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

Abstract

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.

Core Diagnostics for WENDELSTEIN 7-X Steady-State Exploration Until 18 GJ

“…Charge Exchange Neutral Particle Analyzers (CX-NPA) measure the escaping charge exchange neutrals resulting from interaction of a neutral particle beam and hence provide the slowing down distribution of the fast particles in addition to the thermal ion energy distribution. In W7X for geometrical reasons active CX-NPA is not possible at the heating beams and predictive analyses for a dedicated diagnostic beam showed that magnetic configurations with varying FI confinement lead to only small signal differences for the accessible observation geometries there [51]. No further dedicated activities are presently assumed for this project.…”

Review of the NPA Diagnostic Application at Globus-M/M2

“…Neutral particle analyzers (NPAs) are actively used for the ion component study at present tokamaks [1][2][3][4][5][6][7][8][9][10][11][12][13] and stellarators [14][15][16][17][18] and will be utilized at ITER [19,20]. The NPA-based diagnostic utilizes analysis of charge-exchange (CX) fluxes of atoms emitted from the plasma.…”