POPCON, short for plasma operating contour plot, is a design chart that maps where a 📝fusion device can operate in density and temperature coordinates by contouring the auxiliary heating power a steady 📝plasma would require at each point.
A POPCON is built from a zero-dimensional power balance evaluated across a grid of volume-averaged density and temperature. At each grid point the calculation takes fusion power from the 📝D-T reaction rate, retains the fraction carried by 📝alpha heating, subtracts radiated power and the transport loss implied by an 📝energy confinement time (τ_E) drawn from 📝confinement scaling laws, and solves for the external heating power that closes the balance. Contouring that power gives the chart its characteristic form: a closed region where the required power falls to zero — 📝ignition — ringed by contours of increasing auxiliary power, with a saddle through which the lowest-power path into that region runs. Contours of 📝Q (fusion energy gain factor), fusion power and 📝triple product overlay the same axes.
What turns the chart into a design tool is the constraint curves drawn across it. The 📝Greenwald limit cuts off high density, a 📝plasma beta limit cuts off high pressure, the L-H power threshold marks where 📝H-mode becomes accessible, and 📝divertor heat flux and installed heating power bound the remainder. The operating point is chosen inside whatever region survives, with margin on every side. Change the magnetic field, the current or the major radius and the entire picture redraws, which is how a POPCON participates in sizing a machine rather than merely describing one.
The method was introduced by W. A. Houlberg, S. E. Attenberger and L. M. Hively in Nuclear Fusion in 1982; the acronym itself appears in their companion 📝Oak Ridge National Laboratory report, "Fusion reactor plasma-performance modeling: POPCON analysis." Its near neighbour is 📝operational space, the general idea of a bounded region a machine can run inside — a POPCON is one particular two-dimensional slice of that space. The slice is deliberately coarse. Because the underlying model is zero-dimensional it says nothing about profile shapes, 📝pedestal height or where the exhaust heat lands, so its role is to identify which operating points deserve full 📝integrated modeling, not to predict what those points will deliver.