Positive Ion Flow Research Articles

Flow of high-energy positive oxygen ions from plasma onto the substrate in a pulsed magnetron discharge with a hot target.

A group of high-energy positive O^+ ions was detected in a plasma flowhigh-current pulsed magnetron discharge with a hot target in an Ar/O_2 gas mixture. The mechanism for the formation of accelerated O^+ ions may be the conversion of accelerated ones in the cathode layer of negative ions O^– → O^+ in the processes of charge exchange or ionization by electron impact.

Integration of CVD graphene in gaseous electron multipliers for high energy physics experiments

To enhance the performance of micro-patterned gaseous detectors (MPGDs) to meet the challenging requirements of future high energy physics (HEP) experiments, two-dimensional (2D) materials are attractive candidates to address the back flow of positive ions, which affects detector performance by distorting electric field lines. In this context, graphene is promising to work as selective filter for ion back flow suppression, being transparent to electrons while at the same time blocking ions. Also, graphene membranes can physically separate drift and amplification regions of the detectors, offering additional flexibility in the choice of gas mixtures and allowing independent optimizations of detector sensitivity and electron multiplication processes. Here we present an approach to integrate graphene grown via chemical vapor deposition (CVD) on gaseous electron multiplier (GEM) prototypes via a wet transfer procedure in order to suspend graphene over thousands of holes with 60 μm diameter and overcome the challenges encountered due to process steps involving liquids, mostly related with the capillary effects during drying and evaporation of them. In order to overcome the risk of damaging the membrane and decreasing the yield of suspended 2D material membranes, critical point dryer (CPD) and inverted floating method (IFM) procedures are investigated. In addition to the necessity to cover the full holes in the active area, polymeric residuals have to be minimized in order to evaluate the graphene transparency at the electron energies (i.e., < 15 eV) typically obtained in the operating conditions, measurements in these energy ranges are still not deeply investigated.

Study of positive ion transport to the plasma electrode in giant RF negative ion sources

Negative ion sources are fundamental components of neutral beam injectors (NBI), one of the main heating systems for fusion reactors. SPIDER is the full-scale prototype negative ion source for ITER NBIs. It is hosted in Padua as part of the Neutral Beam Test Facility (NBTF). It aims to extract up to 330Am−2 of negative hydrogen ions from an inductively coupled plasma, generated inside 8 cylindrical drivers. The negative ion production is enhanced by caesium evaporation inside the source.In caesium-seeded negative ion sources, negative ions are produced close to the extraction apertures, and they are mainly generated by surface conversion of neutral atoms and positive ions impinging on the ion source walls, particularly on the plasma grid. The conversion yields depend on the energy distribution of these precursors, and so does the energy of those particles which are reflected as negative ions. The positive ion flow in the extraction region may also impact on the extraction probability of negative ions, via momentum transfer. Besides, in giant multi-driver RF sources such as SPIDER, a gradient of plasma potential is present in the expansion region Sartori et al. (2021), affecting the positive ion transport towards the caesiated plasma electrode and their energy.To approach this complex problem, a 3D test-particle Monte Carlo code for tracing plasma motion in SPIDER was developed. Positive ions species are generated in different positions within the plasma source volume and are tracked under the influence of electric and magnetic fields. Then, Monte Carlo collisions are used to simulate the interaction with predetermined backgrounds of plasma and neutrals, with profiles derived from experimental data. The particles are traced until they hit the ion source walls. Finally, the energy distribution of the different particle species impinging on the plasma grid (PG) are determined, and used to assess the generation and the energy distribution of the produced H−.

Beam instability in the vicinity of beam extraction region of negative ion source

Beam instability in the presheath region of negative ion beam extraction is investigated in theoretically and experimentally. The linear stability analysis shows that the beam instability is unstable due to coupling between positive ion flow and negative ion flow. On the other hand, no clear activity can be seen in the experiment in the frequency range predicted by the theory. The beam instability in the presheath region of negative ion beam extraction may not cause the degradation of the beam focusing because of collisional damping and/or Landau damping.

Study of the charged particle flow near the plasma grid in negative ion source

Flows of the charged particles in hydrogen (H) and deuterium (D) plasmas were measured with the single-tip directional Langmuir probe. The flow patterns of positive and negative ions have been constructed from the data by moving the probe near the plasma grid. The positive ion flow from driver to extraction regions showed the different velocity equivalent to the mass difference between H and D. The negative ion flow indicated the surface production of negative ions on the plasma grid, and changed the direction corresponding to the magnitude of the extraction electric field.

