Ion Mean Free Path Research Articles

Numerical investigation of sheath characteristics for electronegative magnetized plasma and dust charging

We have studied the effects of the magnetic field on the active electronegative plasma sheath properties and dust charging process in the sheath region for two different collisional models: constant ion mean free path and constant ion mobility using 1d3v fluid hydrodynamics model. It is found that the magnetic field strength and choice of collisional models have a significant effect on the active plasma sheath characteristics and charging of an isolated dust grain. The sheath criterion for an active electronegative magnetized plasma for both collisional models has been extended, and the effects of neutral gas pressure, source frequency, obliqueness of magnetic field, and initial electric field at sheath edge are graphically illustrated. There are two distinct regions observed in the sheath region: magnetic field and electric field dominant regions. The spatial distribution of plasma sheath parameters is systematically presented. It is found that the evolution of dust surface potential is affected by the magnitude of the magnetic field and collisional models. The stable levitation of dust grains in the sheath region is close to the sheath entrance. Moreover, the total force experienced by an isolated dust grain in the sheath region rapidly increases close to the material surface, and the magnitude of force is higher for larger dust grain.

Stability of a dust cloud in the radio frequency low-pressure gas discharge

We analyze stability of a three-dimensional cloud of the dust particles in the low-pressure radio frequency discharge under microgravity conditions. The parameters of such complex plasma are assumed to conform to the recently developed ionization equation of state that includes the effects of the ion–atom collisions on the particle charge and of the ion–particle collisions on the ion mean free path. Propagation of the particle number density perturbation in a cloud is treated using the fluid approach. The equation for dust cloud dynamics allows for the electric and ion drag forces that are not compensated in a nonstationary state and yields the wave equation for the dust acoustic waves with either a positive or negative effective friction coefficient. The resulting dispersion relation defines a condition of the instability onset, which is written for the maximum ambipolar field and dust number density gradient. Analysis of the available experimental data reveals a satisfactory correspondence with the theory. In particular, it is demonstrated that the maximum dust density gradient in the cloud increases with the increase in the particle diameter and in the pressure of plasma-forming gas.

Influence of the Dust Particle Shape on the Wake Formation in a Complex Plasma

In this article, a numerical model is presented, where the self-consistent spatial distribution of the plasma potential near isolated dust particles is calculated. The dust particles shape is ellipsoid of revolution. This article demonstrates large-scale studies of the characteristics of the “dust particle—ion cloud” system measured for the various parameters of dusty plasma. These parameters are the shape and size of dust particles, the strength of the external electrostatic field, and the ion mean free path. The approximations describing the system characteristics on the plasma parameters are presented.

1D kinetic study of pinch formation in a dense plasma focus: Transition from collisional to collisionless regimes

The pinch-formation stage of a deuterium dense plasma focus, and associated “shock-flash” neutron yield, is studied using 1D kinetic simulations considering a plasma column with initial pressure P, initial radius R, and the compression to be driven by a constant current I. The relative behavior of the compression is shown to be similar for fixed ratios of the characteristic ion mean free path to the radius of the plasma column at stagnation, λst/Rst. This dimensionless parameter is shown to scale like I4/(P3R5). The compression ratio, R/Rst, is found to be a minimum when λst/Rst≈1 and is the largest in the collisionless limit where λst≫Rst. This behavior is in contrast to the analogous planar pinch where R/Rst decreases from one constant for λst/Rst≪1 to a smaller constant for λst/Rst≫1. The yield in the collisionless regime is shown to fall between the two well-known I4 scaling laws. Furthermore, this regime exhibits qualities that potentially make it appealing for radiography applications, such as increased localization in time and space of the neutron formation.

How to Increase Further the Resolving Power of the Ultrahigh Magnetic Field FT ICR Instruments? The New Concept of the FT ICR Cell-the Open Dynamically Harmonized Cell as a Part of the Vacuum System Wall.

