Ion Species In Plasma Research Articles

Cryogenic large volume 3D and TEM sample preparation with multiple ion species plasma FIB

Cryogenic large volume 3D and TEM sample preparation with multiple ion species plasma FIB

Experimentally corroborated model of pressure relaxation limit cycle oscillations in the vicinity of the transition to high confinement in tokamaks

An analytical formula systematically predicts the observed frequency of pressure relaxation limit cycle oscillations in the vicinity of the transition to high confinement in four tokamaks (JET, ASDEX Upgrade, COMPASS, Globus-M). The experimental dataset spans the widest available range of frequencies, machine sizes and plasma ion species. The machine size dependence is explained by the connection length scale of plasma flows parallel to the magnetic field. The model also explains the observed up-down poloidal current asymmetry and the impact of the plasma ion species mass and charge.

Mass Spectra Fitting as Diagnostic Tool for Magnetron Plasmas Generated in Ar and Ar/H2 Gases with Tungsten Targets.

In this work, we describe an ion mass spectra processing method from plasmas generated in Ar and Ar/H2 gases in contact with tungsten surfaces. For this purpose, advanced model functions, i.e., those suitable for fitting the experimental mass peak profiles, are used. In addition, the peak positions, peak shapes, abundances, and ion ratios are the parameters considered for building these functions. In the case of a multielement magnetron target, the calibration of the mass spectra with respect to the peak shape and position on the m/z scale is helpful in reducing the number of free variables during fitting. The mass spectra fitting procedure is validated by the obtained isotopic abundances of W ions in W/Ar magnetron plasmas, which, in turn, are comparable with their natural abundance. Moreover, its usefulness is exemplified by calculating the ratio of WH+/W+ ions in W/Ar/H2 plasma. This work paves the way for obtaining relevant results regarding ion species in plasma even in the case of using general-purpose mass spectrometers with limited resolution and accuracy. Although this method is illustrated for the W/Ar/H2 plasma system, it can be easily extendable to any plasma type.

The capture of injected 1+ ions in charge breeder electron cyclotron resonance ion source plasma

The capture process of the 1+ ions in charge breeder electron cyclotron resonance ion sources has been attributed to cumulative small-angle scattering of the incident ions in ion–ion collisions with the buffer gas ions. The model results in controversy between the plasma potential and ion temperature values derived from simulations, and those found in the experiments. We demonstrate that the 1+ ion capture process is dominated by the slowing down of the incident ions by the ambipolar plasma potential, which has previously been assigned with a secondary importance. In this revised view, supported by experimental data, the 1+ capture does not depend on the mass of the injected and plasma ion species, or the ion temperature, which resolves the disagreement between modelling and experiments.

Effect of negatively biased electrode on two ion species plasma–wall transition, ion drag force, and levitation of dust particle

The effect of negatively biased electrodes on two ion species (argon and xenon) magnetized plasma–wall transition characteristics and the levitation of an isolated dust particle in the sheath region has been investigated using the kinetic trajectory simulation method based on a kinetic theory. It is found that the electrode biasing affects the transition parameters: space charge density, sheath potential, evolution of phase-space, and particle flux toward the electrode. The scale length of the Debye sheath region becomes widened for the increase in negative biasing and the presence of magnetic field as well. The biasing voltage and size of the dust particle have significant effect on the evolution of the dust charge, ion drag force, and levitation of a charged dust grain in the transition region. The dust particle is negatively charged at the particle injection side, and it acquires a positive charge for higher biasing voltage close to the electrode owing to electron depletion in that region. The distance of stable levitation from the electrode increases with the increase in the negative voltage applied to the electrode. Furthermore, the volumetric composition of two species of positive ions influence the dust charging process with the negative charge of the dust particle increasing as the concentration of xenon ions increases.

Characteristics of plasma sheath in multi-component plasmas with three-ion species

The plasma sheath of a three ion species plasma is studied numerically, relying on the results of the experiment by Yip et al. (Phys. Plasmas 23:050703 (2016) to measure the positive ion velocities at the sheath edge. The positive ion species (Ar^+, Kr^+, and Xe^+) are assumed to be singly charged and to be characterized by the same temperature. It is shown that the sheath characteristics, viz. the particle number densities, the electrostatic potential and the space charge density profile in the sheath all depend on the Kr^+ concentration that is gradually added to the argon-xenon plasma as the third positive ion species. Also, the effect of ion-neutral collisions on the sheath properties is investigated numerically. Our results may be extended to a multi-ion plasma with more than two species of positive ions.

