1,454 research outputs found
Ion temperature gradient turbulence in helical and axisymmetric RFP plasmas
Turbulence induced by the ion temperature gradient (ITG) is investigated in
the helical and axisymmetric plasma states of a reversed field pinch device by
means of gyrokinetic calculations. The two magnetic configurations are
systematically compared, both linearly and nonlinearly, in order to evaluate
the impact of the geometry on the instability and its ensuing transport, as
well as on the production of zonal flows. Despite its enhanced confinement, the
high-current helical state demonstrates a lower ITG stability threshold
compared to the axisymmetric state, and ITG turbulence is expected to become an
important contributor to the total heat transport.Comment: 8 pages, to appear in Phys. Plasma
Distinct turbulence saturation regimes in stellarators
In the complex 3D magnetic fields of stellarators, ion-temperature-gradient
turbulence is shown to have two distinct saturation regimes, as revealed by
petascale numerical simulations, and explained by a simple turbulence theory.
The first regime is marked by strong zonal flows, and matches previous
observations in tokamaks. The newly observed second regime, in contrast,
exhibits small- scale quasi-two-dimensional turbulence, negligible zonal flows,
and, surprisingly, a weaker heat flux scaling. Our findings suggest that key
details of the magnetic geometry control turbulence in stellarators.Comment: Erratum added to en
MERLIN imaging of the maser flare in Markarian 348
MERLIN images of Mrk 348 at 22 GHz show water maser emission at 0.02 - 0.11
Jy, within approximately 0.8 pc of the nucleus. This is the first direct
confirmation that molecular material exists close to the Seyfert 2 nucleus. Mrk
348 was observed in 2000 May one month after Falcke et al. (2000) first
identified the maser in single-dish spectra. The peak maser flux density has
increased about threefold. The masing region is < 0.6 pc in radius. The flux
density of radio continuum emission from the core has been rising for about 2
years. The maser-core separation is barely resolved but at the 3 sigma
significance level they are not coincident along the line of sight. The masers
lie in the direction of the northern radio lobes and probably emanate from
material shocked by a jet with velocity close to c. The correlation between the
radio continuum increase and maser flare is explained as arising from high
level nuclear activity through a common excitation mechanism although direct
maser amplification of the core by masers tracing a Keplerian disc is not
completely ruled out.Comment: Accepted by MNRAS Letters, 5 pages, 3 figures, corrections in text
and figur
Signature of a universal statistical description for drift-wave plasma turbulence
This Letter provides a theoretical interpretation of numerically generated
probability density functions (PDFs) of intermittent plasma transport events.
Specifically, nonlinear gyrokinetic simulations of ion-temperature-gradient
turbulence produce time series of heat flux which exhibit manifestly
non-Gaussian PDFs with enhanced tails. It is demonstrated that, after the
removal of autocorrelations, the numerical PDFs can be matched with predictions
from a fluid theoretical setup, based on the instanton method. This result
points to a universality in the modeling of intermittent stochastic process,
offering predictive capability.Comment: 4 pages, 5 figure
Financial crisis, intervention and performance measurement
Aspects of the financial markets that became apparent in the 2008 crisis were exacerbated by the intervention of monetary authorities. Financial markets under stress validate the general concept of Prospect Theory, under certain assumptions about the distributional characteristics of asset returns. This validation points to the need for re-examining performance metrics, such as the Sharpe Ratio and the Information Ratio. This analysis proposes new ratios that accommodate a higher moment of the portfolio return distribution. This alteration is reflected by the qualitative analysis of investment managers, which is performed by the performance evaluation industry, as it pertains to fixed income.peer-reviewe
Collisionless microinstabilities in stellarators II - numerical simulations
Microinstabilities exhibit a rich variety of behavior in stellarators due to
the many degrees of freedom in the magnetic geometry. It has recently been
found that certain stellarators (quasi-isodynamic ones with maximum-
geometry) are partly resilient to trapped-particle instabilities, because
fast-bouncing particles tend to extract energy from these modes near marginal
stability. In reality, stellarators are never perfectly quasi-isodynamic, and
the question thus arises whether they still benefit from enhanced stability.
Here the stability properties of Wendelstein 7-X and a more quasi-isodynamic
configuration, QIPC, are investigated numerically and compared with the
National Compact Stellarator Experiment (NCSX) and the DIII-D tokamak. In
gyrokinetic simulations, performed with the gyrokinetic code GENE in the
electrostatic and collisionless approximation, ion-temperature-gradient modes,
trapped-electron modes and mixed-type instabilities are studied. Wendelstein
7-X and QIPC exhibit significantly reduced growth rates for all simulations
that include kinetic electrons, and the latter are indeed found to be
stabilizing in the energy budget. These results suggest that imperfectly
optimized stellarators can retain most of the stabilizing properties predicted
for perfect maximum- configurations.Comment: 15 pages, 40 figure
Nonlinearity And The Forward Premium Anomaly
Speculative efficiency (the ability to profit without bearing commensurate risk in foreign exchange markets) is predicated on the forward premium anomaly. Market efficiency still manifests itself in the form of nonlinear adjustments, eroding excess profits. The returns on foreign exchange speculation may not behave in a market-efficient manner except at average values of interest-rate differentials. This study suggests that market efficiency holds as a result of a hedging relation between linear and nonlinear responses of returns
Solar System Experiments and the Interpretation of Saa's Model of Gravity with Propagating Torsion as a Theory with Variable Plank "Constant"
It is shown that the recently proposed interpretation of the transposed
equi-affine theory of gravity as a theory with variable Plank "constant" is
inconsistent with basic solar system gravitational experiments.Comment: 6 pages, latex, no figures. Typos correcte
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