The Czech tokamak COMPASS has undergone delicate surgery. A port modification has improved the surveillance of the plasma edge. Only one Czech company was in a position to accomplish this precise mission, after asking its European partners for support.

2 days of work, 9 months of preparation

The completion of the task itself only lasted 30 hours, but the overall preparations took nine months. The Czech tokamak COMPASS underwent a port modification to optimise the measurements for the edge Thomson scattering diagnostic. This is a method which detects the density and the temperature of electrons in the fusion plasma.

Old geometrical constraints

The COMPASS ports either face towards the plasma core or tangentially. However, the Thomson scattering diagnostic equipment also requires one port pointed towards the plasma edge. Until recently, it was not possible to cover the whole edge plasma region due to the geometrical constraints of the existing edge port. “We could only actually see the true edge of the plasma if we generated smaller plasma”, says Martin Hron, the Deputy Head of the Tokamak Department at the Institute of Plasma Physics (IPP) in Prague.

This is a diagnostic to measure temperature and density of the plasma. It uses intense bursts of laser light to interrogate the thermal motion of the plasma electrons by analysing the scattered light signal, performed here on a model of the Joint European Torus. Picture: EUROfusion

This is a diagnostic to measure temperature and density of the plasma. It uses intense bursts of laser light to interrogate the thermal motion of the plasma electrons by analysing the scattered light signal, performed here on a model of the Joint European Torus. Picture: EUROfusion

Keeping the vacuum

In order to optimise the view and enable the observation of the entire edge plasma region, the edge port had to be cut out and a new one at to be welded at a different angle while maintaining the vacuum capabilities. COMPASS’ vessel is made of the superalloy Inconel, a steel material with specific properties. The cut-out for the new port had to be precisely executed without damaging the vessel wall, magnetic coils or the support structure. So, the institute designed the new edge port and issued a call to tender. But only one company was able to meet the demands: IMV Design, a company which specialises in laser and robotic applications.

Shortest outage but highly demanding

“Almost everything in a tokamak is special. In this unique facility, most of the subsystems represent an extreme, such as the achieved temperature values and the intensity of the magnetic field. Of course, the price of such a device also commands respect. The need to ensure the shortest possible downtime of the tokamak was the most complicated requirement to be met. It actually meant that we could remove as few of the peripheral devices as possible“, says IMV Design’s Executive Manager Michal Vychroň.

Practice using a mock-up

The company’s specialists practised the laser cutting procedure on a quarter of a full size mock-up which is usually used for testing and special training. Thanks to the tests, a robotic laser was able to cut out the original port on the tokamak within 30 hours. After that, the new port was welded into place and the vacuum pumping for the tokamak was resumed. After resetting the necessary standard wall conditions, including the vacuum vessel baking, the tokamak was again ready for operation.

Searching and finding pedestals

Glimpse of the manual work inside the vessel of COMPASS. Picture: IPP Prague

Glimpse of the manual work inside the vessel of COMPASS. Picture: IPP Prague

The COMPASS team is happy about the changes: “The modification worked very well, we can resolve the pedestals in all H-mode plasmas now.” Pedestals are a feature within magnetically confined fusion plasma. This term refers to a global increase of a profile, such as pressure. When transiting from the L-mode to the H-mode, the appearance of the pedestal often leaves the profile gradients in the core region intact whereas the edge gradients increase sharply. Thus, it appears as if the core profiles are merely shifted upwards, as if put upon a pedestal.
Overall, the port modification has been a success for the scientists working on COMPASS: “One of the main goals of the modification was to enhance the plasma research in H-mode. Now we can start doing research”, says Hron.

“We can learn a lot from each other”

Michal Vychroň, Picture: IMV Design IMV Design’s Executive Manager Michal Vychroň talks about the challenges of modifying a tokamak. A project which was entirely new for its company.
How did you solve the task?
After a couple of attempts using the mock up, we could specify the appropriate components of the laser system. In this regard we were glad of the help from an international company called IPG Photonics Corporation which is a manufacturer of fibre lasers. We also collaborated with a supplier called Laser Mechanisms. They specialise in cutting heads. The procedure on the tokamak itself took only two days but it was a result of good preparation and collaboration.
What did you learn?
We were able to verify our concept of mobile robotics in practice, making 3D laser cuttings possible wherever and whenever it is needed.
What advice would you give to other fusion laboratories when it comes to an exchange with industry?
We were really proud to be able to cooperate with IPP Prague. So, I do not think that I am the one who should be giving advice to other fusion workplaces. But I am deeply convinced of the mutual benefit of close cooperation. We can learn a lot from each other.