Table of contents

When we form a structure of plasmas distributed in a certain space in which electromagnetic waves propagate, such a plasma structure serves as a different medium from a homogeneous bulk plasma. We can also enhance or generate novel functions of the plasmas when we add other structural materials such as functional components. That is to say, when we estimate such a medium from the material properties such as permittivity, permeability and conductivity, it shows extraordinary and/or functional effects that arise from the synthesis of the structure. We call such an artificial material a plasma metamaterial. In this review, starting from a fundamental understanding of electromagnetic wave propagation in and around plasmas, we review the new functions of plasmas as metamaterials, including a photonic-crystal-like behavior, a negative refractive index state and a nonlinear bifurcated electric response, by describing specific plasma structures. In addition, we survey some specific applications of such media and predict a feasible scientific expansion of this field in the near future.

The velocity and energy distribution functions of ions escaping radially from the magnetic trap region of a HiPIMS discharge have been measured using a retarding field analyzer (RFA). Spatially and angularly resolved measurements recorded at a representative time show more energetic ions detected along a line-of-sight coincident with an oncoming rotating ion fluid, which circulates above the racetrack in the same direction as the electron E × B drift. The difference in the mean ion energies between measurements made into and against the direction of rotation is ∼5 eV. Numerical solutions of the equation of motion for the ions accounting for azimuthal acceleration (modified two-stream instability model used by Lundin et al) have been found. The centripetal force caused by the radial electric field and a drag force term accounting for ion collisions revealed that only a small fraction (typically <5%) of the circulating ion flux can leave the discharge tangentially. Operating the discharge at different background pressures revealed an interplay between the azimuthal acceleration of ions, dominating under low pressure conditions and the scattering of ions into the RFA at higher pressure.

A retarding field energy analyzer is used to characterize an asymmetric, 13.56 MHz driven, capacitively coupled, parallel plate discharge operated at low pressure. The characterization is carried out in argon discharges at 10 and 20 mTorr where the sheaths are assumed to be collisionless. The analyzer is set in the powered electrode where the impacting ion and electron energy distributions are measured for a range of discharge powers. A circuit model of the discharge is used to infer important electrical parameters from the measured energy distributions, including electrode excitation voltages, plasma potential and sheath potentials. Analytical models of the ion energy distribution in a radio-frequency sheath are used to determine plasma parameters such as sheath width, ion transit time, electron temperature and ion flux. A radio-frequency compensated Langmuir probe is used for comparison with the retarding field analyzer measurements.

Laser scattering provides a very direct method for measuring the local densities and temperatures inside a plasma. We present new experimental results of laser scattering on an argon atmospheric pressure microwave plasma jet operating in an air environment. The plasma is very small so a high spatial resolution is required to study the effect of the penetration of air molecules into the plasma. The scattering signal has three overlapping contributions: Rayleigh scattering from heavy particles, Thomson scattering from free electrons and Raman scattering from molecules. The Rayleigh scattering signal is filtered out optically with a triple grating spectrometer. The disentanglement of the Thomson and Raman signals is done with a newly designed fitting method. With a single measurement we determine profiles of the electron temperature, electron density, gas temperature, partial air pressure and the N₂/O₂ ratio, with a spatial resolution of 50 µm, and including absolute calibration.

We have investigated the performance of a microwave-cavity discharge (MWD) operating in tandem with a fast rise-time pulsed dielectric-barrier discharge (DBD). The tandem discharge operated in a helium/oxygen mixture, where metastable molecular oxygen could be produced efficiently using MWD in proportionally large quantities (order of 20% of total oxygen number density). In this new arrangement, a DBD operating at high E/N provided a metastables-rich mixture, thereby modifying the discharge kinetics of the MWD, which operated in the E/N range centered around 10 Td. Both discharges operated in synchronized pulse-repetitive mode, which was tailored to maximize the oxygen metastable production efficiency. The system operated at pressures up to 350 Torr with an average power between 3 and 20 W.

