Now showing items 1-10 of 20
Statistical properties of a filtered Poisson process with additive random noise: distributions, correlations and moment estimation
(Journal article; Tidsskriftartikkel; Peer reviewed, 2017-04-19)
Filtered Poisson processes are often used as reference models for intermittent fluctuations in physical systems. Such a process is here extended by adding a noise term, either as a purely additive term to the process or as a dynamical term in a stochastic differential equation. The lowest order moments, probability density function, auto-correlation function and power spectral density are derived ...
Relationship between frequency power spectra and intermittent, large-amplitude bursts in the Alcator C-Mod scrape-off layer
(Journal article; Tidsskriftartikkel; Peer reviewed, 2017-08-21)
Fluctuations in the boundary region of the Alcator C-Mod tokamak have been analyzed using gas puff imaging data from a set of Ohmically heated plasma density scan experiments. It is found that the relative fluctuation amplitudes are modest and close to normally distributed at the separatrix but become increasingly larger and skewed towards the main chamber wall. The frequency power spectra are ...
Auto-correlation function and frequency spectrum due to a super-position of uncorrelated exponential pulses
(Peer reviewed, 2017-03)
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 ...
Power law spectra and intermittent fluctuations due to uncorrelated Lorentzian pulses
(Peer reviewed, 2017-02)
A stochastic model for intermittent fluctuations due to a super-position of uncorrelated Lorentzian pulses is presented. For a constant pulse duration, this is shown to result in an exponential power spectral density for the stationary process. A random distribution of pulse durations modifies the frequency spectrum, and several examples are shown to result in power law spectra. The distribution of ...
Scrape-off layer turbulence in TCV: Evidence in support of stochastic modelling.
(Journal article; Tidsskriftartikkel; Peer reviewed, 2016-01-28)
Intermittent fluctuations in the TCV scrape-off layer have been investigated by analysing long Langmuir probe data time series under stationary conditions, allowing calculation of fluctuation statistics with high accuracy. The ion saturation current signal is dominated by the frequent occurrence of large-amplitude bursts attributed to filament structures moving through the scrape-off layer. The ...
Probability distribution functions for intermittent scrape-off layer plasma fluctuations
(Journal article; Peer reviewed; Tidsskriftartikkel, 2018-01-24)
A stochastic model for intermittent fluctuations in the scrape-off layer of magnetically confined plasmas has been constructed based on a super-position of uncorrelated pulses arriving according to a Poisson process. In the most common applications of the model, the pulse amplitudes are assumed exponentially distributed, supported by conditional averaging of large-amplitude fluctuations in experimental ...
Level crossings and excess times due to a superposition of uncorrelated exponential pulses
(Journal article; Tidsskriftartikkel; Peer reviewed, 2018-01-10)
A well-known stochastic model for intermittent fluctuations in physical systems is investigated. The model is given by a superposition of uncorrelated exponential pulses, and the degree of pulse overlap is interpreted as an intermittency parameter. Expressions for excess time statistics, that is, the rate of level crossings above a given threshold and the average time spent above the threshold, are ...
Stochastic modeling of scrape-off layer fluctuations
(Conference object; Konferansebidrag, 2017-08-24)
1. Stochastic model of data time series <br> 2. Comparison to experimental measurements
Level crossings, excess times, and transient plasma-wall interactions in fusion plasmas
(Journal article; Tidsskriftartikkel; Peer reviewed, 2016-04-19)
Based on a stochastic model for intermittent fluctuations in the boundary region of magnetically confined plasmas, an expression for the level crossing rate is derived from the joint distribution of the process and its derivative. From this the average time spent by the process above a certain threshold level is obtained. This provides novel predictions of plasma–wall interactions due to transient ...
Stochastic modelling of intermittent fluctuations in the scrape-off layer: Correlations, distributions, level crossings, and moment estimation
(Journal article; Tidsskriftartikkel; Peer reviewed, 2016-05)
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 ...