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Description
The edge velocity-space distribution of the fast ions can play a significant role in the access of some high-confinement modes via an additional plasma rotation or by possible mutual interaction with the edge MHD modes. In addition to their charge-exchange (CX) collisions with main neutral atoms D, edge fast ions collide with working-gas molecules D2. The reactions produce fast neutrals which can escape plasma being measured by a neutral particle analyzer (NPA) or can produce fast-ion D-alpha (FIDA) light. The density of molecules can be an order of magnitude higher than the atomic density outside the
separatrix and it drops faster in comparison with atoms [1]. Therefore, we expect molecules to be a main source of NPA or FIDA signal outside the separatrix and comparable with atoms close to the separatrix. This source of fast neutrals is newly added to the synthetic diagnostic code FIDASIM [2].
An impact on the edge passive FIDA signal is demonstrated on the dataset measured on the DIII-D tokamak. Fast ion tracing code EBdyna [1] includes CX fast ion losses with background neutral atoms and molecules. Therefore we use EBdyna to provide an edge fast ion distribution as input to FIDASIM. Code KN1D [3] provides 1D background neutral density profiles, separately for atoms and molecules of the working gas, an input for EBdyna and FIDASIM. To implement the new cross-sections into FIDASIM, we used measured data from [4, 5] for the beam-target CX cross-sections of fast protons differentiated by principal quantum number. The CX tables are supplemented with the theoretical data [6] when there is a lack of available measured ones.
References
[1] Jaulmes F, Zadvitskiy G, Bogar K et al. 2021 Nuclear Fusion 61 046012
[2] Geiger B, Stagner L, Heidbrink W et al. 2020 Plasma Physics and Controlled Fusion 62 105008
[3] Bombard B 2001 KN1D: A 1-D space, 2-D velocity, kinetic transport algorithm for atomic and molecular hydrogen in an ionizing plasma
[4] Barnett C F, Hunter H T, Fitzpatrick M I et al. 1990 NASA STI/Recon Technical Report N 91 13238
[5] Hughes R H, Dawson H R and Doughty B M 1967 Phys. Rev. 164(1) 166–170
[6] Plowman C T, Abdurakhmanov I B, Bray I et al. 2022 The European Physical Journal D 76 31
Acknowledgement
This work has been carried out within the framework of the EUROfusion Consortium,funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200—EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.Work supported by US DOE under DE-SC0020337 and DE-FC02- 04ER54698.
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