FT ICR mass spectrometry continues to be the leader in the resolving power among all mass spectrometry methods. With the introduction of the dynamically harmonized FT ICR cell, it has become possible to achieve the resolving power of more than 10 million at m/q = 1000 with moderate magnetic fields of about 7 T. A further increase in the mass resolving power is desirable mainly for two reasons: with this we are increasing the number of resolved components in analyses of complex mixtures (like oil and natural organic matter (NOM)) and increasing the number of the mass of molecules for which a fine structure can be resolved. In recent years, some attempts have been made to further increase the mass resolving power by increasing the magnetic field and using the multielectrode detection method. An increase in the magnetic field to 21 T did not show a proportional increase in the mass resolving power. Likely, the reason for this is an insufficiently high vacuum to satisfy the requirement of an increase in the mean free path of ions with the increasing magnetic field power. We offer a new design for the FT ICR cell and the whole mass spectrometer, in which an open, dynamically harmonized FT ICR cell is integrated into a vacuum system with the outer surfaces of the cell electrodes at atmospheric pressure. In this design, the trap electrodes are the walls of the vacuum system and have the minimized active surfaces by combining the vacuum system surface and the cell surface into one. In this design, the pumping process is accelerated, and the factor of insufficient vacuum in FT ICR mass spectrometers with an ultrahigh magnetic field is eliminated.

Investigation of laser‐produced plasma multistructuring by floating probe measurements and optical emission spectroscopy

AbstractWith the continuous development of pulsed laser deposition as a versatile technique for the deposition of complex thin films, there is a need for a better understanding of the role and control of the deposition parameters. The understanding of the particle kinetics and plasma chemistry during the deposition process can greatly improve the properties of the synthesized films. By using the floating voltage regime of the Langmuir probe technique, we performed angular and time‐resolved measurements during laser ablation of an Ag target, which evidenced the structuring of the plasma plume in ultrahigh vacuum conditions. The addition of N2 gas in the pressure range from 5 × 10–5 to 10 Pa leads to more rapid plasma thermalization and the control of its kinetic energy. The electrical measurements were complemented by optical emission spectroscopy, which showcased the presence of neutral and multiple ionized species distributed across the laser‐produced plasma plume. The plasma homogenization resulted in a decrease of the mean free path of Ag ions and atoms, which increased both their excitation temperature and electron density.

Influence of the Ion Mass in the Radial to Orbital Transition in Weakly Collisional Low-Pressure Plasmas Using Cylindrical Langmuir Probes

This paper presents an experimentally observed transition from the validity of the radial theories to the validity of the orbital theories that model the ion current collected by a cylindrical Langmuir probe immersed in low-pressure, low-temperature helium plasma when it is negatively biased with respect to the plasma potential, as a function of the positive ion-neutral collision mean free path to the Debye length ratio Λ=λ+/λD. The study has been also conducted on argon and neon plasmas, which allows a comparison based on the mass of the ions, although no transition has been observed for these gases. As the radial or orbital behavior of the ions is essential to establish the validity of the different sheath theories, a theoretical analysis of such a transition not only as a function of the parameters Λ and β=T+/Te, T+ and Te being the positive ion and electron temperature, respectively, but also as a function of the ion mass is provided. This study allows us to recognize the importance of the mass of the ion as the parameter that explains the transition in helium plasmas. Motivated by these theoretical arguments, a novel set of measurements has been performed to study the relationship between the Λ and β parameters in the transition that demonstrate that the effect of the ion mean free path cannot be completely ignored and also that its influence on the ion current collected by the probe is less important than the effect of the ion temperature.

Investigation of Wide- and Ultrawide-Bandgap Semiconductors From Impact-Ionization Coefficients

We survey impact-ionization coefficients for silicon, wide bandgap semiconductors (gallium nitride and 4H-silicon carbide), and ultrawide-bandgap semiconductors (aluminum gallium nitride, beta-gallium oxide, and diamond). We employ a custom genetic algorithm to fit those existing coefficients into a modified Thornber model. This fitting process leads to an estimation of material properties, such as ionization energy, optical-phonon energy, and mean free path. After evaluating electric-field profiles by solving the Poisson equation, we use the impact-ionization integral to fundamentally calculate the breakdown voltage and the critical field of various p-i-n structures. This work captures how the doping concentration and the thickness of the drift layer shape the breakdown voltage as well as the critical field achievable with a given material. It is observed that a wider bandgap is not the sole requirement for the achievement of a higher breakdown voltage. Impact-ionization coefficients and dielectric constants are additional material properties that influence the calculation of the breakdown voltage. The performance limits of diamond are found to be surprisingly poor for its large bandgap. Al x Ga1– x N and $\beta $ -Ga2O3 have almost the same performance limits, but it is observed that aluminum gallium nitride can outperform gallium oxide at high voltage if its background doping (doping floor) can be reduced.

