title: Density fluctuation measurements by far-forward collective scattering in the MST reversed-field pinch.
abstract: The multichannel polarimeter-interferometer system on the MST reversed-field pinch can be utilized to measure far-forward collective scattering from electron density fluctuations. The collective scattering system has 11 viewing chords with ∼8 cm spacing. The source is a 432 μm (694 GHz) far infrared laser and the scattered power is measured using a heterodyne detection scheme. Collective scattering provides a line-integrated measurement of fluctuations within the divergence of the probe beam covering wavenumber range: k(⊥) < 1.3 cm(-1), corresponding k(⊥)ρ(s) < 1.3 (ρ(s) is the ion-sound Larmor radius), the region of primary interest for turbulent fluctuation-induced transport. The perpendicular wavenumber consists of toroidal, poloidal, and radial contributions, which vary with chord position. Coherent modes associated with tearing instabilities and neutral-beam driven fast particles are observed along with broadband turbulence at frequencies up to 500 kHz. Changes in frequency are consistent with a Doppler shift due to parallel plasma flow.
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.forename: L
authors.initials: L
authors.lastname: Lin
authors.forename: J R
authors.initials: JR
authors.lastname: Duff
authors.forename: D L
authors.initials: DL
authors.lastname: Brower
authors.forename: J S
authors.initials: JS
authors.lastname: Sarff