[Publication] A new technique for tokamak edge density measurement based on microwave interferometer

The diagnostic technique is based on measuring the change in phase and power of a microwave beam passing tangentially through the edge plasma, perpendicular to the background magnetic field.

To interpret the phase and amplitude measurements, refraction effects, phase change, and further interference of the beam fractions have to be taken into account.

A 3D full-wave model is constructed as a synthetic diagnostic for MILS and allows exploring this broad range of wave propagation regimes. The diagnostic parameters, such as its dimensions, frequency, and configuration of the emitter and receiver antennas, should be balanced to meet the target range and location of measurements.

It can therefore be adjusted for various conditions, and here, the diagnostic concept is evaluated on a chosen example, which was taken as suitable to cover densities of ∼10¹⁵ to 10¹⁹ m⁻³ on the edge of the ASDEX Upgrade tokamak at the Max Planck Institute for Plasma and Physik, Garching, Germany.

Once the database was established, a genetic algorithm was developed to reconstruct the density profile from the phase and amplitude measurements provided by the microwave interferometer with an accuracy of 5 to 10% for low density ~10¹⁷ m^-3. This precision is far superior to those usually obtained (~5 times).

This new approach has strong potential and should provide access to measurements of the time-evolution of density profiles during a fast plasma relaxation. It also offers the possibility of precise real-time control of the plasma edge position, which is necessary to save the wall of a future fusion reactor.

Title : A new technique for tokamak edge density measurement based on microwave interferometer

Authors: Mariia Usoltceva, Stéphane Heuraux, Ildar Khabibullin, Helmut Faugel

Journal: Review of Scientific Instruments

Date of publication (online) : January 2022

Lien https://doi.org/10.1063/5.0074838

Image caption: MILS diagnostic picture on ASDEX-Upgrade and raytracing proving multi-sub-beams in interaction