Density fluctuation measurements by far-forward collective scattering in the MST reversed-field pinch

• Published in: Review of scientific instruments 2012-10, Vol.83 (10), p.10E302-10E302-3
• Main Authors: Ding, W. X., Lin, L., Duff, J. R., Brower, D. L., Sarff, J. S.
• Format: Article
• Language: English
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Broadband; Density; DOPPLER EFFECT; ELECTRON DENSITY; Fluctuation; FLUCTUATIONS; Fluid dynamics; INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; INTERFEROMETERS; IONS; LARMOR RADIUS; LASER RADIATION; MAGNETOHYDRODYNAMICS; PLASMA; PLASMA BEAM INJECTION; PLASMA DENSITY; PLASMA DIAGNOSTICS; POLARIMETERS; REVERSE-FIELD PINCH; REVERSED-FIELD PINCH DEVICES; SCATTERING; TEARING INSTABILITY; TURBULENCE; Wavenumber
• ISSN: 0034-6748; 1089-7623
• DOI: 10.1063/1.4728098

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 μ m (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.