Edge turbulence measurements in NSTX by gas puff imaging

Abstract

Turbulent filaments in visible light emission corresponding mainly to density fluctuations at the edge have been observed in large aspect ratio tokamaks: TFTR, ASDEX, Alcator C-Mod, and DIII-D. This article reports on similar turbulent structures observed in the National Spherical Torus Experiment (NSTX) using a fast-framing, intensified, digital visible camera. These filaments were previously detected mainly in high recycling regions, such as at limiters or antennas, where the line emission from neutral atoms was modulated by the fluctuations in local plasma density. However, by introducing controlled edge gas puffs, i.e., gas puff imaging, we have increased the brightness and contrast in the fluctuation images and allowed the turbulent structure to be measured independently of the recycling. A set discrete fiber-optically coupled sight-lines also measured the frequency spectra of these light fluctuations with a 200 kHz bandwidth. Initial results in NSTX show that the turbulent filaments are well aligned with the magnetic field which can be up to 45° from the horizontal at the outer midplane of NSTX. The dominant wavelength perpendicular to the magnetic field is ∼7–11 cm, corresponding to a k⊥ ρs of ∼0.3 at an assumed Te=25 eV, and the frequency spectra has a typical broad shape characteristic of edge turbulence extending to about 100 kHz. By imaging a He gas puff along a magnetic field line the characteristic radial scalelength appears to be in the 3–5 cm range.

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

“…This led to the parallel development and first results of gas puff imaging systems on the NSTX 12,13 and Alcator C-Mod tokamaks. [14][15][16][17] An overview of GPI is given in Sec.…”

“…[12][13][14][15][16][17] Since then, there have been many hardware improvements and successful applications to other magnetic fusion devices, but the basic features of the instrumentation and interpretation of the data remain the same.…”

“…92,93 On the NSTX spherical tokamak in the US, a single GPI sightline views a gas puff manifold located just above the outer midplane through a re-entrant window port. 12,13,44,[52][53][54]58,78,80,94 This sightline has a wide angle field of view covering ∼30 cm in the poloidal direction and ∼24 cm in the radial direction.…”

Plasma tomographic reconstruction from tangentially viewing camera with background subtraction

“…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.…”

“…Usually, cameras are used for observation of plasma-wall interaction, however there are many other applications, e.g., plasma shape, fluctuation, or dust studies. [1][2][3][4][5] The plasma edge emits visible radiation mainly because of interaction of neutral particles flux from wall with plasma. Projections along lines of sight (LoS) of this emissivity are afterwards detected.…”