CO 2 laser based two-volume collective scattering instrument for spatially localized turbulence measurements

Abstract

We describe and demonstrate a two-volume collective scattering system for localized measurements of plasma turbulence. The finite crossfield correlation length of plasma turbulence combined with spatial variations in the magnetic field direction are used to obtain spatially localized turbulence measurements at the Wendelstein 7-AS fusion experiment. By comparing with localized measurements obtained using the single wide beam technique [Truc et al., Rev. Sci. Instrum. 63, 3716 (1992)] we provide a proof-of-principle demonstration of the two-beam method. An optimized configuration with large toroidal separation of the measurement volumes is predicted to be capable of providing spatial resolution better than 5 cm.

Study of intermittent small-scale turbulence in Wendelstein 7-AS plasmas during controlled confinement transitions

“…If the actual sample length is 10 ms, this means that we average over M = ∆ν/δν = 100-1000 points in the spectrum. The power in uncorrelated parts of the spectrum is reduced due to phase mixing by a factor √ M = 10-30 [9,18]. Contributions to the remaining spectrum are due to correlated signals, the power being proportional to the power of the correlated fluctuations.…”

“…The localized turbulence scattering (LOTUS) density fluctuation diagnostic has been described in detail elsewhere [9]. We will therefore limit ourselves to a rudimentary description below.…”

“…Two features exist in SG confinement: A low frequency, high amplitude feature travelling in the ion DD direction and a high frequency, low amplitude feature travelling in the electron DD direction [9,10]. Assuming that the frequency shift of the fluctuations is dominated…”

“…We have previously demonstrated that some spatial localization can be obtained by calculating the crosspower spectrum between two spatially separated, narrow measurement volumes [9,10].…”

“…The diagnostics available for our analysis are Mirnov coil measurements of magnetic fluctuations [8] and collective scattering measurements of density fluctuations using an infrared light source [9].…”