Plasma diagnostics for Wendelstein 7-X

The plasma of Wendelstein 7-X is observed by means of measuring instruments, diagnostics as they are called.

Wendelstein 7-X from above (left side) and from below. (right) The 153 diagnostic ports are marked in red. A diagnostic system can occupy several ports and up to ten diagnostics can share one port.

Of the total of 254 ports that lead into the plasma, 153 ports are available for the various measuring devices.

In the current reconstruction phase, existing measuring instruments are being extensively upgraded and further developed and new measuring systems are being installed in order to be able to comprehensively explore the plasma properties. Since Wendelstein 7-X is designed to investigate reactor-relevant plasmas for 30 minutes, all diagnostics must be able to withstand extreme thermal and mechanical loads. In addition, they must be shielded against microwave radiation from the plasma heating as well as against particle and X-ray radiation.

Ten cameras monitor the entire interior of the machine in order to detect undesired operating conditions, such as the plasma touching structural elements, and transmit this information to the safety control system. The photo on the right was taken by one of the cameras during a plasma discharge.

Approximately 45 diagnostics are used for the Wendelstein 7-X device: They provide data for the scientific investigation of the plasma, control the plasma parameters or ensure the safe operation of the system. The measuring devices use different physical effects for diagnostics. The aim is to determine the properties of the plasma without perturbing it.

The operational diagnostics include various video and infrared cameras that monitor the first wall in real time to prevent it from overheating. Special attention is paid to the most heavily loaded component, the divertor.

Thomson scattering diagnostics: Laser beams are brought to the plasma vessel in protective tubes and guided through the plasma via deflecting mirrors. Two lenses collect the light scattered by the electrons and guide it through optical fibers to the analysis unit. The components can be aligned with millimetre precision using an accessible support structure. The local temperature and the density of the electrons are determined from the spectrum and intensity of the scattered light.

In order to analyse the plasma and to understand its complex behaviour as comprehensively as possible, a large number of different plasma parameters must be determined simultaneously. These are the density of electrons and ions, the type and density of impurities in the plasma, the temperature of the electrons and ions (which can vary considerably depending on the heating scenarios used), the total stored energy, the plasma pressure, the radiation loss, currents and electric fields in the plasma, and many others.

Many parameters depend on the location of the measurement. For example, the electron temperature and density typically reach maximum values in the plasma center, while the values at the plasma edge can be much smaller, depending on the chosen plasma scenario. Therefore, many of the properties of the plasma are determined as profiles over the entire plasma cross section.

20 pinhole cameras measure along 360 lines of sight the X-ray light emitted by impurities in the plasma. Using computer tomography methods, the dynamics of plasma instabilities and the shape of the nested magnetic flux surfaces can be calculated from this data. The right figure shows the four segments of the diagnostic system before it is installed in the plasma vessel.

The diagnostic system is designed redundantly, i.e. a certain physical parameter, e.g. the electron temperature, is measured simultaneously by several different measuring systems based on different physical principles, so that systematic errors can be excluded.

Also important for understanding the processes in the plasma are measurements of the turbulent transport of particles and energy, which must be performed with high temporal and spatial resolution.

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