Cylindrical Plasma Source Research Articles

Influence of electrode transparency on the plasma properties of a cylindrical IEC plasma source

This study investigates the influence of cathode and anode grid transparency on the plasma properties of a Cylindrical Inertial Electrostatic Confinement plasma source. The primary focus is to elucidate how variations in the transparency of the electrodes impact plasma characteristics and the operational performance of the source, particularly during spray jet mode. Employing optical emission spectroscopy (OES) and Langmuir probe diagnostics, the plasma behavior is analyzed comprehensively. The results demonstrate that cathode transparency has a pronounced effect on discharge current and plasma density within the cathode. Conversely, the transparency of the anode has only a little influence the ignition characteristics and operational stability of the plasma source. The findings highlight the importance of utilizing cathode transparency to adjust both the pressure and voltage operating ranges of the IEC source in spray jet mode, optimizing its performance for various applications.

Characterization of a radially emitting spray jet in a cylindrical inertial electrostatic confinement plasma source based on particle in cell simulation

This paper investigates the outflow mechanism of a cylindrical inertial electrostatic confinement plasma source using Particle in Cell plasma simulations. Various phenomena such as ion-wall interactions, impact ionization, excitation, and secondary electron generation are considered. Together with a fine time and cell resolution to reveal relevant phenomena. The results indicate a high electron and ion density in front of the cathode at the widest grid opening, which suggests the formation of a cascade-like charge carrier emission, commonly referred to as the “spray jet”. Initially, a virtual anode is formed within the cathode grid and is shifted toward the grid opening as time elapsed. Ions are accelerated in the plasma sheath around the cathode and collide with the cathode grids or oscillate through the grid opening. Electrons are accelerated and ionize the background gas at the plasma sheath edge. While the electron flow is disordered within the cathode it forms a cascade-like quasi-neutral plasma at the outlet of the plasma source. Through the formation of a virtual anode the discharge becomes self-sustained. With this simulation results the working principle of the inertial electrostatic confinement plasma source can be derived.

Spatial Distribution Analyses of Axially Long Plasmas under a Multi-Cusp Magnetic Field Using a Kinetic Particle Simulation Code KEIO-MARC

To realize the development of a long plasma source with a uniform electron density distribution in the axial direction, the spatial distribution of plasma under a multi-cusp magnetic field was analyzed using a KEIO-MARC code. Considering a cylindrical plasma source with an axial length of 3000 mm and a cross-sectional diameter of 100 mm, in which the filament electrode was the electron source, the electron density distribution was calculated using the residual magnetic flux density, Bres, and the number of permanent magnets installed at different locations surrounding the device, Nmag, as design parameters. The results show that both Bres and Nmag improved the uniformity of the electron density distribution in the axial direction. The maximum axial electron density decreased with increasing Nmag and increased with increasing Bres. These trends can be explained by considering the nature of the multi-cusp field, where particles are mainly confined to the field-free region (FFR) near the center of the plasma column, and the loss of particles due to radial particle transport. The use of multiple filaments at intervals shorter than the plasma decay length dramatically improved axial uniformity. To further improve axial uniformity, the filament length and FFR must be properly set so that electrons are emitted inside the FFR.

Investigation of Fludioxonil Reduction Using Non-Thermal Atmospheric Plasma through Experimental Simulation

In this study, the effect of non-thermal atmospheric plasma (NTAP) treatment on the reduction of residual fludioxonil (C12H6F2N2O2, 4-(2,2-difluoro-1,3-benzodioxol-4-yl)-1H-pyrrole-3-carbonitrile) was investigated through experimental simulation. Fludioxonil is known for its high residual concentration on fruits and vegetables. To simulate residual fludioxonil reduction in the storage location prior to consumption of fruits or vegetables by consumers, we designed an experimental setup utilizing a gas distribution system and a cylindrical dielectric barrier discharge (DBD) plasma source. A cylindrical DBD plasma source was adopted to produce the plasma activated chemical species (O3). To evaluate the effect of plasma treatment on the reduction of residual fludioxonil, experiments were performed under three different conditions: varying concentrations and treatment times of O3, as well as the surface roughness of microscope slide glass. Based on the results, 10 min plasma treatment with an O3 concentration of 11.89 μL/L, which showed a 58.5% reduction rate, is recommended. The O3 concentration has a higher priority than the treatment time for reduction rates of residual fludioxonil.

