Beam emission spectroscopy turbulence imaging system for the MAST spherical tokamak

Field, A. R.; Dunai, D.; Gaffka, R; Ghim, Young-chul; Kiss, I.; Mészáros, B.; Krizsanóczi, T.; Shibaev, S.; Zoletnik, S.

doi:10.1063/1.3669756

Cited by 30 publications

(56 citation statements)

References 16 publications

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“…Beam Emission Spectroscopy (BES) measurements of density fluctuations in tokamak plasmas [17][18][19][20][21] have made it possible to probe ion-scale turbulence in these devices directly. In this Letter, we use such measurements in the MAST tokamak, along with the local equilibrium parameters calculated by other diagnostics, to estimate and compare the characteristic time scales of the turbulent fluctuations in the energy-containing range.…”

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Experimental Signatures of Critically Balanced Turbulence in MAST

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Beam Emission Spectroscopy (BES) measurements of ion-scale density fluctuations in the MAST tokamak are used to show that the turbulence correlation time, the drift time associated with ion temperature or density gradients, the particle (ion) streaming time along the magnetic field and the magnetic drift time are consistently comparable, suggesting a "critically balanced" turbulence determined by the local equilibrium. The resulting scalings of the poloidal and radial correlation lengths are derived and tested. The nonlinear time inferred from the density fluctuations is longer than the other times; its ratio to the correlation time scales as ν −0.8±0.1 * i , where ν * i = ion collision rate/streaming rate. This is consistent with turbulent decorrelation being controlled by a zonal component, invisible to the BES, with an amplitude exceeding the drift waves' by ∼ ν −0.8 * i .Introduction. Microscale turbulence hindering energy confinement in magnetically confined hot plasmas is driven by gradients of equilibrium quantities such as temperature and density. These gradients give rise to instabilities that inject energy into plasma fluctuations ("drift waves") at scales just above the ion Larmor scale. The most effective of these is believed to be the iontemperature-gradient (ITG) instability [1][2][3]. A turbulent state ensues, giving rise to "anomalous transport" of energy [4]. It is of interest, both for practical considerations of improving confinement and for the fundamental understanding of multiscale plasma dynamics, what the structure of this turbulence is and how its amplitude, scale(s) and resulting transport depend on the equilibrium parameters: ion and electron temperatures, density, angular velocity, magnetic geometry, etc.Fluctuations in a magnetized toroidal plasma are subject to a number of distinct physical effects, which can be thought about in terms of various time scales such as the drift times associated with the temperature and density gradients, the particle streaming time along the magnetic field as it takes them around the torus toroidally and poloidally, the magnetic (∇B and curvature) drift times of particles moving across the field, the nonlinear time of the fluctuations being advected across the field by the fluctuating E × B velocity, the time between collisions, the shear time associated with plasma rotation. Some of these time scales and, consequently, the corresponding physics may be irrelevant, while others play a crucial role for the saturation of the linearly unstable fluctuations. There has been a growing understanding [5], driven largely by theory [6][7][8][9], observations [10][11][12] and simulations of magnetohydrodynamic [13][14][15] and kinetic [7,16] plasma turbulence in space, that if a medium can

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“…During the 2011 campaign, density fluctuation data from the BES diagnostic [21] on MAST were collected in a variety of discharges (including L-and H-modes and internal transport barriers). Here we report the data from 39 neutral- beam heated "double-null-diverted" discharges, with no pellet injection and no resonant magnetic perturbations.…”

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Experimental Signatures of Critically Balanced Turbulence in MAST

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“…Aligning the LoS parallel to the local magnetic field at the intersection of the LoS and the neutral heating beam helps minimize the degradation of the spatial resolution. A more detailed description of the 2D BES system on MAST with possible sources of some losses of spatial locality can be found elsewhere [16,20].…”

Section: What Is Measured By the 2d Bes Systemmentioning

confidence: 99%

“…where I ij DC is the DC value -a typical value of 0.8 V is used for all channels [16]. The rest of the terms are as follows.…”

Section: Appendix A2 Synthetic 2d Bes Datamentioning

confidence: 99%

“…With the aim of characterizing the microscale plasma turbulence and searching for correlations between it and the background plasma characteristics, a two-dimensional (8 radial × 4 poloidal channels) beam emission spectroscopy (2D BES) system [16] has been installed on the Mega Amp Spherical Tokamak (MAST). It is able to measure density fluctuations at scales above the ion Larmor radius ρ i , viz., k ⊥ ρ i < 1, where k ⊥ is the wavenumber perpendicular to the magnetic field.…”

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Measurement and physical interpretation of the mean motion of turbulent density patterns detected by the beam emission spectroscopy system on the mega amp spherical tokamak

Ghim¹,

Field²,

Dunai³

et al. 2012

Plasma Phys. Control. Fusion

106

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Abstract. The mean motion of turbulent patterns detected by a two-dimensional (2D) beam emission spectroscopy (BES) diagnostic on the Mega Amp Spherical Tokamak (MAST) is determined using a cross-correlation time delay (CCTD) method. Statistical reliability of the method is studied by means of synthetic data analysis. The experimental measurements on MAST indicate that the apparent mean poloidal motion of the turbulent density patterns in the lab frame arises because the longest correlation direction of the patterns (parallel to the local background magnetic fields) is not parallel to the direction of the fastest mean plasma flows (usually toroidal when strong neutral beam injection is present). The experimental measurements are consistent with the mean motion of plasma being toroidal. The sum of all other contributions (mean poloidal plasma flow, phase velocity of the density patterns in the plasma frame, non-linear effects, etc.) to the apparent mean poloidal velocity of the density patterns is found to be negligible. These results hold in all investigated L-mode, H-mode and internal transport barrier (ITB) discharges. The one exception is a high-poloidal-beta (the ratio of the plasma pressure to the poloidal magnetic field energy density) discharge, where a large magnetic island exists. In this case BES detects very little motion. This effect is currently theoretically unexplained.

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The mega amp spherical tokamak

Meyer

2016

Magnetic Fusion Energy

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Beam emission spectroscopy turbulence imaging system for the MAST spherical tokamak

Cited by 30 publications

References 16 publications

Experimental Signatures of Critically Balanced Turbulence in MAST

Experimental Signatures of Critically Balanced Turbulence in MAST

Measurement and physical interpretation of the mean motion of turbulent density patterns detected by the beam emission spectroscopy system on the mega amp spherical tokamak

The mega amp spherical tokamak

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