Erlangen 2026 – scientific programme

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P: Fachverband Plasmaphysik

P 16: Poster Session Plasma Physics

P 16.100: Poster

Thursday, March 19, 2026, 13:45–15:45, Redoutensaal

High Frequency Dispersion Interferometry for Alfvén Mode Studies — •Vineeta Nair — Max Planck Institute for Plasma Physics, Greifswald

Dispersion interferometry is emerging as a preferred diagnostic for measuring line-integrated electron density in many fusion devices, offering a robust alternative to the traditional dual-color interferometers because its phase measurements are inherently immune to mechanical vibrations of optical components. At Wendelstein 7-X (W7-X), a phase-modulated dispersion interferometer based on a continuous-wave CO2 laser (10.6 µm) serves as the standard electron density diagnostic. Phase modulation in the current diagnostic setup is performed using a photo-elastic modulator, which offers a good modulation depth (ρ), but is limited to modulation frequencies (ωm) of only a few kHz due to its mechanical resonance based modulation. With the current diagnostic, fluctuations in electron density can be measured only up to 50 kHz. To investigate several instabilities*such as MHD modes, kinetic ballooning modes, and fast-ion*driven Alfvén modes*it is essential to extend the capabilities of the standard single-channel dispersion interferometer to a high-frequency, multi-channel system capable of localized electron density measurements. To achieve higher modulation frequency, a 40 MHz GaAs electro-optical modulator is planned for implementation. Using multiple plasma sightlines together with appropriate inversion techniques will allow spatially resolved, high-frequency electron density fluctuation measurements. This poster will outline the project plan, present the current status, and discuss the main technical challenges.

Keywords: Dispersion Interferometry; Electron density profile; Phase modulation; Electro-optical modulator; Plasma instability modes