Biased electrodes are common components of plasma sources and diagnostics. The plasma-electrode interaction is mediated by an intervening sheath structure that influences properties of the electrons and ions contacting the electrode surface, as well as how the electrode influences properties of the bulk plasma. A rich variety of sheath structures have been observed, including ion sheaths, electron sheaths, double sheaths, double layers, anode glow, and fireballs. These represent complex self-organized responses of the plasma that depend not only on the local influence of the electrode, but also on the global properties of the plasma and the other boundaries that it is in contact with. This review summarizes recent advances in understanding the conditions under which each type of sheath forms, what the basic stability criteria and steady-state properties of each are, and the ways in which each can influence plasma-boundary interactions and bulk plasma properties. These results may be of interest to a number of application areas where biased electrodes are used, including diagnostics, plasma modification of materials, plasma sources, electric propulsion, and the interaction of plasmas with objects in space.

Interaction of biased electrodes and plasmas: sheaths, double layers, and fireballs

We discuss analytical fast-ion velocity distribution functions which are useful for basic plasma modelling as illustrated for the tokamak ITER. The Maxwellian is by far the most widespread model for ions and electrons in tokamaks and stellarators. The bi-Maxwellian and the drifting (bi-)Maxwellian are extensions allowing for anisotropy and bulk plasma flow, respectively. For example, fast ions generated by wave heating in the ion cyclotron range of frequencies are often described by bi-Maxwellians or so-called tail temperatures. The ring distribution can serve as a basic building block for arbitrary distributions or as a bump-on-tail in stability studies. The isotropic slowing-down distribution is a good model for fusion α-particles. The anisotropic slowing-down distribution occurs for anisotropic particle sources as is typical for neutral beam injection. We physically motivate these distribution functions and present analytical models in various coordinate systems commonly used by theorists and experimentalists. We further calculate 1D projections of the distribution functions onto a diagnostic line-of-sight to gain insight into measurements relying on the Doppler shift.

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Bi-Maxwellian, slowing-down, and ring velocity distributions of fast ions in magnetized plasmas

Formation of three-dimensional carbon nanotube structures by controllable vapor densification

This study investigated the mutual interaction between the plasma and plasma treated water (PTW). Many works have shown that the plasma treatment decreases the pH of PTW due to nitric oxide electrolyte ion but the interactions between PTW and the plasma are still largely unknown. We investigated the effect of PTW on a plasma as well as the effect of a plasma on PTW using a pin-to-liquid discharge system. It is found that PTW affects not only the chemical properties but also the physical properties of the plasma such as breakdown voltage and concentration of plasma column. The decrease of the liquid surface tension of PTW due to nitric oxide electrolyte ion from the plasma results in the increase of plasma current onto the surface of PTW and vice versa. The feedback process will be continued until the transition from normal discharge to abnormal discharge. These results can be basic data for the development of plasma sources to treat liquids.

/pdf/mutual-interaction-between-plasma-characteristics-and-liquid-37s2jqcsug.pdf

Mutual Interaction between Plasma Characteristics and Liquid Properties in AC-driven Pin-to-Liquid Discharge

Benchmarking and validation are prerequisites for using simulation codes as predictive tools. In this work, we have developed a Global Model for Negative Hydrogen Ion Source (GMNHIS) and performed benchmarking of the GMNHIS against another independently developed code, Global Enhanced Vibrational Kinetic Model (GEVKM). This is the first study to present a quite comprehensive benchmarking test of this kind for models of negative hydrogen ion sources (NHIS), and excellent agreements have been achieved for collisional energy loss per electron-ion pair created, electron number density, electron temperature, densities of H 3 + and H 2 + ions, and densities of H(n = 1–3) atoms. Very small discrepancies in number densities of H − ions and H + ions, as well as the vibrational distribution function of hydrogen molecules, can be attributed to the differences in the chemical reactions datasets. The GEVKM includes additional chemical reactions that are more important at high pressures. In addition, we validated the GMNHIS against experimental data obtained in an electron cyclotron resonance discharge used for H − production. The model qualitatively (and even quantitatively for certain conditions) reproduces the experimental H − number density. The H − number density as a function of pressure first increases at pressures below 1.6 Pa and then saturates for higher pressures. This dependence was analyzed by evaluating contributions from different reaction pathways to the creation and loss of the H − ions. The developed codes can be used for predicting the H − production, improving the performance of NHIS, and ultimately optimizing the parameters of negative ion beams for fusion reactors.