Secondary ion mass spectrometry measurements under ambient and humid conditions using MeV ions

Secondary ion emission with MeV heavy ions provides a unique opportunity for insight into ion collisions and material analysis. MeV-energy ion beams excite electrons near the surface and enhance the ionization of high-mass molecules, enabling the acquisition of secondary ion mass spectrometry (SIMS) spectra of ionized molecules with suppressed fragments. The SIMS technique with MeV-energy heavy ions (MeV-SIMS) has opened new possibilities for investigating the chemical composition and structure of organic and biological materials, as well as for their imaging. The ambient analysis system is essential for analyzing solid–liquid interfaces, because liquid materials evaporate easily in a vacuum. It is difficult to measure volatile liquid (wet) samples using conventional SIMS. The mean free path of ions with energy in the keV range is very short in low vacuum, and these ions cannot penetrate the surface. By contrast, evaporation of liquid materials in He at atmospheric pressure is suppressed and samples containing liquid can be measured using the MeV-SIMS technique without the need for preparing dry samples. Recently, the authors developed a humidity-controlled ambient MeV-SIMS system because the water evaporation rate strongly depends on humidity. To obtain imaging mass spectra of wet samples, the wet sample surface needs to be maintained under ambient and humid conditions for a prolonged time. At 0.04 MPa, under 100% humidity, a sample of aqueous solution of benzoic acid produced water droplets that remained in the target chamber for more than 30 min, and clusters and compounds of benzoic acid with water were detected with high sensitivity by the humidity-controlled ambient MeV-SIMS system.

Observation of shock-front separation in multi-ion-species collisional plasma shocks

We observe shock-front separation and species-dependent shock widths in multi-ion-species collisional plasma shocks, which are produced by obliquely merging plasma jets of a He/Ar mixture (97% He and 3% Ar by initial number density) on the plasma liner experiment [S. C. Hsu et al., IEEE Trans. Plasma Sci. 46, 1951 (2018)]. Visible plasma emission near the He-I 587.6-nm and Ar-II 476.5–514.5-nm lines is simultaneously recorded by splitting a single visible image of the shock into two different fast-framing cameras with different narrow bandpass filters (589 ± 5 nm for observing the He-I line and 500 ± 25 nm for the Ar-II lines). For conditions in these experiments (pre-shock ion and electron densities ≈5×1014 cm−3, ion and electron temperatures of ≈2.2 eV, and relative plasma-merging speed of 22 km/s), the observationally inferred magnitude of He/Ar shock-front separation and the shock widths themselves are <1 cm, which correspond to ∼50 post-shock thermal ion–ion mean free paths. These experimental length scales are in reasonable qualitative and quantitative agreement with results from 1D multi-fluid simulations using the chicago code. However, there are differences between the experimentally inferred and simulation-predicted ionization states and line emission intensities, particularly in the post-shock region. Overall, the experimental and simulation results are consistent with theoretical predictions that the lighter He ions diffuse farther ahead within the overall shock front than the heavier Ar ions.