Kinetic-theory-based investigation of electronegative plasma–wall transition with two populations of electrons

Kinetic theory has been employed to investigate the magnetized plasma-sheath structure and its characteristics in the presence of more than one species of negatively charged particles: hot electrons, cold electrons, and negative ions. The cold electrons and negative ions are considered to obey a Maxwellian distribution, whereas the hot electrons follow a truncated Maxwellian distribution. The Bohm sheath condition has been extended for the case of more than one species of negatively charged particles, in which the concentration of hot electrons has a crucial role in achieving the Bohm velocity. The thermal motion of hot electrons is much higher compared to cold electrons and negative ions, such that the variation of hot electron concentrations and the temperature ratio of hot to cold electrons play a key role in the determination of the plasma-sheath parameters: particle densities, electrostatic potential, the flow of positive ions towards the wall, and sheath thickness. We have estimated the deviation of the resultant drift velocity of positive ions on the plane perpendicular to the wall from the parallel component at the presheath–sheath interface. It is found that the deviation between the two velocity components increases with an increase in the obliqueness of the magnetic field. Furthermore, the results obtained from the kinetic trajectory simulation model are compared with the results obtained using a fluid model; the results are qualitatively similar, although the potential varies by less than 4% in terms of the magnitude at the wall.

Edge of magnetized electronegative plasma ion source in the presence of collisional adiabatic thermal positive ions

This paper develops a theoretical model for formation of multilayers in the magnetized electronegative plasma at the edge of plasma ion source. The impacts of positive ion temperature and collisions are studied and quantified by obtaining the structure of the plasma sheath. By adding the negative ions into the discharge, the solutions of Poisson’s equation become oscillatory. A finite temperature for the positive ions and also the collisions results in a change in the behavior of such oscillatory solutions. Here, it is assumed that the collision frequency depends on the positive ions velocity and thermal positive ions flow is adiabatic. The spatial distribution of the species density, electric potential, and positive ion velocity is calculated for different values of positive ion temperature and negative ion concentration for two limiting cases where the collision frequency either is constant or depends linearly on velocity. In addition, the influence of the plasma parameters such as negative ion density and temperature and positive ion temperature is investigated on the space charge and positive ion flux as well as parameter space region. It is also shown that the presence of the negative ion into the plasma ion source influences the extracted positive ion flux and increases the positive ions intensity.

Theoretical model for magnetically insulated flow with both negative and positive ions

Negative and positive ions crossing the anode-cathode gap of a magnetically insulated transmission line (MITL) can cause non-negligible current loss and energy deposition on the electrodes, which may lead to the formation of anode plasma and the growth of cathode plasma. Furthermore, gap closure could occur due to the expansion of cathode plasma and anode plasma. In this paper, a model for magnetic insulation of both negative ion flow and positive ion flow is developed. The operating voltage V of the MITL is expressed as a function of the total current I0 and the boundary current Ib. The total current and the boundary current of the MITL are derived at saturated and self-limited flows, respectively. In addition, particle-in-cell simulations are implemented for the validation of the theoretical model. The thickness and density of the magnetically insulated ion layers are analyzed, and an empirical expression for space charge factor g is obtained through simulation results. This work extends the understanding of magnetically insulated ion flow in MITLs.

PIC modeling of negative ion sources for fusion

This work represents the first attempt to model the full-size ITER negative ion source prototype including expansion, extraction and part of the acceleration regions keeping the resolution fine enough to resolve every single aperture of the extraction grid. The model consists of a 2.5-dimensional Particle-in-Cell/Monte Carlo Collision representation of the plane perpendicular to the filter field lines. Both the magnetic filter and electron deflection fields have been included. A negative ion current density of produced by neutral conversion from the plasma grid is used as fixed parameter, while negative ions produced by electron dissociative attachment of vibrationally excited molecules and by ionic conversion on plasma grid are self-consistently simulated. Results show the non-ambipolar character of the transport in the expansion region driven by electron magnetic drifts in the plane perpendicular to the filter field. It induces a top-bottom asymmetry detected up to the extraction grid which in turn leads to a tilted positive ion flow hitting the plasma grid and a tilted negative ion flow emitted from the plasma grid. As a consequence, the plasma structure is not uniform around the single aperture: the meniscus assumes a form of asymmetric lobe and a deeper potential well is detected from one side of the aperture relative to the other side. Therefore, the surface-produced contribution to the negative ion extraction is not equally distributed between both the sides around the aperture but it come mainly from the lower side of the grid giving an asymmetrical current distribution in the single beamlet.

