The connection length is understood to be the distance between two points, measured along a magnetic field line passing through these points.
In the fusion context, a distinction is made between closed field lines (inside the Last Closed Flux Surface) and open field lines (outside).
Closed field lines
In a toroidally confined plasma, inside the Last Closed Flux Surface, the connection length is commonly defined as the length, measured along the magnetic field, to complete a poloidal turn.
where q is the safety factor, approximated by .
Open field lines
Outside Last Closed Flux Surface, the connection length associated with a given point is defined as the shortest distance from that point to any material surface measured along the field line through that point.
- K. Miyamoto, Plasma Physics and Controlled Nuclear Fusion, Springer-Verlag (2005) ISBN 3540242171