Alfv\'{e}n eigenmode activity during the sawtooth phase in Alcator C-Mod

Recent current ramp experiments in Alcator C-mod have utilized early ICRF heating (P$_{RF}$ $\sim$ 3 MW) and low densities (n$_{e}$l $\sim $ 0.6x10$^{20}$ m$^{-2})$ to destabilize core localized Alfv\'{e}n eigenmodes [1]. This results in creation of Alfv\'{e}n cascades, chirping modes highly sensitive to small changes in the minimum of the q-profile [2,3]. This great sensitivity makes Alfv\'{e}n cascades particularly useful as a diagnostic and MHD monitor [3]. The phase contrast imaging (PCI) system has observed modes similar to the cascades, but preceding sawteeth in sufficiently low density plasmas. The hybrid kinetic-MHD code NOVA-K [4] has provided results showing the theoretical existence of q=1 cascade modes under plasma conditions taken from actual discharges. Implications for post- crash current profiles will be presented. \newline \newline [1] M. Porkolab \textit{et al}., IEEE Trans. on Plasma Science \textbf{34}, 229 (2006). \newline [2] H. L. Berk \textit{et al.,} Physical Review Letters \textbf {87}, 185002 (2001). \newline [3] S.E. Sharapov \textit{et al}., Physics Letters A \textbf {289}, 127 (2001). \newline [4] C.Z. Cheng and M.S. Chance, J. of Comp. Physics \textbf{71}, 124 (1987).