Ohmic Heating

Heating of JET plasmas

Magnetic fields in a tokamak - the toroidal field is generated by external coils, poloidal by electric current in the plasma

The tokamak concept is a breakthrough in plasma research, but not a complete solution. At millions of degrees and above, plasma is conducting electricity far too well, with very little resistance – which also means with not enough heat produced by the Joule Effect. The unit of electric resistance is the Ohm, so plasma physicists usually say ‘Ohmic heating is ineffective at high temperatures’ where the word ‘high’ refers to the hundreds of millions of degrees required for burning plasmas. In order to attain the target temperatures some sort of ‘additional heating’ is required to supplement the ‘Ohmic heating’ (as a matter of fact, the ‘additional heating’ eventually plays a dominant role). Neutral particle beams (‘NB Heating’) and resonant electromagnetic waves (‘RF Heating’) can do this job.

Furthermore, tokamaks cannot maintain a continuous electrical current in the plasma and this limits the concept of complementing the magnetic field. Tokamaks have a transformer-like electrical setup, with plasma that acts as a single secondary loop – and no transformer can provide continuous direct electric current in its secondary circuit. An additional ‘current drive’ needs to be provided if we wish to confine burning plasma continuously. Electromagnetic wave current drive offers a possible solution.