Alcator C-Mod

Kelly Smith

Updated: 7/12/06

With the recent addition of the Lower Hybrid Current Drive System on Alcator C-Mod, measurements of the poloidal magnetic field and q-profile are increasingly important. One approach to directly measuring the magnetic field in hot fusion plasmas is Faraday rotation, in which laser light traversing the magnetized plasma experiences a rotation in linear polarization proportional to line integrated product of the plasma electron density and magnetic field parallel to the direction of propagation. To make this measurement we plan to implement a multi-chord far-infrared laser polarimeter viewing the plasma with a poloidal fan of beams entering the tokamak through a window on the outboard side and returning along the same path after retro-reflection on the inner wall. Viewing with approximately twenty chords will allow the data to be inverted and current profiles to be determined. This information is essential if the lower hybrid current drive experiments are to be understood.

A polarimeter prototype has already been constructed and implemented on C-Mod. This one-chord experiment has measured electron density and inner wall movement in the poloidally viewing configuration, and has recently been installed on the Two-Color Interferometer (TCI) with a vertical view. At the current wavelength (10.6 m m), rotations of only 0.1 to 0.3 degrees are expected, and good signal-to noise is difficult to obtain. However, preliminary results show reasonable agreement with theoretically predicted values. In addition, analysis and optimization of measurement techniques and assessment of performance of in-vessel optics are useful offshoots of the present work.