Reversed Shear Alfv\'{e}n Eigenmodes in the Presence of Current Ramps and ICRF Minority Tails in Alcator C-Mod*

Alfv\'{e}n cascades (ACs), or equivalently, reverse shear Alfv\'{e}n eigenmodes (RSAE) have been observed in Alcator C-Mod during current ramps with intense ICRF minority heating$^{1}$. Both phase contrast imaging (PCI) and magnetic pick-up coils have been used to study the frequency spectrum. PCI can also give an estimate of the k$_{\theta }$-spectrum. From such data, the evolution of the q-profile has been deduced. The results indicate that up to 0.18 seconds reversed shear should be present with peak electron temperatures approaching 5 keV, offering a reasonable target plasma for off-axis lower hybrid current drive (LHCD) to maintain the reversed shear for AT studies. Without LHCD the current profiles relax to monotonic q-profiles and sawteeth set in by 0.3 sec. Using the ideal MHD code NOVA-K$^{2}$, the frequency chirping characteristics of these modes have been modeled with good accuracy and the code predicts accurately the minimum frequency $\Delta \omega $, which may be interpreted as the geodesic deformation of the Alfv\'{e}n continuum at low frequency$^{3}$. (1) M. Porkolab et al, Proc. 15$^{th}$ Conf. High RF Power in Plasmas, Park City, UT, (2005). (2) C. Z. Cheng, Phys. Rep. 211, \textbf{1} (1992). (3) M.S. Chu, J. M. Greene, L.L. Lao et al., Phys. Fluids B \textbf{4}, 3713 (1992). *Work supported by USDOE Contract DE-AC02-76-CH0-3073 and Cooperative Agreement No. DE-FC02-99-ER54512.