Triangularity

The triangularity refers to the shape of the poloidal cross section of the Last Closed Flux surface (LCFS) or separatrix of a tokamak. Assuming:

• R₀ is the major radius of the midpoint of the 2 intersections of the LCFS or separatrix with the equatorial plane (Z = 0).
• a is the minor radius of the plasma, defined as the radial difference between the 2 intersections of the LCFS or separatrix with the equatorial plane (Z = 0), divided by 2.
• R_upper is the major radius of the upper X-point (in case it exists) or of the highest vertical point of the LCFS.
• R_lower is the major radius of the lower X-point (in case it exists) or of the lowest vertical point of the LCFS.

Sketch of tokamak geometry, including separatrix

The upper triangularity is then defined as follows:

$\delta _{upper}=(R_{0}-R_{upper})/a$

and similar for δ_lower. The overall triangularity is defined as the mean of δ_upper and δ_lower.

See also

• Ellipticity
• Toroidal coordinates
• Effective plasma radius