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research article

Imaging of turbulent structures and tomographic reconstruction of TORPEX plasma emissivity

Iraji, Davoud  
•
Furno, Ivo  
•
Fasoli, Ambrogio  
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2010
Physics of Plasmas

In the TORPEX [ A. Fasoli et al., Phys. Plasmas 13, 055902 (2006) ], a simple magnetized plasma device, low frequency electrostatic fluctuations associated with interchange waves, are routinely measured by means of extensive sets of Langmuir probes. To complement the electrostatic probe measurements of plasma turbulence and study of plasma structures smaller than the spatial resolution of probes array, a nonperturbative direct imaging system has been developed on TORPEX, including a fast framing Photron-APX-RS camera and an image intensifier unit. From the line-integrated camera images, we compute the poloidal emissivity profile of the plasma by applying a tomographic reconstruction technique using a pixel method and solving an overdetermined set of equations by singular value decomposition. This allows comparing statistical, spectral, and spatial properties of visible light radiation with electrostatic fluctuations. The shape and position of the time-averaged reconstructed plasma emissivity are observed to be similar to those of the ion saturation current profile. In the core plasma, excluding the electron cyclotron and upper hybrid resonant layers, the mean value of the plasma emissivity is observed to vary with (Te)^α(ne)^β, in which α = 0.25–0.7 and β = 0.8–1.4, in agreement with collisional radiative model. The tomographic reconstruction is applied to the fast camera movie acquired with 50 kframes/s rate and 2 μs of exposure time to obtain the temporal evolutions of the emissivity fluctuations. Conditional average sampling is also applied to visualize and measure sizes of structures associated with the interchange mode. The ω-time and the two-dimensional k-space Fourier analysis of the reconstructed emissivity fluctuations show the same interchange mode that is detected in the ω and k spectra of the ion saturation current fluctuations measured by probes. Small scale turbulent plasma structures can be detected and tracked in the reconstructed emissivity movies with the spatial resolution down to 2 cm, well beyond the spatial resolution of the probe array.

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Type
research article
DOI
10.1063/1.3523052
Web of Science ID

WOS:000285770500012

Author(s)
Iraji, Davoud  
Furno, Ivo  
Fasoli, Ambrogio  
Theiler, Christian  
Date Issued

2010

Publisher

American Institute of Physics

Published in
Physics of Plasmas
Volume

17

Issue

12

Article Number

122304

Subjects

TORPEX group

Editorial or Peer reviewed

REVIEWED

Written at

EPFL

EPFL units
CRPP  
SPC  
Available on Infoscience
February 9, 2011
Use this identifier to reference this record
https://infoscience.epfl.ch/handle/20.500.14299/64121
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