Table of contents

For unmagnetized low temperature Ar plasmas with plasma density ranging from 3 × 10⁸ to 10¹⁰ cm⁻³ and an electron temperature of ∼1 eV, the expansion of the ion collecting area of a double-sided planar Langmuir probe with respect to probe bias is experimentally investigated, through a systematic scan of plasma parameters. In accordance with many existing numerical studies, the ion collecting area is found to follow a power law for a sufficiently negative probe bias. Within our experimental conditions, the power law coefficient and exponent have been parameterized as a function of the normalized probe radius and compared with numerical results where qualitatively comparable features are identified. However, numerical results underestimate the power law coefficient while the exponent is overestimated. Our experimental measurements also confirm that ion–neutral collisions play a role in determining the expanded ion collecting area, thus changing values of the power law coefficient and exponent. This work suggests that a power law fit to the ion collecting area must be performed solely based on experimentally obtained data rather than using empirical formulae from simulation results since material and cleanness of the probe, type of working gas, and neutral pressure may also affect the expansion of the ion collecting area, factors which are difficult to model in a numerical simulation. A proper scheme of analyzing an I–V characteristic of a Langmuir probe based on a power law fit is also presented.

The benefits of thermionic emission from negatively biased electrodes for perpendicular electric field control in a magnetized plasma are examined through its combined effects on the sheath and on the plasma potential variation along magnetic field lines. By increasing the radial current flowing through the plasma thermionic emission is confirmed to improve control over the plasma potential at the sheath edge compared to the case of a cold electrode. Conversely, thermionic emission is shown to be responsible for an increase of the plasma potential drop along magnetic field lines in the quasi-neutral plasma. These results suggest that there exists a trade-off between electric field longitudinal uniformity and amplitude when using negatively biased emissive electrodes to control the perpendicular electric field in a magnetized plasma.

Low-energy electron beam generation using a DC biased grid was investigated in an inductively coupled plasma (ICP). The electron beam was measured in argon gas at various pressures, ICP source powers, and substrate voltages (V_sub). At a low ICP source power (50 W), an electron beam was generated even at small values of V_sub (10 V), however at a high ICP source power (200 W), an electron beam was only generated when a higher voltage (30 V) was applied due to the short sheath thickness on the grid surface. The sheath on the grid surface is an important factor for generating electron beams because low-energy electrons are blocked. If the sheath thickness to small, a high voltage should be applied to generate an electron beam, as accelerate regions cannot exist without the sheath. At high pressure, since electrons experience numerous neutral collisions, a high substrate voltage is needed to generate an electron beam. However, if the applied substrate voltage becomes too high (40 V) at high pressure, high-energy electrons result in secondary plasma under the grid. Therefore, maintaining a low pressure and low ICP source power is important for generating electron beams.

Capacitively coupled plasma is investigated kinetically utilizing the particle-in-cell technique. The argon (Ar) plasma is generated via two radio frequencies. The plasma bulk density increases by increasing the voltage amplitude of the high frequency (⩾13.56 MHz), which is much greater than the ion plasma frequency. The intermediate radio frequencies (≈1 MHz), which are comparable to the ion plasma frequency, cause a considerable broadening of the ion energy distribution, i.e. ions gain energies higher and lower than the time-averaged energy. The good agreement between published experimental results and our theoretical calculations via the ensemble-in-spacetime model confirms the modulation of ions around time-averaged values. Intermediate frequencies allow ions to partially respond to the instantaneous electric field. The response of ions to the instantaneous electric field is investigated semi-analytically. The dispersion relation of the plasma sheath and bulk are derived. Stable ion acoustic modes are found. Ion-acoustic modes have two different velocities and carry energy from the sheath edge to the electrode. In addition, intermediate frequencies excite solitons in the plasma sheath. The results may help to explain the ion density, flux, and energy modulation, and, consequently, the broadening of the ion energy distribution.

Thus far, effects of secondary γ-electrons emitted from accelerator grids (AGs) of gridded ion sources on ionization in discharge chambers have not been studied. The presence and induced processes of such secondary electrons in a microwave electron cyclotron resonance gridded ion source are confirmed by the consistent explanations of: (1) the observed jump of ion beam current (I_b) in case of a low-density plasma appearing at the chamber's radial center due to the microwave skin effect; (2) the evolution of glow images recorded from the end-view of the ion source during the jump of I_b; (3) the over-large jump step of I_b with increasing microwave power; (4) the pattern appearing on the temperature sticker exposed to the discharge operated in the regime where the arrayed energetic-electron beamlets are injected into the discharge chamber; (5) the measured step-increment in the voltage drop across the screen grid (SG) sheath. A positive feedback loop composed of involved processes is established to elucidate the underlying mechanism. Energetic γ-electrons from the AG and warm δ-electrons from the opposite antenna do not produce direct excitation and ionization, but they enhance the electrical confinement of cold electrons by elevating the voltage drop across the sheaths at the antenna and SG, thus leading to the jump of I_b. The energetic γ-electrons-based model can be also modified to explain abnormal results observed in the other gridded ion sources. Energetic γ-electrons from AGs should be taken into account in understanding gridded ion sources.

