To get fusion reactions going in a tokamak, the plasma must be heated with megawatts of power.
Phase 1: Ohmic heating
A gas of deuterium and tritium is injected into the tokamak. The ohmic coils are ramped down and then back up again. This creates a voltage swing which causes the atoms to dissociate into ions and electrons (plasma breakdown). The plasma is then heated ohmically. Since ohmic heating decreases with temperature, this method will only bring the temperature up a few electron volts
Phase 2: NB and RF heating
Neutral Beams of deuterium atoms are then injected at high speed into the plasma. Electromagnetic waves can be radiated from an antenna to resonate with the ions or electrons and hence deposit energy. Up to 20 MW NB power and 10 MW RF power have been injected in present day tokamaks.
The ion temperature will then strongly peak towards the centre of the plasma and fusion reactions will begin. About 13 MW of fusion power has been 'created'.
A future fusion reactor will need about 60 MW of injected power from a combination of NB and RF in order to achieve plasma ignition.