Speaker
Description
Conventional neoclassical theory for impurity transport is frequently derived under simplifying assumptions such as circular flux surfaces and large aspect ratio, which are not generally satisfied in realistic tokamak equilibria [1]. These approximations become particularly questionable when comparing negative triangularity (NT) and positive triangularity (PT) plasmas: although NT operation has demonstrated improved energy confinement, reduced turbulence, and ELM-free scenarios, a quantitative understanding of how magnetic shaping alone modifies neoclassical impurity transport remains incomplete [2]. For instance, recent TCV experiments have further highlighted that impurity behavior can be strongly geometry-sensitive [3].
In this work, we revisit neoclassical impurity transport in shaped geometry by extending standard formulations from circular to non-circular flux surfaces, explicitly retaining the effects of plasma elongation (κ) and triangularity (δ) through the metric coefficients. The resulting theory shows that departures from circular geometry can significantly modify neoclassical impurity transport via changes in geometric factors, trapped-particle fractions, and impurity poloidal asymmetries set by the parallel momentum balance.
To validate these predictions, we use the global, full-f gyrokinetic code GYSELA [3] to perform nonlinear simulations of deuterium plasmas with trace tungsten in matched NT, symmetric, and PT configurations. The simulations indicate a pronounced sensitivity of neoclassical impurity transport to shaping, leading to enhanced core accumulation in non-circular plasmas, in quantitative agreement with the revised theoretical model.
[1] S.P. Hirshman and D.J. Sigmar. Nucl. Fusion 21, 1079 (1981)
[2] M. Kikuchi et al., Nucl. Fusion 59, 056017 (2019)
[3] F. Bagnato et al. Plasma Phys. Control. Fusion 66, 075019 (2024)
[4] V. Grandgirard et al., Comp. Phys. Comm. 207, 35 (2016)