Plasma diagnostics for Wendelstein 7-X

The plasma of Wendelstein 7-X is to be observed by means of measuring instruments, diagnostics as they are called. Installation of such apparatus is in full swing.

A basic stock of diagnostics must be available for the first operation phase of the device, which startet end of 2015. These serve to control the plasma and record its essential characteristics, including:

  • Interferometers (measurement of the mean electron density)
  • Langmuir probes (electron density and temperature at the plasma edge)
  • Electron cyclotron emission diagnostics (profile of the electron temperature)
  • Video and infrared cameras (plasma edge and heating of components)

The flux surface diagnostics came into play, however, before production of the first plasma to measure the magnetic flux surfaces. The results obtained were awaited with keen interest: this checks whether the superconducting magnets generate the magnetic field with the desired accuracy – the field being the core piece of the Wendelstein device, whose shape governs the quality of the thermal insulation attainable for the plasma.

For this purpose, a thin electron beam traces the magnetic field lines in the evacuated plasma vessel. In a precisely calculated manner, they are to form nested surfaces, i.e. the magnetic flux surfaces. A rod-shaped fluorescence detector sweeps across the cross-section of the magnetic field. Whenever it is impinged on by the electron beam, a small flash of light is emitted and is recorded by a video camera. The electron beam passing along the magnetic field lines eventually traces the complete plasma cross-section. This clarifies whether the specially shaped magnet coils of Wendelstein 7-X form the appropriate magnetic field cage for the plasma. For first result see IPP Info 7/2015.

Installation of the diamagnetic loop in the plasma vessel: the copper loop (foreground) encircles the whole plasma cross-section and records variations of the magnetic field. They are caused by variations of the plasma energy or the distribution of electric currents in the plasma.

Subsequent experimentation phases will enlist many other measuring facilities to determine the plasma properties in every detail:

  • Thomson scattering (profiles of the electron temperature and density)
  • HEXOS VUV/XUV spectrometer (impurities in the plasma core)
  • X-ray spectrometers (electron and ion temperature, plasma rotation, impurity distribution)
  • Spectrometers for the visible wavelength region (impurities at the plasma edge)
  • Diamagnetic loop (total energy of the plasma)
  • Rogowski coils (net current in the plasma)
  • Mirnov coils (variation of the magnetic field)
  • Reflectometers (turbulence, plasma rotation, electron density profile at the plasma edge)
  • Bolometers (electromagnetic waves from the plasma)
  • Thermography (temperature of components at the plasma edge)
  • Charge exchange diagnostics (ion temperature, electric fields)
  • and many others

The measuring facilities use very different physical effects for diagnostics. The aim is to determine the properties of the plasma without perturbing it.

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