Experimental measurement of three-wave coupling and energy cascading

Ritz, Ch. P.; Powers, E.J.; Bengtson, Roger D.

doi:10.1063/1.859082

Cited by 164 publications

(97 citation statements)

References 35 publications

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“…[12,13,14,15,16] The inherently nonstationary nature of the L-H transition, however, renders the use of such methods impossible. Thus the study of this important transition requires the treatment of the simultaneous evolution of turbulent scales and sheared E × B flows.…”

Section: Model Equationsmentioning

confidence: 99%

Zonal flow production in the L–H transition in Alcator C-Mod

Cziegler¹,

Tynan²,

Diamond³

et al. 2014

Plasma Phys. Control. Fusion

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Abstract. Transitions of tokamak confinement regimes from low-to highconfinement are studied on Alcator C-Mod [1] tokamak using gas-puff-imaging (GPI) with a focus on the interaction between the edge drift-turbulence and the local shear flow. Results show that the nonlinear turbulent kinetic energy transfer rate into the shear flow becomes comparable to the estimated value of the drift turbulence growth rate at the time the turbulent kinetic energy starts to drop, leading to a net energy transfer that is comparable to the observed turbulence losses. A corresponding growth is observed in the shear flow kinetic energy. The above behavior is demonstrated across a series of experiments. Thus both the drive of the edge zonal flow and the initial reduction of turbulence fluctuation power are shown to be consistent with a lossless kinetic energy conversion mechanism, which consequently mediates the transition into H-mode. The edge pressure gradient is then observed to build on a slower (1ms) timescale, locking in the H-mode state. These results unambiguously establish the time sequence of the transition as: first the peaking of the normalized Reynolds power, then the collapse of the turbulence, and finally the rise of the diamagnetic electric field shear as the L-H transition occurs.

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Section: Model Equationsmentioning

confidence: 99%

Zonal flow production in the L–H transition in Alcator C-Mod

Cziegler¹,

Tynan²,

Diamond³

et al. 2014

Plasma Phys. Control. Fusion

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show abstract

“…To investigate this mechanism in the multi-scale simulations, the energy transfer between three coupled waves existing at distinct spatial scales, where the relationship evaluated. Such three wave coupling has been studied in both simulated data 32 as well as experimental fluctuation data 33,34 using higher order spectral analysis. In this analysis, we closely follow the approach outlined in Ref.…”

Section: A a Summary Of Transport Changes Found In Multi-scale Simulmentioning

confidence: 99%

Multi-scale gyrokinetic simulations: Comparison with experiment and implications for predicting turbulence and transport

et al. 2016

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To better understand the role of cross-scale coupling in experimental conditions, a series of multi-scale gyrokinetic simulations were performed on Alcator C-Mod, L-mode plasmas. These simulations, performed using all experimental inputs and realistic ion to electron mass ratio ((mi/me)1∕2 = 60.0), simultaneously capture turbulence at the ion (kθρs∼O(1.0)) and electron-scales (kθρe∼O(1.0)). Direct comparison with experimental heat fluxes and electron profile stiffness indicates that Electron Temperature Gradient (ETG) streamers and strong cross-scale turbulence coupling likely exist in both of the experimental conditions studied. The coupling between ion and electron-scales exists in the form of energy cascades, modification of zonal flow dynamics, and the effective shearing of ETG turbulence by long wavelength, Ion Temperature Gradient (ITG) turbulence. The tightly coupled nature of ITG and ETG turbulence in these realistic plasma conditions is shown to have significant implications for the interpretation of experimental transport and fluctuations. Initial attempts are made to develop a “rule of thumb” based on linear physics, to help predict when cross-scale coupling plays an important role and to inform future modeling of experimental discharges. The details of the simulations, comparisons with experimental measurements, and implications for both modeling and experimental interpretation are discussed.

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“…[40] is an early example). Although we do not pursue it here, bispectral techniques can give estimates of the coupling coefficients and mode growth rates [41][42][43].…”

Section: Bispectral Analysismentioning

confidence: 99%

Kinetic enhancement of Raman backscatter, and electron acoustic Thomson scatter

et al. 2007

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1-D Eulerian Vlasov-Maxwell simulations are presented which show kinetic enhancement of stimulated Raman backscatter (SRBS) due to electron trapping in regimes of heavy linear Landau damping. The conventional Raman Langmuir wave is transformed into a set of beam acoustic modes [L. Yin et al., Phys. Rev. E 73, 025401 (2006)]. For the first time, a low phase velocity electron acoustic wave (EAW) is seen developing from the self-consistent Raman physics. Backscatter of the pump laser off the EAW fluctuations is reported and referred to as electron acoustic Thomson scatter. This light is similar in wavelength to, although much lower in amplitude than, the reflected light between the pump and SRBS wavelengths observed in single hot spot experiments, and previously interpreted as stimulated electron acoustic scatter [D. S. Montgomery et al., Phys. Rev. Lett. 87, 155001 (2001)]. The EAW observed in our simulations is strongest well below the phase-matched frequency for electron acoustic scatter, and therefore the EAW is not produced by it. The beating of different beam acoustic modes is proposed as the EAW excitation mechanism, and is called beam acoustic decay. Supporting evidence for this process, including bispectral analysis, is presented. The linear electrostatic modes, found by projecting the numerical distribution function onto a GaussHermite basis, include beam acoustic modes (some of which are unstable even without parametric coupling to light waves) and a strongly-damped EAW similar to the observed one. This linear EAW results from non-Maxwellian features in the electron distribution, rather than nonlinearity due to electron trapping.

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Experimental measurement of three-wave coupling and energy cascading

Cited by 164 publications

References 35 publications

Zonal flow production in the L–H transition in Alcator C-Mod

Zonal flow production in the L–H transition in Alcator C-Mod

Multi-scale gyrokinetic simulations: Comparison with experiment and implications for predicting turbulence and transport

Kinetic enhancement of Raman backscatter, and electron acoustic Thomson scatter

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