Table of contents

For over 80 years emissive probes have been used to measure plasma potential and a wide variety of methods for interpreting probe data now exists. Constructions, heating methods and measurement techniques are reviewed in detail and their various strengths and limitations are compared. Additionally, several novel uses for emissive probes, such as measuring electron temperature are presented. This review also includes tables of recommendations for emissive probe design given the type of plasma and desired measurements.

Axial emission profiles in a parallel plate dc microdischarge (feedgas: argon; discharge gap d = 1 mm; pressure p = 10 Torr) were studied by means of time-resolved imaging with a fast ICCD camera. Additionally, volt–ampere (V–A) characteristics were recorded and Ar* metastable densities were measured by tunable diode laser absorption spectroscopy (TDLAS). Axial emission profiles in the steady-state regime are similar to corresponding profiles in standard size discharges (d ≈ 1 cm, p ≈ 1 Torr). For some discharge conditions relaxation oscillations are present when the microdischarge switches periodically between the low current Townsend-like mode and the normal glow. At the same time the axial emission profile shows transient behavior, starting with peak distribution at the anode, which gradually moves toward the cathode during the normal glow. The development of argon metastable densities highly correlates with the oscillating discharge current. Gas temperatures in the low current Townsend-like mode (T_g = 320–400 K) and the high current glow mode (T_g = 469–526 K) were determined by the broadening of the recorded spectral profiles as a function of the discharge current.

This work is devoted to the analysis of experimental results obtained in dry air at atmospheric pressure in a positive point-to-plane corona discharge under a pulsed applied voltage in the cases of anodic mono- and multi-tips. In the mono-tip case, the peak corona current is analysed as a function of several experimental parameters such as magnitude, frequency and duration of pulsed voltage and gap distance. The variation of the corona discharge current is correlated with the ozone production. Then in the multi-tip case, the electrical behaviour is analysed as a function of the distance between two contiguous tips and the tip number in order to highlight the region of creation active species for the lowest dissipated power. Intensified charge-coupled device pictures and electric field calculations as a function of inter-tip distance are performed to analyse the mutual effect between two contiguous tips. The optical emission spectra are measured in the UV–visible–NIR wavelength range between 200 nm and 800 nm, in order to identify the main excited species formed in an air corona discharge such as the usual first and second positive systems with first negative systems of molecular nitrogen. The identification of atomic species (O triplet and N) and the quenching of NOγ emission bands are also emphasized.

Electron motion in a CF₄ magnetic neutral loop discharge (NLD) plasma was simulated using a Monte Carlo method. The spatial distribution of electrons illustrated the fundamental structure of the NLD plasma, and its dynamics were depicted from the distributions of the mean electron energy, the electron energy gain and the azimuthal electron flux. The peak of mean electron energy appeared at the neutral loop (NL), which confirmed that electrons gain energy near this loop. High mean electron energy was observed not only near the NL but also along the separatrices of the quadrupole magnetic field. Energetic electrons were transported along the separatrices and induced ionization at those locations. However, the electron distribution had valleys along the separatrices, because electrons accelerated near the NL were likely to leave this region where the binding of the magnetic field is weak. The azimuthal electron flux representing the loop plasma current showed that the electron conduction path around the NL, which has conventionally been modelled as a ring conductor, has a particular directionality due to the rectification effect of antiparallel magnetic fields composing the quadrupole magnetic field. The directionality in the upper and lower regions of the quadrupole magnetic field was opposite to that in the inner and outer regions.

Anode striations and wall charges, common phenomena in the plasma display panel (PDP) discharge process, are investigated by simulations in both alternating current coplanar and novel shadow mask PDPs. The formation process of striations is presented and the formation mechanism is investigated. The results reveal that in both structures there is an obvious correspondence between the striation distribution and both the negative and the positive charge accumulation on the dielectric layer above the anode. Each of the two contributions to the total wall charge reaches its peak at the positions where the striations emerge. The total wall charge distribution on the other hand, which is the sum of the wall charge collected from electrons and ions, does not show any striation effect at all.

Low-temperature RF discharges have been widely used in processing applications. Plasma diagnostics provide useful information about the plasma state which is important for processing results. In the deposition process, as the dielectric material is coated onto the probe surface, electrical diagnostic techniques using a dc current cannot be applied. Instead, an ac voltage is applicable for measuring plasma parameters. In this paper, electron temperatures and plasma densities were measured with an anodized aluminum probe using the floating-type harmonic method and the self-bias method. The Al₂O₃ layer on the probe surface and the sheath were modeled as a series connection of a capacitor and a resistor, respectively. The applied ac voltage was divided into the two parts depending on their impedances, and the voltage across the sheath was determined by the phase between the voltage and the current. According to experimental results, the conventional harmonic method, which uses the first and second harmonic current, was not valid to measure the electron temperature when the dielectric layer was thick. In contrast, the electron temperature measured by the self-bias method, which uses only the first harmonic current, was reliable regardless of the thickness of the dielectric layer.

Low-power plasma generators with two kinds of hot anode/nozzle structures, one with a natural radiation-cooled nozzle and the other with a regeneratively cooled nozzle, were designed to investigate the dependence of the volt–ampere characteristics on the anode temperature. Pure argon, nitrogen or hydrogen gas was used as the plasma working gas at input powers from 130 to 1200 W in a plenum chamber kept at a pressure of below 20 Pa. Variations of the arc voltage with changes in arc current, gas flow rate and firing time (anode temperature) were examined, and the effects of the arc volt–ampere characteristics on the properties of the ejected plasma flow from the nozzle exit are discussed with respect to the evaluation of the average plume temperature and flow velocity. Results show that there are definitely non-negligible effects of anode temperature on these characteristics.

