Imaging of turbulent structures and tomographic reconstruction of TORPEX plasma emissivity

Abstract

In the TORPEX ͓A. Fasoli et al., Phys. Plasmas 13, 055902 ͑2006͔͒, 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 ͑T e ͒ ␣ ͑n e ͒ ␤ , in which ␣ = 0.25-0.7 and ␤ = 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 s 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 -time and the two-dimensional k-space Fourier analysis of the reconstructed emissivity fluctuations show the same interchange mode that is detected in the 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.

Plasma turbulence, suprathermal ion dynamics and code validation on the basic plasma physics device TORPEX

“…A gas puff imaging (GPI) system is also available, including a fast piezoelectric valve and a ceramic tube with a 100 μm nozzle. The use of the GPI system results in a five-fold improvement of the light collection level, which allows acquiring at higher acquisition frequencies, up to 50 kframes s −1 (Iraji et al 2010). …”

Invited Review Article: Gas puff imaging diagnostics of edge plasma turbulence in magnetic fusion devices

“…29,30,101 Finally, on the TORPEX toroidal plasma device in Switzerland, an intensified tangential view was interpreted using tomographic image reconstruction, and similar fluctuations were seen in visible light emission and Langmuir probes. 26,27 …”

“…25 Fast passive imaging has also been done in the small toroidal device TORPEX using a hydrogen plasma, but without an interference filter. 26 A 4 kHz interchange mode seen by Langmuir probes was also seen in camera images, and the images were tomographically reconstructed to resolve smallscale structures. A gas puff and a camera intensifier were added to improve the system, and a higher cross correlation (∼0.…”

Plasma tomographic reconstruction from tangentially viewing camera with background subtraction

“…7,15 At the same time, the tangential tomography is based on rather basic methods; e.g., truncated singular value decomposition (SVD) or linear methods have been used. 2,5,6,8,9 One of the reasons are the higher computational demands of the tangential tomography.…”

“…On the other hand, the tangential configuration is often used to observe the plasma edge and divertor regions. [2][3][4]6 The lines of sights tangential to the plasma column lead to higher contrast in the projections. Moreover, numerical tests proved that the local plasma emissivity can be reconstructed from one 2D image camera, hence one camera is sufficient for a reconstruction, even though the coverage in the projection space 7 is rather narrow.…”