# 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^{[1]}:

*R*is the maximum value of_{max}*R*along the LCFS or separatrix.*R*is the minimum value of_{min}*R*along the LCFS or separatrix.*R*is the geometric major radius, defined as_{geo}*(R*._{max}+ R_{min})/2*a*is the minor radius of the plasma, defined as*(R*._{max}- R_{min})/2*R*is the major radius of the highest vertical point of the LCFS or separatrix._{upper}*R*is the major radius of the lowest vertical point of the LCFS or separatrix._{lower}

The upper triangularity is then defined as follows:

and similar for δ_{lower}.
The overall triangularity is defined as the mean of δ_{upper} and δ_{lower}.

Triangularity, especially the triangularity opposite the dominant X-point (so upper triangularity for a lower null plasma), influences the stability and character of the pedestal and ELMs.^{[2]}

Some devices (TCV and DIII-D) can form plasma cross sections with negative triangularity (the X-points are pushed to larger than the center of the plasma), which makes H-mode difficult or impossible to access but improves performance of the L-mode.^{[3]}