Magnetic field pitch angle measurement using Beam Emission Spectroscopy (BES) diagnostic on DIII-D

The magnetic field pitch angle is defined by the slope of the ratio between the poloidal and toroidal magnetic field, α=arctan(B_p/B_t). Multi-point time delay estimation can been applied for calculating the α profile if the line-of-sight is separated toroidally and poloidally. The inclination of turbulent structure is referred to the pitch angle by measuring the propagation of density fluctuation. The Beam Emission Spectroscopy (BES) diagnostic measures the two-dimensional density fluctuation by volume sampling the Doppler-shifted Balmer-alpha beam emission. The measured intensity is a convolution of the emission along the line-of-sight where the spatial resolution depends on the point spread function (PSF), and the observed volume is determined by the intersection of the neutral beam volume and the collection optics. On DIII-D, the tangential neutral beam gives rise to a toroidal separation for the BES measurement, which enable us to calculate the pitch angle assuming a Gaussian distribution of the beam emission in the observed volume. The pitch angle image in the plasma edge region is measured. The α images during the L-H transition and ELM are presented.