# Triangularity

Sketch of tokamak geometry, including separatrix

The triangularity refers to the shape of the poloidal cross section of the Last Closed Flux surface (LCFS) or separatrix of a tokamak. Assuming[1]:

• Rmax is the maximum value of R along the LCFS or separatrix.
• Rmin is the minimum value of R along the LCFS or separatrix.
• Rgeo is the geometric major radius, defined as (Rmax + Rmin)/2.
• a is the minor radius of the plasma, defined as (Rmax - Rmin)/2.
• Rupper is the major radius of the highest vertical point of the LCFS or separatrix.
• Rlower is the major radius of the lowest vertical point of the LCFS or separatrix.

The upper triangularity is then defined as follows:

$\delta_{upper} = (R_{geo}-R_{upper})/a$

and similar for δlower. The overall triangularity is defined as the mean of δupper and δlower.