Spatial transfer function for the beam emission spectroscopy diagnostic on DIII-D

The spatial transfer function for the beam emission spectroscopy (BES) diagnostic is critical to quantitatively interpret local density fluctuation measurements. A three-dimensional geometrical calculation of the spatial transfer function is presented for the upgraded BES diagnostic on DIII-D to determine its spatial resolution and wave-number sensitivity. The spatial transfer function calculation for the BES system on DIII-D incorporates the high speed (f2) collection optics, an optical fiber bundle, neutral beam-sight line geometry, the neutral beam cross-section intensity profile, magnetic field pitch angle, as well as atomic physics of the finite atomic transition time of the collisionally excited beam atoms. The resulting imaged volumes for each BES channel typically have ∼1-2 cm radial and poloidal resolutions. In addition, the viewing volume is nominally aligned along a magnetic field line to minimize spatial smearing of the field-aligned turbulent eddies. This calculation is crucial for performing the proper deconvolution of measured signals to determine correlation lengths and wave-number spectra of the measured density fluctuations, and is also required for applying synthetic diagnostics to nonlinear turbulence simulation codes (e.g., GYRO) to relate code output to measured signals.