Vertical localization of phase contrast imaging diagnostic in Alcator C-Mod

Phase contrast imaging (PCI) diagnostic has been used to study mode conversion physics of ion cyclotron range of frequency waves [E. Nelson-Melby, Phys. Rev. Lett. 90, 155004 (2003)], plasma turbulence [A. Mazurenko, Phys. Rev. Lett. 89, 225004 (2002); N. Basse, Phys. Plasmas 12, 052512 (2005)], and Alfv́n Cascades [M. Porkolab, IEEE Trans. Plasma Sci. 34, 229 (2006)] in Alcator C-Mod. The C-Mod PCI system measures line integrated electron density fluctuations along 32 vertical chords with a sampling frequency of 10 MHz and wavenumber resolution up to 30 cm-1. Although PCI normally lacks localization along the chords, the vertical variation of the magnetic field pitch angle allows for localized measurements for large k⊥ fluctuations. A system consisting of a partially masked phase plate on a rotatable stage has been installed and quasicoherent modes with wave number ∼5 cm-1 associated with the enhanced DαH mode at the top and bottom of the plasma have been differentiated. In future experiments, for k∼30 cm-1, a spatial resolution of ra∼0.3 can be achieved under ideal conditions.