On heavy particle-wall interaction in axisymmetric plasma discharges

The effects of heavy particle-wall interaction on a cylindrical plasma source discharge are investigated, through hybrid particle-in-cell/fluid simulations. The bulk plasma is considered quasineutral with isothermal electrons, and with no secondary electron emission from the walls. The neutral gas wall reflection model is shown to play a major role in determining the conditions for a self-sustained and stationary plasma discharge. A hysteresis cycle on the injection mass flow rate is found when neutrals deviate from a purely diffuse reflection at the walls, with the mass utilization efficiency changing up to 20% between purely specular and diffuse scenarios. However, as the ratio of ionization mean free path to macroscopic length is decreased, the neutral-wall reflection model becomes irrelevant. Finally, even small deviations from unity of the ion energy accommodation coefficient at the walls are seen to have a major impact on both the ion and neutral distribution functions, and ultimately the mass utilization efficiency. This behavior stresses out the importance of a precise experimental determination of this parameter for accurate simulations.

Cylindrical inertial electrostatic confinement plasma source for surface treatment

The inertial electrostatic confinement (IEC) has been commonly researched in the scope of application for space propulsion and fusions reactors since many years. The utilization as thruster is possible due to a free emitting stream in a so-called jet mode of the IEC source. Depending on the setup, this jet can be operated as high energetic electron beam (tight jet) with electron energies of several keV or as low energetic plasma jet close to quasineutrality (spray jet). These modes are of high interest for thin film applications and plasma treatment. However, the IEC source is still not used in this field of application. Relevant application scopes can be electron substrate heating, surface plasma pretreatment, ionized physical vapor deposition and plasma enhanced chemical vapor deposition. In this paper, a cylindrical IEC source is presented that emits the plasma in both known jet modes, spray jet and tight jet. The jet emission occurs in radial outlet direction along the entire source height (“curtain-like” emission). Furthermore, an ignition test and operational behavior test is introduced. A stable spray jet operation region was examined and dependencies on pressure and voltage are investigated.

Firetube formation through sheath-plasma instability in expanding RF plasma

Formations of complex structures observed during the instability instigated expanding capacitive mode of RF discharge in a linear vacuum vessel, in the absence of an externally applied magnetic field or any other additional constraint applied. Systematic measurements of the plasma parameters, using multiple probe diagnostics, carried out to investigate the parametric dependence of sheath development reveal visible cylindrical tube like multiple double-layer formations around a mesh-type cylindrical plasma source. Such firetube formations provide many interesting and vital information on expanding radio frequency (RF) plasma sheath, charge particle trapping and the associated sheath instability with the RF discharge. The sheath structures, glowing spot formations, space charge formations, complex structure and firetube formations in the expanding plasma are some of the features discussed here in relation to such an expanding RF discharge. Low-frequency sheath instabilities are also observed in connection to the firetube formations using a coaxial mesh-type cylindrical plasma source powered at an operating frequency of 13.56 MHz. The ion acoustic frequency-dominated instability is identified as a sheath-plasma instability associated with the RF plasma.