Influence of collisions in a fluid model for the warm-ion sheath around a cylindrical Langmuir probe

A long cylindrical Langmuir probe immersed in a collisional plasma is modelled by using fluid equations in which the collision term is introduced assuming a constant ion mean free path. The physical magnitudes of the system are expanded in a perturbation series in the collision parameter, that is, the ratio between the Debye length and the ion mean free path, and the system is thus separated into zeroth-order and first-order perturbation systems. The ion temperature is arbitrary and a continuous solution from the plasma to the probe is obtained for the perturbation systems. Using this methodology, a continuous solution for the ion sheath around a cylindrical Langmuir probe is obtained for long ion mean free path, for any ion temperature with no need of two-scale study or asymptotic matching. It is found that the collisions and the ion temperature have opposite effects, and these effects are quantified. The solution for cold ions cannot be obtained directly using this method, but it can be recovered from the warm-ion solution. The methodology is usable for long cylindrical probes in which the scale of the probe is much higher than the scale of the sheath, so that the geometry is essentially cylindrical. The model is numerically solved to obtain the potential profile, the ion temperature profile, the ion density profile and the ion velocity profile, and the dependence of the sheath on the ion mean free path and the ion temperature is studied. The ion-current to probe-potential profiles for different probe radius are obtained. Finally, the Sonin plot is calculated, in order to use the solution in plasma diagnosis.

Structure of the Wall Sheath in a Gas-Discharge Plasma for an Arbitrary Orientation of a Flat Probe Relative to the Electric Field in the Plasma

We investigate the structure of the wall sheath of a gas discharge near a flat surface at a negative potential for high mean electron energy. It is shown that in the conditions where the mean energy of ions in the plasma is much lower than the mean electron energy, the parameters of the wall sheath weakly depend on the mutual orientation of the normal to the surface and the electric field in the plasma for an arbitrary ratio of the Debye radius to the ion mean free path relative to the resonant charge exchange process. It is found that for inert gases (He, Ar) for ratio E/P of the electric field to pressure exceeding 10 V/(cm Torr) in the plasma, the disregard of ionization in the perturbed wall sheath can lead to substantial errors in the calculation of its parameters. It is shown that the ionization leads to an increase in the electric field in the wall sheath and, as a consequence, to an increase in the mean velocity of ions at the boundary between the quasi-neutral presheath and the part of the perturbed wall sheath in which quasi-neutrality is substantially violated. The parameters of the wall sheath where quasi-neutrality is significantly violated depend on the ionization rate much less strongly than the corresponding parameters of the quasi-neutral presheath. We have determined the relation for concentration of charged particles in the unperturbed plasma from the ion saturation current considering the actual ion energy distribution function in the plasma as well as ionization in the presheath and the part of the perturbed wall sheath in which quasi-neutrality is violated significantly.

Chemical state effects on the Auger transitions in Cr, Fe, and Cu compounds

As a sophisticated surface elemental analysis tool, electron-excited Auger electron spectroscopy offers excellent spatial resolution (<10 nm), high surface sensitivity (∼2 nm in depth), and efficient detection (about 0.1 atm%) of all elements heavier than helium, which makes scanning Auger nanoprobe especially suitable for small feature analysis. However, elemental composition can generally only be done in a semi-quantitative way. In addition, the chemical-state identification based on peak position often fails due to the complex surface environment. In order to improve the accuracy of quantitative Auger analysis, we systematically investigated the influence of the oxidation states and primary beam on the sensitivity factors for Auger transitions in 3d transition metal Cr/Fe/Cu compounds. Relativistic Dirac-Fock-Slater calculations were performed to illustrate the electronic structure, electron-impacted ionization, and Auger transition dynamics at an atomistic and electronic level. It was found that a new set of chemical state-based sensitivity factors are required for highly accurate quantification, because the chemical states of an element directly affect all four major components in its sensitivity factor, i.e., ionization cross section, backscattering factor, Auger yield, and mean free path. In this study, the chemical state and environment can be affected by the oxidation state, the number and types of coordinating atoms, the spin state, and magnetic environment; the focus here will be on the oxidation state. The electron configurations of different oxidation states directly affect the Coulomb and Breit interactions differently in Auger transitions. The intensity ratios and the energy gaps between different Auger transitions were found to change with oxidation state in these 3d transition metal compounds. These findings may provide a new guide to improve the accuracy of Auger quantification and help the oxidation state identification in complex materials.

