How to see inside the ITER Vacuum Vessel?

Representation of the IVVS probe deployed inside the ITER Vacuum Vessel. © F4E

The In-Vessel Viewing System (IVVS) will scan the interior of ITER’s Vacuum Vessel to obtain information about the state of the plasma-facing components, looking for damage and erosion. The system will have six cartridges located in different vacuum vessel port extensions. A probe will be deployed from each cartridge, like a sophisticated periscope, to see where our eyes cannot.

The IVVS will launch a laser to scan the surfaces of the blanket and divertor. This is the function of the pan & tilt head. A portion of the backscattered light from the target surfaces will retrace its way through the head, then into the probe body, made of a complex assembly of windows, lenses, mirrors, fibres and other components – the IVVS Measurement System. In a nutshell, by comparing the received light to the emitted, the system is able to reconstruct both 2D and 3D images of the interior of the machine.

Following the approval of the preliminary design of the IVVS in 2021, F4E has just signed a contract with Bertin Technologies to develop the final design of the IVVS Measurement System. This design phase will build up on lessons learned from the full-scale prototyping and testing activities started in late 2020, already with Bertin Technologies, in parallel with the Preliminary Design Review.

Latest design of the IVVS Measurement System probe body.
Latest design of the IVVS Measurement System probe body. © F4E

“The F4E IVVS team and Bertin have developed a very constructive way of working together. They are doing some great work with the prototypes, our teams look forward to resolving few outstanding issues of our system,” says Gregory Dubus, F4E Project Manager for the IVVS. Julien Marque, Bertin Project Leader, also shares his thoughts. “This is a nice opportunity to demonstrate that we are able to work together from the conceptual design phase of a unique instrument to its manufacturing. In particular, we are proud to address the challenging requirements from 1 down to 0.1 mm positioning accuracy, given the vacuum vessel constraints and the complex mechanical deployment of the IVVS probe.”

Another challenge within the IVVS Measurement System is related to the pieces of a motor used to steer the laser beam so that it points where appropriate. Due to the in-vessel high magnetic field during IVVS operations, a standard electromagnetic actuator would not be able to perform such tasks. By contrast, piezoelectric materials contract or expand depending on the voltage applied on them and are insensitive to magnetic fields. However, these motors wear away with use, so each IVVS probe will be replaced at least once during ITER’s lifetime. This operation will be carried out via remote handling means. “As we have built a full-scale prototype of the IVVS Measurement System in a previous contract, we want to conduct a live demonstration to check how this remote operation works. It’s the fastest and most effective way to convince everybody of the remote maintenance of our system,” explains Gregory Dubus.

Prototype of the IVVS Measurement System probe, during assembly.
Prototype of the IVVS Measurement System probe, during assembly. © F4E

Final design will also include activities related to validating the robustness of the IVVS Measurement System with respect to changes in temperature. Although the IVVS will not function during plasma operations, it will be exposed to temperatures ranging from 20 °C to 120 °C. Engineers have to make sure that this does not change the alignment of the optical components, preventing the system from getting reliable measurements.

Besides the Measurement System, the contract also covers tasks related to other IVVS sub-systems – such as the IVVS Confinement System, a vacuum-tight feedthrough enabling the transmission of electrical and optical signals from and to each IVVS cartridge. Ultimately, the goal is to bring the IVVS Measurement System to a Final Design Review in the second half of 2023. The Final Design Review of the Confinement System will follow shortly after.