Fluctuation statistics in the scrape-off layer of Alcator C-Mod

Kube, Ralph; Theodorsen, Audun; Garcia, Odd Erik; LaBombard, B.; Terry, J. L.

doi:10.1088/0741-3335/58/5/054001

Cited by 32 publications

(86 citation statements)

References 72 publications

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“…The ion temperature at the LCFS is assumed to be T T i e 0 0 = , since no ion temperature measurement was available for that discharge. Resistivity can affect substantially blobs dynamics by electrically disconnecting from the target sheaths and subsequently increasing their convective velocity, as predicted in [25] and experimentally measured in [4,27,28]. The influence of resistivity on blob dynamics has been investigated with seeded simulations in [17,29].…”

Section: Nonlinear Simulations With the Gbs Codementioning

confidence: 93%

Blob properties in full-turbulence simulations of the TCV scrape-off layer

Nespoli

Furno

Labit

et al. 2017

Plasma Phys. Control. Fusion

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To investigate blob properties in the tokamak scrape-off layer (SOL), we perform dedicated numerical nonlinear simulations of plasma turbulence in the SOL of a TCV discharge using the Global Braginskii Solver code. A blob detection technique is used for the first time in a threedimensional (3D) full-turbulence simulation to track the motion of the filaments in the SOL. The specific size, density amplitude and radial velocity of the blobs are computed, with the typical values being 7.4 s r , n 0.33 e and c 0.016 s , respectively. The analysis of blob structure in the parallel direction shows that the blobs are partially detached from the limiter. The cross correlation analysis shows how the blobs are born all along the entire field line, not being generated primarily on the low field side SOL and expanding towards the limiter. The blob radial velocity agrees well with the inertial branch of the existing scaling law. The radial particle and heat fluxes given by blobs are shown to be responsible of up to 100% and 70% of the turbulent particle and heat flux in the far SOL, respectively. The results of a second simulation with a 40 times higher resistivity are also discussed.

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Section: Nonlinear Simulations With the Gbs Codementioning

confidence: 93%

Blob properties in full-turbulence simulations of the TCV scrape-off layer

Nespoli

Furno

Labit

et al. 2017

Plasma Phys. Control. Fusion

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“…The statistical properties of these bursts in the SOL of several tokamak plasmas have recently been elucidated by the analysis of long data time series from probe measurements and gas puff imaging. [11][12][13][14][15] Conditional averaging has revealed exponentially distributed burst amplitudes and waiting times, while the burst duration is found to be constant. The wave-form for largeamplitude bursts in the time series is well described by an exponential function.…”

Section: Introductionmentioning

confidence: 97%

“…[6][7][8][9][10] The average SOL plasma density and the particle and heat fluxes due to the radial motion of plasma filaments depend on their velocity, amplitude, and frequency of occurrence. [11][12][13][14][15] The turbulence-driven transport results in broad plasma profiles in the SOL and enhanced levels of plasma--wall interactions that may be an issue for the next generation plasma confinement experiments and future fusion power reactors. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] The radial motion of blob-like structures leads to largeamplitude bursts in single-point measurements.…”

Section: Introductionmentioning

confidence: 99%

“…The wave-form for largeamplitude bursts in the time series is well described by an exponential function. [11][12][13][14][15][23][24][25][26][27][28][29][30][31][32] The present study incorporates these features in a stochastic model for intermittent plasma fluctuations in the SOL, described as a super-position of uncorrelated pulses. [34][35][36][37][38][39] This model explains many of the salient experimental findings and empirical scaling relations, including broad plasma profiles and large fluctuation levels, skewed and flattened probability density functions, and a parabolic relation between the skewness and flatness moments.…”

Section: Introductionmentioning

confidence: 99%

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Stochastic modelling of intermittent fluctuations in the scrape-off layer: Correlations, distributions, level crossings, and moment estimation

et al. 2016

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A stochastic model is presented for intermittent fluctuations in the scrape-off layer of magnetically confined plasmas. The fluctuations in the plasma density are modeled by a super-position of uncorrelated pulses with fixed shape and duration, describing radial motion of blob-like structures. In the case of an exponential pulse shape and exponentially distributed pulse amplitudes, predictions are given for the lowest order moments, probability density function, auto-correlation function, level crossings, and average times for periods spent above and below a given threshold level. Also, the mean squared errors on estimators of sample mean and variance for realizations of the process by finite time series are obtained. These results are discussed in the context of single-point measurements of fluctuations in the scrape-off layer, broad density profiles, and implications for plasma--wall interactions due to the transient transport events in fusion grade plasmas. The results may also have wide applications for modelling fluctuations in other magnetized plasmas such as basic laboratory experiments and ionospheric irregularities. Published by AIP Publishing.

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“…[78][79][80][81][82][83][84] From single-point measurements it has been demonstrated that there is an exponential distribution of both the peak amplitudes and the waiting times between them. Moreover, the average duration time does not depend on the amplitude and also appears to be robust against changes in plasma parameters.…”

Section: Introductionmentioning

confidence: 99%

Auto-correlation function and frequency spectrum due to a super-position of uncorrelated exponential pulses

Garcia

Theodorsen

2017

Physics of Plasmas

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The auto-correlation function and the frequency power spectral density due to a super-position of uncorrelated exponential pulses are considered. These are shown to be independent of the degree of pulse overlap and thereby the intermittency of the stochastic process. For constant pulse duration and a one-sided exponential pulse shape, the power spectral density has a Lorentzian shape which is flat for low frequencies and a power law at high frequencies. The algebraic tail is demonstrated to result from the discontinuity in the pulse function. For a strongly asymmetric two-sided exponential pulse shape, the frequency spectrum is a broken power law with two scaling regions. In the case of a symmetric pulse shape, the power spectral density is the square of a Lorentzian function. The steep algebraic tail at high frequencies in these cases is demonstrated to follow from the discontinuity in the derivative of the pulse function. A random distribution of pulse durations is shown to result in apparently longer correlation times but has no influence on the asymptotic power law tail of the frequency spectrum. The effect of additional random noise is also discussed, leading to a flat spectrum for high frequencies. The probability density function for the fluctuations is shown to be independent of the distribution of pulse durations. The predictions of this model describe the variety of auto-correlation functions and power spectral densities reported from experimental measurements in the scrape-off layer of magnetically confined plasmas.

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Fluctuation statistics in the scrape-off layer of Alcator C-Mod

Cited by 32 publications

References 72 publications

Blob properties in full-turbulence simulations of the TCV scrape-off layer

Blob properties in full-turbulence simulations of the TCV scrape-off layer

Stochastic modelling of intermittent fluctuations in the scrape-off layer: Correlations, distributions, level crossings, and moment estimation

Auto-correlation function and frequency spectrum due to a super-position of uncorrelated exponential pulses

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