Modernization of the filtration system for the Doppler reflectometry diagnostics of the L-2M stellarator for operation in regimes high-power ECR heating

The commissioning of the MIG-3 gyrotron complex at the L-2M stellarator has made it possible to reach a record power density input into plasma of 3.4 MW/m3 during the ECR heating of the plasma. This power input, however, resulted in noisy Doppler reflectometry signals (with a high-amplitude component at the gyrotron working frequency of 75.3 GHz), which could not be processed and analyzed. We modernized the filtering system of the Doppler reflectometry diagnostics, which had consisted of two pin-type band stop waveguide filters. Another pin-type filter was added to the filter system and a new compact bandstop filter was designed based on the Fabry-Perot resonator (BFFPR) to provide high suppression at the frequency of 75.3 GHz and low attenuation at the 30–40 GHz diagnostic frequency. We optimized the cavity length of the filter and calculated the characteristics of filters with different numbers of resonators by using numerical simulations. The filter layout was designed, and its characteristics were measured. The results of experimental measurements and numerical simulations have a good match. The redesigned filters were used to suppress the noise from the MIG-3 gyrotron complex.