The suite of methods used to raise fusion plasma to temperatures exceeding 100 million degrees — far hotter than the core of the Sun — so that nuclei can overcome their electrostatic repulsion and fuse.
For fusion reactions to occur at useful rates, the fuel ions must collide with enough energy to overcome the Coulomb barrier — the electrostatic repulsion between positively charged nuclei. In a deuterium-tritium (D-T) plasma, peak reactivity occurs at ion temperatures around 10–20 keV, corresponding to roughly 100–200 million degrees Celsius. No material can withstand such temperatures; instead, the plasma is magnetically confined and heated by external systems until it approaches or reaches ignition.[1]
In a tokamak, the plasma itself carries a large toroidal current. Because the plasma has finite electrical resistivity, this current dissipates energy as heat — the same principle as a toaster wire. Ohmic heating is effective at lower temperatures, but plasma resistivity drops as temperature rises (proportional to T−3/2), so it cannot alone reach fusion-relevant conditions. Typically, ohmic heating saturates around 20–30 million degrees.[2]
High-energy neutral atoms — usually deuterium — are accelerated as ions, then neutralized and injected into the plasma at energies of 80–1,000 keV. Once inside the magnetic field, the fast neutrals are re-ionized by collisions and transfer their energy to the bulk plasma through Coulomb interactions. NBI also drives plasma current and rotation, which can stabilize certain instabilities.[1]
Electromagnetic waves at specific frequencies can resonantly transfer energy to plasma particles:
Ion Cyclotron Resonance Heating (ICRH) uses waves in the 30–120 MHz range to accelerate ions gyrating around magnetic field lines. Electron Cyclotron Resonance Heating (ECRH) employs millimeter waves (100–170 GHz) to heat electrons at their cyclotron frequency. Lower Hybrid Heating (LH) operates in the 1–8 GHz range and is especially effective at driving non-inductive plasma current.[3]
In a burning D-T plasma, each fusion reaction produces a 3.5 MeV alpha particle (helium-4 nucleus) that remains confined by the magnetic field. These fast alphas slow down and deposit their energy in the plasma, providing internal self-heating. When alpha heating dominates over external heating, the plasma is said to be "burning" — the defining milestone for a fusion power plant.