Kinetic Theory of the Wall Sheath for Arbitrary Conditions in a Gas-Discharge Plasma

The self-consistent problem of the structure of a perturbed wall sheath in a dc gas-discharge plasma near a flat surface under negative potential relative to the plasma has been solved for an arbitrary relation between the Debye radius and the ion mean free path. The solution has been obtained without artificial separation of this layer into the quasi-neutral “presheath” and the wall sheath in which quasi-neutrality is violated substantially. The actual ion distribution function in the unperturbed plasma, the dependence of the charge-exchange cross section on the ion energy, and the nonzero electric field in the unperturbed plasma have been considered. It is shown that when the average electron energy is conserved the structure of the perturbed wall sheath weakly depends on the form of the electron distribution function. It has been established that the mean energy of ions in the unperturbed plasma substantially affects the structure of the quasi-neutral presheath as well as the structure of a part of the wall sheath in which quasi-neutrality is not observed even under the assumption that the mean electron energy is much higher than the mean energy of ions. The calculations of ion flow parameters and the structure of the perturbed wall sheath are in conformity with experimental data obtained by other authors, which could not be adequately interpreted earlier.

Effect of ion mean free path length on plasma polarization behind a dust particle in an external electric field

In this paper, a self‐consistent numerical model that describes the behavior of plasma around an isolated, highly charged dust particle is presented. Using the developed model, self‐consistent distributions of the space charge density and plasma potential in the presence of an external electric field are obtained. These distributions are thoroughly analysed though Legendre decomposition. For different dust plasma parameters, such as the radius of the dust particle, the amplitude of the external field, and the mean free path of ions, the dipole moment of the ion cloud surrounding the dust particle is calculated. It turns out that the dependencies of the dipole moment on the value of the external electric field obtained for different parameters are reduced to a single curve by simple scaling.

Структура призондового слоя в газоразрядной плазме при произвольной ориентации плоского зонда относительно электрического поля в плазме

AbstractWe investigate the structure of the wall sheath of a gas discharge near a flat surface at a negative potential for high mean electron energy. It is shown that in the conditions where the mean energy of ions in the plasma is much lower than the mean electron energy, the parameters of the wall sheath weakly depend on the mutual orientation of the normal to the surface and the electric field in the plasma for an arbitrary ratio of the Debye radius to the ion mean free path relative to the resonant charge exchange process. It is found that for inert gases (He, Ar) for ratio E / P of the electric field to pressure exceeding 10 V/(cm Torr) in the plasma, the disregard of ionization in the perturbed wall sheath can lead to substantial errors in the calculation of its parameters. It is shown that the ionization leads to an increase in the electric field in the wall sheath and, as a consequence, to an increase in the mean velocity of ions at the boundary between the quasi-neutral presheath and the part of the perturbed wall sheath in which quasi-neutrality is substantially violated. The parameters of the wall sheath where quasi-neutrality is significantly violated depend on the ionization rate much less strongly than the corresponding parameters of the quasi-neutral presheath. We have determined the relation for concentration of charged particles in the unperturbed plasma from the ion saturation current considering the actual ion energy distribution function in the plasma as well as ionization in the presheath and the part of the perturbed wall sheath in which quasi-neutrality is violated significantly.

Characterization of the helicon plasma flux to the target of Proto-MPEX

The ITER divertor will have to handle the boundary plasma power exhaust in the form of high heat and particle flux. The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) is a plasma source development device for the proposed Material Plasma Exposure eXperiment (MPEX), which is intended to create fusion reactor divertor-like plasma conditions for studying plasma material interactions. Here, ion fluxes to various targets within Proto-MPEX are found to be 5 ± 2.5 × 1023 m−2s-1 when using only a helicon plasma source. The fluxes along the flux tube are compared for both 30 cm in front of the target and a few cm in front of the target. The emission intensity fall-off distance in front of the target is studied via Balmer series from a visible camera looking at the target. This fall-off distance of ∼2–4 cm and the dissociation and ionization mean free path of D2 being over 0.5 m leads to a conclusion that Proto-MPEX is not in a high recycling regime with helicon only plasmas.

