615 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
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
The Sin of Exclusion: Applicability of Trials Encouraging Omission of Radiation Therapy to Nonwhite Patients With Breast Cancer
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
Are realized volatility models good candidates for alternative Value at Risk prediction strategies?
In this paper, we assess the Value at Risk (VaR) prediction accuracy and efficiency of six ARCH-type models, six realized volatility models and two GARCH models augmented with realized volatility regressors. The α-th quantile of the innovation’s distribution is estimated with the fully parametric method using either the normal or the skewed student distributions and also with the Filtered Historical Simulation (FHS), or the Extreme Value Theory (EVT) methods. Our analysis is based on two S&P 500 cash index out-of-sample forecasting periods, one of which covers exclusively the recent 2007-2009 financial crisis. Using an extensive array of statistical and regulatory risk management loss functions, we find that the realized volatility and the augmented GARCH models with the FHS or the EVT quantile estimation methods produce superior VaR forecasts and allow for more efficient regulatory capital allocations. The skewed student distribution is also an attractive alternative, especially during periods of high market volatility.High frequency intraday data; Filtered Historical Simulation; Extreme Value Theory; Value-at-Risk forecasting; Financial crisis.
MERLIN observations of Stephan's Quintet
We present MERLIN L-band images of the compact galaxy group, Stephan's
Quintet. The Seyfert 2 galaxy, NGC 7319, the brightest member of the compact
group, is seen to have a triple radio structure typical of many extra-galactic
radio sources which have a flat spectrum core and two steep spectrum lobes with
hot spots. The two lobes are asymmetrically distributed on opposite sides of
the core along the minor axis of the galaxy. Ultraviolet emission revealed in a
high resolution HRC/ACS HST image is strongly aligned with the radio plasma and
we interpret the intense star formation in the core and north lobe as an event
induced by the collision of the north radio jet with over-dense ambient
material. In addition, a re-mapping of archive VLA L-band observations reveals
more extended emission along the major axis of the galaxy which is aligned with
the optical axis. Images formed from the combined MERLIN and archive VLA data
reveal more detailed structure of the two lobes and hot spots.Comment: Completely revised version with new HST data included, to appear in
MNRA
Gyrokinetic studies of the effect of beta on drift-wave stability in NCSX
The gyrokinetic turbulence code GS2 was used to investigate the effects of
plasma beta on linear, collisionless ion temperature gradient (ITG) modes and
trapped electron modes (TEM) in National Compact Stellarator Experiment (NCSX)
geometry. Plasma beta affects stability in two ways: through the equilibrium
and through magnetic fluctuations. The first was studied here by comparing ITG
and TEM stability in two NCSX equilibria of differing beta values, revealing
that the high beta equilibrium was marginally more stable than the low beta
equilibrium in the adiabatic-electron ITG mode case. However, the high beta
case had a lower kinetic-electron ITG mode critical gradient. Electrostatic and
electromagnetic ITG and TEM mode growth rate dependencies on temperature
gradient and density gradient were qualitatively similar. The second beta
effect is demonstrated via electromagnetic ITG growth rates' dependency on
GS2's beta input parameter. A linear benchmark with gyrokinetic codes GENE and
GKV-X is also presented.Comment: Submitted to Physics of Plasmas. 9 pages, 27 figure
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