Asymmetric Salt Profiles Expand Reaction Conditions for Nanopore Sequencing with MspA

MspA is a biological porin that has demonstrated use in nanopore DNA sequencing. In this technique, an electric field is applied across a lipid bilayer containing MspA, and the flow of ions through the pore produces a measurable current. Negatively charged single stranded DNA molecules are driven through MspA according to the electric field. A polymerase or helicase bound to the DNA molecule controls the polymers’ translocation. The different DNA nucleotides uniquely modulate the flow of ions as the DNA translocates, allowing a sequence to be interpreted from the changes in current. As the observed current is due to the flow of positive and negative ions within MspA, varying the buffer salt concentration can have dramatic effects on the signal. With salt concentrations below 100 mM on both sides of the membrane, signal is reduced making sequencing challenging. In this study, we varied the concentrations on either side of the membrane (cis and trans) independently, creating asymmetric salt profiles. We observed the effects of these asymmetries on open pore current, DNA interaction rate, and DNA blockage currents during controlled translocation by phi29 DNA polymerase. MspA is highly cation selective during DNA translocation with the majority of current produced by the flow of K+ ions from trans to cis. Varying trans [K+] has dramatic effects on the signal magnitude, whereas changing cis [Cl−] produces only small effects. This means that useful signals can be gained with negligible cis [KCl], if the concentration of [KCl] on the trans side is kept high. Many DNA motor enzymes, whose properties may be ideal for sequencing, loose function at appreciable salt concentrations, and these results reveal that such enzymes could still be used in nanopore DNA sequencing with MspA.

An Investigation of Ionic Flows in a Sphere-Plate Electrode Gap

This paper presents analyses of ion flow characteristics and ion discharge pulses in a sphere-ground plate electrode system. As a result of variation in electric field intensity in the electrode gap, the ion flows towards electrodes generate non-uniform discharging pulses. Inspection of these pulses provides useful information on ionic stream kinetics, the effective thickness of ion cover around electrodes, and the timing of ion clouds discharge pulse sequences. A finite difference time domain (FDTD) based space-charge motion simulation is used for the numerical analysis of the spatio-temporal development of ionic flows following the first Townsend avalanche, and the simulation results demonstrate expansion of the positive ion flow and compression of the negative ion flow, which results in non-uniform discharge pulse characteristics.

Earthquake forewarning — A multidisciplinary challenge from the ground up to space

Most destructive earthquakes nucleate at between 5–7 km and about 35–40 km depth. Before earthquakes, rocks are subjected to increasing stress. Not every stress increase leads to rupture. To understand pre-earthquake phenomena we note that igneous and high-grade metamorphic rocks contain defects which, upon stressing, release defect electrons in the oxygen anion sublattice, known as positive holes. These charge carriers are highly mobile, able to flow out of stressed rocks into surrounding unstressed rocks. They form electric currents, which emit electromagnetic radiation, sometimes in pulses, sometimes sustained. The arrival of positive holes at the ground-air interface can lead to air ionization, often exclusively positive. Ionized air rising upward can lead to cloud condensation. The upward flow of positive ions can lead to instabilities in the mesosphere, to mesospheric lightning, to changes in the Total Electron Content (TEC) at the lower edge of the ionosphere, and electric field turbulences. Advances in deciphering the earthquake process can only be achieved in a broadly multidisciplinary spirit.

THE VLASOV–POISSON–BOLTZMANN SYSTEM FOR SOFT POTENTIALS

An important physical model describing the dynamics of dilute weakly ionized plasmas in the collisional kinetic theory is the Vlasov–Poisson–Boltzmann system for which the plasma responds strongly to the self-consistent electrostatic force. This paper is concerned with the electron dynamics of kinetic plasmas in the whole space when the positive charged ion flow provides a spatially uniform background. We establish the global existence and optimal convergence rates of solutions near a global Maxwellian to the Cauchy problem on the Vlasov–Poisson–Boltzmann system for angular cutoff soft potentials with -2 ≤ γ &lt; 0. The main idea is to introduce a time-dependent weight function in the velocity variable to capture the singularity of the cross-section at zero relative velocity.

Dissipative Property of the Vlasov–Maxwell–Boltzmann System with a Uniform Ionic Background

In this paper we discuss the dissipative property of near-equilibrium classical solutions to the Cauchy problem of the Vlasov-Maxwell-Boltzmann System in the whole space $\R^3$ when the positive charged ion flow provides a spatially uniform background. The most key point of studying this coupled degenerately dissipative system here is to establish the dissipation of the electromagnetic field which turns out to be of the regularity-loss type. Precisely, for the linearized non-homogeneous system, some $L^2$ energy functionals and $L^2$ time-frequency functionals which are equivalent with the naturally existing ones are designed to capture the optimal dissipation rate of the system, which in turn yields the optimal $L^p$-$L^q$ type time-decay estimates of the corresponding linearized solution operator. These results show a special feature of the one-species Vlasov-Maxwell-Boltzmann system different from the case of two-species, that is, the dissipation of the magnetic field in one-species is strictly weaker than the one in two-species. As a by-product, the global existence of solutions to the nonlinear Cauchy problem is also proved by constructing some similar energy functionals but the time-decay rates of the obtained solution still remain open.

