275 research outputs found
New Hypothesis for Cause of Epidemic among Native Americans, New England, 1616–1619
This epidemic may have been leptospirosis complicated by Weil syndrome
Transition from no-ELM response to pellet ELM triggering during pedestal build-up—insights from extended MHD simulations
Pellet edge localized mode (ELM) triggering is a well-established scheme for decreasing the time between two successive ELM crashes below its natural value. Reliable ELM pacing has been demonstrated experimentally in several devices, increasing the ELM frequency considerably. However, it was also shown that the frequency cannot be increased arbitrarily due to a so-called lag-time. During this time, after a preceding natural or triggered ELM crash, neither a natural ELM crash occurs nor is it possible to trigger an ELM crash by pellet injection. For this article, pellet ELM triggering simulations are advanced beyond previous studies in two ways. Firstly, realistic E B and diamagnetic background flows are included. And secondly, the pellet is injected at different stages of the pedestal build-up. This allows us to recover the lag time for the first time in simulations and investigate it in detail. A series of nonlinear extended MHD simulations is performed to investigate the plasma dynamics resulting from an injection at different time points during the pedestal build-up. The experimentally observed lag-time is qualitatively reproduced. In particular, a sharp transition is observed between the regime where no ELMs can be triggered and the regime where pellet injection causes an ELM crash. Via variations of pellet parameters and injection time, the two regimes are studied and compared in detail, revealing pronounced differences in the nonlinear dynamics. The toroidal mode spectrum is significantly broader when an ELM crash is triggered, enhancing the stochasticity and therefore also the losses of thermal energy along magnetic field lines. In the heat fluxes to the divertor targets, pronounced toroidal asymmetries are observed. In the case of high injection velocities leading to deep penetration, the excitation of core modes like the 2/1 neoclassical tearing mode is also observed
Optical Multicolor WBVR-Observations of the X-Ray Star V1341 Cyg = Cyg X-2 in 1986-1992
We present the results of observations of the low-mass X-ray binary
V1341 X--2. Our observations include a total of
2375 individual measurements in four bands on 478 nights in 1986-1992. We tied
the comparison and check stars used for the binary to the catalog using
their magnitudes. The uncertainty of this procedure was 3 in the
and bands and 8%-10% for the and bands. In quiescence, the
amplitude of the periodic component in the binary's brightness variations
is within ( in ); this is due
to the ellipsoidal shape of the optical component, which is distorted with
gravitational forces from the X-ray component. Some of the system's active
states (long flares) may be due to instabilities in the accretion disk, and
possibly to instabilities of gas flows and other accretion structures. The
binary possesses a low-luminosity accretion disk. The light curves reveal no
indications of an eclipse near the phases of the upper and lower conjunctions
in quiescence or in active states during the observed intervals. We conclude
that the optical star in the close binary V1341
X-2 is a red giant rather than a blue straggler. We studied the long-term
variability of the binary during the seven years covered by our observations.
The optical observations presented in this study are compared to X-ray data
from the Ginga observatory for the same time intervals.Comment: 35 pages, 8 figure
Probing non-linear MHD stability of the EDA H-mode in ASDEX Upgrade
Regimes of operation in tokamaks that are devoid of large ELMs have to be
better understood to extrapolate their applicability to reactor-relevant
devices. This paper describes non-linear extended MHD simulations that use an
experimental equilibrium from an EDA H-mode in ASDEX Upgrade. Linear ideal MHD
analysis indicates that the operational point lies slightly inside of the
stable region. The non-linear simulations with the visco-resistive extended MHD
code, JOREK, sustain non-axisymmetric perturbations that are linearly most
unstable with toroidal mode numbers of n = \{6 \dots 9\}, but non-linearly
higher and lower n become driven and the low-n become dominant. The poloidal
mode velocity during the linear phase is found to correspond to the expected
velocity for resistive ballooning modes. The perturbations that exist in the
simulations have somewhat smaller poloidal wavenumbers (k_{\theta} \sim 0.1 to
0.5 cm^{-1} ) than the experimental expectations for the quasi-coherent mode in
EDA, and cause non-negligible transport in both the heat and particle channels.
In the transition from linear to non-linear phase, the mode frequency chirps
down from approximately 35 kHz to 13 kHz, which corresponds approximately to
the lower end of frequencies that are typically observed in EDA H-modes in
ASDEX Upgrade
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