The information about the electron population of a helicon source plasma that expands along a magnetic nozzle is important for understanding the plasma acceleration across the potential drop that forms in the nozzle. The electrons need an energy higher than the potential drop to escape from the source. At these energies the signal of a Langmuir probe is less accurate. An inverted RFEA measures the high-energy tail of the electrons. To reach the probe, they must have energies above the plasma potential V_P, which can vary over the region of the measurement. By constructing a full distribution by applying the electron temperature T_e obtained from the electron IV-curve and the V_P obtained from the ion collecting RFEA or an emissive probe, a density measure of the hot electron distribution independent of V_P can be obtained. The variation of the high-energy tail of the EEDF in both radial and axial directions, in the two different cases of (1) a purely expanding magnetic field nozzle, and (2) a more constricted one by applying current in a third, downstream coil was investigated. The electron densities and temperatures from the source are then compared to two analytic models of the downstream development of the electron density. The first model considers the development for a pure Boltzmann distribution while the second model takes an additional magnetic field expansion into account. A good match between the measured densities and the second model was found for both configurations. The RFEA probe also allows for directional measurement of the electron current to the probe. This property is used to compare the densities from the downstream and upstream directions, showing a much lower contribution of downstream electrons into the source for a purely expanding magnetic field in comparison to the confined magnetic field configuration.

Radio frequency capacitively coupled plasmas (RF CCPs) sustained in fluorocarbon gases or their mixtures with argon are widely used in plasma-enhanced etching. In this work, we conduct studies on instabilities in a capacitive CF₄/Ar (1:9) plasma driven at 13.56 MHz at a pressure of 150 mTorr, by using a one-dimensional fluid/Monte-Carlo (MC) hybrid model. Fluctuations are observed in densities and fluxes of charged particles, electric field, as well as electron impact reaction rates, especially in the bulk. As the gap distance between the electrodes increases from 2.8 cm to 3.8 cm, the fluctuation amplitudes become smaller gradually and the instability period gets longer, as the driving power density ranges from 250 to 300 W m⁻². The instabilities are on a time scale of 16–20 RF periods, much shorter than those millisecond periodic instabilities observed experimentally owing to attachment/detachment in electronegative plasmas. At smaller electrode gap, a positive feedback to the instability generation is induced by the enhanced bulk electric field in the highly electronegative mode, by which the electron temperature keeps strongly oscillating. Electrons at high energy are mostly consumed by ionization rather than attachment process, making the electron density increase and overshoot to a much higher value. And then, the discharge becomes weakly electronegative and the bulk electric field becomes weak gradually, resulting in the continuous decrease of the electron density as the electron temperature keeps at a much lower mean value. Until the electron density attains its minimum value again, the instability cycle is formed. The ionization of Ar metastables and dissociative attachment of CF₄ are noticed to play minor roles compared with the Ar ionization and excitation at this stage in this mixture discharge. The variations of electron outflow from and negative ion inflow to the discharge center need to be taken into account in the electron density fluctuations, apart from the corresponding electron impact reaction rates. We also notice more than 20% change of the Ar⁺ ion flux to the powered electrode and about 16% difference in the etching rate due to the instabilities in the case of 2.8 cm gap distance, which is worthy of more attention for improvement of etching technology.

The plasma potential at a typical substrate position is studied during the positive pulse of a bipolar high-power impulse magnetron sputtering (bipolar HiPIMS) discharge with a Cu target. The goal of the study is to identify suitable conditions for achieving ion acceleration independent on substrate grounding. We find that the time-evolution of the plasma potential during the positive pulse can be separated into several distinct phases, which are highly dependent on the discharge conditions. This includes exploring the influence of the working gas pressure (0.3–2 Pa), HiPIMS peak current (10–70 A corresponding to 0.5–3.5 A cm⁻²), HiPIMS pulse length (5–60 μs) and the amplitude of the positive voltage U₊ applied during the positive pulse (0–150 V). At low enough pressure, high enough HiPIMS peak current and long enough HiPIMS pulse length, the plasma potential at a typical substrate position is seen to be close to 0 V for a certain time interval (denoted phase B) during the positive pulse. At the same time, spatial mapping of the plasma potential inside the magnetic trap region revealed an elevated value of the plasma potential during phase B. These two plasma potential characteristics are identified as suitable for achieving ion acceleration in the target region. Moreover, by investigating the target current and ion saturation current at the chamber walls, we describe a simple theory linking the value of the plasma potential profile to the ratio of the available target electron current and ion saturation current at the wall.

