Thomson scattering upgrades on Alcator C-Mod

The Thomson scattering (TS) diagnostics set has been upgraded to deliver measurements of electron temperature (T-e) and density (n(e)) with improved temporal and spatial resolution. Two 30 Hz, 1.3 J Nd-doped yttrium-aluminum-garnet (Nd:YAG) lasers share a vertical beam path through the tokamak plasma and are fired in an alternating fashion, allowing the collection of scattered light with a 60 Hz repetition rate. Eight compact polychromators have been added to the system, better than doubling the spatial resolution of core plasma TS measurements and allowing better diagnosis of core ne and T-e gradients. Each fiber holder can. be repositioned easily, making-it possible to concentrate core measurements in the regions of particular interest with a spatial resolution of similar to1, cm. The newly designed mount can be positioned with submillimeter precision, and its alignment with respect to the Nd:YAG beam can be adjusted between discharges using remote controlled stepper motors. Cross calibration of TS ne measurements with electron cyclotron emission (ECE) diagnostics is performed using dynamic ramps of both plasma density and magnetic field to force ECE signal cutoffs at known values of n(e). This technique supplements an absolute calibration based on Raman scattering in hydrogen and deuterium gas. (C) 2003 American Institute of Physics.