The interplay of gas flow and depletion by plasma dissociation determines the spatial distribution of species and the deposition uniformity in a plasma source. Many plasma reactors use a gas showerhead and the design of the flow dynamics is a critical aspect of the reactor performance. In this paper, plasma deposition is considered as chemically reacting gas flow in an ideal showerhead reactor. The gas fluid flow is described by finite-gap stagnation-point creeping flow. The distribution of neutral species across the electrode gap is determined by diffusion equations, whereas their lateral transport is purely convective. Parameters relevant to large-area radio-frequency plasma deposition are particularly suitable for a complete analytical solution of the multi-component transport. A representative reaction scheme for hydrogen/silane plasma deposition is used for an analytical example from first principles which shows good agreement with numerical simulation. For a laterally uniform plasma, apart from edge effects, the deposition uniformity is limited only by the lateral uniformity of the pressure: if the electrode gap is very small in a large-area reactor, the pressure and deposition rate will be non-uniform even for a uniform showerhead. The deposition mass flux is self-consistently accounted for by the Stefan velocity for arbitrary levels of gas concentration and depletion, and its influence on streamlines and fluid velocity is shown.

Plasmas generated inside oxygen bubbles in water have been developed for water purification. Zero-dimensional numerical simulations were used to investigate the chemical reactions in plasmas driven by dc voltage. The numerical and experimental results of the concentrations of hydrogen peroxide and ozone in the solution were compared with a discharge current between 1 and 7 mA. Upon increasing the water vapour concentration inside bubbles, we saw from the numerical results that the concentration of hydrogen peroxide increased with discharge current, whereas the concentration of ozone decreased. This finding agreed with the experimental results. With an increase in the discharge current, the heat flux from the plasma to the solution increased, and a large amount of water was probably vaporized into the bubbles.

A radio-frequency (RF) helicon plasma reaction chamber (HPRC) is developed and used to decompose methane gas into high-purity hydrogen gas and solid carbon in the form of graphite. A single-turn (m = 0) helicon antenna, operated at 13.56 MHz, and a 100 G dipole magnetic field are used to excite a helicon mode in a nonthermal plasma, creating plasma densities exceeding 10¹³ cm⁻³ using 8–20 SCCM methane gas at up to 1300 W of RF power. The HPRC device takes advantage of a uniform large amplitude electron sheath across the exit aperture. At this aperture, all of the incident electron flux from the plasma is extracted and all ions are reflected back into the source. In this way, only neutrals and electrons are allowed out of the reaction chamber, enhancing the breakdown of methane into deposited carbon and hydrogen gas that escapes. A methane decomposition percentage of 99.99 ± 0.06% is demonstrated using 1300 W of RF power and a methane gas flow rate of 8 SCCM. A total nonambipolar flow of particles maximizes the recirculation of ions, and leads to the very high degree of molecular decomposition achieved in this proof-of-concept device. The HPRC in its present proof-of-concept form requires 37× more energy per kg of H₂ produced, compared with steam-methane reformation, though this energy comparison does not include the energy required to sequester the emitted CO₂ during the steam–methane reformation cycle.

The experimental and calculated results of uniformity in a glow dielectric barrier discharge (DBD) under sub-atmospheric pressures are reported. Driven by a square-wave power source, the discharge in a parallel-electrode DBD system shows uniform or various lateral structures under different conditions. There exists a critical frequency below which the DBD is uniform for almost all the applied voltages. Above the critical frequency, a non-uniform (patterned) discharge is observed and the patterned structures change with frequency and voltage. A two-dimensional fluid modeling is performed on this DBD system which shows similar results in agreement with the experiments. The simulations reveal that the distribution of the space electron density at the beginning of each voltage pulse plays an important role in achieving the uniformity. Uniform space charge results in a uniform DBD. The patterned DBD always evolves from the initial uniform state to the eventual non-uniform one. During this process, the space electrons form a patterned distribution ahead of the surface charges and lead to non-uniform discharge channels.

