Probability distribution functions for intermittent scrape-off layer plasma fluctuations

Theodorsen, Audun; Garcia, Odd Erik

doi:10.1088/1361-6587/aa9f9c

Cited by 13 publications

(19 citation statements)

References 60 publications

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“…Radial motion of blob-like structures results in single-point recordings dominated by largeamplitude bursts. Recently, a stochastic model was introduced, describing the fluctuations as a super-position of uncorrelated pulses with an exponential shape and constant duration [16][17][18][19][20]. Predictions of this model, including the probability density function and the frequency power spectral density, are in excellent agreement with Langmuir probe and gas puff imaging (GPI) measurements obtained in ohmic and low confinement modes (L-modes) of several tokamak devices [21][22][23][24][25][26][27][28].…”

Section: Introductionmentioning

confidence: 73%

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Universality of Poisson-driven plasma fluctuations in the Alcator C-Mod scrape-off layer

et al. 2018

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Large-amplitude, intermittent fluctuations are ubiquitous in the boundary region of magnetically confined plasmas and lead to detrimental plasma-wall interactions in the next-generation, high duty cycle fusion power experiments. Using gas puff imaging data time series from the scrape-off layer in the Alcator C-Mod device, it is here demonstrated that the large-amplitude fluctuations can be described as a super-position of pulses with fixed shape and constant duration. By applying a new deconvolution algorithm on the data time series with a two-sided exponential pulse function, the arrival times and amplitudes of the pulses can be estimated and the measurement time series can be reconstructed with high accuracy. The pulse amplitudes are shown to follow an exponential distribution. The waiting times between pulses are uncorrelated, their distribution has an exponential tail, and the number of arrivals is a linear function of time. This demonstrates that pulse arrivals follow a homogeneous Poisson process. Identical statistical properties apply to both ohmic and high confinement mode plasmas, clearly demonstrating universality of the fluctuation statistics in the boundary region of Alcator C-Mod. * audun.theodorsen@uit.no † odd.erik.garcia@uit.no

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Section: Introductionmentioning

confidence: 73%

Section: Introductionmentioning

confidence: 99%

Universality of Poisson-driven plasma fluctuations in the Alcator C-Mod scrape-off layer

et al. 2018

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“…In the following, the numerical simulation data will be compared to predictions of a stochastic model which describes the fluctuations as a super-position of uncorrelated pulses with fixed shape and constant duration. This is written as [43][44][45][46][47][48][49][50][51][52][53]…”

Section: Model Equationsmentioning

confidence: 99%

“…[28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] A statistical framework based on filtered Poisson processes has proven an accurate description of both average radial profiles and fluctuations in the boundary of magnetically confined plasma. [43][44][45][46][47][48][49][50][51][52][53] So far, this stochastic model has not been utilized to analyze fluctuation data from numerical turbulence simulations of the boundary region of magnetized plasmas. In order to obtain statistically significant results, long simulation data time series or a large ensemble are required, equivalent to several hundred milliseconds in experiments with medium-sized magnetically confined plasma.…”

Section: Introductionmentioning

confidence: 99%

Numerical turbulence simulations of intermittent fluctuations in the scrape-off layer of magnetized plasmas

Decristoforo,

Theodorsen,

Omotani

et al. 2021

Preprint

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Intermittent fluctuations in the boundary of magnetically confined plasmas are investigated by numerical turbulence simulations of a reduced fluid model describing the evolution of the plasma density and electric drift vorticity in the two-dimensional plane perpendicular to the magnetic field. Two different cases are considered, one describing resistive drift waves in the edge region and another including only the interchange instability due to unfavorable magnetic field curvature in the scrape-off layer. Analysis of long data time series obtained by single-point recordings are compared to predictions of a stochastic model describing the plasma fluctuations as a super-position of uncorrelated pulses. For both cases investigated, the radial particle density profile in the scrape-off layer is exponential with a radially constant scale length. The probability density function for the particle density fluctuations in the far scrape-off layer has an exponential tail. Radial motion of blob-like structures leads to large-amplitude bursts with an exponential distribution of peak amplitudes and the waiting times between them. The average burst shape is well described by a two-sided exponential function. The frequency power spectral density of the particle density is simply that of the average burst shape and is the same for all radial positions in the scrape-off layer. The fluctuation statistics obtained from the numerical simulations are in excellent agreement with recent experimental measurements on magnetically confined plasmas. The statistical framework defines a new validation metric for boundary turbulence simulations.

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“…An exponential frequency power spectrum is a signature of deterministic chaos and has been attributed to Lorentzian-shaped pulses in the underlying time series. In order to demonstrate this, consider the stochastic process that gives a superposition of pulses with fixed shape ϕ and duration τ d , [106][107][108][109][110][111][112][113][114][115][116][117]…”

Section: Lorentzian Pulsesmentioning

confidence: 99%

Intermittent fluctuations due to Lorentzian pulses in turbulent thermal convection

2020

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Turbulent motions due to flux-driven thermal convection are investigated by numerical simulations and stochastic modeling. Tilting of convection cells leads to the formation of sheared flows and quasi-periodic relaxation oscillations for the energy integrals far from the threshold for linear instability. The probability density function for the temperature and radial velocity fluctuations in the fluid layer changes from a normal distribution at the onset of turbulence to a distribution with an exponential tail for large fluctuation amplitudes for strongly driven systems. The frequency power spectral density has an exponential shape, which is a signature of deterministic chaos. By use of a novel deconvolution method, this is shown to result from the presence of Lorentzian pulses in the underlying time series, demonstrating that exponential frequency spectra can also persist in turbulent flow regimes.

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Probability distribution functions for intermittent scrape-off layer plasma fluctuations

Cited by 13 publications

References 60 publications

Universality of Poisson-driven plasma fluctuations in the Alcator C-Mod scrape-off layer

Universality of Poisson-driven plasma fluctuations in the Alcator C-Mod scrape-off layer

Numerical turbulence simulations of intermittent fluctuations in the scrape-off layer of magnetized plasmas

Intermittent fluctuations due to Lorentzian pulses in turbulent thermal convection

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