Resonant Charge Exchange Research Articles

Two-Electrode Ion-Extraction System of a Radio-Frequency Ion Source: Numerical and Experimental Studies of Erosion of the Accelerating Electrode

The results of numerical and experimental studies of the erosion rate of the accelerating electrode of an ion thruster are presented. When the ion beam propagates beyond the edge of the thruster, the process of resonant charge-exchange leads to the formation of slow ions in the engine volume. Under the action of negative electric potential of the accelerating electrode, ions bombard the electrode surface which faces the outgoing beam. Erosion due to the bombardment of the accelerating electrode by charge-exchange ions is the main factor limiting the ion-thruster lifetime. A 25-layer coating of optically contrasting copper and chromium layers, which is applied to the outer electrode surface, is used to diagnose the sputtering rate. The total time of exposure is 14 h. The time dependence of the growing cavity depth is obtained by recording the cavity profile every 3–5 hours. The results obtained in the experiment are used as the initial data for the numerical 3D modeling of multilayer-coating erosion under conditions corresponding to the experiment. Based on the results, the lifetime of the test thruster is estimated at 13 800 hours in respect to the wear of the titanium electrode at its potential of –500 V.

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.

Nearly monoenergetic positive ion beam with self-neutralized space charge extracted from a pulsed plasma

A nearly monoenergetic Ar+ ion beam (100 eV peak energy, 3.5 eV full width at half maximum), with self-neutralized space charge, was extracted from a power modulated (pulsed) inductively coupled plasma (ICP), through a single grounded grid. The beam energy was set by applying a synchronous DC bias voltage to a boundary electrode in contact with the plasma, in the late afterglow of the power pulse. A retarding field energy analyzer and a movable Faraday cup were used to measure the ion energy distribution (IED) and the time resolved ion and electron current densities, as a function of position along the beam axis, to unravel the mechanism of self-neutralization of the space charge of the beam. When a +100 V DC bias was applied to the boundary electrode during the afterglow, a ~100 eV beam (ion density ~109 cm−3) emerged that was space-charge neutralized by periodic injection of electrons, mainly during the early afterglow, into the region downstream of the extraction grid. A background plasma (electron density ~1010 cm−3) was formed by these electrons, in conjunction with relatively low-energy ions (1–10 eV) that flew out of the ICP when the DC bias was off, as well as formed by resonant charge exchange of the 100 eV beam ions with the background Ar in the downstream region near the extraction grid. The behavior of the beam IED as a function of pressure and power of the ICP as well as the timing of application of the DC bias voltage was also studied.

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.

Ionospheric O+ Momentum Balance Through Charge Exchange With Thermospheric O Atoms

AbstractOne of the most notorious uncertainties in ionosphere/thermosphere physics concerns the cross section for resonant charge exchange between neutral atomic and ionized oxygen, O and O+, the principle constituents between ∼200–500 km in the terrestrial atmosphere. O − O+ charge exchange plays a vital role in both momentum and energy exchange between the thermosphere and ionosphere, such that the value of the cross section, , strongly influences calculations of plasma drift speeds, diffusion coefficients, and electron density distributions. We present an analysis of the nighttime O+ momentum budget in the F region ionosphere, using an unprecedented 27‐year baseline of observations from Arecibo Observatory, as a means to assess the agreement between the data, recent theoretical calculations of , and NRLMSISE‐00 model predictions of O density, [O]. We find evidence for local time, seasonal, and solar cycle variation between the derived and modeled O − O+ collision frequency, with the best agreement observed near midnight at solar maximum. The data overall support our conclusion that recent theory regarding the magnitude of is likely accurate. Although biases in the momentum balance technique may account in part or in full for the local time and seasonal dependencies in observed O+ momentum imbalances, the most likely source of the observed solar cycle variation is MSIS model underestimation of [O]. These findings serve to establish the O+ momentum and energy balance techniques as a valuable means of remotely sensing thermospheric [O] in support of future model validation and development.

Velocity distribution of titanium neutrals in the target region of high power impulse magnetron sputtering discharges

The velocity distribution function of titanium neutrals in the target region of a high power impulse magnetron sputtering discharge was investigated by optical emission spectroscopy. A high-resolution plane grating spectrograph combined with a fast, gated, intensified CCD camera was used to study the shape of selected optical emission lines. Doppler broadening and shift were analyzed to gain information about the velocity distribution of sputtered titanium neutrals. The velocity distribution function was found to depend on the discharge power for target power densities up to 0.6 kW cm−2. Above that value, the velocity distribution was constant. The collision processes of sputtered neutrals close to the target were found to be describable using a modified version of the Krook collisional operator. Using this interpretation, evidence for strong scattering of the titanium neutrals in the target region was found. This scattering can be explained by resonant charge exchange with previously scattered titanium ions.

