557 research outputs found
Exploration of Non-Resonant Divertor Features on the Compact Toroidal Hybrid
Non-resonant divertors (NRDs) separate the confined plasma from the
surrounding plasma facing components (PFCs). The resulting striking field line
intersection pattern on these PFCs is insensitive to plasma equilibrium
effects. However, a complex scrape-off layer (SOL), created by chaotic magnetic
topology in the plasma edge, connects the core plasma to the PFCs through
varying magnetic flux tubes. The Compact Toroidal Hybrid (CTH) serves as a
test-bed to study this by scanning across its inductive current. Simulations
observe a significant change of the chaotic edge structure and an effective
distance between the confined plasma and the instrumented wall targets. The
intersection pattern is observed to be a narrow helical band, which we claim is
a resilient strike line pattern. However, signatures of finger-like structures,
defined as heteroclinic tangles in chaotic domains, within the plasma edge
connect the island chains to this resilient pattern. The dominant connection
length field lines intersecting the targets are observed via heat flux
modelling with EMC3-EIRENE. At low inductive current levels, the excursion of
the field lines resembles a limited plasma wall scenario. At high currents, a
private flux region is created in the area where the helical strike line
pattern splits into two bands. These bands are divertor legs with distinct SOL
parallel particle flow channels. The results demonstrate the NRD strike line
pattern resiliency within CTH, but also show the underlying chaotic edge
structure determining if the configuration is diverted or limited. This work
supports future design efforts for a mechanical structure for the NRD.Comment: 26 pages, 16 figure
Zeno and anti-Zeno effects for photon polarization dephasing
We discuss a simple, experimentally feasible scheme, which elucidates the
principles of controlling ("engineering") the reservoir spectrum and the
spectral broadening incurred by repeated measurements. This control can yield
either the inhibition (Zeno effect) or the acceleration (anti-Zeno effect) of
the quasi-exponential decay of the observed state by means of frequent
measurements. In the discussed scheme, a photon is bouncing back and forth
between two perfect mirrors, each time passing a polarization rotator. The
horizontal and vertical polarizations can be viewed as analogs of an excited
and a ground state of a two level system (TLS). A polarization beam splitter
and an absorber for the vertically polarized photon are inserted between the
mirrors, and effect measurements of the polarization. The polarization angle
acquired in the electrooptic polarization rotator can fluctuate randomly, e.g.,
via noisy modulation. In the absence of an absorber the polarization
randomization corresponds to TLS decay into an infinite-temperature reservoir.
The non-Markovian nature of the decay stems from the many round-trips required
for the randomization. We consider the influence of the polarization
measurements by the absorber on this non-Markovian decay, and develop a theory
of the Zeno and anti-Zeno effects in this system.Comment: 11 pages, 4 figure
Numerical analysis of heat load distribution in Heliotron J with magnetic field tracing and plasma transport modeling
Helical modulation of the electrostatic plasma potential due to edge magnetic islands induced by resonant magnetic perturbation fields at TEXTOR
The electrostatic response of the edge plasma to a magnet ic island induced by resonant magnet ic
perturbations to the plasma edge of the circular limiter tokamak TEXT OR is analyzed.
Measurem ents of plasma potential are interprete d by simulations wit h the Hamilton ian guiding
center code ORBIT. We find a strong correlation between the magnetic field topology and the
poloidal modulation of the measured plasma potential. The ion and electron drifts yield a
predominantly electron driven radial diffusion when approaching the island X-point while ion diffusivities are generally an order of magnitude smaller. This causes a strong radial electric field
structure pointing outward from the island O-point. The good agreement found between measured
and modeled plasma potential connected to the enhanced radial particle diffusivities supports that a
magnetic island in the edge of a tokamak plasma can act as convective cell. We show in detail that
the particular, non-ambipolar drifts of electrons and ions in a 3D magnetic topology account for
these effects. An analytical model for the plasma potential is implemented in the code ORBIT, and
analyses of ion and electron radial diffusion show that both ion- and electron-dominated transport
regimes can exist, which are known as ion and electron root solutions in stellarators. This finding
and comparison with reversed field pinch studies and stellarator literature suggest that the role of
magnetic islands as convective cells and hence as major radial particle transport drivers could be a
generic mechanism in 3D plasma boundary layers
Reflection and Transmission in a Neutron-Spin Test of the Quantum Zeno Effect
The dynamics of a quantum system undergoing frequent "measurements", leading
to the so-called quantum Zeno effect, is examined on the basis of a
neutron-spin experiment recently proposed for its demonstration. When the
spatial degrees of freedom are duely taken into account, neutron-reflection
effects become very important and may lead to an evolution which is totally
different from the ideal case.Comment: 26 pages, 6 figure
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The use of the Kalman filter in the automated segmentation of EIT lung images