Benchmarking and validation of global model code for negative hydrogen ion sources

A global model was developed to investigate the densities of negative ions and the other species in a low-pressure inductively coupled hydrogen plasma with a bi-Maxwellian electron energy distribution. Compared to a Maxwellian plasma, bi-Maxwellian plasmas have higher populations of low-energy electrons and highly vibrationally excited hydrogen molecules that are generated efficiently by high-energy electrons. This leads to a higher reaction rate of the dissociative electron attachment responsible for negative ion production. The model indicated that the bi-Maxwellian electron energy distribution at low pressures is favorable for the creation of negative ions. In addition, the electron temperature, electron density, and negative ion density calculated using the model were compared with the experimental data. In the low-pressure regime, the model results of the bi-Maxwellian electron energy distributions agreed well quantitatively with the experimental measurements, unlike those of the assumed Maxwellian electron energy distributions that had discrepancies.

Global model analysis of negative ion generation in low-pressure inductively coupled hydrogen plasmas with bi-Maxwellian electron energy distributions

Recent studies have demonstrated that the two-ion-stream-instability occurs near the plasma boundary and makes the ions reach the 'modified Bohm velocity' at the sheath edge. In most low-temperature plasmas, however, the ion-neutral collisions can disturb the growth of instability to occur frequently, and the ions exit the plasma boundary with their own Bohm velocities. We report some experimental observations regarding this issue. The spatial variations of the ion drift velocities and the space potential near the sheath edge were measured in Ar/Xe mixture plasmas by increasing the total pressure in the range of 0.5–2.1 mTorr. The results show that the instability cannot occur above a certain pressure condition and that is consistent with the theoretically driven pressure criteria for the onset of the instability.

https://iopscience.iop.org/article/10.1088/1361-6595/aa67c4/pdf

Ion-neutral collision effect on ion-ion two-stream-instability near sheath-presheath boundary in two-ion-species plasmas

This paper introduces how to determine concentrations of ion species in a mixed gas plasma that are not linearly proportional to their neutral partial pressures. A particle balance model was developed to predict the relative ion concentrations in multiple-ion-species plasmas, considering their ionization rates and loss fluxes to the wall. Analysis is carried out especially with Ar/Xe and Ar/He multi-dipole plasmas in which the neutral gases are directly ionized by the mono-energetic primary electrons. The experimental data of ion concentrations were obtained using the ion acoustic wave measurement method of the concentration of two ion species. The comparison reveals that the ion concentration ratio is linearly proportional to the ratio of the ionization cross sections and the ion loss velocity between two gas species. Especially, the model prediction is improved with using the two-ion-species sheath model (recently reported by Baalrud and Hegna) for obtaining the ion loss velocity at the sheath boundary.

https://iopscience.iop.org/article/10.1088/0022-3727/48/22/225201/pdf

How to determine the relative ion concentrations of multiple-ion-species plasmas generated in the multi-dipole filament source

The effect of a standing wave on plasma density is observed in an inductively coupled plasma (ICP) with a single turn antenna whose circumferential length is much shorter than the wavelength of the driving frequency. The experiment is performed in an argon plasma operated at 5 mTorr with 13.56 MHz power applied to the single turn antenna with diameter of 300 mm. Measured plasma density near the ground termination of the antenna is higher than near the power input, which degrades the uniformity in the ICP source. It is analysed with the standing wave effect of the current and voltage on the antenna, which is carried out with a transmission line model based on the transformer circuit model of the ICP. The wavelength and the distribution of the inductively and capacitively coupled powers along the antenna are obtained from the model with the measured impedance of the antenna. The expected density distribution agrees well with the measured one, revealing that the wavelength is shortened and the nonuniformity of the plasma density becomes severe with increase of the input power.

http://iopscience.iop.org/article/10.1088/0022-3727/47/1/015205/pdf

Sung-Ryul Huh

Papers

Global model analysis of negative ion generation in low-pressure inductively coupled hydrogen plasmas with bi-Maxwellian electron energy distributions

Ion-neutral collision effect on ion-ion two-stream-instability near sheath-presheath boundary in two-ion-species plasmas

How to determine the relative ion concentrations of multiple-ion-species plasmas generated in the multi-dipole filament source

Standing wave effect on plasma distribution in an inductively coupled plasma source with a short antenna

Mechanism of cone-shaped carbon nanotube bundle formation by plasma treatment