Description
Dispersion interferometry has established itself as a common diagnostic for measuring the line-integrated electron density in many fusion devices, offering a robust alternative to the traditional two-color interferometer, which requires two separate interferometric setups for the two wavelengths (colors). It is a self-referencing interferometer in which a probing laser beam co-propagates with its second harmonic, orthogonally polarized relative to the fundamental beam. At Wendelstein 7-X (W7-X), a phase-modulated dispersion interferometer based on a continuous-wave CO¬2¬ laser (10.6 μm) serves as the standard line integrated electron density diagnostic. Its bandwidth, however, is limited to 25 kHz, owing to the limited modulation frequency of the optical phase modulator used in the system. Moreover, the current diagnostic cannot provide localized electron density measurements. Previous experiments at W7-X have shown that rapid fluctuations in electron density impact the device performance, highlighting the need for a better understanding of these fluctuations. Hence, it is essential to extend the capabilities of the standard single-channel dispersion interferometer to a better bandwidth and spatially resolved electron density measurements. The aim of this project is to develop an electron density diagnostic addressing these limitations, by building a high bandwidth, multi-channel interferometer. Multiple plasma sightlines together with appropriate inversion techniques will allow spatially resolved measurements. A 10-channel dispersion interferometer is being developed. For the first upcoming W7-X experimental campaign in 2026, at least four channels are planned to be operational. To overcome the bandwidth limitation, a 40 MHz GaAs electro-optical modulator (EOM) is planned to be used for phase modulation, allowing the measurement of fast density fluctuations that can potentially aid in understanding fast instabilities like fast-ion-driven Alfvén modes. This work presents the design and the initial qualification of the system.