Measurement and modeling of aluminum sputtering and ionization in the DIII-D divertor including magnetic pre-sheath effects

We present analysis and modeling of Al sputtering and ionization in attached, low-power L-mode plasmas near the outer divertor strike point of the DIII-D tokamak. Al serves as a useful proxy for Be, the low-Z main wall material for ITER and JET that will undergo significant divertor plasma contact upon migrating from the first wall to the divertor. Al is easily distinguishable from background sources in DIII-D (namely C and B), has a high physical sputtering yield similar to Be, and has a long ionization mean free path compared to its gyro radius (). Using neutral Al emission imaging techniques, we measured a toroidal and radial asymmetry in the shape of the photo-emission plumes of sputtered neutral Al that was consistent with previously observed asymmetry in the distribution of redeposited Al in these experiments. We propose that the main cause of the emission and redeposition asymmetry is due to a sputtering anisotropy caused by near-grazing angle incident ions. The observed emission asymmetry was reproduced using a simple emission/ionization model that included full angular distributions of sputtering yield and energy calculated by SDTRIM.SP, but not when symmetric, mono-energetic cosine sputtering distributions were assumed. We used an ion orbit tracking model to calculate the distributions of ion impact energies through the potential gradient in the magnetic pre-sheath and Debye sheath. We found that with the magnetic field pitch angle (1.5°–2° with respect to the surface plane), the majority of ions strike the surface at <15° with respect to the surface plane, leading to angular sputtering yield and energy distributions with significant forward-scattering bias. We also observed surface microstructure consistent with directional sputtering and ion flux shadowing expected from the calculated ion incidence angles.

Thrust performance, propellant ionization, and thruster erosion of an external discharge plasma thruster

It is quite a challenge to design low power Hall thrusters with a long lifetime and high efficiency because of the large surface area to volume ratio and physical limits to the magnetic circuit miniaturization. As a potential solution to this problem, we experimentally investigated the external discharge plasma thruster (XPT). The XPT produces and sustains a plasma discharge completely in the open space outside of the thruster structure through a magnetic mirror configuration. It eliminates the very fundamental component of Hall thrusters, discharge channel side walls, and its magnetic circuit consists solely of a pair of hollow cylindrical permanent magnets. Thrust, low frequency discharge current oscillation, ion beam current, and plasma property measurements were conducted to characterize the manufactured prototype thruster for the proof of concept. The thrust performance, propellant ionization, and thruster erosion were discussed. Thrust generated by the XPT was on par with conventional Hall thrusters [stationary plasma thruster (SPT) or thruster with anode layer] at the same power level (∼11 mN at 250 W with 25% anode efficiency without any optimization), and discharge current had SPT-level stability (Δ &amp;lt; 0.2). Faraday probe measurements revealed that ion beams are finely collimated, and plumes have Gaussian distributions. Mass utilization efficiencies, beam utilization efficiencies, and plume divergence efficiencies ranged from 28 to 62%, 78 to 99%, and 40 to 48%, respectively. Electron densities and electron temperatures were found to reach 4 × 1018 m−3 (∂ne/ne = ±52%) and 15 eV (∂Te/Te = ±10%–30%), respectively, at 10 mm axial distance from the anode centerline. An ionization mean free path analysis revealed that electron density in the ionization region is substantially higher than the conventional Hall thrusters, which explain why the XPT is as efficient as conventional ones even without a physical ionization chamber. Our findings propose an alternative approach for low power Hall thruster design and provide a successful proof of concept experiment of the XPT.

Hot accretion flow with anisotropic viscosity

In extremely low accretion rate systems, the ion mean-free path can be much larger than the gyroradius. Therefore, gas pressure is anisotropic with respect to magnetic field lines. The effects of pressure anisotropy can be modeled by an anisotropic viscosity with respect to magnetic field lines. Angular momentum can be transferred by anisotropic viscosity. In this paper, we investigate hot accretion flow with anisotropic viscosity. We consider the case that anisotropic viscous stress is much larger than Maxwell stress. We find that the flow is convectively unstable. We also find that the mass inflow rate decreases towards a black hole. Wind is very weak; its mass flux is 10−15% of the mass inflow rate. The inward decrease of inflow rate is mainly due to convective motions. This result may be useful to understand the accretion flow in the Galactic Center Sgr A* and M 87 galaxy.