Markov Properties of Electrical Discharge Current Fluctuations in Plasma

Using the Markovian method, we study the stochastic nature of electrical discharge current fluctuations in the Helium plasma. Sinusoidal trends are extracted from the data set by the Fourier-Detrended Fluctuation analysis and consequently cleaned data is retrieved. We determine the Markov time scale of the detrended data set by using likelihood analysis. We also estimate the Kramers-Moyal's coefficients of the discharge current fluctuations and derive the corresponding Fokker-Planck equation. In addition, the obtained Langevin equation enables us to reconstruct discharge time series with similar statistical properties compared with the observed in the experiment. We also provide an exact decomposition of temporal correlation function by using Kramers-Moyal's coefficients. We show that for the stationary time series, the two point temporal correlation function has an exponential decaying behavior with a characteristic correlation time scale. Our results confirm that, there is no definite relation between correlation and Markov time scales. However both of them behave as monotonic increasing function of discharge current intensity. Finally to complete our analysis, the multifractal behavior of reconstructed time series using its Keramers-Moyal's coefficients and original data set are investigated. Extended self similarity analysis demonstrates that fluctuations in our experimental setup deviates from Kolmogorov (K41) theory for fully developed turbulence regime.Comment: 25 pages, 9 figures and 4 tables. V3: Added comments, references, figures and major correction

Function Parametrization - a Fast Inverse Mapping Method

Function parametrization (FP) is a method to invert computer models that map physical parameters describing the state of a physical system onto measurements. It find a mapping of the measurements onto the physical parameters that requires little computing time to evaluate. The major advantages of FP over other analysis methods are: it is quite general; it is fast, allowing real-time control of experiments; it allows a thorough error analysis; it can provide insight into the structure of the computer program used to model the experiment; it can be used to analyze sets of dissimilar measurements; it can be used to study the adequacy of certain new measurements for determination of specific physical parameters. FP is tested on the reconstruction of plasma equilibria from magnetic measurements. As a result, some important parameters describing the plasma state are shown to be recoverable in a fast and reliable manner

Application of Function Parametrization to the Analysis of Polarimetry and Interferometry Data in Textor

Function parametrization (FP) provides a way to perform complex data analysis in a fast and reliable manner that allows inter-shot analysis. The method has been used to analyse polarimetry and interferometry data in TEXTOR with the purpose of obtaining spatial distributions of the electron density and the toroidal plasma current. A standard TEXTOR discharge is investigated and the results from FP are compared with results obtained by conventional methods of data analysis. Agreement between the two methods is generally good. For the central safety factor the value of q0 = 0.8 +/- 0.1 is in accordance with previous calculations. The new method can easily be modified to incorporate additional data

Shear Reversal and Mhd Activity During Pellet Enhanced Performance Pulses in Jet

Analysis of MHD activity in Pellet Enhanced Performance (PEP) pulses is used to determine the position of rational surfaces associated with the safety factor q. This gives evidence for negative shear in the central region of the plasma. The plasma equilibrium calculated from the measured q values yields a Shafranov shift in reasonable agreement with the experimental value of about 0.2 m. The corresponding current profile has two large off-axis maxima in agreement with the bootstrap current calculated from the electron temperature and density measurements. A transport simulation shows that the bootstrap current is driven by the steep density gradient, which results from improved confinement in the plasma core where the shear is negative. During the PEP phase, (m, n) = (1, 1) fast MHD events are correlated with collapses in the neutron rate. The dominant mode preceding these events usually is n = 3, whereas the mode following them is dominantly n = 2. Toroidal linear MHD stability calculations assuming a non-monotonic q-profile with an off-axis minimum decreasing from above 1 to below 1 describe this sequence of modes (n = 3, 1, 2), but always give a larger growth rate for the n = 1 mode than for the n = 2 mode. This large growth rate is due to the high central poloidal beta of 1.5 observed in the PEP pulses. Finally, a rotating (m, n) = (1, 1) mode is observed as a hot spot with a ballooning character on the low field side. The hot spot has some of the properties of a 'hot' island consistent with the presence of a region of negative shear