MeV-range velocity-space tomography from gamma-ray and neutron emission spectrometry measurements at JET

Abstract

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Recent progress in fast-ion diagnostics for magnetically confined plasmas

“…Velocity-space tomography is a method to obtain a 2D image of the fast-ion-velocity distribution function based on the available fast-ion measurements (Egedal and Bindslev 2004;Salewski et al 2011Salewski et al , 2012Salewski et al , 2013Salewski et al , 2014bSalewski et al , 2015bSalewski et al , 2016bSalewski et al , 2017Salewski et al , 2018Geiger et al 2015a;Weiland et al 2016;Jacobsen et al 2016a, b; Jaulmes et al 2016; Rasmussen et al 2016). This 2D image is the best useful fit to all measurements from the available diagnostics.…”

“…The figure is taken from Salewski et al (2015a) ill-conditioned, noise in the measurements would be strongly amplified in the calculation of F from S. This makes it necessary to add some form of regularization to the problem as is done in the standard position-space tomography. To date, velocity-space tomography has employed the standard inversion methods singular value decomposition (SVD), the maximum entropy method, and variants of the Tikhonov regularization (Salewski et al 2014b(Salewski et al , 2015bWeiland et al 2016;Jacobsen et al 2016a;Salewski et al 2016b).…”

“…The installation of additional fast-ion diagnostics is a very effective remedy against artefacts (Salewski et al 2012;Weiland et al 2016). High-definition tomography techniques make use of additional prior information one might have which also decreases artefacts and improves the inversions (Salewski et al 2016b). Potent prior information are the measured absence of ions, the position of the injection sources in velocity space, the non-negativity constraint, the smoothness of the velocity distribution function, or, if available, a numeric simulation of the discharge (Salewski et al 2016b).…”

“…High-definition tomography techniques make use of additional prior information one might have which also decreases artefacts and improves the inversions (Salewski et al 2016b). Potent prior information are the measured absence of ions, the position of the injection sources in velocity space, the non-negativity constraint, the smoothness of the velocity distribution function, or, if available, a numeric simulation of the discharge (Salewski et al 2016b). These high-definition tomography techniques improved results for the five-view FIDA diagnostic at the tokamak ASDEX Upgrade and further allow the use of inversion techniques for more common FIDA systems with two or three views.…”

“…Magnetohydrodynamic (MHD) fluctuations are the main cause of fast-ion transport (García-Muñoz et al 2009a). Among others, Alfvén Eigenmodes (AEs) Wong et al (1991); Heidbrink et al (1991); Kimura et al (1998); Berk et al (2001); Sharapov et al (2001); Shinohara et al (2004); Snipes et al (2005); Van Zeeland et al (2006); García-Muñoz et al (2011), Neoclassical Tearing Modes (NTMs) (Zweben et al 1999;Carolipio 2002b;García-Muñoz et al 2007), sawtooth crashes (Kolesnichenko and Yakovenko 1996;Van Zeeland et al 2010;Salewski et al 2016b), and Edge Localized Modes (ELMs) (García-Muñoz et al 2013a, b) can reduce the fast-ion density by up to 50% of the classically expected density. The wave-particle interaction causing this fast-ion transport depends on the nature of the fluctuations and fast-ion orbital characteristics.…”