Abstract
Neutral particle analyzers (NPA) detect neutralized energetic particles that escape from plasmas. Geometric factors relate the counting rate of the detectors to the intensity of the particle source. Accurate geometric factors enable quick simulation of geometric effects without the need to resort to slower Monte Carlo methods. Previously derived expressions [G. R. Thomas and D. M. Willis, "Analytical derivation of the geometric factor of a particle detector having circular or rectangular geometry," J. Phys. E: Sci. Instrum. 5(3), 260 (1972); J. D. Sullivan, "Geometric factor and directional response of single and multi-element particle telescopes," Nucl. Instrum. Methods 95(1), 5-11 (1971)] for the geometric factor implicitly assume that the particle source is very far away from the detector (far-field); this excludes applications close to the detector (near-field). The far-field assumption does not hold in most fusion applications of NPA detectors. We derive, from probability theory, a generalized framework for deriving geometric factors that are valid for both near and far-field applications as well as for non-isotropic sources and nonlinear particle trajectories.
Highlights
Neutral particle analyzers (NPA) detect neutralized energetic particles (EP) that escape from plasmas
A geometric factor relates the particle source intensity and the counting rate of the detector by a multiplicative scale factor that depends on the detector geometry
II we show that previously derived expressions1,2 for the geometric factor, while perfectly adequate for astrophysical systems, cannot be applied to NPA detectors due to near-field effects
Summary
Neutral particle analyzers (NPA) detect neutralized energetic particles (EP) that escape from plasmas. Previous expressions for geometric factors can only be applied to systems with isotropic particle sources that move in straight lines. For an isotropic distribution of incident particles, the total geometric factor of the detector is given by θm fg = 2π S(θ ) sin θ cos θ dθ, (2)
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