Experiments and Modeling of Gas Heating in a Radio-Frequency Plasma Jet

Small plasma sources have been developed for a variety of applications and are often known as plasma jets (e.g. biomedical applications using atmospheric pressure helium or argon plasmas), as micro-plasma sources or arrays [1] (e.g. surface processing using lower operating pressure plasmas), or as micro-thrusters (e.g. electric propulsion using hollow cathode thrusters or neutralizers). At the ANU we have designed a capacitively coupled radiofrequency (13.56 MHz) cylindrical plasma source which currently operates at pressures around a few Torr and at powers ranging from 1-100 Watts with or without a pressure gradient imposed along the 2 cm long cavity [2]. The plasma diameter ranges from 1 to 6 mm before expanding in a larger chamber. Neutral gas heating is measured for a variety of gases (argon, nitrogen, xenon) and cavity diameters using optical emission spectroscopy. The results are combined with electric probe measurements to develop a plasma heating model where the heating is assumed to come from ion-neutral collisions in the maximum of the discharge under the powered electrode. A peaked density on axis is well described by the plasma model [2]. Typically, gas temperatures around 1000K are measured for N2 and Ar in agreement with theoretical predictions from Fruchtman [3]. The results show that this type of source can be tailored to suit a variety of plasma applications: its effectiveness in dissociating species means that it can be used for remote plasma etching [1] or for surface passivation from atomic nitrogen using N2. It can also be developed as an electrothermal plasma micro-thruster for space propulsion[2]. All of these applications benefit from the neutral gas heating and from the ability of the source to be developed as an array of sources. REFERENCES: [1]S. Dixon, C. Charles, R. Boswell, W. Cox, J. Holland and R. Gottscho, “Interactions between arrayed hollow cathodes”, J. Phys. D: Appl. Phys. 46, 145204, 2013[2]C. Charles and R.W. Boswell, “Measurement and modeling of a radiofrequency micro-thruster”, Plasma Sources Sci. Technol. 21, 022002, 2012[3]A. Fruchtman, “Energizing and depletion of neutrals by a collisional plasma”, Plasma Sources Sci. Technol. 21, 022002, 2012

Ray-tracing WKB analysis of Whistler waves in non-uniform magnetic fields applied to space thrusters

Radiofrequency magnetized cylindrical plasma sources are proposed for the development of space thrusters, whose thrust efficiency and specific impulse depend on the power coupled into the plasma. At this stage of research, emphasis has been on the absorption of Whistler wave energy by non-uniform plasmas but not much on the role played by the magneto-static confinement field, considered uniform, constant and aligned with the axis of the source. We present RAYWh (RAY-tracing Whistler), a three-dimensional (3D) ray-tracing solver for electromagnetic propagation and power deposition in cylindrical plasma sources for space plasma thrusters, where actual magnetic confinement configurations along with plasma density profiles are included. The propagation and absorption of Whistler waves are investigated by solving the 3D Maxwell–Vlasov model equations by a Wentzel–Kramers–Brillouin (WKB) asymptotic expansion. The reduced set of equations for the wave phase and for the square amplitude of the electric field is solved numerically by means of a modified Runge–Kutta algorithm. Unexpected cut-offs, resonances, radial reflections, mode conversions and power deposition profile of the excited waves are found, when realistic confinement magnetic fields are considered. An analysis of the influence of axial wavenumbers and the axial length of the system on the power deposition is presented.

Ferroelectric plasma sources for NDCX-II and heavy ion drivers

A barium titanate ferroelectric cylindrical plasma source has been developed, tested and delivered for the Neutralized Drift Compression Experiment NDCX-II at Lawrence Berkeley National Laboratory (LBNL). The plasma source design is based on the successful design of the NDCX-I plasma source. A 7kV pulse applied across the 3.8mm-thick ceramic cylinder wall produces a large polarization surface charge density that leads to breakdown and plasma formation. The plasma that fills the NDCX-II drift section upstream of the final-focusing solenoid has a plasma number density exceeding 1010cm−3 and an electron temperature of several eV. The operating principle of the ferroelectric plasma source are reviewed and a detailed description of the installation plans is presented. The criteria for plasma sources with larger number density will be given, and concepts will be presented for plasma sources for driver applications. Plasma sources for drivers will need to be highly reliable, and operate at several Hz for millions of shots.

