[Paper Review] Experimental Inference of Neutral and Impurity Transport in Alcator C-Mod Using High-Resolution X-Ray and Ultra-Violet Spectra

F. Sciortino, N. T. Howard+11 more|arXiv (Cornell University)|Jul 28, 2021

Magnetic confinement fusion research121 references12 citations

TL;DR

This study presents a novel Bayesian inference framework using high-resolution X-ray (XICS) and extreme ultraviolet (EUV) spectroscopy to experimentally constrain neutral and impurity transport in Alcator C-Mod plasmas. By incorporating full Ca Kα spectral modeling—including satellite lines—and charge exchange between edge neutrals and impurities, it achieves improved accuracy in inferred diffusion and convection profiles, showing good agreement with turbulent transport models at midradius but revealing discrepancies in convection predictions.

ABSTRACT

We present experimental inferences of cross-field impurity transport coefficients for Alcator C-Mod plasmas using a novel forward model for the entire Ca K-alpha spectrum, including satellite lines within the spectral range, to compare to high-resolution X-ray Imaging Crystal Spectroscopy (XICS). These measurements are complemented by Extreme Ultra-Violet (EUV) spectroscopy that constrains transport closer to the edge. Using new atomic data sets for both XICS and EUV analysis has enabled consideration of line ratios across both spectral ranges and has increased the accuracy of inferred transport coefficients. Inclusion of charge exchange between edge thermal neutrals and impurities is shown to be extremely important in C-Mod pedestals. We obtain D atomic neutral densities from experimental D Ly-alpha measurements at the midplane and compare these to SOLPS-ITER simulations, finding good agreement. Bayesian inferences of impurity transport coefficients are presented for L-, EDA H-, and I-mode discharges, making use of the Aurora package for forward modeling and combining our spectroscopic constraints. Experimentally inferred diffusion profiles are found to match turbulent transport models at midradius within uncertainties, using both quasilinear gyro-fluid TGLF SAT-1 and nonlinear ion-scale gyrokinetic CGYRO simulations. Significant discrepancies in convection are observed in some cases, suggesting difficulties in predictions of flat or hollow impurity profiles.

Motivation & Objective

• To improve experimental inference of cross-field impurity transport coefficients in tokamaks by leveraging high-resolution X-ray and EUV spectroscopy.
• To develop a comprehensive forward model for the entire Ca Kα spectrum, including satellite lines, to enhance spectral fidelity and transport constraint accuracy.
• To incorporate charge exchange between thermal neutrals and impurities into the transport inference framework for more realistic pedestal transport modeling.
• To validate transport models by comparing experimental spectroscopic constraints with SOLPS-ITER simulations and gyrokinetic simulations.
• To assess the predictive capability of turbulent transport models (TGLF and CGYRO) against experimentally inferred diffusion and convection profiles.

Proposed method

• Utilizes high-resolution X-ray Imaging Crystal Spectroscopy (XICS) to measure the full Ca Kα spectrum, including resonance, forbidden, intercombination, and satellite lines.
• Employs a new atomic data compilation from the atomDB database, including radiative recombination, dielectronic recombination, electron impact excitation, and inner-shell processes.
• Applies a fully-Bayesian inference workflow using the Aurora 1.5D package to iteratively adjust radial profiles of diffusion (D) and convection (v) until synthetic spectra match experimental data.
• Complements XICS with Extreme Ultra-Violet (EUV) spectroscopy to constrain impurity transport closer to the plasma edge.
• Incorporates charge exchange effects between edge thermal neutrals and impurities using D Lyα measurements and SOLPS-ITER/EIRENE simulations.
• Validates inferred transport profiles against quasilinear (TGLF SAT-1) and nonlinear (CGYRO) gyrokinetic simulations.

Experimental results

Research questions

• RQ1 How do experimentally inferred radial profiles of impurity diffusion and convection compare to predictions from turbulent transport models in Alcator C-Mod?
• RQ2 To what extent does including charge exchange between edge neutrals and impurities improve the accuracy of transport coefficient inference?
• RQ3 How well do synthetic spectra generated from the new forward model (including satellite lines) reproduce measured XICS data?
• RQ4 What is the agreement between experimental D Lyα measurements and SOLPS-ITER simulations of neutral density near the last closed flux surface?
• RQ5 In what ways do discrepancies between inferred and modeled convection profiles suggest limitations in current transport model predictions?

Key findings

• Experimentally inferred radial diffusion profiles for impurities in L-, EDA H-, and I-mode discharges show good agreement with both quasilinear TGLF SAT-1 and nonlinear CGYRO gyrokinetic simulations at midradius within uncertainties.
• Significant discrepancies in inferred convection profiles are observed compared to model predictions, suggesting challenges in modeling flat or hollow impurity density profiles.
• Inclusion of charge exchange between edge thermal neutrals and impurities is found to be essential for accurate modeling of transport in the C-Mod pedestal region.
• The new forward model for the full Ca Kα spectrum, including satellite lines, significantly improves spectral fit quality and transport coefficient accuracy compared to previous models.
• Experimental D Lyα measurements at the midplane show good agreement with SOLPS-ITER simulations using the EIRENE neutral model, validating the neutral density predictions used in the inference framework.
• The use of updated atomic data sets across both XICS and EUV spectral ranges enables robust line ratio analysis, enhancing the reliability of inferred transport coefficients.