Charge Exchange Loss Research Articles

Wall Conditioning and Neutral Gas Transport at the GDT Facility

The effect of wall conditioning on a plasma was studied in the Gas Dynamic Trap (GDT) facility by means of titanium coating of the containment wall. After depositing titanium on the inner surface of the vacuum chamber the charge-exchange losses of the fast ions turned out to be much less than in former, non-conditioned discharges. The temporal and spatial variation of the neutral gas density was measured during typical shots and calculated by means of a Monte Carlo transport code. The comparison of the results numerical simulation with the experimental data are presented.

The role of precipitation losses in producing the rapid early recovery phase of the Great Magnetic Storm of February 1986

The possible role of precipitation losses in eroding stormtime ring current is subject to debate. To explore this controversy, the recovery phase of the February 6–10, 1986, great magnetic storm is examined, when intense ion precipitation was observed at midlatitudes by NOAA‐6 and DMSP satellites. This storm period is particularly interesting because the ring current exhibits distinctive two‐phase decay as seen in the Dst index, the early rapid timescale decay corresponding to the intense ion precipitation period described above. Hamilton et al. [1988] concluded, from close agreement between the observed timescale for ring current decay and the theoretical timescale for O+ charge exchange loss, that rapid early recovery phase of this storm resulted from the charge exchange loss of high energy O+; the second and longer decay phase was equated with H+ charge exchange loss. A model of the ring current evolution during this great magnetic storm [Fok et al., 1995] failed to reproduce the observed ring current decay rates, a puzzling result because charge exchange losses were well represented in the ring current model and initial and boundary conditions were taken from the same data set used in the Hamilton et al. [1988] study. A simple energy balance calculation for the global ring current is carried out using (1) either an energy input predicted from upstream solar wind parameters or one calculated from the drift‐loss model output, (2) collisional loss timescales extracted from the drift‐loss model, and (3) precipitation losses estimated from NOAA‐6 and DMSP observations. The energy balance model replicates the evolution of the ring current energy content derived from Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) observations when ion precipitation losses are included and model energy input function is reduced to agree with predictions based upon upstream solar wind parameters. The O+ charge exchange losses and observed global precipitation losses were of equal magnitude in early recovery of the ring current during this great magnetic storm. Later longer decay timescales in the model resulted from a combination of O+ and H+ charge exchange losses; O+ charge exchange losses remained important throughout the model time interval. The present model produces agreement with the AMPTE/CCE estimates of ring current kinetic energy content versus time. Disagreement between the Dst* inferred from the AMPTE/CCE particle measurements and the observed Dst* is an interesting issue needing further explanation.

Energy loss and charge exchange: Statistics and atomistics

This is a brief summary of recent work by the authors on (I) statistical aspects of the interplay between charge exchange and electronic energy loss and (II) atomistics of charge-dependent stopping of heavy ions. (I) A matrix formalism has been developed that specifies energy-loss spectra differential in entrance and exit state. Examples refer to a simple two-state system where experimental results exist. The precise definition of energy loss needs attention when particles change identity. (II) The relation between stopping cross section and ion charge has been studied on the basis of the Bohr theory. A dimensionless parameter has been identified which governs the significance of projectile screening, indicating that the stopping cross section for target excitation is as sensitive to the target as to the projectile. The contribution of projectile excitation to electronic stopping is discussed qualitatively. Comparison with measured fixedcharge stopping cross sections shows good to satisfactory agreement.

Galileo‐measured depletion of near‐Io hot ring current plasmas since the Voyager epoch

The first mass‐discriminated, hot ion distribution moments (pressure, energy intensity) are determined for hot >50‐keV ions in Jupiter's inner magnetosphere at the outer edge of Io's plasma torus by using the Galileo energetic particle detector (EPD) data. These hot plasmas were significantly depleted during the Galileo encounter in 1995 as compared with the Voyager epoch of 1979. The depletion of the hot ions is apparently caused by enhanced charge exchange losses of hot ions, perhaps associated with enhanced emissions of neutral gases from the volcanoes of Io. Such neutral gas enhancements could simultaneously explain increases, reported elsewhere, in the densities of the cooler Io torus plasmas. The hot plasma changes may explain why radial transport interchange turbulence has been observed by Galileo in the Io torus regions, whereas such turbulence was not apparent during the Voyager encounters in 1979. The hot ion depletion could also play a role in explaining the apparent differences between the Jovian auroral configuration observed in recent years by the Hubble space telescope and ground observers and the configuration observed by Voyager. This possibility is much less certain, however.

