Comparing linear ion-temperature-gradient-driven mode stability of the National Compact Stellarator Experiment and a shaped tokamak

TL;DR

This study compares the linear stability of ion-temperature-gradient-driven modes in the NCSX stellarator and a shaped tokamak using gyrokinetic simulations, revealing that NCSX's stellarator configuration offers competitive or superior critical gradients.

Contribution

It provides the first detailed comparison of ITG mode stability between NCSX and a shaped tokamak, highlighting stabilizing effects unique to stellarator geometry.

Findings

NCSX has competitive or better normalized critical gradients than the tokamak.

For kinetic ITG modes, NCSX shows higher critical gradients and lower growth rates up to a certain threshold.

NCSX's stellarator configuration offsets the destabilizing effects of narrow cross sections.

Abstract

One metric for comparing confinement properties of different magnetic fusion energy configurations is the linear critical gradient of drift wave modes. The critical gradient scale length determines the ratio of the core to pedestal temperature when a plasma is limited to marginal stability in the plasma core. The gyrokinetic turbulence code GS2 was used to calculate critical temperature gradients for the linear, collisionless ion temperature gradient (ITG) mode in the National Compact Stellarator Experiment (NCSX) and a prototypical shaped tokamak, based on the profiles of a JET H-mode shot and the stronger shaping of ARIES-AT. While a concern was that the narrow cross section of NCSX at some toroidal locations would result in steep gradients that drive instabilities more easily, it is found that other stabilizing effects of the stellarator configuration offset this so that the normalized critical gradients for NCSX are competitive with or even better than for the tokamak. For the adiabatic ITG mode, NCSX and the tokamak had similar critical gradients, though beyond marginal stability, NCSX had larger growth rates. However, for the kinetic ITG mode, NCSX had a higher critical gradient and lower growth rates until a/L_T is approximately 1.5 times a/L_{T,crit}, when it surpassed the tokamak's. A discussion of the results presented with respect to a/L_T vs. R/L_T is included.