The radius of the circular orbit traced by a charged particle gyrating around a magnetic field line — a fundamental length scale governing particle confinement and transport in magnetized plasmas.
The Larmor radius, also called the gyroradius or cyclotron radius, is the radius of the helical path a charged particle follows as it spirals around a magnetic field line. Named after the Irish physicist Joseph Larmor, it is one of the most basic parameters in plasma physics and determines how tightly a particle is bound to a field line.[1]
The Larmor radius is given by rL = mv⊥ / (|q|B), where m is the particle mass, v⊥ is the component of velocity perpendicular to the magnetic field, q is the particle charge, and B is the magnetic field strength. For a 10 keV deuterium ion in a 5 T tokamak field, the Larmor radius is approximately 4 mm; for an electron at the same temperature and field, it is roughly 0.07 mm.[2]
The Larmor radius sets the spatial scale below which a particle "feels" the magnetic field structure. Plasma transport theories distinguish between classical transport (governed by collisions displacing particles by one Larmor radius) and anomalous or turbulent transport, which can move particles across distances many times the Larmor radius. Gyrokinetic simulations, the primary computational tool for studying turbulent transport in tokamaks and stellarators, operate in the framework where fluctuations vary slowly compared to the cyclotron frequency and smoothly over the Larmor radius scale.[3]
The cyclotron frequency, ωc = |q|B/m, is the angular frequency of the gyration. Together, the Larmor radius and cyclotron frequency completely describe the circular motion component of a charged particle in a uniform magnetic field. Resonant heating at the cyclotron frequency — ion cyclotron resonance heating (ICRH) — is a standard plasma heating method in fusion experiments.[2]