Neutral Atoms in Controlled Fusion and Space Plasmas

Dear Colleagues,

Techniques dealing with fluxes of fast suprathermal neutral particles in plasma are at the frontier of controlled fusion research and space exploration. Neutral beam injection (NBI) is currently the main method for heating fusion plasmas in flagship magnetic confinement devices. Neutral particle analysis (NPA) is the most direct method for diagnosing fast ions in a magnetically confined fusion plasma by detecting the escaping neutral atoms formed of these ions due to charge changing atomic reactions. In astrophysics and planetary science, measurements of this kind are typically referred to as energetic neutral atom (ENA) imaging or sensing of space plasmas.

NBI systems based on positive ion sources are suitable at beam energies around 150 keV or lower. To produce higher energy beams at 500 keV or above, required for enhancing fusion plasma performance, negative ions are used to maintain an optimum neutralization fraction. NBI at 1 MeV for D⁰ beams based on a negative ion source is one of the basic systems required to reach ITER operation goals [1]. The deuterium NBI is the most powerful heating system at JT-60SA, including both positive ion-source-based injection at 85 keV and negative ion-source-based injection at 500 keV [2].

Numerical modeling of NBI and the related diagnostics certainly require the knowledge of cross-sections of all relevant atomic reactions. It is noted, first, that many of the required cross-sections are not well defined, with uncertainties of over 10%, second, that the lack of experimental data in energy regions of interest necessitates some of cross-sections to be extrapolated, and third, that fewer data are available for explicit deuterium reactions; thus, protium cross sections are used [3].

The main NPA purpose on ITER is to measure the deuterium/tritium fuel isotope ratio in the plasma core using energy distributions of escaping D⁰ and T⁰ atoms in the MeV range [4]. Advanced NPA diagnostics in the MeV energy range were used on JT-60U [5] and are foreseen at JT-60SA [6]. Measurements of kinetic energy resolved fluxes of neutral hydrogen and helium atoms are possible. Thus, cross-section data are required for all the atomic reactions that lead to a change of the electric charge state of hydrogen and helium particles in plasma in the presence of impurities. These reactions determine both the source function of neutral atoms within the plasma and the attenuation of the neutral flux in the plasma between the birth location and the periphery. The attenuation of the neutral flux from plasma towards the NPA diagnostic device is governed by the same physics as the penetration of fast atoms from the NBI into plasma.

ENA sensors provide information on composition, spatial, temporal, and energy distributions of plasma particles around the Earth and other planets, at the heliospheric boundary, and in the interstellar medium [7,8]. Analogously, ENAs originate from energetic ions through charge-changing collisions with the constituents of the ambient space plasma of interest.

This Special Issue is dedicated to all aspects of atomic physics, atomic, and molecular data and numerical modeling involved in the development of NBI, NPA, and ENA systems, as well as in theoretical and experimental research work associated with these systems. The purpose is to facilitate the tasks of plasma and fusion scientists. Review articles, original research articles, and data tables are welcome, aiming to extend the renowned sources such as [9] and the subsequent volumes [10] and further develop the approach [11,12]. In addition, multidisciplinary papers illustrating the importance of the subject in both plasma physics and astrophysics would be of interest.

[1] M.J. Singh et al. Heating neutral beams for ITER: negative ion sources to tune fusion plasmas. New J. Phys. 19 055004 (2017) https://doi.org/10.1088/1367-2630/aa639d

[2] G. Giruzzi et al. Physics and operation oriented activities in preparation of the JT-60SA tokamak exploitation. Nucl. Fusion 57 085001 (2017) https://doi.org/10.1088/1741-4326/aa7962

[3] A. Hurlbatt et al. The particle tracking code BBCNI for large negative ion beams and their diagnostics. Plasma Phys. Control. Fusion 61 105012 (2019) https://doi.org/10.1088/1361-6587/ab3c13

[4] M.I. Mironov et al. Sawtooth mixing of alphas, knock-on D, and T ions, and its influence on NPA spectra in ITER plasma. Nucl. Fusion 58 082030 (2018) https://doi.org/10.1088/1741-4326/aab678

[5] Y. Kusama et al. Charge‐exchange neutral particle measurement in MeV energy range on JT‐60U. Rev. Sci. Instrum. 66 339 (1995) https://doi.org/10.1063/1.1146405

[6] N. Aiba et al. JT-60SA Research Plan. JT-60SA Research Unit (2018) http://www.jt60sa.org/pdfs/JT-60SA_Res_Plan.pdf

[7] K.C. Hsieh, E. Möbius. Energetic Neutral Atoms in Space. A Diagnostic Tool for Space Plasmas. World Scientific (2020, in press) https://doi.org/10.1142/11241

[8] E.E. Scime, A.M. Keesee. Enhanced Energetic Neutral Atom Imaging. Front. Astron. Space Sci. 6:9 (2019) https://doi.org/10.3389/fspas.2019.00009

[9] Atomic and Plasma–Material Interaction Data for Fusion No. 1, IAEA, Vienna (1991)

[10] Atomic Data for Fusion, vol. 1, ORNL-6086 (1990)

[11] R.K. Janev et al. Penetration of energetic neutral beams into fusion plasmas. Nucl. Fusion 29 2125 (1989) https://doi.org/10.1088/0029-5515/29/12/006

[12] S. Suzuki et al. Attenuation of high-energy neutral hydrogen beams in high-density plasmas. Plasma Phys. Control. Fusion 40 2097 (1998) https://doi.org/10.1088/0741-3335/40/12/009

Dr. Pavel Goncharov
Dr. Christian Hill
Dr. Kalle Heinola
Guest Editors

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• Neutral beam injection
• Neutral particle analysis
• Energetic neutral atoms
• Atomic data for fusion
• Diagnostics and modeling of fusion plasmas
• Remote sensing of space plasmas