title: An upgraded interferometer-polarimeter system for broadband fluctuation measurements.
abstract: Measuring high-frequency fluctuations (above tearing mode frequencies) is important for diagnosing instabilities and transport phenomena. The Madison Symmetric Torus interferometer-polarimeter system has been upgraded to utilize improved planar-diode mixer technology. The new mixers reduce phase noise and allow more sensitive measurements of fluctuations at high frequency. Typical polarimeter rms phase noise values of 0.05°-0.07° are obtained with 400 kHz bandwidth. The low phase noise enables the resolution of fluctuations up to 250 kHz for polarimetry and 600 kHz for interferometry. The importance of probe beam alignment for polarimetry is also verified; previously reported tolerances of ≤0.1 mm displacement for equilibrium and tearing mode measurements minimize contamination due to spatial misalignment to within acceptable levels for chords near the magnetic axis.
authors.affiliation: Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, California 90095, USA.
authors.forename: E
authors.initials: E
authors.lastname: Parke
authors.affiliation: Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, California 90095, USA.
authors.forename: W X
authors.initials: WX
authors.lastname: Ding
authors.affiliation: Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: J
authors.initials: J
authors.lastname: Duff
authors.affiliation: Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, California 90095, USA.
authors.forename: D L
authors.initials: DL
authors.lastname: Brower