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Charge exchange in ion-ion collisions
Published in S Svanberg, C-G Wahlström, X-ray Lasers 1996, 2020
In addition to the established pumping schemes for x-ray lasers like collisional excitation and three-body-recombination non-resonant charge exchange reactions are possible which could lead to selective population of excited levels. In general, non-resonant charge exchange collisions are described by the following scheme: AZ++BZ'+⇒A(Z−m)+*+B(Z'+m)++Δε where the colliding ions A and B have charges Z, Z’ > 0. During the collision m electrons change from ion B to ion A which is in an excited state after this. Δε is a small energy defect. For certain collision processes theoretical calculations predict large cross sections above 10-16cm2 in spite of the strong Coulomb repulsion of both ions. In colliding plasmas, such processes could become dominant if the relative velocities of the ions are high enough. To investigate charge exchange reactions between highly charged ions and ions of lower ionisation state, the interaction zone of two, colliding plasmas with different temperatures is observed.
Ion Implantation
Published in Robert Doering, Yoshio Nishi, Handbook of Semiconductor Manufacturing Technology, 2017
Michael Ameen, Ivan Berry, Walter Class, Hans-Joachim Gossmann, Leonard Rubin
A common cause of implant dose errors is the charge exchange between ions in the beam, and atoms and molecules of residual gas in the implanter beamline [130]. This phenomenon is especially important in implanters where the total beam power into a photoresist coated wafer results in significant pressure rise in the implant chamber. Here, high current and high energy implanters are the most susceptible to dose error. Mid-current implanters have traditionally not been as susceptible but with the drive for increased implanter productivity, many mid-current machines now provide beam currents where pressure induced dose error are becoming manifest. Charge exchange can result in the neutralization of the ionized particle (for low and medium energy ions) or an increase in the ion charge state due to electron stripping (for high energy ions). Since virtually all implanters use accumulated ion charge measured by a Faraday detector to measure accumulated dose, changes in charge state will result in dose errors, e.g., neutralized ions will not be detected and over-dosing will occur. The momentum and energy of the accelerated dopant atom remains virtually unchanged from such interactions which implies that charge exchange interactions that occur upstream of the mass analysis magnet will be removed. However, interactions that occur downstream of the mass analysis magnet can result in dose as well as energy errors. For those implanters that do not use post-analysis ion acceleration, the deviations are confined to dose errors. For those implanters that use post-analysis acceleration or decelaration, both energy and dose errors may occur. It is also noted that systems which use magnetic or electrostatic beam parallelizing components (see Section 7.4.6.1 of this chapter) are subject to dose shifts across the wafer as well as the implantation of off-angle components. A common problem with ion implanters that use post-analysis deceleration to produce very shallow p-n junctions, is the contribution from more energetic fast neutrals that yields an unwanted deep tail to the implant profile.
Reduced graphene oxide foils for ion stripping applications
Published in Radiation Effects and Defects in Solids, 2019
L. Torrisi, L. Silipigni, V. Havranek, M. Cutroneo, A. Torrisi, G. Salvato
The principles on which the stripper foils are based concern the processes of ionization, charge exchange and ion recombination. The first process has a cross section decreasing with the ion energy for ions having energy above the ionization potential (25). The process of charge exchange depends on the ion collisions with electrons and on the cross sections of single electron loss σ i, i+1 and single electron capture σ i, i−1 by the energetic ions crossing the thin stripper foil. Thus, the initial charge state, the ion atomic number, its velocity and penetration in the stripper foil material influence strongly the charge exchange process (26). The ion recombination phenomenon is due to the fast electron-ion recombination occurring through the photoionization and electronic recombination processes. Generally, its cross section is high at low ion energy and decreases increasing the ion energy (27). Thus, the transmitted charge state by the thin irradiated stripper foil is the result of an ionization balance in radiatively and collisionally ionized plasmas that involves many parameters such as the incident ion mass and its velocity, the stripper foil electron density, its electrical conductivity and its thickness.
Ion Fluxes and Neutral Gas Ionization Efficiency of the 100-kW Light-Ion Helicon Plasma Source Concept for the Material Plasma Exposure eXperiment
Published in Fusion Science and Technology, 2019
J. F. Caneses, P. A. Piotrowicz, T. M. Biewer, R. H. Goulding, C. Lau, M. Showers, J. Rapp
In Ref. 11, we discuss the conditions needed in Proto-MPEX for effective electron and ion heating: high-density plasmas (2 × 1019 to 6 and low neutral gas pressures (0.02 to 0.05 Pa) in the heating sections. The presence of higher neutral gas pressures leads to excessive collisional losses, to plasma production instead of heating, and, in the case of ion heating, to charge exchange interactions. Through the use of differential pumping systems, this “optimal” mode of operation has been attained in Proto-MPEX (Ref. 11). In fact, it was under these operating conditions that the first evidence of electron heating was observed in Proto-MPEX (Ref. 12). In this paper, we investigate quantitatively the performance of the Proto-MPEX plasma source and answer questions such as the following: How much of the input neutral gas is converted into plasma? What is the ionization cost for this source? How much plasma in ions per second is delivered at the target? What aspects of the source can be improved?
Modeling the Edge-Plasma Interface for Liquid-Lithium Walls in FNSF
Published in Fusion Science and Technology, 2019
M. E. Rensink, T. D. Rognlien, C. E. Kessel
The velocities for the neutrals are obtained from momentum equations. For hydrogenic (DT) neutrals, ion-atom charge exchange is the dominant collision process, and for high DT density detached plasmas, this is a controlling mechanism for atom transport. In that case, the parallel momentum equation as in Eq. (2) is solved but without any electric field or thermal force terms. The ion-neutral friction term becomes , where nDT is the hydrogenic ion density.