A time-dependent plasma discharge model has been developed for the ionization region in a high-power impulse magnetron sputtering (HiPIMS) discharge. It provides a flexible modeling tool to explore, e.g., the temporal variations of the ionized fractions of the working gas and the sputtered vapor, the electron density and temperature, and the gas rarefaction and refill processes. A separation is made between aspects that can be followed with a certain precision, based on known data, such as excitation rates, sputtering and secondary emission yield, and aspects that need to be treated as uncertain and defined by assumptions. The input parameters in the model can be changed to fit different specific applications. Examples of such changes are the gas and target material, the electric pulse forms of current and voltage, and the device geometry. A basic version, ionization region model I, using a thermal electron population, singly charged ions, and ion losses by isotropic diffusion is described here. It is fitted to the experimental data from a HiPIMS discharge in argon operated with 100 µs long pulses and a 15 cm diameter aluminum target. Already this basic version gives a close fit to the experimentally observed current waveform, and values of electron density n_e, the electron temperature T_e, the degree of gas rarefaction, and the degree of ionization of the sputtered metal that are consistent with experimental data. We take some selected examples to illustrate how the model can be used to throw light on the internal workings of these discharges: the effect of varying power efficiency, the gas rarefaction and refill during a HiPIMS pulse, and the mechanisms determining the electron temperature.

There is an increasing concern that high power impulse magnetron sputtering (HiPIMS) systems are being disadvantaged by relatively low deposition rates compared with traditional dc magnetrons operating at the same average power input. Nevertheless, a minimization of the losses of ionized and neutral species is possible in HiPIMS discharges. In the described magnetron configuration, the majority of metal ions escaping the discharge volume can be either used for deposition or, if they fail to reach the substrate, for discharge maintenance when charge redistribution from one ionization area to the other takes place. The anode-to-cathode configuration allows the neutrals, which are not deposited on the substrate, to be guided to the other instantaneous cathode and be ionized or re-sputtered. For the same average power input, it becomes possible to increase the self-sputtering efficiency, deposition rate and obtain a versatile control over the film microstructure.

An atmospheric-pressure room-temperature plasma brush, which can deliver uniform surface treatment effects, is reported. The plasma structure, which includes the negative glow, Faraday dark space and positive column, is clearly visible to the naked eye. The width of the Faraday dark space diminishes with decreasing gap distance and this phenomenon is different from that observed from low-pressure glow discharge plasmas. High-speed photographs taken at an exposure time of 2.5 ns show that the plasma propagates from the nozzle to the object in about 100 ns and 10 ns for gap distances of 6 mm and 2 mm, respectively, and the results are consistent with electric measurements. The emission spectra reveal N₂(B–A) bands in addition to those of O, , N₂(C–B) and He, indicating that the plasma source is reactive and suitable for applications such as surface modification and materials processing.

A set of diagnostic methods to obtain the plasma parameters including power dissipation, gas temperature and electron density is evaluated for an atmospheric pressure helium or argon radio frequency (RF) plasma needle for biomedical applications operated in open air. The power density of the plasma is more or less constant and equal to 1.3 × 10⁹ W m⁻³. Different methods are investigated and evaluated to obtain the gas temperature. In this paper the gas temperatures obtained by rotational spectra of OH(A–X) and (B–X) are compared with Rayleigh scattering measurements and measurements of the line broadening of hydrogen and helium emission lines. The obtained gas temperature ranges from 300 to 650 K, depending on the gas. The electron densities are estimated from the Stark broadening of the hydrogen α and β lines which yield values between 10¹⁹ and 10²⁰ m⁻³. In the case of helium, this is an overestimate as is shown by a power balance from the measured power density in the plasma jet. The obtained plasma parameters enable us to explain the radial contraction of the argon plasma compared with the more diffuse helium plasma. The accuracy of all considered diagnostics is discussed in detail.

A low-frequency oscillation (<100 Hz) has been observed in a low-pressure (1–50 mTorr) radio-frequency (RF) inductively coupled plasma, produced in sulfur hexafluoride. Langmuir probe studies have characterized this oscillation with respect to RF power, gas pressure and probe proximity to the antenna. The experimental parameter space within which this oscillation is observed is mapped with respect to power and pressure for the reaction chamber in use. The oscillation is observed in Langmuir probe currents for positive probe bias, and has a strong dependence on experimental conditions, as well as probe position within the chamber. The propagation speed of the instability away from the source is found to be 16 m s⁻¹.

The far-field plume of a 1.5 kW Hall effect thruster is mapped with a Langmuir probe and an emissive probe. Time-averaged measurements of the plasma potential, the electron temperature and the electron number density are performed for different operating conditions of the thruster. The influence of the discharge voltage, the cathode mass flow rate as well as the magnetic field strength is investigated. The plasma potential decreases from 30 V at 300 mm on the thruster axis to 5 V at 660 mm and at 60°, the electron temperature decreases from 5 to 1.5 eV. The electron number density drops from 3.5 × 10¹⁶ to 1 × 10¹⁵ m⁻³ in the far-field plume. The values of the plasma potential and electron temperature measured with the Langmuir probe and the emissive probe are in good agreement.

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