The tokamak that changes shape

The idea in one paragraph

Inside a tokamak the plasma forms a ring. Cut that ring through and you see its cross-section: it can be round, stretched upwards, shaped like the letter D, or something more unusual. That shape matters a great deal, because it changes how well the plasma keeps its heat, how stable it is and how its exhaust reaches the wall (see Why the shape matters). Most machines can only vary it a little. TCV was designed so that the shape itself becomes the thing under study.

A tall vessel and many coils

The key to TCV is its vacuum vessel, which is three times taller than it is wide, and the coils around it. The height was not an accident: calculations made in the time of the earlier tokamak TCA had shown that stretched plasmas were advantageous, and that stable plasmas were limited to an elongation of about 3, which set the design of the new chamber.

Sixteen poloidal coils, in two stacks on either side of the vessel, shape the plasma, while the ohmic coil, the OH coil, drives an electric current through it by induction. According to the SPC, TCV has more independently powered shaping coils than any comparable machine, so the plasma can be pulled, stretched and bent into very different forms.

An archived presentation of TCV from the CRPP, last updated in 2007, gives its main dimensions: a major radius of 0.88 metres, a minor radius of 0.25 metres and a toroidal magnetic field of up to 1.54 tesla. The plasma current was designed for 1.2 megaamperes; in the first year of operation it reached about 1 megaampere. The plasma could be stretched vertically to an elongation of 2.8, and its triangularity, a measure of how pointed its D shape is, could range from −0.7 to 1. The SPC’s current pages describe the vessel and the heating systems but do not repeat these figures, so they are given here as historical values.

How TCV heats its plasma

Besides the current driven through the plasma by the OH coil, TCV uses two heating systems. The first is electron cyclotron heating, which uses beams of microwaves to heat the electrons. TCV can use up to seven separate beams, steered in real time, which can heat the plasma or drive a current locally, where they are aimed. The second is neutral beam injection, which heats the ions; TCV has two such injectors. Controlling these systems in real time has opened up new ways of running the machine.

Thirty years of experiments

TCV followed the CRPP’s earlier tokamak, TCA, which ran from 1980 to 1992 to test heating with Alfvén waves; after it was decommissioned, its parts were rebuilt in São Paulo as TCABR. Construction of TCV began in 1989, its first plasma came in November 1992 and scientific campaigns started in June 1993.

Heating came in stages. In 1998 a first electron cyclotron resonance heating system (ECRH) of 1.5 megawatts at 82.7 gigahertz was installed, and the electron temperature reached 6 keV. In 1999 a plasma current driven only by microwaves was sustained for 1.9 seconds. In 2000 the microwave power rose to 3 megawatts, plus 0.5 megawatts at 118 gigahertz, and in 2002 the electron temperature reached 18 keV.

Between 2014 and 2015 the machine was shut down to install a neutral beam injector (NBI) and to upgrade diagnostics; afterwards TCV joined the EUROfusion programme, with work on disruptions, runaway electrons and alternative divertors. In 2016 the NBI added 1 megawatt and two further ECRH sources of 0.75 megawatts each came into use; that year the ion temperature reached 2.5 keV. TCV also gained optional baffles, which allow different exhaust geometries. A second neutral beam injector was delivered in 2023.

Shapes no one had made before

In 2008 TCV produced the first experimental snowflake divertor. A divertor is the part of a tokamak where the edge of the plasma is led away to absorb its heat. In a snowflake, two points where the magnetic field cancels out are brought together, which spreads the exhaust over more of the wall. A more recent variant, the “jellyfish”, adds a third such point to distribute the exhaust power even more effectively.

TCV has also studied negative triangularity: a D shape turned the other way round, with its point facing the centre of the machine. The SPC reports that this shape suppresses turbulence, on TCV and on other tokamaks. TCV experiments have also shown that extreme elongation helps increase plasma pressure while keeping the plasma stable, which the SPC calls critical for reactor-scale tokamaks. TCV can even hold two independent plasmas in its vessel at the same time, a configuration known as droplets.

When TCV celebrated its thirtieth anniversary in 2023, Stefano Coda, head of TCV physics, named the snowflake and negative triangularity among the plasma shapes first made on this machine.

A plasma controlled by artificial intelligence

In February 2022 the SPC and DeepMind published in Nature the first demonstration of magnetic control of a tokamak plasma by deep reinforcement learning. The voltages on a tokamak’s coils must be controlled very carefully, or the plasma can hit the wall. Instead of calculating each control setting in advance, the controller learned by trying many strategies on the SPC’s plasma simulator, which the SPC’s Federico Felici described as based on more than 20 years of research.

The controller was then used on TCV, where it produced and held a range of plasma shapes, including two separate plasmas in the vessel at the same time.

Still being rebuilt

TCV keeps changing. The SPC describes a major infrastructure upgrade that adds a coil to limit runaway electrons (REMC) and a tightly baffled, long-legged divertor (TBLLD), together with new diagnostics. For the latest status of the machine, see the SPC’s own pages (status as of 3 November 2026).

Groups can see TCV on a visit to the Swiss Plasma Center; see Science Travel.