Effect of a non-uniform magnetic field on two ion species plasma-wall transition

Fluid theory has been employed to investigate the magnetized plasma-wall transition properties for two ion species plasmas with a uniform background of neutral gas density in the presence of an external magnetic field. The external applied magnetic field is parallel to the surface and its magnitude varies in the direction perpendicular to the surface. The governing equations of ion and electron fluids include ionization and collision with neutral atoms. A comparative study of transition parameters for non-uniform and uniform magnetic fields is performed at equal values of the magnetic flux density at x = 0. This study shows that the sheath region shrinks for the non-uniform magnetic field case, essentially in reason of the lower value of the average magnetic field intensity in the plasma-wall transition region. We introduce a figure of merit to quantify the nonuniformity of the magnetic field , and show that for its value 0.21 it is possible to model the plasma-wall transition region considering the magnetic field as uniform and equal to its average value. Furthermore, we find that the density distribution of electrons close to the surface deviates from the Boltzmann distribution due to the influence of a strong magnetic field.

Neoclassical transport simulations with an improved model collision operator

The improved model collision operator proposed by Sugama et al. [Phys. Plasmas 26, 102108 (2019)], in which the original Sugama collision operator [Phys. Plasmas 16, 112503 (2009)] is extended to recover the friction–flow relation of the linearized Landau (exact) collision operator, is newly implemented in a global full-f gyrokinetic simulation code, GT5D. Neoclassical transport simulations of a single ion species plasma in a circular concentric tokamak are performed over the wide collisionality regime. The improved operator is verified to reproduce the neoclassical thermal diffusivity of Braginskii precisely in the Pfirsch–Schlüter regime, where a friction–flow relation of higher accuracy is required than in the lower collisional regime. In addition, it is found in all collisionality regimes that the overestimation of the neoclassical thermal diffusivity and the magnitude of the parallel flow coefficient observed for the original Sugama operator can be eliminated by using the improved operator, demonstrating that collisional processes described by the exact operator are correctly retained in the improved operator.

Plasma-wall transition in two ion species plasma with bi-Maxwellian electrons

Presheath and sheath structures of collisional two ion species (helium and argon) plasma in the presence of bi-Maxwellian electrons have been investigated by using a fluid model. As the thermal energy of hot electrons is higher than cold electrons, the electron impact ionization process is governed by the concentration of hot electrons. The velocity of positive ions at the sheath boundary, i.e., the Bohm criterion, gets modified in the presence of ion–neutral drag force, source term, and bi-Maxwellian electrons. It is found that the ion–neutral drag force, ionization rates, and volumetric composition of electrons affect the characteristics of the presheath and sheath. The scale length of the sheath region widens from about 1.09 mm to 5.80 mm with the increase in the concentration of hot electrons. The acoustic speed of helium ions at the sheath boundary is higher than its common speed, whereas the acoustic speed of argon ions is lower than its common speed. The common speed of positive ions at the sheath boundary is slower by 13% in magnitude than in the collisionless case. Furthermore, the effect of ion–neutral drag force on streaming instability for two ion species plasma has been systematically presented.

Parametric instabilities in a two ion species plasma as a driver of super Alfvénic waves

Linear dispersion relation of parametric instabilities of a left hand circularly polarized wave in a three species plasma consisting electrons, protons, and α particles is derived and the obtained dispersion relation is numerically solved. It is shown that the so-called super Alfvénic waves, which are thought to play important roles in preferential acceleration of heavy ions, can get excited through a parametric decay instability when the parent wave is on the proton cyclotron branch and the plasma beta is sufficiently low. A variety of other parametric instabilities are also generated.

H-mode power threshold studies in mixed ion species plasmas at ASDEX Upgrade

Understanding the dependence of the H-mode power threshold, on the main ion composition is crucial for H-mode operation in ITER. In this work, H-mode power threshold studies conducted at ASDEX Upgrade are presented. These were performed in hydrogen plasmas with helium doping and in mixed hydrogen-deuterium plasmas. stays constant on the level of pure hydrogen in hydrogen plasmas with helium doping up to . Power balance calculations with the transport code ASTRA show that the edge ion heat flux at the L-H transition is independent of helium concentration and applied auxiliary heating, supporting that a critical edge ion heat flux is needed to enter H-mode. A non-linear dependence of on the relative hydrogen content is found. and the edge ion heat flux are constant at the level of pure deuterium for between 0 and 0.5 and constant on the level of pure hydrogen for between 0.8 and 1.