An atmospheric steam plasma jet generated by an original dc water plasma torch is investigated using electrical and spectroscopic techniques. Because it directly uses the water used for cooling electrodes as the plasma-forming gas, the water plasma torch has high thermal efficiency and a compact structure. The operational features of the water plasma torch and the generation of the steam plasma jet are analyzed based on the temporal evolution of voltage, current and steam pressure in the arc chamber. The influence of the output characteristics of the power source, the fluctuation of the arc and current intensity on the unsteadiness of the steam plasma jet is studied. The restrike mode is identified as the fluctuation characteristic of the steam arc, which contributes significantly to the instabilities of the steam plasma jet. In addition, the emission spectroscopic technique is employed to diagnose the steam plasma. The axial distributions of plasma parameters in the steam plasma jet, such as gas temperature, excitation temperature and electron number density, are determined by the diatomic molecule OH fitting method, Boltzmann slope method and H_β Stark broadening, respectively. The steam plasma jet at atmospheric pressure is found to be close to the local thermodynamic equilibrium (LTE) state by comparing the measured electron density with the threshold value of electron density for the LTE state. Moreover, based on the assumption of LTE, the axial distributions of reactive species in the steam plasma jet are estimated, which indicates that the steam plasma has high chemical activity.

Dual frequency capacitively coupled plasmas (CCPs) are widely used in (large area) etching and plasma enhanced chemical vapor deposition processes. However, applications in physical vapor deposition (PVD) are still sparse due to the well-established dc magnetron cathode discharges. Nevertheless, there exist critical applications such as ferromagnetic or ceramic thin film deposition which are difficult to handle even for dc magnetron systems. For these materials systems dual frequency CCPs pose a good alternative, because for insulators charging can be avoided and for ferromagnetic materials the target thickness becomes independent of the magnetron configuration at comparable deposition rates.

In this work we investigate two separate subjects. First, in dual frequency capacitive discharges a complex coupling of the applied excitation frequencies can be observed, which from a plasma parameter point of view limits the separability of ion flux (usually controlled by frequencies >60 MHz) and ion bombarding energy (usually controlled by frequency <15 MHz) onto the sputter target. By performing deposition experiments it was found that by following simple tuning guidelines a very good degree of separability is achievable. Additionally, the deposition homogeneity is not affected.

Second, we correlate the growth conditions with crystalline and magnetic properties as well as the degree of O content for Fe and Ni films. Therefore, we applied different signals as a substrate bias to influence thin film growth. It was found that the crystalline and magnetic properties can be influenced for both Fe and Ni films but is more pronounced for Ni.

In this work, we compare measurements of electron density (n_e), plasma potential (φ_p), electron temperature (T_e), and electron energy distribution function (EEDF or f(E)) made with a dc biased RF probe and a Langmuir probe. The measurements show good agreement between the two probes across an order of magnitude in plasma density.

Fireballs are luminous regions produced by double layers in front of positively biased electrodes in plasmas. Although fireballs have been investigated previously there are a great variety of unexplained nonlinear phenomena, some of which are addressed in this work. First, it is shown that a fireball is not an isolated local phenomenon but an integral part of the entire discharge plasma. Current closure and limits are discussed. Fireballs with currents from milliamperes to tens of amperes are created depending on whether the electron source is temperature limited, space-charge limited or limited by ion currents in afterglow plasmas. Fireballs are created with highly transparent grids which allow electron transmission through the electrodes and optimize the ionization efficiency. The physics of pulsating fireballs is investigated. Fireballs disrupt when density outflow exceeds production, leading to density collapse and current disruption when the electron drift exceeds the Buneman limit. The current disruption causes a density decay in the entire discharge causing the electrode sheath to widen, starting sheath ionization and the formation of a new fireball. Finally, novel fireball properties have been observed in nonuniform magnetic fields of dipole, mirror and cusp topologies.

