Conditional eddies in plasma turbulence

Johnsen, H.; Pécseli, H. L.; Trulsen, J.

doi:10.1063/1.866158

Cited by 113 publications

(83 citation statements)

References 46 publications

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“…The conditional average (CA) technique [11] is particularly suited to study the twodimensional spatio-temporal dynamics of blobs, structures of increased plasma density, that move from the source region of TORPEX, where the plasma density is higher, to the low field side of the machine, characterized by lower plasma density [12]. Similarly to the plasma edge of all laboratory devices, blobs give TORPEX transport an intermittent character.…”

Section: B Experimental Primacy Level:mentioning

confidence: 99%

Langmuir probe-based observables for plasma-turbulence code validation and application to the TORPEX basic plasma physics experiment

et al. 2009

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The methodology for plasma-turbulence code validation is discussed, with focus on the quantities to use for the simulation-experiment comparison, i.e. the validation observables, and application to the TORPEX basic plasma physics experiment [A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)]. The considered validation observables are deduced from Langmuir probe measurements and are ordered into a primacy hierarchy, according to the number of model assumptions and to the combinations of measurements needed to form each of them. The lowest levels of the primacy hierarchy correspond to observables that require the lowest number of model assumptions and measurement combinations, such as the statistical and spectral properties of the ion saturation current time trace, while, at the highest levels, quantities such as particle transport are considered. The comparison of the observables at the lowest levels in the hierarchy is more stringent than at the highest levels. Examples of the use of the proposed observables are applied to a specific TORPEX plasma configuration characterized by interchange-driven turbulence. * Electronic address: paolo.ricci@epfl.ch 2

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Section: B Experimental Primacy Level:mentioning

confidence: 99%

Langmuir probe-based observables for plasma-turbulence code validation and application to the TORPEX basic plasma physics experiment

et al. 2009

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“…Time-resolved measurements of ion saturation current and floating potential V fl provide information on the blob motion. These data are conditionally sampled and averaged [18] around the blob detection time (τ ¼ 0 μs) over a time window of 100 μs, determined using as a trigger the HEXTIP LP located at ½r ≃ 7; z ≃ 0 cm. An amplitude threshold corresponding to 4 times the standard deviation of the J sat signal is chosen for the structure detection.…”

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confidence: 99%

X-Point Effect on Plasma Blob Dynamics

et al. 2016

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Plasma blob dynamics on the high-field side in the proximity of a magnetic field null (X point) is investigated in TORPEX. A significant acceleration of the blobs towards the X point is observed. Close to the X point the blobs break apart. The E × B drifts associated with the blobs are measured, isolating the background drift component from the fluctuating contribution of the blob internal potential dipole. The time evolution of the latter is consistent with the fast blob dynamics. An analytical model based on charge conservation is derived for the potential dipole, including ion polarization, diamagnetic, and parallel currents. In the vicinity of the X point, a crucial role in determining the blob motion is played by the decrease of the poloidal magnetic field intensity. This variation increases the connection length that short circuits the potential dipole of the blob. Good quantitative agreement is found between the model and the experimental data in the initial accelerating phase of the blob dynamics. DOI: 10.1103/PhysRevLett.116.105001 Magnetic field nulls (X points) are ubiquitous in space and laboratory plasmas [1][2][3]. The transport of particles and heat across them is of utmost importance for a variety of systems [4]. For example, much effort has been devoted by the fusion community in the past decades to investigate diverted magnetic geometries [5], which allow channeling through an X point a significant fraction of the exhaust power to material surfaces. Cross-field drifts associated with the steady-state and turbulence-induced electric fields play an important role for plasma transport in the X-point region, as indicated by numerical and experimental studies [6,7]. However, experimental investigations close to X points are difficult, limiting the progress in the understanding of the X-point dynamics and simulationexperiment comparisons. In fusion plasmas, the diagnostic accessibility is challenged by the high power flux so that the generation and propagation of intermittent plasma blobs in the vicinity of an X point is largely unexplored [8][9][10].In this Letter, we present the first spatial and temporaldependent in situ measurements of turbulence-generated plasma blob dynamics around the X-point region. The blob motion towards the X point is tracked and analyzed, showing an acceleration in the initial phase that can be directly linked to the background radial flow and to the measured blob electric potential dipole. The acceleration of the blob towards the X point can be quantitatively described by an analytical model that includes the dominant perpendicular and parallel current contributions. In particular, a crucial role is played by the geometrical parameter L ∥ , expressing the length of the current path parallel to the magnetic field, along which the blob potential dipole is short-circuited.The measurements are performed on the Toroidal Plasma Experiment (TORPEX) [11] with diverted magnetic geometries. Tokamaklike configurations, such as first-order (X point) or second-order (snowflake) nu...

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“…The latter could be regarded as a consequence of the reduction in turbulence amplitudes by ZFs, which is expected to be reflected in transport, too. In order to quantify the transport reduction by the ZF, a conditional averaging technique [33] is applied to the time series of total transport and potential. To this end, short time windows with a length of 512 µs are stored whenever the chosen threshold value of 2σ is reached in the Φ z (t) time series.…”

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confidence: 99%

Experimental Evidence of Turbulent Transport Regulation by Zonal Flows

et al. 2013

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Abstract. The regulation of turbulent transport by zonal flows is studied experimentally on a flux surface of the stellarator experiment TJ-K. Data of 128 Langmuir probes at different toroidal and poloidal positions on a single flux surface enable to measure simultaneously the zonal flow activity and the turbulent transport in great detail. A reduction of turbulent transport by 30 % during the zonal flow phase is found. It is shown that the reduction process is initiated by a modification in the cross phase between density and electric field followed by a reduction in the fluctuation levels, which sustain low transport levels on larger time scales than the zonal flow life time.

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Conditional eddies in plasma turbulence

Cited by 113 publications

References 46 publications

Langmuir probe-based observables for plasma-turbulence code validation and application to the TORPEX basic plasma physics experiment

Langmuir probe-based observables for plasma-turbulence code validation and application to the TORPEX basic plasma physics experiment

X-Point Effect on Plasma Blob Dynamics

Experimental Evidence of Turbulent Transport Regulation by Zonal Flows

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