In this paper, we present a new pipeline for the fast and accurate segmentation of impedance images of the lungs using electrical impedance tomography (EIT). EIT is an emerging, promising, non-invasive imaging modality that produces real-time, low spatial but high temporal resolution images of impedance inside a body. Recovering impedance itself constitutes a nonlinear ill-posed inverse problem, therefore the problem is usually linearized, which produces impedance-change images, rather than static impedance ones. Such images are highly blurry and fuzzy along object boundaries. We provide a mathematical reasoning behind the high suitability of the Kalman filter when it comes to segmenting and tracking conductivity changes in EIT lung images. Next, we use a two-fold approach to tackle the segmentation problem. First, we construct a global lung shape to restrict the search region of the Kalman filter. Next, we proceed with augmenting the Kalman filter by incorporating an adaptive foreground detection system to provide the boundary contours for the Kalman filter to carry out the tracking of the conductivity changes as the lungs undergo deformation in a respiratory cycle. The proposed method has been validated by using performance statistics such as misclassified area, and false positive rate, and compared to previous approaches. The results show that the proposed automated method can be a fast and reliable segmentation tool for EIT imaging
Quantum anti-Zeno effect without wave function reduction
We study the measurement-induced enhancement of the spontaneous decay (called
quantum anti-Zeno effect) for a two-level subsystem, where measurements are
treated as couplings between the excited state and an auxiliary state rather
than the von Neumann's wave function reduction. The photon radiated in a fast
decay of the atom, from the auxiliary state to the excited state, triggers a
quasi-measurement, as opposed to a projection measurement. Our use of the term
"quasi-measurement" refers to a "coupling-based measurement". Such frequent
quasi-measurements result in an exponential decay of the survival probability
of atomic initial state with a photon emission following each
quasi-measurement. Our calculations show that the effective decay rate is of
the same form as the one based on projection measurements. What is more
important, the survival probability of the atomic initial state which is
obtained by tracing over all the photon states is equivalent to the survival
probability of the atomic initial state with a photon emission following each
quasi-measurement to the order under consideration. That is because the
contributions from those states with photon number less than the number of
quasi-measurements originate from higher-order processes.Comment: 7 pages, 3 figure
Lung volume calculated from electrical impedance tomography in ICU patients at different PEEP levels
Purpose: To study and compare the relationship between end-expiratory lung volume (EELV) and changes in end-expiratory lung impedance (EELI) measured with electrical impedance tomography (EIT) at the basal part of the lung at different PEEP levels in a mixed ICU population. Methods: End-expiratory lung volume, EELI and tidal impedance variation were determined at four PEEP levels (15-10-5-0 cm H2O) in 25 ventilated ICU patients. The tidal impedance variation and tidal volume at 5 cm H2O PEEP were used to calculate change in impedance per ml; this ratio was then used to calculate change in lung volume from change in EELI. To evaluate repeatability, EELV was measured in quadruplicate in five additional patients. Results: There was a significant but relatively low correlation (r = 0.79; R2= 0.62) and moderate agreement (bias 194 ml, SD 323 ml) between ÎEELV and change in lung volume calculated from the ÎEELI. The ratio of tidal impedance variation and tidal volume differed between patients and also varied at different PEEP levels. Good agreement was found between repeated EELV measurements and washin/washout of a simulated nitrogen washout technique. Conclusion: During a PEEP trial, the assumption of a linear relationship between change in global tidal impedance and tidal volume cannot be used to calculate EELV when impedance is measured at only one thoracic level just above the diaphragm
Projection Postulate and Atomic Quantum Zeno Effect
The projection postulate has been used to predict a slow-down of the time
evolution of the state of a system under rapidly repeated measurements, and
ultimately a freezing of the state. To test this so-called quantum Zeno effect
an experiment was performed by Itano et al. (Phys. Rev. A 41, 2295 (1990)) in
which an atomic-level measurement was realized by means of a short laser pulse.
The relevance of the results has given rise to controversies in the literature.
In particular the projection postulate and its applicability in this experiment
have been cast into doubt. In this paper we show analytically that for a wide
range of parameters such a short laser pulse acts as an effective level
measurement to which the usual projection postulate applies with high accuracy.
The corrections to the ideal reductions and their accumulation over n pulses
are calculated. Our conclusion is that the projection postulate is an excellent
pragmatic tool for a quick and simple understanding of the slow-down of time
evolution in experiments of this type. However, corrections have to be
included, and an actual freezing does not seem possible because of the finite
duration of measurements.Comment: 25 pages, LaTeX, no figures; to appear in Phys. Rev.
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