Electrical and optical characteristics of cylindrical non-thermal atmospheric-pressure dielectric barrier discharge plasma sources

Two types of cylindrical non-thermal dielectric barrier discharge plasma sources working at atmospheric pressure have been investigated and the electrical and optical characteristics were determined. The configurations compared are ring-and-rod and double-ring. At 30kHz frequency and 62W fixed input power, the electrical characteristics: V–I and V–Q Lissajous curves were measured. The capacitances, energies dissipated, and powers consumed for plasma generation have been quantified. Light emission characteristics of the two configurations were also measured at the main discharge region between the electrodes as well as at the afterglow plasma region. The propagation speed of ionization front of afterglow plasma has been estimated to be 0.5–1.3cm/μs in this experiment.

On the Measurement of Subsonic Flow in a Capacitively Coupled Helicon Plasma Source

Plasma parameters and the subsonic flow from a capacitively coupled, cylindrical plasma source of the Njord helicon device are investigated by means of a Mach probe and a retarding field energy analyzer (RFEA). 13.56 MHz and 600 W RF power is inserted into the argon working gas under low-pressure conditions and moderate magnetic field. By means of a downstream field coil, the magnetic field is shaped from a purely expanding field to a configuration with more parallel field lines. It is shown that the downstream plasma density along the outer rim of the source increases significantly and there is a sudden increase by nearly 20 V in the plasma potential already after a moderate increase in the downstream magnetic field. The investigation of the flow indicates that current ratios derived from the Mach probe result in an apparent flow in the direction towards the source, while the current bratios derived from the RFEA indicate a flow in the direction away from the source. PIC simulations demonstrate that the acceptance angle of the probes, being nearly 180o for the Mach probe, and about 45o for the RFEA, can critically affect the current ratios and hence the subsonic flow measured by the probes in the weakly magnetized plasma in our device. The first section in your paper

Radial‐Profile Control of Cylindrical Plasma Source With Multiple Low‐Inductance Antenna Units

AbstractRadial‐profile control of cylindrical plasma source with 500 mm in diameter has been carried out using multiple low‐inductance antenna (LIA) units. Measurements of ion saturation current in the cylindrical chamber demonstrated an excellent control capability of radial distribution by adjusting the power‐deposition profile. The non‐uniformity of radial distributions was improved from 8.3% to 5.4% with the plasma control technologies. The results indicate that the plasma control technologies with multiple LIA units are quite attractive as a plasma source for large‐area wafer processes over 450 mm in diameter, which inherently require excellent profile control capabilities.

Uniformity of 500-mm cylindrical plasma source sustained with multiple low-inductance antenna units

Uniformity of plasma density and 4s excited Ar (Ar⁎) density profiles in a 500-mm cylindrical plasma source sustained with eight LIA units have been investigated with a fluid-simulation code. Uniformity of the simulated radial profiles of plasma density on the substrate holder located at a distance of 300 mm from a top flange became better with increasing Ar pressure from 1.3 Pa to 6.7 Pa. The radial uniformity of plasma density in the central 300 mm region was evaluated to be approximately 4.0% at 6.7 Pa. Uniformity of the radial profiles of Ar⁎ density became better with increasing distance of the substrate holder to the down-stream region. In the present antenna configuration, fairly uniform radial profiles of Ar⁎ density can be obtained at a distance of 300 mm from the top flange.

Properties of Inductively-Coupled Plasmas Driven by Multiple Low-Inductance Internal-Antenna Units

We have developed cylindrical RF plasma sources sustained with multiple low-inductance antenna (LIA) units and have measured plasma parameters for argon plasmas. The plasma source could be operated at 2 000 W RF powers to attain plasma densities as high as 1012 cm−3. The plasma potential as low as 13–14 V in high-density plasmas has been achieved and the potential drop from plasma potential to floating potential was almost constant at 10 V. Kinetic energy distributions of Ar ions were measured at argon pressures around 1.3 Pa in an RF power range of 200–2 000 W, which have peaks which almost correspond to the magnitude of the plasma potential.

