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A preliminary study investigating capabilities of a planned  but not yet installed neutral particle analyzer  (NPA) [1,2] system combined with a  diagnostic neutral beam at Wendelstein 7-X  (W7-X) [3] is  presented. Additionally two NPAs viewing the neutral beam injection  (NBI) [4] source 8 beam and using it as neutral  source are studied. The main focus is laid on what information about  NBI fast-ion slowing down distributions can be inferred from active  NPA measurements when altering the magnetic configuration, plasma  β, density, electron temperature or radial electric field. For  an order of magnitude estimation of the passive signal a model for  penetration of neutral hydrogen recycling from the first wall is  implemented. For the active signal a diagnostic neutral beam  injector was simulated using FIDASIM [5]. The  fast-ion slowing down distributions were calculated with  ASCOT [6].	  The synthetic NPA signal is found in general to be sensitive to  changes in the fast-ion distribution function. Distinct features can  be seen in the high energy active signal in the high-mirror  configuration when changing β, especially, when looking at  deeply trapped fast particles. However, for the initially planned  installation geometry of the NPA diagnostic most fast-ion  distributions exhibit only small differences in the magnitude and  especially shape. In a high density case in the standard magnetic  configuration with a central β of 8%, the fast-ion density in  the core region is too low to provide a measurable flux of charge  exchanged neutrals. The passive signal from the inner plasma regions  is found to be negligible compared to the active signal.

Feasibility of neutral particle analysis for fast-ion measurements at W7-X