Performance analysis of the vertical viewing ECE diagnostic for non-thermal electron distribution measurement

The vertical viewing electron cyclotron emission (VECE) system on Alcator C tokamak is designed to analyze the non-thermal electron velocity distribution during lower hybrid current drive. The system employs vertical chord viewing through the center of the plasma to eliminate magnetic field variation along the line of sight, and a viewing dump made of Pyrex or Macor to decrease the effects of wall reflection and polarization scrambling. A rapid-scanning Michelson interferometer is used to obtain the frequency spectra. In principle, this design makes the frequency spectrum predominantly a function of the electron energy due to the relativistic mass shift. Effectiveness of the dump and the collection optics is evaluated by comparing the second harmonic thermal emission with and without the viewing dump. Results are summarized below. These results clearly show the polarized nature of the thermal emission in the second harmonic, and demonstrate the possibility of decreasing both the wall reflection and polariation scrambling by use of a compact (7cm x 7cm) viewing dump. The effectiveness of the viewing dump in reducing the background level is found to be approximately 90% for both modes in the second harmonic frequency range. Preliminary inspection of the non-thermal spectra in the second harmonic show the effect of background absorption to vanish with the installation of the dump