This paper presents the self-consistent modelling of argon micro-plasmas, produced by a microwave source (2.45 GHz) at atmospheric pressure. The source is a microstrip-like transmission line, with a 50–200 µm final gap where the micro-plasmas are created. Simulations use a one-dimensional, stationary code that solves the fluid-type transport equations for electrons, positive ions Ar⁺ and ${\rm Ar}^+_2$, and the electron mean energy; the rate balance equations for the main neutral species; Poisson's equation for the space-charge electrostatic field; Maxwell's equations for the electromagnetic excitation field; the gas energy balance equation for its temperature distribution; and the kinetic electron Boltzmann equation considering several direct and stepwize electron collision processes. The model uses a kinetic scheme that considers the atomic excited states Ar(4s) and Ar(4p), two excimer states ${\rm Ar}_2^\star$ and ${\rm Ar}_2^{\star\star}$, and two ionization states associated with the atomic and the molecular ions. The model predicts average gas temperatures of ∼600 K (similar to the measured rotational temperatures), power densities of 1–10 kW cm⁻³ (which overestimate measurements at low gap sizes), and excitation temperatures of ∼0.5–0.8 eV (which, at short gap sizes, are slightly above the experimental value of ∼0.5 eV, obtained by optical emission spectroscopy measurements), for on-axis electron densities of 5 × 10¹³, 10¹⁴, 5 × 10¹⁴ cm⁻³ and wall gas temperatures of 500, 550, 600 K, at gap sizes of 150, 100, 50 µm.

F⁻ mobility was calculated in the gas mixtures SF₆–Xe, SF₆–Ar and SF₆–He from a Monte Carlo code for ion transport simulation. The elastic momentum transfer cross sections were determined from the semi-classical JWKB approximation while the inelastic ones (detachment and charge transfer) were taken from the literature. The resulting collision cross section sets were validated from the good agreement between the calculated F⁻ mobilities in the individual gases and the measured ones. Moreover, the longitudinal and transverse density normalized diffusion coefficients and the reaction rate coefficients were calculated in these mixtures for the case where the share of SF₆ is 50%. Finally, a comparison of our calculated F⁻ mobilities in these mixtures obtained from the Monte Carlo code and those obtained from linear Blanc's law showed good agreement in the case of SF₆–Xe at low field but strong disagreement in the case of SF₆–He mixture. However, at 2000 Td, deviation from Blanc's law is obtained for F⁻ mobility in both mixtures.

The influence of a small addition of argon (2–5%) on the parameters of a strongly non-uniform microwave discharge (with the electrode microwave discharge as an example) in nitrogen at reduced pressures was studied. Experiments showed that the small addition of Ar strongly affected the discharge: it increased in size, and the power absorbed in the plasma and the emission intensities of nitrogen bands reduced. A self-consistent 2D modeling of the discharge was carried out. The model included the Maxwell, Poisson and Boltzmann equations and a set of balance equations for neutral excited and charged plasma species. The processes involving vibrationally excited ground state molecular nitrogen were taken into account by the well-known analytic expression for the vibrational distribution of molecules in the diffusion approximation. The results of modeling and experiment were in qualitative agreement. Additional information about the discharge allowed us to explain the experimental results. It was shown that this effect was influenced by a strong spatial non-uniformity of direct electron impact ionization rate, difference in diffusion and mobility coefficients, and difference in diffusion and volume processes of the loss of the main ions. This effect can be observed in all types of discharges if these conditions are satisfied.

The formation and expansion dynamics of laser-produced plasmas was studied by means of the effect of a second delayed laser pulse upon the ion kinetic energy of the plasma created by the first one. Two types of measurements were carried out: ion kinetic energy distributions and overall ion time of flight (t.o.f.). Ion energy distributions were obtained with an electrostatic energy analyser, which allowed the observation of the energy distributions of each charge state separately while the ion t.o.f. signal was measured with an ion probe. Laser power densities ranged from 2 × 10⁸ to 2 × 10⁹ W cm⁻² with 532 nm photons, and studies were extended to Al, Cu and Co targets. The effects of the second laser pulse on the plasma created by the first were very different depending on the interpulse delay. At low delays (from 0 to 10 ns) the second pulse produced an increase of the plasma average charge state and the maximum ion kinetic energy, while at higher delays (>20 ns) it produced a strong enhancement on the low charge yield (especially the single-charge ions) leaving unaltered the energies and yields of the high charges. An analysis of variations in both ion yield and kinetic energy produced interesting results regarding fundamental understanding of plasma formation and expansion dynamics, as well as possible improvements in technological applications based on laser-produced plasmas.