Magnetic confinement

The TJ-II plasma

The TJ-II plasma (pink) and a confining magnetic field line (red ribbon), wrapped around the helical field coil (yellow).

Research and investigation at the Spanish National Fusion Laboratory is focused mainly on magnetic confinement, one of the main techniques on which a future fusion reactor may be based. Magnetic confinement is achieved by generating strong toroidal (doughnut-shaped) magnetic fields using external field coils through which large currents flow.

In order to produce a fusion plasma, hydrogen or deuterium (an isotope of hydrogen) gas is injected into the vacuum chamber within this magnetic field is. The gas then heated using microwave radiation. When its temperature becomes high enough the gas becomes ionized, i.e., the gas molecules are disassociated into atoms which then lose their corresponding electrons.

The resulting ionized gas is called a plasma. The ionized particles are strongly affected by the magnetic field and they travel around the machine following these field lines. As the field lines are shaped to return onto themselves within the doughnut-shaped vessel, the particles cannot escape and are confined. Additional heating, in the form of accelerated neutral particle beams, can further raise the plasma density and temperature.

Several designs are used to achieve the magnetic field structure. The most common type, and best developed, is called a tokamak, in which part of the confining magnetic field is created by the electric currents that flow within the plasma itself. The world’s largest operational tokamak is JET (located in the UK), while the international project ITER (which is under construction in the south of France) is also based on this design.
Another popular design is the stellarator. In this case, the principal magnetic fields are generated by external field coils, thereby providing more control over the plasma. The TJ-II project (located at the LNF, Ciemat) falls into this category, as does W7-X (located in Greifswald, Germany). W7-X created its first plasma on December 10, 2015 and is routinely operated since 2016.