ohmic heating is the resistive heating a 📝plasma receives from its own current, which weakens as the plasma gets hotter.
The name comes from Ohm's law: the mechanism is the same one that warms a toaster wire. In a 📝Tokamak, the megaampere-scale 📝plasma current driven by the 📝central solenoid flows through a medium with finite electrical resistance, and the dissipated power heats the plasma directly. It is the first heating every tokamak plasma receives, and it comes free with the current that confinement already requires.
What distinguishes ohmic heating from every auxiliary method is that it is self-limiting. A plasma's resistivity falls steeply as its electrons get hotter — roughly with the inverse three-halves power of temperature — so the harder ohmic heating works, the less effective it becomes. In practice it stalls at a few tens of millions of degrees, well short of the more than 100 million degrees 📝fusion requires. Reaching fusion conditions therefore depends on external systems: neutral beams and radio-frequency waves that finish the climb ohmic heating begins.