Enhanced collective stopping and drift of electron beams in fusion plasmas with heavy-ion species

The transport and energy deposition of relativistic electron beams in transversely nonuniform plasmas are investigated with two-dimensional electromagnetic particle-in-cell simulations. For the beam with radius much larger than plasma skin depth, the current filamentation instability excited by the electron beam can be observed, which breaks the beam into filaments and leads to the formation of strong magnetic fields consequently. The effects of plasma ion species are significant and asymmetric transverse magnetic fields are formed in plasmas with heavy-ion species due to the asymmetric neutralization of beam space charge by plasma ions. The asymmetric transverse magnetic fields contribute to the directional drift of beam electrons to lower plasma density regions, which may accelerate the filaments merger process and lead to highly localized beam-energy deposition in plasmas.

Anisotropic ultracold plasma expansion

Based on fluid model, the three-dimensional expansion of ultracold plasma is studied. The model considered both thermal pressure and ambipolar potential rising due to the local charge separation. By using isothermal assumption and quasi-neutrality condition, the time dependent asymetric expansion parametric investigation is conducted for two different ultracold plasma ion species (Sr+ and Ca+). It is found that ion temperature, in the range of ultracold limit, did not affect strongly the expanding profile but decreases the expanding density by an amount of ∼0.5% to ∼7%.The ion inertia is also found to enhance the asymetric aspect of the anisotrpic plasma expansion.

Latest Developments in Multiple Ion Species Plasma FIB Technology

Journal Article Latest Developments in Multiple Ion Species Plasma FIB Technology Get access Brandon Van Leer, Brandon Van Leer Thermo Fisher Scientific, Materials & Structural Analysis, Eindhoven, the Netherlands Search for other works by this author on: Oxford Academic Google Scholar Mikhail Dutka Mikhail Dutka Thermo Fisher Scientific, Materials & Structural Analysis, Eindhoven, the NetherlandsThermo Fisher Scientific, Hillsboro, OR, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S2, 1 August 2019, Pages 570–571, https://doi.org/10.1017/S1431927619003581 Published: 01 August 2019

Observation of enhanced ion particle transport in mixed H/D isotope plasmas on JET

Particle transport in tokamak plasmas has been intensively studied in the past, particularly in relation to density peaking and the presence of anomalous inward particle convection in L- and H-modes. While in the L-mode case the presence of the anomalous inward pinch has previously been unambiguously demonstrated, particle transport in the H-mode was unclear. The main difficulty of such studies is that particle diffusion and convection could not be measured independently in steady-state conditions in the presence of a core particle flux. Therefore, it is usually not possible to separate the transport effect(inward convection), from the source effect (slow diffusion of particles introduced to the plasma core by neutral beam injection heating).In this work we describe experiments done on JET with mixtures of two hydrogenic isotopes: H and D. It is demonstrated that in the case of several ion species, convection and diffusion can be separated in a steady plasma without implementation of perturbative techniques such as gas puff modulation. Previous H-mode density peaking studies suggested that for this relatively high electron collisionality plasma scenario, the observed density gradient is mostly driven by particle source and low particle diffusivity D < 0.5 * χeff. Transport coefficients derived from observation of the isotope profiles in the new experiments far exceed that value—ion particle diffusion is found to be as high as D ⩾ 2 * χeff, combined with a strong inward convection. Apparent disagreement with previous findings was explained by significantly faster transport of ion components with respect to the electrons, which could not be observed in a single main ion species plasma. This conclusion is confirmed by quasilinear gyrokinetic simulations.

Dual Ion Species Plasma Expansion from Isotopically Layered Cryogenic Targets.

A dual ion species plasma expansion scheme from a novel target structure is introduced, in which a nanometer-thick layer of pure deuterium exists as a buffer species at the target-vacuum interface of a hydrogen plasma. Modeling shows that by controlling the deuterium layer thickness, a composite H^{+}/D^{+} ion beam can be produced by target normal sheath acceleration (TNSA), with an adjustable ratio of ion densities, as high energy proton acceleration is suppressed by the acceleration of a spectrally peaked deuteron beam. Particle in cell modeling shows that a (4.3±0.7) MeV per nucleon deuteron beam is accelerated, in a directional cone of half angle 9°. Experimentally, this was investigated using state of the art cryogenic targetry and a spectrally peaked deuteron beam of (3.4±0.7) MeV per nucleon was measured in a cone of half angle 7°-9°, while maintaining a significant TNSA proton component.

