banana orbit is the banana-shaped path traced by a charged particle trapped on the outboard side of a tokamak's magnetic field, where the field is too weak to carry the particle all the way around the torus.
The shape follows from geometry. A 📝Tokamak's 📝toroidal field is stronger on the inboard side of the doughnut than the outboard side, so a particle spiraling along a field line climbs into rising field strength as it circulates. If its velocity along the field is small, the magnetic mirror effect reflects it before it reaches the high-field side, and it bounces back and forth within the outboard region instead. Slow cross-field drifts displace the bouncing particle off its original surface, and its projection onto the tokamak's cross-section traces a closed, banana-shaped curve.
Trapped particles are no curiosity: in a typical 📝plasma they make up a substantial fraction of the population, and the banana's width is many times a particle's 📝gyroradius. A single collision can therefore displace a trapped particle across the full banana width — the geometric origin of the enhanced collisional loss known as 📝neoclassical transport — and the same trapped population, colliding with passing electrons, gives rise to the 📝bootstrap current.