gyroradius, also called the Larmor radius, is the radius of the circular spiral a charged particle traces around a magnetic field line, set by the particle's momentum, its charge, and the field strength.
The alternative name honors Irish physicist Joseph Larmor, who analyzed the motion of orbiting charges in the 1890s. A charged particle in a magnetic field streams freely along a field line but is bent into a circle across it; the combination is a helix. The gyroradius equals the particle's perpendicular momentum divided by the product of its charge and the field strength, so heavier and hotter particles trace wider circles while stronger fields wind the spiral tighter. In a fusion plasma the two species differ sharply: a fuel ion's gyroradius is measured in millimeters, an electron's in tens of micrometers.
The gyroradius is the reason magnetic confinement works at all — a particle that would cross a 📝Tokamak in microseconds is instead tethered to a field line, its excursion across the field limited to millimeters. It is also a fundamental design scale: the ion gyroradius sets the size of the turbulent eddies that leak heat from a confined 📝plasma, so the ratio of machine size to gyroradius is one of the parameters that determines confinement quality — and raising the magnetic field shrinks the gyroradius in direct proportion.