Despite facing several challenges, project leader Gábor Kocsis is satisfied after all. At the end of June, he and his team from the Hungar ian Wigner Research Centre for Physics, delivered the final parts of their unique ten-channel camera system to Wendelstein 7-X on time. Their ‘Event Detection Intelligent Camera’ (EDICAM), the special eye on the stellarator wall, is designed to detect radiation of plasma particles and local overheating and instantly feed this information back to the control system. The Hungarian project will help the stellarator to start operating accurately at the end of this year.

One camera to meet all requirements

The camera system must, primarily, protect the Wendelstein 7-X by detecting dangerous operational situations. Thererefore, the main challenge the researchers faced was inventing a prototype camera system to protect the stellarator wall. Hence, it must handle several requests: firstly, it must provide an overview video of the entire cross-section of the stellarator. Secondly, it must be fast enough to record plasma phenomena that changes at a speed greater than one-thousandth of a second as the plasma touchesthe structural elements of the vessel wall. At the same time,
the entire plasma discharge must be covered, for up to half-an-hour without storage problems. Thirdly, all of these events must be communicated promptly to the plasma control system. Additionally, the cameras must operate in the harsh environment of a high magnetic field, while being exposed to radioactivity and consideration heat radiation by the hot plasma. Above all this, the camera system will also be used for scientific investigations.

Camera now makes decisions on the fly

Draft of videodiagnostics in the W 7-X vessel.

Videodiagnostics in the Wendelstein 7-X stellarator vessel. (Image: IPP)

The protection of the stellarator wall involves a wide angle view, recordings of up to 10,000 frames per second (fps) and footage lasting more than half an hour. This takes up storage space. Supported mostly by software, even contradictory requirements can be handled. EDICAM provides both a full-frame size overview and the fast observation of a small area. With the help of a special chip, large areas need to be read out at a slow speed, whereas small areas need to be monitored at high speed. These somewhat contradictory functions make it possible for the camera to
make decisions on the fly. Therefore, footage is analysed in real-time. This functionality inspired the name of this device: Event Detection Intelligent CAMera, or EDICAM for short.
Also, the detected events, if considered dangerous, must be able to influence the plasma control system. Thus, the controller then must take measures to protect the fusion machine. The storage problem has been solved by using powerful data acquisition technology: all sensor data from the EDICAM is streamed directly to SSD disks instead of using the limited camera memory like most cameras.

Watching a football match from four different angles

The special Hungarian camera system runs at 400 fps at full resolution, but is able to speed up the readout of interesting areas if pre-defined hazardous events occur. It was designed to simultaneously record the equivalent of a small movie, containing only a part of the whole image, while a full-sized movie is being recorded. It is like a TV-camera, transmitting a football match, with four integrated slowmotion cameras designed to zoom into various sections of the playing field. For the stellarator, this means that the EDICAM is capable of handling up to four so-called regions-of-interests (ROIs).

This has been realised by the use of a special complementary metal-oxide-semiconductor sensor with non-destructive readout capability. Ordinary camera sensors erase the data when the image is read out, and when only a region-of-interest is read out. Contrary to that, EDICAM is capable of reading out (part of) the data from the sensor without erasing it. It is like using “copy-paste” rather than “cut-paste”.

Hungarian scientist Tamás Szepesi adjusts the camera properly.

Hungarian scientist Tamás Szepesi adjusts the camera properly.

4096 Shades of grey

Besides the speed and the non-destructive readout capability, light intensity is more important to scientists than colour. Therefore the sensor of the Hungarian camera provides a black-and-white image. However, it differentiates between 4096 shades, 16 times more than an ordinary camera. Additionally, interference filters can be used for colour selection. Processing fast camera video streams in real-time requires very high computational power and the EDICAM system uses Field Programmable Gate Arrays for data processing and for camera control. These advanced microchips are as fast as real hardware chips, but their internal structure, and hence functionality, is determined by a programme code loaded at start up. This enables the development of new features and the incorporation of such without the need for hardware modifications.

Camera system plays a key role during the upcoming experimental campaign

Video diagnostics project leader Gábor Kocsis is very happy with the system that his team achieved to mitigate scientific risks at the stellarator: “The video diagnostic system is one of the largest development projects of the Hungarian Research Unit. We hope it will play a key role in several investigations during the forthcoming experimental campaigns both with regard to the scientific field and the safety of the stellarator itself. Based on this successful cooperation between the Wigner RCP and Max Planck Institute for Plasma Physics, new projects have already been started.”

The Wendelstein 7-X stellarator is the cornerstone experiment of Mission 8 within the European fusion roadmap. The mission is dedicated to developing the stellarator line to maturity as an alternative way to achieving fusion energy. The Hungarian Wigner Research Center for Physics is a signatory to the EUROfusion consortium and specialises in diagnostics.