Multi-diagnostic characterization of neutrals in the confined region of DIII-D using interpretive DEGAS2 simulations

Le résumé fourni par la source

Abstract Measurements from multiple diagnostics are combined to constrain the density of neutrals in the confined plasma and improve our understanding of edge particle sources. Passive D α emission spectrum measurements are obtained along tangential views at the plasma midplane and near the X-point. Spectral D α measurements provide a strong constraint on the neutral population through energy information in the wavelength distribution of emission. The two-dimensional distribution of neutrals is calculated using interpretive DEGAS2 neutral transport simulations with a plasma background based largely on 1D profiles and magnetic equilibrium reconstruction. DEGAS2 is used as a forward model to predict the emission measured along various lines of sight. We demonstrate two approaches for calibrating DEGAS2 simulations to match spectral emission measurements: (1) fitting the strength of neutral sources at the simulation boundary, and (2) optimizing the plasma background in the pedestal/SOL. Traditional filter-based measurements of D α and Ly α emission are used to validate the calibrated DEGAS2 case at multiple poloidal locations. Once calibrated, DEGAS2 is able to match the measurements generally within a factor of 2, garnering confidence in our diagnostic models and the physics included in DEGAS2. The experimentally constrained 2D neutral distribution is used to quantitatively study particle transport. We report the flux surface averaged neutral density and (main ion) particle source for a standard DIII-D H-mode plasma. In the pedestal, the particle source is found to be primarily driven by divertor neutral sources (recycling). However, the neutrals originating in the main chamber play a significant role further inside the plasma. We find the global (main ion) particle confinement time to be τ p , D + ≈ 130 ms ( τ p , D + ≈ τ E / 2 ). Finally, we present evidence for poloidal asymmetries in the plasma, including decreased main ion temperature above the X-point, and a high density region above the inner target.

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