Impact of Molecules on Edge Passive Fast-ion D-alpha Signals*

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 order higher outside the separatrix and it drops faster in comparison with atoms [2]. 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 [1].

An impact on the edge passive FIDA signal is demonstrated on the dataset measured on the DIII-D tokamak. Fast ion tracing codes EBdyna [2] and Ascot [3] including CX fast ion losses simulate edge fast ion distribution. Code KN1D [4] provides 1D background neutral density profiles, separately for atoms and molecules of the working gas.

To implement the new cross-sections into FIDASIM, we used measured data from [5,6] for the beam-target CX cross-sections of fast protons differentiated by principal quantum number. The CX tables are supplemented with the theoretical data [7] when there is a lack of available measured ones.

[1] B. Geiger, 2020 PPCF 62 105008

[2] F. Jaulmes, 2021 NF 61 046012

[3] P. Ollus, 2022 PPCF 64 035014

[4] B. LaBombard, report, PSFC/RR-01-3, 2001

[5] C.F. Barnett, report, 91:13238, 1990

[6] R. H. Hughes,1967 PRL 164 166

[7] C. T. Plowman, 2022 EPJ D 76(2):31