Electron and proton radiation belt dynamic simulations during storm periods: A new asymmetric convection‐diffusion model

Using a convection‐diffusion theory, we give the first results from a four‐dimensional model of electron and proton radiation belts. This work is based on the numerical solution of a convection‐diffusion equation taking into account (1) for protons, the deceleration of protons by the free and bounded thermospheric and ionospheric electrons, the charge exchange loss process, radial and azimuthal transports, and (2) for electrons, the deceleration of electrons by the free and bounded electrons of the medium, pitch angle diffusion by Coulomb and wave‐particle interactions, radial and azimuthal transport. This model allows for simulation of a magnetic storm effects by increasing convective electric field and injecting particles with keV range energies in the nightside region. Particles in the energy range 50 – 100 keV are “created” by acceleration of slower particles in the L = 4 region. Four hours are needed for ring current formation. The calculated particle distribution at 6.6 Earth radii as well as at low altitude are in good agreement with those deduced from ATS 6 measurements (the drift echo is well reproduced at this altitude) and from statistical studies of the precipitation by the DMSP satellites, respectively.

A theoretical model of H-mode transition triggered by condensed neutrals near X-point

The effects of neutrals near the X-point of tokamaks on the L/H transition are examined. Four mechanisms for bipolar losses, i.e. the loss cone loss, the bulk viscosity loss, the charge-exchange (CX) loss between ions and neutrals and the anomalous loss, occur simultaneously in high-temperature plasmas. Neutral particles near the X-point are found to trigger the H-mode transition. They cause an additional ion loss cone loss via collisions. The lower limit of the neutral density near the X-point for the onset of the H-mode transition is estimated. Neutral particles at the main plasma are shown to prevent the H-mode transition because of CX loss.

Ring current H atom precipitation on the outer planets

Voyager has revealed the presence of an extended neutral gas corona on Saturn and Uranus. This observed feature leads to the occurrence of charge-exchange loss of energetic charged particles of the ring current: causing important energetic and dynamic effects in the upper atmospheres of these planets through precipitation. This paper explores the importance of this process in producing H Lyα, Hα, Hβ, and H 2 band emissions on the outer planets. This mechanism is found to be an important source of optically thin H Lyα emissions from the outer planets.

Ring current development during storm main phase

The development of the ring current ions in the inner magnetosphere during the main phase of a magnetic storm is studied. The temporal and spatial evolution of the ion phase space densities in a dipole field are calculated using a three dimensional ring current model, considering charge exchange and Coulomb losses along drift paths. The simulation starts with a quiet time distribution. The model is tested by comparing calculated ion fluxes with Active Magnetospheric Particle Tracer Explorers/CCE measurement during the storm main phase on May 2, 1986. Most of the calculated omnidirectional fluxes are in good agreement with the data except on the dayside inner edge (L < 2.5) of the ring current, where the ion fluxes are underestimated. The model also reproduces the measured pitch angle distributions of ions with energies below 10 keV. At higher energy, an additional diffusion in pitch angle is necessary in order to fit the data. The role of the induced electric field on the ring current dynamics is also examined by simulating a series of substorm activities represented by stretching and collapsing the magnetic field lines. In response to the impulsively changing fields, the calculated ion energy content fluctuates about a mean value that grows steadily with the enhanced quiescent field.

