The physics mechanisms of the weakly coherent mode in the Alcator C-Mod Tokamak

The I-mode is an attractive confinement regime for future tokamak based fusion reactors. A model is presented which explains the I-mode regime by the reduction of ITG turbulence near the separatrix at low collisionality, where the separatrix ion temperature can exceed the electron temperature. Drift-Alfvénturbulence develops, with large and small-scale fluctuations being suppressed by phase randomization and finite-Larmor-radius effects, respectively. The intermediate scales form a broad peak in the frequency spectrum, which features the same properties as the characteristic weakly coherent mode. The model, which is studied by means of gyrofluid simulations, reproduces a number of other experimental I-mode observations, such as decoupled energy and particle transport, intermittent turbulent bursts with precursors, an operational window widening with magnetic field strength, and the challenges met when detaching the plasma.

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“…2) as measured in the experiment [7]. The WCM shows drift-Alfvéninterchange characteristics as found in previous simulation results [30].…”

Section: Discussionsupporting

confidence: 85%

Physical mechanism behind and access to the I-mode confinement regime in tokamaks

et al. 2020

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“…n QMN = 18, and to the WCM toroidal mode number measured in C-Mod, i.e. n WCM 20 [42]. In addition, the precursor propagates poloidally in the EDD direction in both devices, but with different frequencies, i.e.…”

Section: Plasma Edge Evolution During Presmentioning

confidence: 93%

I-mode pedestal relaxation events at ASDEX Upgrade

et al. 2020

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In some conditions, I-mode plasmas can feature pedestal relaxation events (PREs) that transiently enhance the energy reaching the divertor target plates. To shed light into their appearance, characteristics and energy reaching the divertor targets, a comparative study between two tokamaks -Alcator C-Mod and ASDEX Upgrade -is carried out. It is found that PREs appear only in a subset of I-mode discharges, mainly when the plasma is close to the H-mode transition. Also, the nature of the triggering instability is discussed by comparing measurements close to the separatrix in both devices. The PRE relative energy loss from the confined region increases with decreasing pedestal top collisionality ν * ped . In addition, the relative electron temperature drop at the pedestal top, which is related to the conductive energy loss, rises with decreasing ν * ped . Finally, the peak parallel energy fluence due to the PRE measured on the divertor in both devices is compared to the model introduced in [1] for type-I ELMs. The model is shown to provide an upper boundary for PRE energy fluence data, while a lower boundary is found by dividing the model by three. These two boundaries are used to make projections to future devices such as DEMO and ARC.

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“…The GAM advects the WCM and leads to broadening through the Doppler effect [12]; this constitutes the energy transfer as seen in [5]. The basic instability of the WCM is still unknown, but recent simulations with BOUT++ indicate a simple drift-Alfénic instability [17].…”

Section: Frequency Spectra Of Fluctuations During I-modementioning

confidence: 99%

Turbulence characteristics of the I-mode confinement regime in ASDEX Upgrade

et al. 2017

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The I-mode is an improved confinement regime of tokamak plasmas operating in the unfavourable ion ∇B-drift direction combining H-mode-like energy confinement with L-mode-like particle and impurity transport r1s. To qualify the I-mode as an operating scenario for ITER threshold and accessibility studies r2s also on a multimachine basis r3s are needed. Even if the I-mode might not be considered as a promising scenario for ITER, studies of turbulence in I-mode may offer a better understanding of the physics of the interaction of energy and particle transport barriers in general. The mechanism which selectively reduces only one of the transport channels is not understood. In the present contribution the turbulence characteristics of I-mode plasmas in ASDEX-Upgrade (AUG) have been studied in detail. In AUG the WCM appear at frequencies around 100 kHz with a width of a few 10 kHz r4s. The GAM advects the WCM and leads to the broadening through the Doppler effect r4s. In AUG magnetic fluctuations already present in L-mode close to the WCM frequency are amplified during the I-mode. Using the measured background profiles the gyrokinetic eigenvalue solver LIGKA is used to determine the kinetic continuum branches of the GAM and the geodesic Alfvénic mode (GAlf). The frequency of the magnetic fluctuations close to the WCM frequency coincides with the GAlf frequency r4s. Comparisons with the Mirnov coils show the characteristic toroidal and poloidal mode numbers of the GAlf of zero and one, respectively r4s. The origin of the WCM lies in the intermittent events, which exhibit a wavelet-like temporal structure where the characteristic frequency coincides with the centre-of-mass frequency of the WCM r5s. Possible generation scenarios for the bursts are discussed. The nonlinear interaction between GAM, WCM and the bursts will be discussed based on a detailed bispectral analysis of nonlocal interactions in frequency space of the experimental data. The importance of the solitary events for a selective density transport remains an open issue.Published as a journal article in Nuclear Fusion http://iopscience.iop.org/article/Abstract:Turbulence in the I-mode confinement regime of ASDEX Upgrade exhibits beside strong geodesic acoustic mode (GAM) activity two prominent features, the weakly coherent mode (WCM) and strongly intermittent solitary density perturbations. The nonlinear interaction between these structures is studied in detail by means of a conditional averaged waveletbicoherence analysis. The wavelet analysis reveals that the WCM appears as these density bursts. The GAM is coupled to all frequency scales of the velocity fluctuations via a modulational instability. The WCM is broadened through the Doppler effect by the GAM flow. The WCM shows coupling to higher frequencies prior to the bursts indicating a wavesteepening-like process.

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The physics mechanisms of the weakly coherent mode in the Alcator C-Mod Tokamak

Cited by 27 publications

References 20 publications

Physical mechanism behind and access to the I-mode confinement regime in tokamaks

Physical mechanism behind and access to the I-mode confinement regime in tokamaks

I-mode pedestal relaxation events at ASDEX Upgrade

Turbulence characteristics of the I-mode confinement regime in ASDEX Upgrade

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