2012 Volume 7 Pages 2402031
Observations of the spectral broadening of lower hybrid (LH) waves in Alcator C-Mod are presented. We report the dependenceof the pump broadeningon two plasma parameters: plasma density and magnetic topology. As the plasma density was raised, we observed a significant increase of the pump broadening at 10 dB below the maximum peak. However, at the constant plasma density, when the magnetic topology was changed from the diverted plasma to the inner wall limited plasma, the spectral broadening was reduced. In an attempt to understand the causes of this variation, we report the change of density profiles in front of the LH launcher in two different magnetic topologies. The model analyses indicate that both the scattering process and parametric decay instability can contribute to the spectral broadening. Further experimental investigations are necessary to identify the causality between spectral broadening and the degradation of lower hybrid current drive efficiency at high density plasmas.