Statistics of projectile states and correlated phenomena for penetrating ions

The first part of the paper summarizes a recently developed formalism for the statistical description of projectile-state-dependent penetration phenomena. The central point is a generalization of the Bothe-Landau scheme governing multiple scattering and stopping under the condition of small relative energy loss. Tools are listed that may be employed in the theoretical analysis and reference is made to recently published applications. The second part reports new applications which include i) charge-dependent multiple scattering, in particular the transient behavior after few charge exchanges, and ii) expressions for the energy-loss spectrum under similar conditions. Comments are made on iii) the interference of charge exchange and energy loss in a multicomponent material and on iv) fast pre-equilibrium charge states observed for heavy ions penetrating thin films.

Population of the near‐Earth magnetotail from the auroral zone

This paper reports the development and performance of a large scale kinetic simulation using a three‐dimensional model of the terrestrial electric and magnetic fields in an effort to reach a better understanding of the ionospheric contribution to the near‐Earth (x < 10 RE) region during quiet and slightly disturbed times. The simulation employed the Tsyganenko [1989] magnetic field model and an electric field derived from the Heppner and Maynard [1987] ionospheric potentials. For the conditions considered in this study (southward interplanetary magnetic field (IMF), ϕXT = 20–40 kV), it was found that the cleft ion fountain plays a relatively minor role in supplying particles to the near‐Earth region. The ionospheric contribution to the near‐Earth proton population is significant during quiet times with the bulk of the O+ ions in the near‐Earth region coming from the auroral zone up welling region. However, the plasma mantle becomes the dominant hot proton source during more active times. Using the nightside auroral zone as a source, we launched distributions of H+, He+, and O+ ions and calculated densities, pressures, and other bulk parameters in the near‐Earth plasma sheet and partial ring current. Because of the static nature of the model, ionospheric ions had very limited access to the trapped ring current, but the ions formed a reservoir of energetic particles just outside this region that in theory could act as a source for the ring current during more active times. The residence time of ions in the model is too short for charge exchange losses to become significant, and the principal loss mechanism is through the dusk flank of the magnetopause, with precipitation into the ionosphere playing a minor role.

High-pressure effects in miniature arrays of quadrupole analyzers for residual gas analysis from 10−9 to 10−2 Torr

The design, construction, and characterization of a miniature array of 16 rods forming nine quadrupole mass analyzers for residual gas analysis are presented. The novel design takes advantage of the reduction of the dimensions and hence the mean free path to broaden the dynamic range by extending the operating pressure range up to 10 mTorr at the high end while maintaining the low-end sensitivity and mass resolution. The small size, low weight, low power consumption, and low cost of this mass analyzer enable many new applications such as in situ process control in high pressure processes. Applications which take advantage of the capability of operating the source pressure two orders of magnitude higher than conventional devices such as high-pressure contaminant monitoring are discussed. Data on high-pressure effects on the ionizer lifetime in various atmospheres and nonlinearities of the signal are presented and correlated with theoretical analysis. Ray tracing simulations and Monte Carlo statistical calculations are used to provide an understanding of the sensitivity reduction at pressures substantially higher than 10 mTorr. These calculations take into account the ion charge density in the source and the dissociation, charge exchange, and energy loss by the ions when colliding with residual molecules along the length of the rods.

An improved kinetic model for the polar outflow of a minor ion

We have studied the escape of a minor ion from the terrestrial atmosphere by outflow along geomagnetic field lines that are open to the interplanetary medium. Coulomb collisions with the background ions are treated rigorously in the Fokker‐Planck transport equation which is adopted in the transition region between the collision‐dominated low‐altitude regime (treated by moment equations) and the collisionless regime at very high altitudes. Sample solutions are presented for both H+ and He+ ions in an O+ background. In the transition region and above, the distribution functions are not well represented by low‐order approximations, such as drifting Maxwellian distributions. For He+, while the densities and drift speeds are sensitive to the velocity distribution, the flux is a robust quantity that depends primarily on the altitude of the transition between the region of chemical equilibrium and no charge exchange loss.

