magnetic shear is the change in the pitch of magnetic field lines from one nested flux surface to the next in a confined plasma — a twist gradient that stabilizes many instabilities.
In a 📝Tokamak, field lines wind helically around nested toroidal surfaces, and the pitch of the winding is measured by the safety factor q. Magnetic shear is the radial variation of that pitch. Its stabilizing power is geometric: a perturbation that aligns itself with the field on one surface finds itself misaligned on neighboring surfaces, so it cannot grow coherently across the 📝plasma — shear localizes and damps pressure-driven instabilities such as ballooning modes, and it decorrelates turbulent eddies that would otherwise span many surfaces.
The profile of shear is a control knob. Ordinary tokamak operation has q rising toward the edge — positive shear — but driving current off-axis can invert the profile in the core, producing reversed (negative) shear, a configuration closely associated with the formation of internal 📝transport barriers. Magnetic shear should not be confused with flow shear, the velocity-gradient analogue in which sheared E×B rotation tears turbulence apart; the two mechanisms are distinct but often work together in the best-confined plasmas.