Experimental Signatures of Critically Balanced Turbulence in MAST

Abstract

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

Recent progress on spherical torus research

“…In the core region of the MAST plasma, highly non-linear saturated "critically balanced" turbulence behavior was observed. 173 Recently, the ion-scale density turbulence measured by BES was compared with a non-linear, global, gyro-kinetic simulations. 174 The results indicate that while there is some degree of agreement seen in the mid-radius region, there is a significant shortfall, for example, in predicted fluctuation levels and ion heat flux in the peripheral region.…”

Experimental determination of the correlation properties of plasma turbulence using 2D BES systems

“…For example, of particular interest is the effect of velocity shear on the spatial structure of turbulence [4,5,6] and the relationship between this and improved confinement regimes [7,8]. There are also interesting questions of a fundamental nature, e.g., whether the turbulence is in critical balance [9,10,11], or how the transition to turbulence occurs [12].…”

E × B flow velocity deduced from the poloidal motion of fluctuation patterns in neutral beam injected L-mode plasmas on KSTAR

“…25,29 On the other hand, the correlation time in this experiment is about a third of the drift time and is much smaller than the streaming time and the magnetic drift time as s st $ s M $ 17 s c . Since s st $ s M as shown in Figs.…”

“…is the ion thermal speed; 25 correlation time of ion-scale turbulence is comparable with three parameters as s c $ s à $ s st $ s M , and the measured turbulence was called critically balanced. 25,29 On the other hand, the correlation time in this experiment is about a third of the drift time and is much smaller than the streaming time and the magnetic drift time as s st $ s M $ 17 s c .…”

“…The poloidal correlation length (' h ) of the density turbulence is determined by fitting a function f ðDZÞ ¼ cosðk h DZÞ expðÀjDZj=' h Þ to the cross correlations with zero time lag, CðDZ 6 ¼ 0; Dt ¼ 0Þ. 25 Note that ' ? ¼ ' h cos a % ' h since a < 7 .…”