Speaker
Description
Fast neutral beam injection into magnetically confined plasmas produces pairs of fast ions and electrons at their ionization location. While the electrons remain close to the ionization flux surface, fast ions experience substantially larger drift orbits, resulting in a radial displacement between positive and negative charges. This charge separation contributes to the formation of a radial electric field Er [1]. Additional charge separation arises from fast ion losses. Fast ions born inside the confined plasma region may be lost to plasma-facing components, leaving the associated electrons confined within the plasma. Furthermore, charge-exchange (CX) fast ion losses typically occur at larger radii than the original fast ion ionization locations, producing thermal ions and additional radial redistribution of charge. The combined effect of these mechanisms leads to the generation of an NBI-induced radial electric field.
Fast ion ionization sources, distributions, and losses are evaluated using the FIDASIM [2] and EBdyna [3] codes. The resulting radial electric field is estimated using a charge-separation based formulation. Since the radial electric field modifies fast ion trajectories through orbit squeezing effects [4], the calculated Er is subsequently included in a new EBdyna simulation and the radial electric field is re-evaluated. This iterative procedure is repeated several times in order to assess the self-consistent evolution of the fast-ion-induced radial electric field.
The analysis focuses on a QH-mode discharge in the DIII-D tokamak, where plasma access is governed by the E × B shear threshold [5]. Charge-exchange losses are evaluated using a two-dimensional neutral density distribution obtained from DEGAS2 [6] simulations constrained by experimental measurements of background neutrals.
References
[1] Xingyuan Xu, Yingfeng Xu, Xiaodong Zhang, and Youjun Hu. Simulations of the radial electric field induced by neutral beam injection in a tokamak. Nuclear Fusion, 61(8):086002, jun 2021.
[2] Benedikt Geiger, Luke Stagner, William W Heidbrink, Ralph Dux, Rainer Fischer, Yutaka Fujiwara, Alvin Garcia, Asger Schou Jacobsen, Anton Jansen vanVuuren, Alexander N Karpushov, Deyong Liu, Philip Adrian Schneider, Igor Sfiligoi, Peter Zsolt Poloskei, and Markus Weiland. Progress in modelling fast-ion d-alpha spectra and neutral particle analyzer fluxes using fidasim. Plasma Physics and Controlled Fusion, 2020.
[3] F. Jaulmes, G. Zadvitskiy, K. Bogar, M. Imrisek, J. Hromadka, S.Y. Cats, J. Varju, M. Komm, and R. Panek.Modelling of charge-exchange induced nbi losses in the compass upgrade tokamak. Nuclear Fusion, 61(4):046012, 2021.
[4] G.J. Kramer, A. Bortolon, A. Diallo, and R. Maingi. The formation of an radial edge electric field due to finite ion orbit width effects is the possible root cause of the h-mode edge. Nuclear Fusion, 64(10):106035, 2024.
[5] TM Wilks, AM Garofalo, PH Diamond, ZB Guo, JW Hughes, KH Burrell, and Xi Chen. Scaling trends of the critical e× b shear for edge harmonic oscillation onset in diii-d quiescent h-mode plasmas. Nuclear Fusion,
58(11):112002, 2018.
[6] Daren Stotler and Charles Karney. Neutral gas transport modeling with degas 2. Contributions to Plasma Physics, 34(2-3):392–397, 1994.
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 Com-
mission. 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.