found to be a tool especially well-suited, since the in situ examination of various locations of interest (plasma, ion beam, and vicinity of the target) is feasible nonintrusively.Nonstoichiometric NiO x thin films were grown on single crystal substrates of c-plane ( 0001 ) sapphires by ion-beam sputter-deposition (IBSD) of a Ni metal target in a mixed argon and oxygen atmosphere. Structural characterization was carried out by X-ray diffraction and scanning electron microscopy. All samples grew ( 111 )-oriented out-of-plane and with a defined in-plane orientation relationship relative to the crystalline substrate. The chemical bonding information of the films was examined by X-ray photoelectron spectroscopy showing that the composition x could be varied by adjusting the oxygen-to-argon ratio in the IBSD process. However, a growth anomaly was detected for a certain range of synthesis parameters, standing out due to an enhanced growth rate, nickel excess, and unusually elongated surface structures. With joint solid-state and plasma diagnostic tools, the underlying processes on the atomic scale were studied. An increased proportion of atomic oxygen species in the intermedi...

The electrical breakdown of a gas subject to an up-ramping external voltage is studied, experimentally and theoretically, under conditions leading to the appearance of a positive corona at the anode in the pre-breakdown regime. Experimentally, voltage ramps with various rates k in the interval ranging from 0.3 V/s to 26 kV/s are applied to the diode, with the histogram of breakdown voltages being recorded for each rate. The theoretical model gives attention to the pre-breakdown multiplication causing the corona, which tends to reduce the statistical time delay $t_{S}$ before the primary electron is released and hence to make $t_{S}$ comparable to the formative time $t_{F}$ . The multiplication being therefore expected to affect the voltage dependence of the electron yield, a nonlinear equation relating the yield to the overvoltage is introduced. The resulting theoretical expression for the breakdown voltage distribution agrees well with the experimental histograms. Especially noteworthy is the good agreement with the low-voltage tail of the distribution, a segment of the data that has challenged previous theoretical analyses of the problem.

The effect of a levitating cloud of microparticles on the parameters of a radiofrequency (RF) plasma has been studied by means of two experimental techniques. Axial distributions of 1 s excited states of argon were measured by a self-absorption method. A correction of a standard self-absorption method for the extinction of the light by the levitating microparticles is proposed. In addition the electron temperature was estimated using the optical emission spectroscopy. Measurements at the same discharge conditions in a microparticle-free discharge and discharge, containing a cloud of levitating microparticles, revealed the non-local influence of the microparticle cloud on the discharge plasma. The most probable cause of this influence is the disturbance of the ionization balance by the levitating microparticles.

Experiments with complex plasmas in a dc discharge using the PK-4 facility will be discussed, focusing on the formation of strings corresponding to an electrorheological fluid observed in recent parabolic flight experiments.

A simple model, allowing to determine the electron temperature in a steady-state low-pressure plasma, is proposed. The model makes use of optical cross-sections and therefore takes into account direct and cascade excitation from ground and metastable states. Spectroscopic data from Mitic et al. (New J. Phys. 11, 083020 (2009)) are used to illustrate the performance of the method.

The results are presented of investigating temporal and spatial development of electrical glow discharge in a neon filled tube under 4mbar pressure. Linear increasing voltage (at 5 V/s increasing voltage rate) is applied to the gas diode. Time dependence of 585.2 nm line light emitted from negative glow is observed from various positions in the diode during formation of electrical discharge. The results show that the development of glow discharge starts in the gap, and propagates to the cathode and in the space around and behind the cathode. An unexpected two-step current rise is found. In the stationary regime, most of the emitted light occupied the cathode carrier rod. This indicates the position where the secondary electron emission is intensive. It corresponds to the second step in the current increase app. 3 ms after the breakdown has already taken place. It is assumed that this step originates from different surface characteristics of the rode material. The analysis of time dependencies of the current and light from the negative glow, from different positions in the gas diode, suggests that the observation of deexcitation processes in gas can be used for determination of early discharge formative processes, as well as processes that lead to the stationary regime in the gas diode tube.

