# Magnetic flux pumping in 3D nonlinear magnetohydrodynamic simulations

## TL;DR

This paper investigates a magnetic flux pumping mechanism in tokamaks through nonlinear 3D MHD simulations, revealing how a dynamo effect driven by a specific instability prevents sawtooth oscillations by maintaining the safety factor near unity.

## Contribution

It demonstrates how a saturated $(m=1,n=1)$ quasi-interchange instability generates a dynamo effect that self-regulates the core safety factor in tokamaks, a novel insight into flux pumping.

## Key findings

- The flux pumping mechanism is driven by a dynamo effect from a specific instability.
- Higher plasma beta enhances the flux pumping by driving the instability.
- The peakedness of heat sources influences the current profile and flux pumping effectiveness.

## Abstract

A self-regulating magnetic flux pumping mechanism in tokamaks that maintains the core safety factor at $q\approx 1$, thus preventing sawteeth, is analyzed in nonlinear 3D magnetohydrodynamic simulations using the M3D-C$^1$ code. In these simulations, the most important mechanism responsible for the flux pumping is that a saturated $(m=1,n=1)$ quasi-interchange instability generates an effective negative loop voltage in the plasma center via a dynamo effect. It is shown that sawtoothing is prevented in the simulations if $\beta$ is sufficiently high to provide the necessary drive for the $(m=1,n=1)$ instability that generates the dynamo loop voltage. The necessary amount of dynamo loop voltage is determined by the tendency of the current density profile to centrally peak which, in our simulations, is controlled by the peakedness of the applied heat source profile.