Core Plasma Diagnostics

The group develops and operates plasma diagnostics that allow quantifying the success of the confinement optimization. For that, the profiles of important plasma quantities – their radial distribution from the plasma center to the edge – are measured. A further optimization to be tested focuses on the confinement of fast particles representing the confinement of later fusion products.


 

In the second operation phase (OP2) scheduled to start late 2022, the diagnostics listed below provide profiles of the electron density, the electron- and ion temperature, some important plasma impurities, and of the radial electric field. The latter results from differences in electron and ion particle transport such that the plasma charges up.

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• Laser Interferometry uses the innovative technique of Dispersion Interferometry for a continuous tracking of the average electron density and thus provides the signal for density control. In a muti-sightline system under development, the shape of the density may be tracked to control the density profile shape as well.
• Thomson Scattering launches high-power laser pulses and derives the local electron density and local electron temperature from intensity and spectrum of the laser light scattered at the plasma electrons.
• The Electron Cyclotron Emission Diagnostic (ECE) derives continuously the local radiation temperature of the electrons by measuring the intensity of the microwave emission resulting from the gyro-motion of the electrons around the magnetic field lines.
• Reflectometry probes the electron density distribution in the plasma by means of microwave radar techniques.
• Two Imaging x-ray Spectrometers use the x-ray line emission caused by high-Z plasma impurities to derive quantities like the local impurity density in the plasma, the radial electric field and also the temperature of the exciting plasma electrons
• The Beam Emission - and Charge Exchange Resonance Spectroscopy (CXRS) observes visible light emitted along the trajectories of the neutral particle heating beams resulting from interaction of the neutral hydrogen atoms with plasma particles. From that, the local temperature of the plasma ions, the local electric field and other quantities can be derived.
• Passive spectroscopy along several sightlines and angles observes the emission from plasmas impurities penetrating the edge regions of the plasma and from that derives their propagating velocities and temperature.
• Calibrated Neutron Counters observe the eventual emission of neutrons as they will result from later deuterium-deuterium fusion processes in the plasma.

One goal of Wendelstein 7-X optimization is an improved confinement of fast ions as they resemble fusion products in a later HELIAS reactor. Their good confinement is necessary for the self-heating of the plasma in a reactor but also avoids localized overheating of wall elements by escaping particles. To mimic the later alpha particles of a reactor Wendelstein 7-X uses the fast ions resulting from NBI or ICRH heating. A set of diagnostics is being developed to assess their confinement. The observations are compared with code predictions.

• A set of diagnostics for fast particles is being developed which - together with numerical modelling - allows to study their birth profile, their slowing-down by collisions with particles from the thermal plasma and monitors and quantifies respective local wall loads. Collaborations with the Princeton Plasma Physics Laboratory (PPPL), USA and NIFS, Japan support these investigations.