# Ellipticity

Sketch of tokamak geometry

The ellipticity (also referred to as elongation[1]) refers to the shape of the poloidal cross section of the Last Closed Flux surface 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.
• Zmax is the maximum value of Z along the LCFS or separatrix.
• Zmin is the minimum value of Z along the LCFS or separatrix.
• a is the minor radius of the plasma, defined as (Rmax - Rmin)/2.

The ellipticity is then defined as follows:

$\kappa = (Z_{max}-Z_{min})/2a$