While plasma–liquid interactions have been an important focus in the plasma research community, the impact of the strong coupling between plasma and liquid on plasma properties and processes remains not fully understood. In this work, we report on the impact of the applied voltage, pulse width and liquid conductivity on the plasma morphology and the OH generation for a positive pulsed DC atmospheric pressure plasma jet with He–0.1% H₂O mixture interacting with a liquid cathode. We adopted diagnostic techniques of fast imaging, 2D laser induced fluorescence of OH and Thomson scattering spectroscopy. We show that plasma instabilities and enhanced evaporation occur and have a significant impact on the OH generation. At elevated plasma energies, it is found that the plasma contracts due to a thermal instability through Ohmic heating and the contraction coincides with a depletion in the OH density in the core due to electron impact dissociation. For lower plasma energies, the instability is suppressed/delayed by the equivalent series resistor of the liquid electrode. An estimation of the energy flux from the plasma to the liquid shows that the energy flux of the ions released into the liquid by positive ion hydration is dominant, and significantly larger than the energy needed to evaporate sufficient amount of water to account for the measured H₂O concentration increase near the plasma–liquid interface.

Plasma-based microwave power limitation in a suspended microstrip transmission line integrating a micro hollow cathode discharge (MHCD) in its centre is experimentally and numerically studied. Transient and steady state microwave power measurements exhibit a limitation threshold of 28 dBm and time responses of 25 microseconds. Intensified charge-coupled device imaging shows that microwave breakdown occurs at the top of the MHCD. The plasma then extends towards the microwave source within the suspended microstrip transmission line. Besides, a self-consistent model is proposed to simulate the non-linear interaction between microwave and plasma. It gives numerical results in agreement with the measurements, and show that the plasma expansion during the transient response is related to a shift between the ionization source term and the electron density maximum. The propagation speed, under the tested conditions, depends mainly on the stepwise ionization from the excited states.

Negative ions are an important constituent of the spatial afterglow of atmospheric pressure plasmas, where the fundamental plasma-substrate interactions take place that are vital for applications such as biomedicine, material synthesis, and ambient air treatment. In this work, we use laser-induced photodetachment to liberate electrons from negative ions in the afterglow region of an atmospheric pressure plasma jet interacting with an argon-oxygen mixture, and microwave cavity resonance spectroscopy to detect the photodetached electrons. This diagnostic technique allows for the determination of the electron density and the effective collision frequency before, during and after the laser pulse was shot through the measurement volume with nanosecond time resolution. From a laser saturation study, it is concluded that O⁻ is the dominant negative ion in the afterglow. Moreover, the decay of the photodetached electron density is found to be dominantly driven by the (re)formation of O⁻ by dissociative attachment of electrons with O₂. As a consequence, we identified the species and process responsible for the formation of negative ions in the spatial afterglow in our experiment.

In this paper we compared the fast Intensified Charge Coupled Device (ICCD) imaging with the newly developed diagnostic method that utilizes laser induced breakdown in plasma jet. Our helium plasma jet was powered by an 80 kHz high-voltage sine wave and propagated into the ambient air. Pulsed laser beam 1064 nm (4 ns pulse duration and 5 Hz repetition rate) was focused with the lens into the plasma jet at energy below breakdown threshold in helium. Laser pulses and the jet powering signal were synchronized. Laser induced plasma is highly dependent on the concentration of seed electrons and other charged particles in the plasma jet channel. We compared the radial profiles of the plasma jet obtained with these two methods. For laser induced breakdown it was ±0.5 mm and for ICCD measurement it was ±1.75 mm, while the ionization wave velocities obtained with these two methods were 15 km s⁻¹ and 20 km s⁻¹ respectively. Electrical characteristics of the plasma jet were also presented and one can see a large hysteresis effect when the applied power to the plasma jet was reducing. We show that the laser induced breakdown spectroscopy can be used as a complementary diagnostics technique with ICCD measurements.