Extraction of neutral particles from a gas discharge

In this paper a version of the extraction of neutral particles from a gas discharge using a gas-distributing disk is proposed, which allows creating a pressure drop between the source volume and the working zone of the vacuum chamber.The gas distributor ensures a uniform supply of gas to the ionization zone and resonant charge exchange, which will make it possible to achieve uniform combustion of the discharge in the entire volume of the source. Also the gas distributor allows to increase the speed of gas flow, due to pressure drop.An increase in the rate of gas flow in the volume of the source makes it possible to increase the number of ionization events per unit of time. Consequently, it makes it possible to increase the number of ions generated in the gas discharge, that is, the ionization coefficient of the working gas increases. In turn, an increase in the number of ions generated in the discharge leads to an increase in the number of neutral atoms in the output beam.

The accuracy of collision cross sections in particle modeling on copper vacuum arcs

Direct Simulation Monte Carlo and Particle-in-Cell (DSMC-PIC) method is a powerful tool in studying metal vapor vacuum arcs (VA). However, the collision cross sections (Xs) for metal atoms/ions are not evaluated yet, compared to the already developed databases for gas discharges. The Xs are subject to uncertainties which influence the accuracy and predictive ability of particle models. In this paper, the common electrode material copper (Cu) is taken as an example for survey of Xs. Various experimental data, theoretical calculation, and analytical fitting formula are reviewed. The electron-impact Cu elastic, excitation, and ionization Xs are found to have an overall uncertainty within 15%. The Cu-Cu+ resonant charge-exchange Xs are not as accurate as that of e-Cu collision. The Cu-Cu interaction Xs by variable hard sphere model requires further validation. Based on the evaluated Xs, the mean free path and collision frequency are derived in copper VA plasmas, and compared to the Debye length and plasma frequency, respectively. The influences of those spatial and time scales are discussed in developing DSMC-PIC codes for modeling of copper VA.

Cooling of trapped ions by resonant charge exchange

The two most widely used ion cooling methods are laser cooling and sympathetic cooling by elastic collisions (ECs). Here we demonstrate another method of cooling ions that is based on resonant charge exchange (RCE) between the trapped ion and the ultracold parent atom. Specifically, trapped Cs$^{+}$ ions are cooled by collisions with co-trapped, ultracold Cs atoms and, separately, by collisions with co-trapped, ultracold Rb atoms. We observe that the cooling of Cs$^{+}$ ions by Cs atoms is more efficient than cooling of Cs$^{+}$ ions by Rb atoms. This signals the presence of a cooling mechanism apart from the elastic ion-atom collision channel for the Cs-Cs$^{+}$ case, which is cooling by RCE. The efficiency of cooling by RCE is experimentally determined and the per-collision cooling is found to be two orders of magnitude higher than cooling by EC. The result provides the experimental basis for future studies on charge transport by electron hopping in atom-ion hybrid systems.

Incoherent Scatter Spectra Based On Monte Carlo Simulations of Ion Velocity Distributions Under Strong Ion Frictional Heating

AbstractUnder strong electric field conditions often found at high latitudes, the ion velocity distribution of the weakly ionized F region plasma can differ enough from a Maxwellian shape to substantially change incoherent scatter (IS) spectra and thus the analysis of those spectra. With the goal to provide a quantitative and reliable description of the IS spectra, this study directly uses for the first time an advanced Monte Carlo calculation of the ion velocity distribution to derive IS spectra for a range of electric fields and aspect angles. For most cases the spectra associated with NO+ maintains a shape that closely resembles that of a spectrum derived from a Maxwellian distribution with the same line‐of‐sight ion temperature as the equivalent Monte Carlo simulated distribution. This study also fully characterizes the spectral shape as well as the ion temperature and its anisotropy for two different models of the resonant charge exchange between O+ and O. It confirms that the distortions from the Maxwellian shape can be substantial for this particular interaction. The distortions are also such that along the magnetic field direction, the extracted apparent electron temperature is always greater than the real temperature. This work also includes a determination of the stability of the plasma against magnetic field‐aligned electrostatic instabilities. It is found that the NO+ distribution is always stable, whereas the O+ distribution may or may not be stable, depending on the model chosen for the resonant charge exchange cross section in collisions with the background atomic oxygen gas.

