What are flux surfaces and why are they important?
Magnetic flux surfaces are the surfaces formed by the magnetic field as it wraps many times around the torus. The confined plasma tends to form isobars or constant pressure surfaces that coincide with these magnetic surfaces. If the magnetic surfaces are not closed or connect to the wall, then the hot plasma will not be well confined and escape rapidly to the wall and extinguish itself (also possibly damaging the wall). Stellarator designers must work hard to produce devices with good closed magnetic surfaces. Small asymmetries or manufacturing defects in the magnetic field coils can lead to open magnetic surfaces.