The properties of Ar plasma generated by electron beam with initial energy of 45 keV passing through a 5 μm-thick diamond film window was investigated by experimental diagnostic and Monte Carlo simulation. It is found that the plasma light emission intensity enhances with increasing the electron beam current, while the plasma shape has no significant change. When the gas pressure increases, the plasma shrinks and becomes brighter, and its shape gradually changes from cone-shape to semi prolate spheroid. The electron density increases with increasing gas pressure and electron current. When the gas pressure is higher than 10 kPa, the electron density can reach the order of 10¹⁰ cm⁻³ at an electron current of 0.3 mA. Under high-pressure conditions, the plasma range with respect to gas pressure satisfies well a simple inverse relationship. The electron energy deposition distribution obtained by Monte Carlo simulation is consistent with the measured plasma light emission intensity distribution. Optical emission spectroscopy was used to analyze collision process in the electron beam plasma. The line intensities of the 2p₂, 2p₆ and 2p₁₀ levels grow relatively with increasing gas pressure, indicating that the atom-atom collisional processes are enhanced.

A two-electron temperature plasma is produced by the method of diffusion of two different plasmas with distinct temperatures and densities. The method is simple and provides an adequate control over the plasma parameters. The study reveals that the temperature and density of both the electron groups can be effectively controlled by just changing the discharge currents of both the plasmas. An ion-acoustic wave is excited in the plasma and is detected using a planar Langmuir probe. The damped amplitude of the wave is measured and is used as a diagnostic tool for establishing the presence of two-electron components. This production method can be helpful in controlling the hot electron density and temperature in plasma processing industries.

Fluid models are frequently used to describe a non-thermal plasma, such as a streamer discharge. The required electron transport data and rate coefficients for the fluid model are parametrized using local field approximation in first-order models and local-mean-energy approximation in second-order models. We performed Monte Carlo simulations in nitrogen gas with step changes in the E/N (reduced electric field) to study the behavior of the transport properties in the transient phase. During the transient phase of the simulation, we extract the instantaneous electron mean energy, which is different to the steady-state mean electron energy, and the corresponding transport parameters and rate coefficients. Our results indicate that the mean electron energy is not a suitable parameter for mobility/drift of electrons due to the large difference in momentum relaxation and energy relaxation. However, the high-energy threshold rates, such as ionization, show a strong correlation to mean electron energy. In second-order models where the energy-balance equation is solved, we suggest that it would be appropriate to rather use the local electric field to find electron drift velocity in gases such as nitrogen, and the local mean electron energy to determine the ionization and excitation rates.

We demonstrate an effective method for realization of robust, tailorable annular plasma photonic crystals (PPC) in dielectric barrier discharge with two water electrodes. Fast reconfiguration between triangular lattice, annular lattice, core-annular lattice and concentric-annular lattice has been achieved. An active control on the structure of plasma elements is realized by solely changing the applied voltage. The changes of photonic band gaps with reconfiguration of different annular PPCs have been studied both experimentally and numerically. The band gaps between 28.0–30.0 GHz for the core-annular lattice and the concentric-annular lattice are experimentally verified. A phenomenological reaction–diffusion model with two nonlinear-coupled interacting layers is established to mimic the formation of various plasma structures. Experimental observations and numerical simulation are in good agreement. Our approach provides a unique strategy to create reversibly deformable annular PPCs, which may offer new capabilities and serve as a promising platform for various applications.

Investigating the ion dynamics in the emerging bipolar pulse high power impulse magnetron sputtering (BP-HiPIMS) discharge is necessary and important for broadening its industrial applications. Recently, an optimized plasma source operating the BP-HiPIMS with an auxiliary anode and a solenoidal coil is proposed to enhance the plasma flux and energy, named as ACBP-HiPIMS ('A'-anode, 'C'-coil). In the present work, the temporal evolutions of the ion velocity distribution functions (IVDF) in BP-HiPIMS and ACBP-HiPIMS discharges are measured using a retarding field energy analyser (RFEA). For the BP-HiPIMS discharge, operated at various positive pulse voltages U₊, the temporal evolutions of IVDFs illustrate that there are two high-energy peaks, E₁ and E₂, which are both lower than the applied U₊. The ratio of the mean ion energy E_i,mean to the applied U₊ is around 0.55–0.6 at various U₊. In ACBP-HiPIMS discharge, the IVDF evolution shows three distinguishable stages which has the similar evolution trend with the floating potential V_f on the RFEA frontplate: (i) the stable stage with two high-energy peaks (E₂ and E₃ with energy respectively lower and higher than the applied U₊ amplitude) when the floating potential V_f is close to the applied positive pulse voltage; (ii) the transition stage with low-energy populations when the V_f drops by ∼20 V within ∼10 μs; and (iii) the oscillation stage with alternating E₂ and E₃ populations and ever-present E₁ population when the V_f slightly decreases until to the end of positive pulse. The comparison of IVDFs in BP-HiPIMS and ACBP-HiPIMS suggests that both the mean ion energy and high-energy ion flux have been effectively improved in ACBP-HiPIMS discharge. The formation of floating potential drop is explored using the Langmuir probe which may be attributed to the establishment of anode double layer structure. The acceleration of ion at the double layer boundary is analysed using a theoretical model, in this way to clarify the oscillation in IVDF evolutions in ACBP-HiPIMS discharge.