Effect of Elastic Collisions on the Ion Distribution Function in Parent Gas Discharge Plasma

An analytical solution is obtained for the Boltzmann kinetic equation for ions in the plasma of its gas with allowance for the processes of resonant charge exchange and elastic ion scattering on the atom. The cross section of differential elastic scattering was assumed to be isotropic in the system of the mass center, and the resonant charge exchange process is independent of the elastic scattering. It is shown that the ion velocity distribution function is determined by two parameters and differs significantly from the Maxwellian one. The allowance for elastic scattering with these assumptions leads to a change in the ion angular distribution and also to a decrease in the average ion energy due to the transfer of part of the ion energy to atoms upon elastic collisions. The calculated values of the drift velocity, the average energy, and the coefficient of transverse diffusion of He+ ions in He, Ar+ ions in Ar are compared with the known experimental data and the results of Monte Carlo calculations; they show good agreement.

AEgIS at ELENA: outlook for physics with a pulsed cold antihydrogen beam.

The efficient production of cold antihydrogen atoms in particle traps at CERN's Antiproton Decelerator has opened up the possibility of performing direct measurements of the Earth's gravitational acceleration on purely antimatter bodies. The goal of the AEgIS collaboration is to measure the value of g for antimatter using a pulsed source of cold antihydrogen and a Moiré deflectometer/Talbot-Lau interferometer. The same antihydrogen beam is also very well suited to measuring precisely the ground-state hyperfine splitting of the anti-atom. The antihydrogen formation mechanism chosen by AEgIS is resonant charge exchange between cold antiprotons and Rydberg positronium. A series of technical developments regarding positrons and positronium (Ps formation in a dedicated room-temperature target, spectroscopy of the n=1-3 and n=3-15 transitions in Ps, Ps formation in a target at 10 K inside the 1 T magnetic field of the experiment) as well as antiprotons (high-efficiency trapping of [Formula: see text], radial compression to sub-millimetre radii of mixed [Formula: see text] plasmas in 1 T field, high-efficiency transfer of [Formula: see text] to the antihydrogen production trap using an in-flight launch and recapture procedure) were successfully implemented. Two further critical steps that are germane mainly to charge exchange formation of antihydrogen-cooling of antiprotons and formation of a beam of antihydrogen-are being addressed in parallel. The coming of ELENA will allow, in the very near future, the number of trappable antiprotons to be increased by more than a factor of 50. For the antihydrogen production scheme chosen by AEgIS, this will be reflected in a corresponding increase of produced antihydrogen atoms, leading to a significant reduction of measurement times and providing a path towards high-precision measurements.This article is part of the Theo Murphy meeting issue 'Antiproton physics in the ELENA era'.

Determination of Anisotropic Ion Velocity Distribution Function in Intrinsic Gas Plasma. Theory.

The first seven coefficients of the expansion of the energy and angular distribution functions in Legendre polynomials for Hg+ ions in Hg vapor plasma with the parameter E/P ≈ 400 V/(cm Torr) are measured for the first time using a planar one-sided probe. The analytic solution to the Boltzmann kinetic equation for ions in the plasma of their parent gas is obtained in the conditions when the resonant charge exchange is the predominant process, and ions acquire on their mean free path a velocity much higher than the characteristic velocity of thermal motion of atoms. The presence of an ambipolar field of an arbitrary strength is taken into account. It is shown that the ion velocity distribution function is determined by two parameters and differs substantially from the Maxwellian distribution. Comparison of the results of calculation of the drift velocity of He+ ions in He, Ar+ in Ar, and Hg+ in Hg with the available experimental data shows their conformity. The results of the calculation of the ion distribution function correctly describe the experimental data obtained from its measurement. Analysis of the result shows that in spite of the presence of the strong field, the ion velocity distribution functions are isotropic for ion velocities lower than the average thermal velocity of atoms. With increasing ion velocity, the distribution becomes more and more extended in the direction of the electric field.

Nonstationary Model of a Low-Density Plasma Jet Ejected from a Stationary Plasma Thruster

A time-dependent problem on finding the parameters of a low-density plasma jet ejected from a stationary plasma thruster is analyzed. In contrast to earlier works in which a stationary problem was solved using a Crook-type kinetic model, a set of kinetic equations is derived at the accuracy level accepted in the kinetic theory with allowance for resonant charge-exchange interaction between ions and neutrals. A method to solve the obtained set of equations is developed and used to perform test simulations, some results of which are presented.

Resonant charge exchange for H–H+ in Debye plasmas

The dynamics of resonant charge exchange in proton–hydrogen collisions embedded in plasma is investigated in the framework of the asymptotic approach, modified to account for the effect of Debye–Huckel screening in particle interactions. The cross sections exhibit a marked dependence on the Debye length in regimes of severe plasma confinement. Processes involving excited states H(n)–H+ are also discussed.

