70,381 research outputs found
Convergence of statistical moments of particle density time series in scrape-off layer plasmas
Particle density fluctuations in the scrape-off layer of magnetically
confined plasmas, as measured by gas-puff imaging or Langmuir probes, are
modeled as the realization of a stochastic process in which a superposition of
pulses with a fixed shape, an exponential distribution of waiting times and
amplitudes represents the radial motion of blob-like structures. With an
analytic formulation of the process at hand, we derive expressions for the
mean-squared error on estimators of sample mean and sample variance as a
function of sample length, sampling frequency, and the parameters of the
stochastic process. % Employing that the probability distribution function of a
particularly relevant shot noise process is given by the gamma distribution, we
derive estimators for sample skewness and kurtosis, and expressions for the
mean-squared error on these estimators.
Numerically generated synthetic time series are used to verify the proposed
estimators, the sample length dependency of their mean-squared errors, and
their performance.
We find that estimators for sample skewness and kurtosis based on the gamma
distribution are more precise and more accurate than common estimators based on
the method of moments.Comment: 31 pages, 10 figure
State-Space Interpretation of Model Predictive Control
A model predictive control technique based on a step response model is developed using state estimation techniques. The standard step response model is extended so that integrating systems can be treated within the same framework. Based on the modified step response model, it is shown how the state estimation techniques from stochastic optimal control can be used to construct the optimal prediction vector without introducing significant additional numerical complexity. In the case of integrated or double integrated white noise disturbances filtered through general first-order dynamics and white measurement noise, the optimal filter gain is parametrized explicitly in terms of a single parameter between 0 and 1, thus removing the requirement for solving a Riccati equation and equipping the control system with useful on-line tuning parameters. Parallels are drawn to the existing MPC techniques such as Dynamic Matrix Control (DMC), Internal Model Control (IMC) and Generalized Predictive Control (GPC)
Hall response of interacting bosonic atoms in strong gauge fields: from condensed to FQH states
Interacting bosonic atoms under strong gauge fields undergo a series of phase
transitions that take the cloud from a simple Bose-Einstein condensate all the
way to a family of fractional-quantum-Hall-type states [M. Popp, B. Paredes,
and J. I. Cirac, Phys. Rev. A 70, 053612 (2004)]. In this work we demonstrate
that the Hall response of the atoms can be used to locate the phase transitions
and characterize the ground state of the many-body state. Moreover, the same
response function reveals within some regions of the parameter space, the
structure of the spectrum and the allowed transitions to excited states. We
verify numerically these ideas using exact diagonalization for a small number
of atoms, and provide an experimental protocol to implement the gauge fields
and probe the linear response using a periodically driven optical lattice.
Finally, we discuss our theoretical results in relation to recent experiments
with condensates in artificial magnetic fields [ L. J. LeBlanc, K.
Jimenez-Garcia, R. A. Williams, M. C. Beeler, A. R. Perry, W. D. Phillips, and
I. B. Spielman, Proc. Natl. Acad. Sci. USA 109, 10811 (2012)] and we analyze
the role played by vortex states in the Hall response.Comment: 10 pages, 7 figure
HD 85567: A Herbig B[e] star or an interacting B[e] binary
Context. HD 85567 is an enigmatic object exhibiting the B[e] phenomenon, i.e.
an infrared excess and forbidden emission lines in the optical. The object's
evolutionary status is uncertain and there are conflicting claims that it is
either a young stellar object or an evolved, interacting binary.
Aims. To elucidate the reason for the B[e] behaviour of HD 85567, we have
observed it with the VLTI and AMBER.
Methods. Our observations were conducted in the K-band with moderate spectral
resolution (R~1500, i.e. 200 km/s). The spectrum of HD 85567 exhibits Br gamma
and CO overtone bandhead emission. The interferometric data obtained consist of
spectrally dispersed visibilities, closure phases and differential phases
across these spectral features and the K-band continuum.
Results. The closure phase observations do not reveal evidence of asymmetry.
The apparent size of HD 85567 in the K-band was determined by fitting the
visibilities with a ring model. The best fitting radius, 0.8 +/- 0.3 AU, is
relatively small making HD 85567 undersized in comparison to the
size-luminosity relationship based on YSOs of low and intermediate luminosity.
This has previously been found to be the case for luminous YSOs, and it has
been proposed that this is due to the presence of an optically thick gaseous
disc. We demonstrate that the differential phase observations over the CO
bandhead emission are indeed consistent with the presence of a compact (~1 AU)
gaseous disc interior to the dust sublimation radius.
Conclusions. The observations reveal no sign of binarity. However, the data
do indicate the presence of a gaseous disc interior to the dust sublimation
radius. We conclude that the data are consistent with the hypothesis that HD
85567 is a YSO with an optically thick gaseous disc within a larger dust disc
that is being photo-evaporated from the outer edge.Comment: Accepted for publication in A &
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