Dielectronic recombination of the open $4d$-shell of Tungsten: W$^{37+}$ - W$^{28+}$

TL;DR

This study provides detailed dielectronic recombination rate coefficients for tungsten ions W$^{37+}$ to W$^{28+}$, highlighting the importance of relativistic effects and revising ionization fractions crucial for fusion plasma modeling.

Contribution

The paper introduces new relativistic calculations of DR rate coefficients for tungsten ions, significantly improving the accuracy of plasma modeling in fusion devices.

Findings

Relativistic configuration mixing significantly affects DR rate coefficients.

Partial DR rate coefficients can differ by up to 75% depending on resolution.

Revised ionization fractions shift peak temperatures to lower values.

Abstract

Tungsten is an important element for magnetically confined fusion plasmas but has the potential to cool, or even quench the plasma due to it being an efficient radiator. Total and level-resolved dielectronic recombination (DR) rate coefficients, for all ionization stages, are essential to model tungsten. We describe a set calculations performed using the distorted wave code AUTOSTRUCTURE for the tungsten ions W$^{37+}$ to W$^{28+}$. We demonstrate the importance of relativistic configuration mixing in such calculations. In particular, we show that the partial DR rate coefficients calculated in level and configuration resolution can differ by as little as 5%, and up to as much as 75%. Using the new data, we calculate a revised steady-state ionization fraction for tungsten. We find that, relative to the ionization fraction calculated using the recombination rate coefficients of Putterich et al. (Plasma Phys. Control. Fusion, 50, 085016), the peak temperatures of W$^{37+}$ to W$^{28+}$ ionization states are shifted to lower temperatures spanning 0.9-1.6keV. This temperature range is important for understanding the performance of large tokamaks, such as ITER, because the temperatures in the pedestal, edge, scrape-off-layer and divertor region fall in this range.