Optimal geometry for neutral-beam-based optical diagnostics in tokamaks

The key to obtaining good spatial resolution for neutral-beam-based optical diagnostics in tokamak plasmas is that the narrow optical sightline must cross the wide neutral beam path at a point where the sightline is tangent to the toroidal direction. For a single one-dimensional pinhole camera arrangement, this condition can be met at two points of interest in the plasma, such as the magnetic axis and the plasma edge. The camera location required to provide optimized spatial resolution is calculated. We find that the camera must be placed closer to the plasma edge as the neutral beam is aimed more tangentially.