Extreme plasma convection and frictional heating of the ionosphere: ISR observations

AbstractExtremely elevated ion temperatures observed with the Poker Flat and Resolute Bay incoherent scatter radars in the E and F regions of the polar ionosphere are discussed. Our observations include one‐dimensional, line‐of‐sight ion temperatures (Ti,1d) that at times can rise to up to ∼8000°K. While examining the accuracy of the derived temperatures is difficult due to the several potential sources of error and uncertainty, we find that at altitudes of ∼130–250 km the line‐of‐sight ion temperatures obtained at relatively small aspect angles (for instance, at 22.5° away from the magnetic field direction) are below the expected values for the average or three‐dimensional ion temperatures (Ti). We find that this difference matches well with the theoretical expectation from anisotropic ion velocity distributions that emerge from polarization elastic scattering collisions between NO+ and N2. These results are therefore the clearest detection of anisotropic ion velocity distributions with the Poker Flat Incoherent Scatter Radar and Resolute Bay Incoherent Scatter Radar facilities. Moreover, at higher altitudes (>∼300 km), where the resonant charge exchange collisions prevail, and for a very high ion temperature event (Ti∼8000°K), we observe that the incoherent scatter radar (ISR) spectra obtained at large aspect angles (e.g., 55°) look like those expected from non‐Maxwellian plasma. At similar altitudes, the measured one‐dimensional temperatures at an aspect angle of 22.5° are found to exceed the expected values from the frictional heating and resonant charge exchange collisions by about 1000°K. A portion of this offset is thought to reflect the role of Coulomb collisions.

Ion velocity distribution function in intrinsic gas at cryogenic gas temperatures

The influence of elastic scattering on the ion distribution function in the plasma of an intrinsic gas in weak fields has been considered. An analytical expression valid for cryogenic temperatures of atoms has been obtained. The reduced He+–He, Ar+–Ar mobilities as functions of the temperature of atoms in a range of 4–1000 K have been calculated in the approximation of the zero field taking into account elastic collisions; the calculated dependences well agree with the available experimental data. It has been demonstrated that elastic collisions play an important role in the formation of the ion distribution function at low temperatures. The results of measurement of the ion mobility in the limit of the zero field at low temperatures can be used to obtain data on the ratio of elastic scattering and resonance charge exchange cross sections.

Radiative transitions in quasimolecules: H+H- collisions

The H + H- ⟶ H- + H + ħω, H + H- +ħω⟶ H + H + e reactions give examples of bound-bound and bound-free electron transitions in the H + H– quasi-molecules temporally formed during collisions. The present work is aimed at the calculations of spectral profiles produced in the reactions of resonant charge exchange H + H-. The oscillator strength of photodetachment of quasimolecules has been calculated for the first time.

Large numbers of cold positronium atoms created in laser-selected Rydberg states using resonant charge exchange

Lasers are used to control the production of highly excited positronium atoms (Ps*). The laser light excites Cs atoms to Rydberg states that have a large cross section for resonant charge-exchange collisions with cold trapped positrons. For each trial with 30 million trapped positrons, more than 700 000 of the created Ps* have trajectories near the axis of the apparatus, and are detected using Stark ionization. This number of Ps* is 500 times higher than realized in an earlier proof-of-principle demonstration (2004 Phys. Lett. B 597 257). A second charge exchange of these near-axis Ps* with trapped antiprotons could be used to produce cold antihydrogen, and this antihydrogen production is expected to be increased by a similar factor.

Status of the SPI for the JINR Accelerator Complex

The Source of Polarized Ions (SPI) of deuterons and protons with nearly resonant charge-exchange plasma ionizer is under construction at the LHEP, JINR. The [Formula: see text]D[Formula: see text] ([Formula: see text]H[Formula: see text]) current output of the source is expected to be at a level of 10[Formula: see text]mA. The polarization will be up to 90% of the maximal vector (±1) for [Formula: see text]D[Formula: see text] ([Formula: see text]H[Formula: see text]) and tensor ([Formula: see text]1, [Formula: see text]) polarization for [Formula: see text]D[Formula: see text]. Realization of the project is carried out in close cooperation with INR of the RAS (Moscow). The equipment available from the CIPIOS ion source (IUCF, Bloomington, USA) is partially used for the Dubna setup. The new source at the JINR NUCLOTRON accelerator facility will make it possible to increase the polarized deuteron beam intensity up to the level of 10[Formula: see text] [Formula: see text]/pulse.