Control of Asymmetric Magnetic Perturbations in Tokamaks

TLDR

Tokamak plasmas are highly sensitive to tiny deviations in the magnetic field (~10⁻⁴), and recent work has shown that such perturbations significantly affect plasma equilibrium even far from stability limits. The paper aims to explain the counterintuitive findings regarding asymmetric magnetic perturbations in tokamaks. Experiments show that reducing the harmful effects of asymmetries often requires stronger external fields that interact more with the equilibrium, and the ideal perturbed equilibrium explanation highlights key implications for controlling field errors in toroidal plasmas.

Abstract

The sensitivity of tokamak plasmas to very small deviations from the axisymmetry of the magnetic field |deltaB/B| approximately 10{-4} is well known. What was not understood until very recently is the importance of the perturbation to the plasma equilibrium in assessing the effects of externally produced asymmetries in the magnetic field, even far from a stability limit. DIII-D and NSTX experiments find that when the deleterious effects of asymmetries are mitigated, the external asymmetric field was often made stronger and had an increased interaction with the magnetic field of the unperturbed equilibrium. This Letter explains these counterintuitive results. The explanation using ideal perturbed equilibria has important implications for the control of field errors in all toroidal plasmas.