Microwave scattering diagnostic on the Advanced Toroidal Facility torsatrona)

Abstract

A 2 mm scattering diagnostic used to study density fluctuations of the electron cyclotron resonance heated plasma in the Advanced Toroidal Facility is described. A four‐channel flexible antenna design was chosen to monitor fluctuations within the wave number range of 3<k⊥<20 cm−1 and to locate the scattering volume anywhere in the stellarator minor cross section. Special attention was paid to the system calibration and to the normalization of the k spectra. Analysis of the diagnostic noise showed that the main source of spurious signal was a scattering from high‐level edge turbulence driven by the second‐harmonic radiation from the gyrotron used for plasma heating. This spurious signal was suppressed using waveguide filters and rearrangement of the microwave circuit elements. Evidence in support of direct scattering from a well‐defined plasma volume was found. Good correlation of the scattered waves’ intensity with the density scale length was observed. Use of a dual‐homodyne circuit configuration made is possible to determine that the plasma density fluctuations were propagating in the electron diamagnetic direction.