Dust and powder in fusion plasmas: recent developments in theory, modeling, and experiments

Reviews of Modern Plasma Physics (2022) 6:20 https://doi.org/10.1007/s41614-022-00081-5 REVIEW PAPER Dust and powder in fusion plasmas: recent developments in theory, modeling, and experiments S. Ratynskaia1 · A. Bortolon2 · S. I. Krasheninnikov3 Received: 1 April 2022 / Accepted: 26 June 2022 © The Author(s) 2022 Abstract In this paper, we present a brief historic overview of the research on dust in fusion devices with carbon plasma-facing components and then highlight the most recent developments in the post-carbon era of the field. In particular, we consider how the metallic dust form, mobilize, and interact with fusion plasmas and plasma facing components. Achievements in wall conditioning and associated anomalous plasma transport modification, including ELM suppression, with the powder injection technique is another focus of the paper. Capabilities of the state-of-art simulation tools to describe different aspects of dust in fusion devices are exemplified and new directions for future dust studies are brought forward. Keywords Metallic dust particles · Powder injection · Dust formation · Dust-plasma and dust-vessel wall interactions · Fusion reactors with metallic armour 1 Introduction The documented history of the study of dust-related phenomena in magnetic fusion devices has started about 45 years ago with the paper by Ohkawa (1977). In Ohkawa (1977), the author has assessed the role of dust in core plasma pollution with impurities originated as a result of the vaporization of some “particulates that might have been produces by blistering or abrasion of wall in previous discharges”. The results found in Ohkawa (1977) did not contradict to available, at that time, rather sparse data on impurity fraction in core plasma and it was concluded that “It is important to test the above hypothesis by further experimental observations”. Nearly four * S. Ratynskaia 1 KTH Royal Institute of Technology, Stockholm, Sweden 2 Princeton Plasma Physics Laboratory, Princeton, USA 3 University California San Diego, La Jolla, USA 13 Vol.:(0123456789) 20 Page 2 of 50 Reviews of Modern Plasma Physics (2022) 6:20 Fig. 1  Dust particles collected in TEXTOR tokamak (Winter 1998). Reproduced with the permission from Winter (1998), © IOP Publishing decades later, there is still no consensus on the role of dust in the contamination of plasma with impurity in magnetic fusion devices. Soon after Ohkawa’s paper, “high speed cine photography” was introduced in many magnetic fusion devices to study different processes ranging from plasma interactions with material surfaces surrounding plasma to the “UFO” observations (“UFO” being the “nick-name” for glowing dust particles heated up by plasma). Inspection of the plasma facing components (PFCs) during ventilation events also revealed the presence of a large amount of dust particles of different sizes, shapes, and composition. This marked the beginning of systematic investigation of dust in magnetic fusion devices; in the TEXTOR tokamak (Winter 1998) followed by studies from the JET (Peacock et al. 1999), TFTR, DIII-D, Alcator C-Mod (Carmack et al. 2000), Tore-Supra (Chappuis et al. 2001), and ASDEX-Upgrade (Sharpe 2001; Rohde et al. 2009) tokamaks. The results of such "postmortem" analysis of dust showed a whole "zoo" of dust particles with sizes ranging from sub-mm to 100’s of nm, see Fig. 1. Survey of the postmortem analysis of dust particle collected in different magnetic fusion devices can be found in the review papers (Federici et al. 2001; Rubel et al. 2001; Sharpe et al. 2002; Winter 2004; Krasheninnikov et al. 2011), and the Chapter 5 of the book (Krasheninnikov et al. 2020). Low sensitivity of the “high speed cine photography” and limitations of postmortem analysis to yield information on the dust number density, size distribution, and dynamics in plasma volume called for development of other experimental techniques. It turned out that the signal from the Rayleigh channel of the lasers, typically used in tokamaks for Thomson scattering measurements of plasma density and electron temperature, is a very useful tool for in-situ measurements of both dust particle density and size distribution. First in-situ detection of dust particles with the laser was reported for JIPPT-IIU tokamak (Narihara et al. 1997). This was followed by rather detailed studies of dust density and size distribution in both mid-plane and divertor regions of the DIII-D tokamak (West et al. 2006; West and Bray 2007), see Fig. 2. Later on the laser diagnostic was employed for dust studies in other magnetic fusion devices (Krasheninnikov et al. 2020). Analysis of data of laser dust detection in DIII-D tokamak with the Mie scattering theory, accounting for the dust grain ablation under intense laser radiation, 13 Reviews of Modern Plasma Physics (2022) 6:20 Page 3 of 50 20 Fig. 2  Dust particle density in a low divertor of the DIII-D tokamak. Reprinted from West and Bray (2007), with the permission from Elsevier has shown size distribution close to the power-law, with the exponent 𝛼 ≈ 2.6–2.7 (Smirnov et al. 2007a). Interestingly, similar power-law distribution with 𝛼 ≈ 2.5 was found in the LHD stellarator (Koga et al. 2009) as a result of the analysis of dust particles in the range 10 ÷ 104 nm collected from the PFCs. In-situ dust observations with fast framing CCD cameras, which in the beginning of twenty-first century started to be widely used for different applications in fusion devices, played a pivotal role in the development of both theoretical models and numerical codes focused on deeper understanding of the role of dust in fusion plasmas. In particular, tracking of dust with multiple cameras facilitated reconstruction of 3D dust trajectories (Roquemore et al. 2007), which were used for the benchmarking of dust transport codes. Some trajectories appear to be very peculiar (e.g. see Fig. 3), indicating a complex grain shapes and composition affecting dust dynamics. In addition, fast cameras were used for study of dust creation events, see e.g. Hong et al. (2010). Detailed analysis of imaging diagnostics with fast cameras (Smirnov et al. 2009) has shown that the sensitivity of the cameras does not allow to detect dust grains with the size below ∼ 1 μ m. The description of other diagnostics used for dust studies in fusion plasmas, such as electrostatic and microbalance detectors, dust capturing with aerogel, and others can be found in the reviews (Ratynskaia et al. 2008; Rudakov et al. 2008; Krasheninnikov et al. 2011) and the references therein. Analysis of available experimental data and the need for assessment of dust impact in both contemporary and future devices has prompted progress of theoretical understanding of dust–plasma interaction in fusion plasmas. Although the initial modelling development has greatly benefited from a significant body of work in the field of dusty- or complex-plasmas (e.g. see Tsytovich 1997; Shukla and Mamun (...truncated)