TCV (Tokamak à Configuration Variable)

Switzerland's shape-shifting tokamak — the world's most flexible plasma laboratory and the birthplace of negative-triangularity research.

TCV — the Tokamak à Configuration Variable — has operated at the Swiss Plasma Center (SPC, formerly CRPP) at EPFL in Lausanne, Switzerland, since 1992. As its name declares, TCV was designed for one overriding purpose: to explore how plasma shape affects confinement, stability, and performance. With an unprecedented array of independently powered poloidal field coils, TCV can produce plasma cross-sections ranging from conventional positive-triangularity D-shapes to highly elongated, diverted, and negative-triangularity configurations that no other tokamak can match.

Design Philosophy

TCV has a major radius of 0.88 m, a minor radius of 0.25 m, and a toroidal field of up to 1.54 T. Its vacuum vessel is unusually tall — with an elongation capability exceeding 2.8 — to accommodate the extreme vertical stretching its shaping experiments require. The machine's 16 independently controlled poloidal field coils give it unmatched flexibility in sculpting the plasma boundary. Plasma currents can reach 1 MA under favorable conditions.¹

Negative Triangularity Pioneer

TCV's most consequential contribution to contemporary fusion science is its systematic exploration of negative-triangularity (NT) plasmas. In a conventional tokamak, the D-shaped plasma cross-section leans outward (positive triangularity). TCV demonstrated that reversing this lean — creating plasmas where the triangularity parameter delta is negative — can suppress edge turbulence and produce H-mode-level confinement without the sharp edge pedestal and damaging ELMs that normally accompany H-mode.²

This finding has profound implications for reactor design. ELMs deposit intense, pulsed heat loads on plasma-facing components, and mitigating them is one of the most challenging engineering problems facing ITER and future power plants. If negative triangularity can deliver comparable confinement without ELMs, it could simplify reactor engineering dramatically. TCV's NT results have inspired dedicated negative-triangularity campaigns on DIII-D in the United States and are informing next-generation reactor concepts worldwide.³

Broader Research Portfolio

TCV's flexibility extends well beyond triangularity studies. The machine has been a testbed for advanced divertor configurations including the snowflake divertor, where multiple X-points spread exhaust heat over a larger wetted area. TCV has also contributed significantly to electron cyclotron heating and current drive (ECRH/ECCD) physics, operating with up to 4.5 MW of ECH power — an unusually high ratio of heating power to plasma volume that enables detailed studies of wave-plasma interactions.⁴

In recent years, TCV has also served as a platform for testing machine-learning-based plasma control algorithms, including a collaboration with DeepMind that demonstrated reinforcement-learning controllers maintaining multiple plasma configurations in real time. The machine continues to operate as one of the most scientifically productive medium-sized tokamaks in the world, regularly contributing to EUROfusion and international collaborative programs.⁵

Sources

1. Hofmann, F. et al., 'Creation and Control of Variably Shaped Plasmas in TCV,' Plasma Physics and Controlled Fusion, Vol. 36, No. 12B, B277, 1994.
2. Camenen, Y. et al., 'Impact of Plasma Triangularity and Collisionality on Electron Heat Transport in TCV L-Mode Plasmas,' Nuclear Fusion, Vol. 47, No. 7, pp. 510–516, 2007.
3. Austin, M.E. et al., 'Achievement of Reactor-Relevant Performance in Negative Triangularity Shape in the DIII-D Tokamak,' Physical Review Letters, Vol. 122, No. 11, 115001, 2019.
4. Reimerdes, H. et al., 'TCV Experiments Towards the Development of a Plasma Exhaust Solution,' Nuclear Fusion, Vol. 57, No. 12, 126007, 2017.
5. Degrave, J. et al., 'Magnetic Control of Tokamak Plasmas Through Deep Reinforcement Learning,' Nature, Vol. 602, pp. 414–419, 2022.

Related

Negative Triangularity DIII-D Plasma Shaping Snowflake Divertor ASDEX (Original)