Experimental studies of ion flow near the sheath edge in multiple ion species plasma including argon, xenon and neon

The Bohm sheath criterion was studied with laser-induced fluorescence (LIF) in three ion species plasmas using two tunable diode lasers. It was found in the first LIF studies of three ion species plasma (Yip et al 2016 Phys. Plasmas 23 050703) in which krypton was added to a mixture of argon and xenon plasma confined in a multidipole, dc hot filament discharge, that the addition of krypton served to turn off instability enhanced collisional friction (IEF) found in two ion species plasma (Yip et al 2010 Phys. Plasmas). In this study, neon, a less massive atomic gas than argon was added. Argon and xenon ion velocity distribution functions (IVDFs) were measured at the sheath-presheath boundary near a negatively biased boundary plate, and the Ne+ density was systematically increased. We found in both cases that once the added ion density significantly exceeded the density of the other two ions, IVDF measurements consistent with the absence of the instability were obtained, and the measured ion sheath edge speeds tended toward their individual Bohm velocities. For all other relative concentrations, the ions reached the sheath edge neither at their Bohm speeds nor the ion sound speed of the system, consistent, qualitatively, with the action of the IEF.

Observation of ion-ion counter streaming instability in presheath-sheath region of a mesh grid immersed in low temperature plasma

Instabilities in the sheath-presheath region formed in plasma-boundary layers are known to modify the particle flow velocities and their distribution functions, hence influencing the particle transport in this region significantly. In this paper, experimental observations of the ion-ion counter streaming instability excited in the sheath-presheath region of Argon (Ar), Helium (He), and Ar + He plasmas have been reported. These instabilities are excited in the vicinity of a stainless steel mesh grid placed inside the plasma. Floating potential (FP) fluctuations from the grid and from a movable Langmuir probe placed in the sheath-presheath region are measured. The frequency spectra of FP fluctuations in an argon plasma show a dominant broad peak in the range of 10–20 kHz centering around 15 kHz, which is identified as due to the ion-ion counter streaming instability. This frequency peak exists only in the sheath-presheath region and ceases to exist when the mesh grid is covered with a thin metal foil from one side, which restricts the counter streaming of the ions. The measured wave number, k, of the wave matches quite well with the calculated one from the dispersion relation of ion-ion counter streaming instability. The experiments are repeated to study the instability in He and Ar + He (two ion species) plasmas in similar experimental conditions. The neutral pressure threshold for sustenance of this instability has also been observed.

Bohm criterion in dusty plasmas with two species of positive ions and non-thermal electrons

Bohm criterion is revisited in the framework of a one-dimensional stationary and unmagnetized low-pressure argon and helium plasma mixture containing non-thermal electrons and dust grains model. Dust grains, electron non-thermality, and ions density ratio effects on Bohm velocities along with dust grain charge number |Φd0|, are investigated. It is revealed that Bohm velocities are highly correlated, with their profiles exhibiting a moving peak. Mixing the two gases allowed the appearance of subsonic (argon) as well as supersonic (helium) Bohm velocities. The present model may be generalized to many ion species plasmas.

Laser-induced fluorescence measurements of argon and xenon ion velocities near the sheath boundary in 3 ion species plasmas

The Bohm sheath criterion is studied with laser-induced fluorescence in three ion species plasmas using two tunable diode lasers. Krypton is added to a low pressure unmagnetized DC hot filament discharge in a mixture of argon and xenon gas confined by surface multi-dipole magnetic fields. The argon and xenon ion velocity distribution functions are measured at the sheath-presheath boundary near a negatively biased boundary plate. The potential structures of the plasma sheath and presheath are measured by an emissive probe. Results are compared with previous experiments with Ar–Xe plasmas, where the two ion species were observed to reach the sheath edge at nearly the same speed. This speed was the ion sound speed of the system, which is consistent with the generalized Bohm criterion. In such two ion species plasmas, instability enhanced collisional friction was demonstrated [Hershkowitz et al., Phys. Plasmas 18(5), 057102 (2011).] to exist which accounted for the observed results. When three ion species are present, it is demonstrated under most circumstances the ions do not fall out of the plasma at their individual Bohm velocities. It is also shown that under most circumstances the ions do not fall out of the plasma at the system sound speed. These observations are also consistent with the presence of the instabilities.