Collisional losses of ring current ions

The time evolution of the ring current population during the recovery phase of a typical moderate magnetic storm is studied, using a newly developed kinetic model for H+, He+ and O+ ions which includes nonequatorially mirroring particles. The bounce‐averaged distribution function is defined for variables that are accessible to direct measurement, and some useful formulas for calculating the total energy and number density of the ring current are derived. The bounce‐averaged kinetic equation is solved, including losses due to charge exchange with neutral hydrogen and Coulomb collisions with thermal plasma along ion drift paths. Time‐dependent magnetospheric electric fields and anisotropic initial pitch angle distributions are considered. The generation of ion precipitating fluxes is addressed, a process that is still not completely understood. It is shown that both the decrease of the distribution function due to charge exchange losses and the buildup of a low‐energy population caused by Coulomb collisions proceed faster for particles with smaller pitch angles. The maximum of the equatorial precipitating fluxes occurs on the nightside during the early recovery phase and is found to be of the order of 104–105 cm−2sr−1s−1keV−1. The mechanisms considered in this paper indicate that magnetospheric convection plays the predominant role in causing ion precipitation; Coulomb scattering contributes significantly to the low‐energy ion precipitation inside the plasmasphere.

SALAMMBO: A three‐dimensional simulation of the proton radiation belt

Using diffusion theory, we give the first results for a three‐dimensional model of the proton radiation belt. This is based on the numerical solution of a diffusion equation which takes into account the cosmic ray albedo neutron decay source, deceleration of protons by the free and bounded electrons of the medium, the charge exchange loss process, and radial diffusion by magnetic and electric field perturbations. This model used in static conditions gives results which are in a good agreement with those of NASA's AP8 model. However, a better knowledge of thermospheric densities, radial diffusion, and improved measurements of proton flux near synchronous orbit altitudes may be necessary to improve future model results. We prove that an external boundary condition, which plays the part of an injection source, is essential to radiation belt existence and explains the effect of solar activity and magnetotail dynamics on radiation belt dynamics.

Particle confinement study in hot ion H mode discharges of JT-60U

In order to understand the particle confinement properties of the hot ion H mode plasmas of JT-60U, the particle confinement time was estimated through analyses of the neutral particle behaviour based on measurements of the line emission intensity from the neutral particles combined with neutral particle transport simulations. The relationship between the properties of the particle confinement and the energy confinement was then examined. It was found that: (a) the particle confinement time depended strongly on the averaged electron density, (b) the ratio of the particle confinement time to the energy confinement time was in the range of 1-2 and (c) the particle confinement time was well correlated with the edge ion temperature. Furthermore, in order to clarify the roles of the particle confinement in the edge ion power balance, the charge exchange and convective losses were quantitatively evaluated using the distribution of the neutral particle density and the charged particle outflux, both of which were obtained through an analysis of the neutral particle behaviour. The conductive loss was estimated on the basis of the charge exchange and convective losses and the input power, and then the thermal diffusivity was estimated. From these estimates, it was found that the contributions of the charge exchange and convective losses in the edge power balance were small, and the increase of the edge ion temperature was caused by a decrease of the thermal diffusivity at the plasma edge

Three‐Dimensional Ring Current Decay Model

This work is an extension of a previous ring current decay model. In the previous work, a two‐dimensional kinetic model was constructed to study the temporal variations of the equatorially mirroring ring current ions, considering charge exchange and Coulomb drag losses along drift paths in a magnetic dipole field. In this work, particles with arbitrary pitch angle are considered. By bounce averaging the kinetic equation of the phase space density, information along magnetic field lines can be inferred from the equator. The three‐dimensional model is used to simulate the recovery phase of a model great magnetic storm, similar to that which occurred in early February 1986. The initial distribution of ring current ions (at the minimum Dst) is extrapolated to all local times from AMPTE/CCE spacecraft observations on the dawnside and duskside of the inner magnetosphere spanning the L value range L = 2.25 to 6.75. Observations by AMPTE/CCE of ring current distributions over subsequent orbits during the storm recovery phase are compared to model outputs. In general, the calculated ion fluxes are consistent with observations, except for H+ fluxes at tens of keV, which are always overestimated. A newly invented visualization idea, designated as a chromogram, is used to display the spatial and energy dependence of the ring current ion differential flux. Important features of storm time ring current, such as day‐night asymmetry during injection and drift hole on the dayside at low energies (<10 keV), are manifested in the chromogram representation. The pitch angle distribution is well fit by the function, jo(1 + Ayn), where y is sine of the equatorial pitch angle. The evolution of the index n is a combined effect of charge exchange loss and particle drift. At low energies (<30 keV), both drift dispersion and charge exchange are important in determining n.

