Scenario development for the measurement of Helicon waves with Phase Contrast Imaging on DIII-D

An optimized plasma scenario has been developed to validate the predictions for Helicon waves in DIII-D[1] by the All ORders Spectral Algorithm (AORSA) full wave code[2] against measurements from a phase contrast imaging (PCI) diagnostic. AORSA includes the hot-plasma effects needed to compute the density perturbations for the synthetic PCI diagnostic in the helicon wave frequency regime. Modeling of the PCI measurement requires prediction of the wave path in 3D as it propagates 120 degrees around the torus and inward toward the plasma center. This is accomplished by stacking 11 2D AORSA calculations for different toroidal modes. COMSOL is used to model coupling and propagation through the scrape-off layer and provide a boundary condition represented as a synthetic antenna in AORSA. Analysis with a synthetic diagnostic will be presented showing that if the intersection between the Helicon and the PCI beam occurs at the midplane of the plasma, the predicted signal level is orders of magnitude larger than the expected turbulent density fluctuation background. Optimizing the discharge for PCI measurement of the Helicon wave is consistent with previously achieved DIII-D parameters.

[1] R. Prater et al, Nucl. Fusion 54, 083024 (2014).

[2] E. F. Jaeger et al, Phys. Plasmas 15, 072513 (2008).