Tech transfer: harnessing turbulence

Fusion in space? Lifting a complete power plant the size of ITER into orbit may be out of the realm of possibilities, but fusion still has contributions to make. Scientists from the Spanish EUROfusion member CIEMAT helped propulsion experts at the Plasma & Space Propulsion Team of Universidad Carlos III de Madrid (UC3M) to understand the turbulence that limits the performance of their plasma thrusters.

At first sight, the 150-million degree plasma roaring inside the magnetic cage of a tokamak or stellarator could not be more different than the soft plume of ionised gas coming out of a plasma thruster. Plasma thrusters use electric and magnetic fields to accelerate charged particles. These systems already fly on satellites as an extremely energy-efficient way to build up high exhaust speeds, beyond the limits of traditional chemical rockets.

The EP2-UC3M Helicon Plasma Thruster. A cylindrical device photographed inside a vacuum chamber, producing a white-purple beam of plasma that will act as space propulsion. Source: EP2-UC3M
The EP2-UC3M Helicon Plasma Thruster. Source: EP2-UC3M

Aerospace engineer Jaume Navarro is part of UC3M-EP2, the biggest university group for plasma propulsion in Europe and thinks that extra understanding can help plasma thrusters go a long way. “Even though plasma thrusters already work, they are still limited by fundamental processes like turbulence and plasma-wall-contact, which are not completely understood”, he explains. Fusion researchers at the national fusion laboratory in CIEMAT study the same processes to optimise future power plants. Could the two fields help each other out?

Common language

Navarro got in contact with plasma expert Carlos Hidalgo at CIEMAT, the Spanish member of EUROfusion. With support from EUROfusion’s technology transfer programme FUTTA, the two groups worked together to transfer the plasma expertise from CIEMAT and apply that to plasma thrusters.

“Even though we both work on plasmas, our technical terminology and our ways of looking at things are quite different”, recalls Navarro. “Once we had brought each other up to speed, we understood each other’s language and what challenges we have in common.”

Part of the work was to transfer theoretical knowledge and know-how on diagnostics from fusion to plasma thrusters, explains Hidalgo. For instance, by applying a fast camera that can sense processes in a fusion plasma at three different wavelengths to the plasma thruster experiments. This diagnostic let Navarro and his colleagues discover turbulent structures in their plasmas at a scale smaller than they had ever seen.

“The diagnostic camera shows us how this turbulence behaves in our thrusters; the next step is to learn how to control it and improve thruster performances.”

New spectroscopic system that allows simultaneous 2D-imaging of density, electron temperature and neutral dynamics (left), Filtered images in the TJ-II stellarator (right) – (Source: CIEMAT)
New spectroscopic system that allows simultaneous 2D-imaging of density, electron temperature and neutral dynamics (left), Filtered images in the TJ-II stellarator (right) – (Source: CIEMAT)

Two-way street

For Carlos Hidalgo, the tech transfer project was a two-way street. With both fields interested in observing and understanding plasma turbulence, the collaboration gives the researchers access to turbulence data in two very different regimes, he explains: “fusion plasmas are extremely hot and highly magnetised; in plasma thrusters the magnetic fields and temperatures are much lower. But you get turbulence in both, leaking heat from a fusion plasma and lowering the performance of a plasma thruster. It is very interesting to those side-by-side.”

In this case, collaboration resulted in a broader basis to validate computer models under conditions that the two groups would never have access to on their own. That lets Hidalgo’s group improve their predictions for turbulence control in future fusion power plants.

Images measured in the EP2-UC3M Helicon Plasma Thruster (Source: EP2-UC3M/CIEMAT)
Images measured in the EP2-UC3M Helicon Plasma Thruster (Source: EP2-UC3M/CIEMAT)

Aspirations

Although the formal project for their technology transfer has ended, the two groups are eager to keep working together. One topic they want to explore is the influence of neutral particles — complete atoms instead of ionised ones — in their two fields. Neutral particles fly into the plasma after it contacts the wall of the experiment, diluting and cooling the plasma. “If we can study these phenomena under the conditions of fusion and of space propulsion, we get a much more complete picture. Everyone benefits.”

Funding for such a new collaboration is still being sought, but the will is there. Hidalgo: “It helps that both are fields are very practical in our outlook. We study and model these phenomena because in the end, we want to know how to control them and improve our devices.”

But working with people from outside your field is about more than solving specific problems, adds the plasma physicist: “I wish every researcher the opportunity to look outside their own field. Every time you work with people from another discipline, you learn something new and get inspired by their creative approaches.”

Top view of the TJ-II stellarator: flagship project of the LNF CIEMAT with the objective to study the physics of magnetically confined plasmas (Source: CIEMAT)
Top view of the TJ-II stellarator: flagship project of the LNF CIEMAT with the objective to study the physics of magnetically confined plasmas (Source: CIEMAT)
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