Calculations of energy losses due to atomic processes in tokamaks with applications to the International Thermonuclear Experimental Reactor divertor

Reduction of the peak heat loads on the plasma facing components is essential for the success of the next generation of high fusion power tokamaks such as the International Thermonuclear Experimental Reactor (ITER) [Rebut et al., Plasma Physics and Controlled Nuclear Fusion Research (International Atomic Energy Agency, Vienna, in press)]. Many present concepts for accomplishing this involve the use of atomic processes to transfer the heat from the plasma to the main chamber and divertor chamber walls and much of the experimental and theoretical physics research in the fusion program is directed toward this issue. The results of these experiments and calculations depend upon a complex interplay of many processes. In order to identify the key features of these experiments and calculations and the relative role of the primary atomic processes, simple quasianalytic models and the latest atomic physics rate coefficients and cross sections have been used to assess the relative roles of central radiation losses through bremsstrahlung, impurity radiation losses from the plasma edge, charge exchange and hydrogen radiation losses from the scrape-off layer, and divertor plasma and impurity radiation losses from the divertor plasma. This analysis indicates that bremsstrahlung from the plasma center and impurity radiation from the plasma edge and divertor plasma can each play a significant role in reducing the power to the divertor plates, and identifies many of the factors which determine the relative role of each process. For instance, for radiation losses in the divertor to be large enough to radiate the power in the divertor for high power experiments, a neutral fraction of 10−3 to 10−2 and an impurity recycling rate of neτrecycle of ∼1016 s m−3 will be required in the divertor.

Probing surfaces with low-energy ions

In this paper a brief review is given of areas in surface science in which low-energy ions (with a few keV of kinetic energy) are used successfully for investigating surface properties. These studies include composition analysis in the monolayer regime, structure analysis of single crystal surfaces and thin films, surface modification due to sputtering and equilibration, and inelastic processes due to charge exchange and energy loss. Examples from recent studies are given to discuss the relevant physical processes.

Power balance in DIII-D during single-null ELMing H-mode plasmas

We account for ≥85% of the injected power in DIII-D during single-null diverted ELMing H-mode discharges. Core plasma radiation accounts for ≤18% with the rest flowing through the scrape-off layer to the inboard and outboard divertors. The total power roughly splits in an in: out ratio of 1:1.3. Of the power entering the outboard divertor 75% leaves as direct heat flux to the divertor target plates with the rest in the form of radiation. In the inboard divertor, however, 75% of the power leaves as radiation with the rest as divertor target plate heat flux and charge exchange losses. The in: out ratio remains constant over a range of injected power and plasma current and even as divertor conditions change due to gas puffing.

Ion cyclotron range of frequency heating with hydrogen and helium minority species in Heliotron E

Abstract In ion cyclotron range of frequency heating of Heliotron E, hydrogen and helium 3 were used as minority species. From the charge exchange neutral particle energy measurement, the bulk deuteron temperature was higher in the helium minority case by 20%–40% in the high power region. Helium minority has an advantage in this experiment because its loss region in the velocity space is small, the ion-ion collision cross-section is large and its charge exchange loss ratio is less than that of hydrogen. In this paper, we discuss the comparison of the heating efficiencies and the power loss by escaping particles between the experimental data and the calculation by the Monte Carlo code. The results of the calculation showed that approximately 40% higher efficiency was obtained in the helium minority case. This results agreed with the experimental data in the higher power region.