Characterization of Inductively-Coupled RF Plasma Sources with Multiple Low-Inductance Antenna Units

We have developed a cylindrical RF plasma source by the inductive coupling of multiple low-inductance antenna (LIA) units and analyzed the plasma density profile of this source using fluid simulation. Experiments using four LIA units showed a stable source operation even at 2000 W RF power, attaining plasma densities as high as 1011–1012 cm-3 in an argon pressure range of 0.67–2.6 Pa. The amplitude of antenna RF voltage was measured to be less than 600 V, which is considerably smaller than those obtained using conventional ICP antennas. The radial distribution of plasma density sustained using four LIA units showed excellent agreement with profiles numerically predicted using a fluid-simulation code.

Plasma ionization by annularly bounded helicon waves

The general solution to the electrostatic and magnetic fields is derived with respect to the boundary conditions of a coaxial helicon plasma source. The electric field contours suggest that a simple antenna design can ionize the gas in a coaxial configuration. In addition, the power deposition as a function of excitation frequency is derived. The solution is validated by comparison with the standard cylindrical helicon plasma source. Further, a parametric study of source length, channel radius, channel width, and antenna excitation frequency are presented. This study suggests that it is possible to create a helicon plasma source with a coaxial configuration.

Low-Power RF plasma sources for technological applications: III. helicon plasma sources

The third part of the paper is devoted to an investigation of the so-called helicon plasma sources. These are RF devices operating with a relatively weak external magnetic field, which is, at the same time, strong enough for the resonant electron gyrofrequency to be much higher than the industrial frequency (ω=8.52×107 s−1). As in [1, 2], a study is made of elongated cylindrical plasma sources in a longitudinal (directed along the cylinder axis) magnetic field and planar disk-shaped plasma sources in a transverse (perpendicular to the plane of the disk) magnetic field. A comparison of the present results with the results that were obtained in [3] without using the helicon approximation leads to the conclusion that elongated helicon sources hold great promise for plasma technologies.

Low-power rf plasma sources for technological applications: I. Plasma sources without a magnetic field

A general analytical theory is developed and numerical simulations are carried out of cylindrical plasma sources operating at an industrial frequency of f=13.56 MHz ω=8.52×107 s−1). Purely inductive surface exciters of electromagnetic fields (exciting antennas) are considered; the exciters are positioned either at the side surface of the cylinder or at one of its end surfaces. In the latter case, the plasma flows out of the source through the opposite end of the cylinder. A study is made of both elongated systems in which the length L of the cylinder exceeds its diameter 2R and planar disk-shaped systems with L<2R. The electromagnetic fields excited by the antenna in the plasma of the source are determined, and the equivalent plasma resistance, as well as the equivalent rf power deposited in the plasma, is calculated.

Hybrid Simulation of Space Plasmas: Models with Massless Fluid Representation of Electrons. IV. Kelvin–Helmholtz Instability

This is a survey of the literature on hybrid simulation of the Kelvin–Helmholtz instability. We start with a brief review of the theory: the simplest model of the instability—a transition layer in the form of a tangential discontinuity; compressibility of the medium; finite size of the velocity shear region; pressure anisotropy. We then describe the electromagnetic hybrid model (ions as particles and electrons as a massless fluid) and the main numerical schemes. We review the studies on two-dimensional and three-dimensional hybrid simulation of the process of particle mixing across the magnetopause shear layer driven by the onset of a Kelvin–Helmholtz instability. The article concludes with a survey of literature on hybrid simulation of the Kelvin–Helmholtz instability in finite-size objects: jets moving across the magnetic field in the middle of the field reversal layer; interaction between a magnetized plasma flow and a cylindrical plasma source with zero own magnetic field.