Mitic et al (2011 Opt. Lett. 36 3699) proposed a spectroscopic method of the in situ measurement of the size and optical properties of spherical micro- and nanoparticles with monotonically variable size. The method requires three optical channels: one for the illumination of a particle suspension by white light and two for the measurements of the spectra of scattered light. This allows one to determine the optical properties of the particles in a wide spectral range. In this work an extended description of the experimental setup and data analysis technique are given. Performance of the method is illustrated on particle suspensions in plasmas, exhibiting increase and decrease of particle size.

International audience; A hypothesis on the physical mechanism generating the heartbeat instability in complex (dusty) plasmas is presented. It is suggested that the instability occurs due to the periodically repeated critical transformation on the boundary between the microparticle-free area (void) and the complex plasma. The critical transformation is supposed to be analogous to the formation of the sheath in the vicinity of an electrode. The origin of the transformation is the loss of the electrons and ions on microparticles surrounding the void. We have shown that this hypothesis is consistent with the experimentally measured stability parameter range, with the evolution of the plasma glow intensity and microparticle dynamics during the instability, as well as with the observed excitation of the heartbeat instability by an intensity-modulated laser beam (inducing the modulation of plasma density).

International audience; A simple method of characterization of suspensions of spherical nanoparticles with monotonically variable size is proposed. It allows for the in situ measurement of the particle size as well as spectral dependence of their refractive indices. The method requires three optical channels: one for the illumination of a suspension by white light and two for the measurements of the spectra of scattered light. Parameters of the particles are determined by fitting the measured temporal spectral surfaces by the calculated Mie scattering functions. The method is applied to the particles being grown in a low-pressure reactive plasma of a discharge in an acetylene-argon mixture

We report on the first three-dimensional (3D) complex plasma structure analysis for an experiment that was performed in an elongated discharge tube in the absence of striations. The low frequency discharge was established with 1 kHz alternating dc current through a cylindrical glass tube filled with neon at 30 Pa. The injected particle cloud consisted of monodisperse microparticles. A scanning laser sheet and a camera were used to determine the particle position in 3D. The observed cylindrical-shaped particle cloud showed an ordered structure with a distinct outer particle shell. The observations are in agreement with performed molecular dynamics simulations.

Steady-state clouds of microparticles were observed, levitating in a low-frequency glow discharge generated in an elongated vertical glass tube. A heated ring was attached to the tube wall outside, so that the particles, exhibiting a global convective motion, were confined vertically in the region above the location of the heater. It is shown that the particle vortices were induced by the convection of neutral gas, and the mechanism responsible for the gas convection was the thermal creep along the inhomogeneously heated tube walls. The phenomenon of thermal creep, which commonly occurs in rarefied gases under the presence of thermal gradients, should generally play a substantial role in experiments with complex plasmas.

We report on the first detailed analysis of large three dimensional (3D) complex plasma structures in experiments performed in pure rf and combined rf+dc discharge modes. Inductively coupled plasma (ICP) is generated by an rf coil wrapped around the vertically positioned cylindrical glass tube at a pressure of 0.3 mbar. In addition, dc plasma can be generated by applying voltage to the electrodes at the ends of the tube far from the rf coil. The injected monodisperse particles are levitated in the plasma below the coil. A scanning laser sheet and a high resolution camera are used to determine the 3D positions of about $10^5$ particles. The observed bowl-shaped particle clouds reveal coexistence of various structures, including well-distinguished solid-like, less ordered liquid-like, and pronounced string-like phases. New criteria to identify string-like structures are proposed., Comment: 6 pages, 7 figures

Axial distributions of 1s excited states of argon were measured in a radiofrequency (RF) discharge by a self-absorption method. Experiments were performed in the PK-3+ chamber, designed for microgravity experiments in complex (dusty) plasmas on board the International Space Station. A correction of a standard self-absorption method for the extinction of the light by the levi- tating microparticles is proposed. Distributions, measured at the same discharge conditions in a microparticle-free discharge and a discharge containing a cloud of levitating microparticles, revealed the non-local influence of the microparticle cloud on the discharge plasma. The most probable cause of this influence is the disturbance of the ionization balance by the levitating microparticles.