Ion-induced secondary electron emission at a target surface is an essential mechanism for laboratory plasmas, i.e. magnetron sputtering discharges. Electron emission, however, is strongly affected by the target condition itself such as oxidation. Data of oxidized targets, however, are very sparse and prone to significant systematic errors, because they were often determined by modeling the complex behavior of the plasma. Thus, it is difficult to isolate the process of ion-induced electron emission from all other plasma-surface-interactions. By utilizing ion beams, the complex plasma environment is avoided and electron yields are determined with higher accuracy. In this study, ion-induced secondary electron emission coefficients (SEECs) of clean, untreated (air-exposed), and intentionally oxidized copper and nickel surfaces were investigated in such a particle beam experiment. Pristine and oxidized metal foils were exposed to beams of singly charged argon ions with energies of 0.2 keV - 10 keV. After the ion beam treatment, the surface conditions were analyzed by ex-situ x-ray photoelectron spectroscopy measurements. Further, a model for the electron emission of a partly oxidized surface is presented, which is in agreement with the experimental data. It was found, that oxidized and untreated/air-exposed surfaces do not show the same SEEC: for intentionally oxidized targets, the electron yields were smaller by a factor of 2 than for untreated/air-exposed surfaces. SEECs of oxides were found to be between the values for clean and for untreated metal surfaces. Further, the SEEC was at maximum for untreated/air-exposed surfaces and at minimum for clean surfaces; the electron yields of untreated/air-exposed and clean surfaces were in agreement with values reported in the literature.

A first-principles approach to obtain the attachment length within a hollow cathode with a constrictive orifice, and its scaling with internal cathode pressure, is developed. This parameter, defined herein as the plasma density decay length scale upstream of (away from) the cathode orifice, is critical because it controls the utilization of the hollow cathode insert and influences cathode life. A two-dimensional framework is developed from the ambipolar diffusion equation for the insert-region plasma. A closed-form solution for the plasma density is obtained using standard partial differential equation techniques by applying an approximate boundary condition at the cathode orifice plane. This approach also yields the attachment length and electron temperature without reliance on measured plasma property data or complex computational models. The predicted plasma density profile is validated against measurements from the NSTAR discharge cathode, and calculated electron temperatures and attachment lengths agree with published values. Nondimensionalization of the governing equations reveals that the solution depends almost exclusively on the neutral pressure-diameter product in the insert plasma region. Evaluation of analytical results over a wide range of input parameters yields scaling relations for the variation of the attachment length and electron temperature with the pressure-diameter product. For the range of orifice-to-insert diameter ratio studied, the influence of orifice size is shown to be small except through its effect on insert pressure, and the attachment length is shown to be proportional to the insert inner radius, suggesting high-pressure cathodes should be constructed with larger-diameter inserts.

The generation and enhancement of active species in non-thermal plasmas are always decisive issues with respect to their successful applications. In this work, an atmospheric pressure plasma jet (APPJ) is generated in Ar + 1% CH₄ gas flow by a bipolar nanosecond high-voltage (HV) source with a maximum pulse repetition rate up to 1 MHz (i.e. minimum pulse interval ΔT = 1 μs) in burst mode. The absolute density of hydrogen atom at ground state is measured by the two-photon absorption laser-induced fluorescence method. It is observed that with ΔT = 1 μs, the H atom density keeps increasing during the first eight HV pulses and later on, the H atom density is maintained at a quasi-stable value while more HV pulses are applied. When decreasing ΔT from 10 to 1 μs, while keeping the total number of HV pulses the same (with similar coupled energy), the peak H atom density increases by a factor of more than four times, but the decay of H atom density after the pulse burst with ΔT = 1 μs is faster. Another effect of short ΔT is to extend the axial distribution of the H atom outside the APPJ's nozzle, and the ΔT = 2 μs case has the highest averaged H atom density when taking its temporal evolution and axial distribution into consideration. In this work, we propose that the intensive nanosecond HV burst is an efficient approach to enhance the active species density in non-thermal plasmas when a rapid response is required.