Session XF1: Perspective in Current Trends and Future of Plasma Science II

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Accurate data for electron and heavy-particle collisions with atoms, ions, and molecules are required for many modelling applications in plasma science. Depending on the particular application and the electron or ion temperature in the plasma, the energy range of the projectile may cover a very large range, from a few meV to many MeV. \\ Since it is virtually impossible to measure all the data needed for state-of-the-art collisional radiative models (CRMs), much of the responsibility for generating sufficiently comprehensive datasets has been put on theory. There is a vast variety of methods available to generate the data, ranging from classical to semi-classical to fully quantal approaches, with the latter based on the first- and sometimes second-order plane-wave or distorted-wave Born approximation as well as non-perturbative close-coupling methods that can be systematically driven to convergence for relatively simple collision problems. For complex targets, such as large molecules, often only comparatively simple methods are available, but then the total (integrated over all scattering angles) cross sections or even rate coefficients (integrated over the collision energies with some prescribed weight function) are usually required. Clearly, estimating the uncertainty in such calculations is essential, as discussed in detail in Ref.~[1]. It is worth noting that experimental data, too, have uncertainties. Especially when it comes to absolute cross sections, these uncertainties may be difficult to quantify, certainly more difficult than, for example, the relative energy or angular dependence of a particular cross section.\\ For some of these methods, computer codes of vastly varying complexity are publicly available. In all but the simplest cases, running these codes is far from trivial for non-experts. Hence, many databases exist around the world, in which the original data (energy levels, oscillator strengths, cross sections) are stored and utility codes are provided to extract the data and perform calculations of the parameters of interest for the modeler. One of many such databases is LXCat~[2], which is widely used for modelling electron collisions in low-temperature plasmas.\\ In this contribution, I will give a (necessarily incomplete) overview of what is currently available, both regarding the methods and the resulting data. Recently, the idea of machine-learning to generate new data from and/or assess the accuracy of existing datasets has been put forward. Time permitting, I may discuss some of these ideas. \\ ~[1] H.K. Chung {\it et al.}, Journal of Physics D {\bf 49} (2016) 363002. [2] https://us.lxcat.net/home/   

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The development of novel positron traps and beams has enabled new investigations with antimatter. This talk will discuss recent successes, the tools that enabled them, and prospects for further progress. Antiparticles are of interest for a range of applications, including fundamental tests of gravity and the symmetries predicted by field theories (e.g., CPT), astrophysical processes, and the characterization of materials. However, unlike electrons, positrons are scarce (e.g., currents of picoamps instead of amps) and must be used efficiently. This, and the need to keep positrons isolated from ordinary matter, has motivated the development of new methods to manipulate them in vacuum in the form of single-component plasmas. Three decades of development has enabled specially designed electromagnetic traps for long-term (e.g., weeks or more) antimatter confinement, cryogenically cooled and high-density plasmas, finely focused beams, methods to deliver large bursts and/or short temporal bursts of antiparticles, and guided positronium (Ps) atom beams.\footnote{J. Fajans and C. M. Surko, {\it Phys. Plasmas} {\bf 27}, 030601 (2020).}$^,$\footnote{D. B. Cassidy, {\it Euro. Phys. J. D} {\bf 72}, 53 (2018).} Scientific progress includes the creation and study of antihydrogen; formation of the positronium molecule (the first many-electron, many-positron state, e$^+$e$^-$e$^+$e$^-$); and understanding Feshbach-resonances in positron annihilation and positron-molecule bound states.\footnote{J. R. Danielson, et al., {\it Rev. Mod. Phys.} {\bf 87}, 247 (2015).} Outstanding challenges will be discussed. One goal is study of many-body physics in the electron-positron system. Prospects and progress on this topic will be discussed in both the classical and quantum regimes: a positronium-atom Bose-Einstein condensed gas (BEC) and a classical ``pair'' (i.e., e$^+$- e$^-$) plasma.$^{4,}$\footnote{T. S. Pedersen, et al., {\it New J. Phys.} {\bf 14}, 035010 (2012).}   

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Non-equilibrium plasma (NEP) has a remarkable and impressive history of a series of important applications supported by foundational physics understanding. Plasma is an enabling technology in one of the most important current and future strategic industries: manufacturing integrated circuits. This technology is at the heart of the on-going revolution in communication, information processing and artificial intelligence. Future extensions to quantum materials and quantum devices appear promising and well worthy of investment in fundamental and applied plasma research. A second strategic area of emerging importance in NEP science is healthcare. Obviously, infectious disease has not been fully eradicated and plasma has a role to play in this grand challenge. There is a need for a new focus on the plasma science associated with infection control and associated biomedical applications. Finally, the decade of the 2020s will almost certainly see a rapid rise in actions to develop a more sustainable global society. The fact that chemically active plasma can be powered by renewably generated electricity makes it an attractive technology for a number of thorny problems involving large scale material and chemical processing. Recent advances in high performance computing, data science, machine learning and advanced control in the context of NEP encompass all of these emerging applications. In each of these areas, and probably others as well, NEP can and will play an important role. The sometimes bewildering diversity of topics associated with NEP applications demands strong multi-disciplinary collaborations, and this can be a significant challenge. All things considered, it is a good time to be in the field of NEP science and technology!