Electromotive Forces and the Meissner Effect Puzzle

In a voltaic cell, positive (negative) ions flow from the low (high) potential electrode to the high (low) potential electrode, driven by an `electromotive force' which points in opposite direction and overcomes the electric force. Similarly in a superconductor charge flows in direction opposite to that dictated by the Faraday electric field as the magnetic field is expelled in the Meissner effect. The puzzle is the same in both cases: what drives electric charges against electromagnetic forces? I propose that the answer is also the same in both cases: kinetic energy lowering, or `quantum pressure'.

Effect of Positive-Ion Temperature in the Sheaths Surrounding Cylindrical and Spherical Probes in Electronegative Plasmas

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper analyzes the effect of positive-ion temperature on the sheath that surrounds a cylindrical or spherical probe in an electronegative plasma. We developed a theoretical model that considers a hydrodynamic approach for positive-ion motion. In order to introduce the positive-ion temperature into the model, we consider that positive ions flow adiabatically toward the probe. This model allows us to obtain the electrical potential profile surrounding the probe and the current-to-voltage characteristic curves. These <formula formulatype="inline"> <tex Notation="TeX">$I$</tex></formula>–<formula formulatype="inline"><tex Notation="TeX">$V$</tex></formula> curves were fitted as functions of the probe radius, the probe biasing potential, and the positive-ion temperature. Finally, these expressions allow us to obtain the floating potential of the probe for different electronegativities and positive-ion temperatures. </para>

Parallel-velocity-shear-modified drift wave in negative ion plasmas

A systematic investigation of the effects of a parallel velocity shear and negative ions on the collisionless drift wave instability has for the first time been realized by simultaneously using a segmented tungsten hot plate of a Q-machine and sulfur hexafluoride (SF6) gas in a magnetized potassium plasma. The parallel velocity shear of the positive ion flow tends to decrease the fluctuation level of the drift wave. The introduction of negative ions first increases the fluctuation level and then starts to decrease it at the negative ion exchange fraction of around 10%, while keeping the above-mentioned shear effect qualitatively. In addition, a simple dispersion relation based on the local model has been calculated to show that it can predict wave characteristics similar to the experimental results. Our findings provide a potential for gaining a more profound insight into the physics of space/circumterrestrial plasmas.

Enhancement and Suppression of Velocity-Shear-Driven Drift Instability due to Negative Ions

The collisionless electron drift wave instability in a plasma involving sheared magnetic-field-aligned positive-ion flow and negative ion species has been experimentally investigated. Surveying wide ranges of the shear strength and of the negative ion exchange fraction, which was first made possible by our new apparatus mounted in a Q machine, reveals detailed characteristics of the instability. The kinetic dispersion relation suggests that the wave observed for positive shear is the current-driven shear-modified drift wave. However, for negative shear the wave exhibits peculiar behavior which cannot be directly interpreted by the linear local theory. We also found that negative ions tend to stabilize the instability.

Equilibrium model for two low-pressure electronegative plasmas connected by a double layer

Plihon et al. [J. Appl. Phys. 98, 023306 (2005)] have recently shown that double layers usually form during the expansion of a low pressure electronegative plasma. These double layers act as permeable internal boundaries between the source (upstream) plasma and the downstream expanding plasma; positive ions flow from upstream to downstream whereas negative ions flow in the opposite direction. So far, the detailed physical mechanisms leading to their formation have not been identified. In this paper, we develop a model for the two plasma equilibria, upstream and downstream, assuming that the double layer exists and couples the two plasmas. At very low pressure, typically 0.5mTorr, the coupling is strong and acts both ways. The negative ions created downstream contributes to the upstream equilibrium as well as the upstream positive ions contribute to the downstream equilibrium. As the pressure increases, the situation becomes asymmetric. The source plasma is not affected by the negative ions flowing from downstream, whereas the positive ions coming from the source control the downstream plasma equilibrium, where local ionization is negligible.

Ordering and behavior of fine particles in plasmas under gravity and microgravity conditions

Experiments of fine-particle plasma have been performed using a drop experiment facility in order to analyze the ordering and behavior of fine particles under microgravity and during the change of gravitational conditions. Aligned fine particles in the radial directions were observed under microgravity, although they were aligned in the vertical direction under gravity. It has been suggested that the vertical alignment of fine particles under gravity is caused by the mutual influence between positive ion flow and negative fine particles following the deformation of plasma.