The global power bookkeeping that determines whether a magnetically confined plasma heats itself, cools down, or ignites.
A fusion plasma is a thermodynamic system with well-defined energy inputs and losses. In steady state the global energy balance requires that the total heating power equals the total loss power:1
Ohmic heating provides the initial energy input from resistive dissipation of the plasma current. However, ohmic power decreases as Te−3/2, making it negligible above a few keV. Auxiliary heating—neutral beam injection, ion and electron cyclotron resonance heating, and lower hybrid current drive—raises the plasma into the thermonuclear-relevant regime of 10–25 keV.2
In a burning D-T plasma, alpha particles born at 3.52 MeV slow down on electrons and ions, depositing their energy in the core. When Pα dominates over Paux, the plasma is said to be self-heated. The fusion gain parameter Q = Pfus/Paux quantifies how far along this path the plasma has progressed: Q = 1 is breakeven, Q = 10 is ITER's goal, and Q → ∞ is ignition.1
Energy escapes the confined plasma through three main routes. Transport losses (conduction and convection through turbulent and neoclassical channels) are characterized by τE. Radiation losses—bremsstrahlung, cyclotron emission, and line radiation from impurities—can be volumetric or concentrated at the edge. Particle losses carry kinetic energy with escaping ions and electrons, including ELM-driven bursts in H-mode.3
Understanding and closing the energy balance is the central challenge of fusion science. Every advance in confinement, density, or impurity control shifts the ledger toward net energy gain.