High-k scattering of low frequency MHD activities in NSTX

Low frequency MHD modes in tokamaks usually have long wavelengths outside the detection range of high-k scattering diagnostics. However, data from the NSTX high-k scattering system exhibit many narrow low frequency (10-100 kHz) peaks associated with various low frequency MHD activities, from fishbones to Alfv\'{e}n eigenmodes. These MHD modes are basically shear Alfv\'{e}n waves, they can convert into short wavelength kinetic Alfv\'{e}n waves near Alfv\'{e}n resonance and thus become detectable by high-k scattering. Such observation was first reported in TFTR.\footnote{K.L. Wong et al., Phys. Lett. A \textbf{224} (1996) 99.} Since the NSTX scattering system mainly detects the radial component of \textbf{k}, it can see more detailed features of these mode-converted kinetic Alfv\'{e}n waves. Corroborating data from soft X-ray camera, Mirnov coils and detailed analysis of these data will be presented.