Editing Stellarator (section)

=== Heating ===
Unlike the [[z-pinch]] or tokamak, the stellarator has no induced electrical current within the plasma – at a macroscopic level, the plasma is neutral and unmoving, in spite of the individual particles within it rapidly circulating. In pinch machines, the current itself is one of the primary methods of heating the plasma. In the stellarator, no such natural heating source is present.

Early stellarator designs used a system similar to those in the pinch devices to provide the initial heating to bring the gas to plasma temperatures. This consisted of a single set of windings from a [[transformer]], with the plasma itself forming the secondary set. When energized with a pulse of current, the particles in the region are rapidly energized and begin to move. This brings additional gas into the region, quickly ionizing the entire mass of gas. This concept was referred to as ''ohmic heating'' because it relied on the resistance of the gas to create heat, in a fashion not unlike a conventional [[electric heating|resistance heater]]. As the temperature of the gas increases, the conductivity of the plasma improves. This makes the ohmic heating process less and less effective, and this system is limited to temperatures of about 1 million kelvins.{{sfn|Spitzer|1958|p=187}}

To heat the plasma to higher temperatures, Spitzer proposed a second heat source, the ''magnetic pumping'' system. This consisted of radio-frequency source fed through a coil spread along the vacuum chamber. The frequency is chosen to be similar to the natural frequency of the particles around the magnetic lines of force, the ''[[cyclotron frequency]]''. This causes the particles in the area to gain energy, which causes them to orbit in a wider radius. Since other particles are orbiting their own lines nearby, at a macroscopic level, this change in energy appears as an increase in pressure.{{sfn|Spitzer|1958|p=188}} According to the [[ideal gas law]], this results in an increase in temperature. Like ohmic heating, this process also becomes less efficient as the temperature increases, but is still capable of creating very high temperatures. When the frequency is deliberately set close to that of the ion circulation, this is known as ''ion-cyclotron resonance heating'',{{sfn|Spitzer|1958|p=189}} although this term was not widely used at the time.