The rate at which the pitch of magnetic field lines changes from one flux surface to the next, a critical factor in suppressing turbulence and stabilizing MHD modes.
Magnetic shear quantifies how rapidly the direction of the magnetic field—specifically, the safety factor q—changes across nested flux surfaces in a magnetically confined plasma. It is one of the principal levers that determine both macroscopic MHD stability and microscopic turbulent transport.
Magnetic shear has a powerful stabilizing influence on pressure-driven interchange and ballooning modes. On surfaces with strong positive shear, neighboring field lines diverge rapidly, which limits the radial extent of perturbations and raises the critical pressure gradient for instability onset. This is the essence of the Mercier criterion for interchange stability and the first ballooning stability boundary. Conversely, regions of low or zero shear can be vulnerable to infernal modes—pressure-driven instabilities with global structure.2
Some of the most promising advanced tokamak scenarios exploit reversed magnetic shear. When a hollow current profile produces a region where dq/dr < 0, the resulting negative shear strongly suppresses ion-temperature-gradient (ITG) turbulence. This can form an internal transport barrier (ITB) with dramatically steepened pressure gradients and improved energy confinement. Experiments on JT-60U, DIII-D, and JET have demonstrated that reversed-shear ITBs can raise the confinement time by factors of two or more compared with standard H-mode.3
Beyond its MHD role, magnetic shear interacts with flow shear (the radial gradient of plasma rotation) to govern microinstability growth rates. The E × B flow-shear stabilization criterion, formulated by Waltz and others, shows that turbulence is suppressed when the E × B shearing rate exceeds the maximum linear growth rate of the dominant microinstability. The magnetic shear enters because it sets the radial correlation length of turbulent eddies: stronger magnetic shear narrows the eddies radially, making them easier to shear apart.
The magnetic shear profile is inferred from the q profile, which is reconstructed from internal magnetic-field measurements. Motional Stark Effect (MSE) polarimetry is the standard diagnostic, measuring the pitch angle of the field via Doppler-shifted Balmer-alpha emission from injected neutral beams.