198 research outputs found
A reanalysis of a strong-flow gyrokinetic formalism
We reanalyse an arbitrary-wavelength gyrokinetic formalism [A. M. Dimits, Phys. Plasmas 17, 055901 (2010)], which orders only the vorticity to be small and allows strong, time-varying flows on medium and long wavelengths. We obtain a simpler gyrocentre Lagrangian up to second order. In addition, the gyrokinetic Poisson equation, derived either via variation of the system Lagrangian or explicit density calculation, is consistent with that of the weak-flow gyrokinetic formalism [T. S. Hahm, Phys. Fluids 31, 2670 (1988)] at all wavelengths in the weak flow limit. The reanalysed formalism has been numerically implemented as a particle-in-cell code. An iterative scheme is described which allows for numerical solution of this system of equations, given the implicit dependence of the Euler-Lagrange equations on the time derivative of the potential
A nonlinear approach to transition in subcritical plasmas with sheared flow
In many plasma systems, introducing a small background shear flow is enough
to stabilize the system linearly. The nonlinear dynamics are much less
sensitive to sheared flows than the average linear growthrates, and very small
amplitude perturbations can lead to sustained turbulence. We explore the
general problem of characterizing how and when the transition from near-laminar
states to sustained turbulence occurs; a model of the interchange instability
being used as a concrete example. These questions are fundamentally nonlinear,
and the answers must go beyond the linear transient amplification of small
perturbations. Two methods that account for nonlinear interactions are
therefore explored here. The first method explored is edge tracking, which
identifies the boundary between the basins of attraction of the laminar and
turbulent states. Here, the edge is found to be structured around an exact,
localized, traveling wave solution; a solution that is qualitatively similar to
avalanche-like bursts seen in the turbulent regime. The second method is an
application of nonlinear, non-modal stability theory which allows us to
identify the smallest disturbances which can trigger turbulence (the minimal
seed for the problem) and hence to quantify how stable the laminar regime is.
The results obtained from these fully nonlinear methods provides confidence in
the derivation of a semi-analytic approximation for the minimal seed
Simple advecting structures and the edge of chaos in subcritical tokamak plasmas
In tokamak plasmas, sheared flows perpendicular to the driving temperature
gradients can strongly stabilize linear modes. While the system is linearly
stable, regimes with persistent nonlinear turbulence may develop, i.e. the
system is subcritical. A perturbation with small but finite amplitude may be
sufficient to push the plasma into a regime where nonlinear effects are
dominant and thus allow sustained turbulence. The minimum threshold for
nonlinear instability to be triggered provides a criterion for assessing
whether a tokamak is likely to stay in the quiescent (laminar) regime. At the
critical amplitude, instead of transitioning to the turbulent regime or
decaying to a laminar state, the trajectory will map out the edge of chaos.
Surprisingly, a quasi-traveling-wave solution is found as an attractor on this
edge manifold. This simple advecting solution is qualitatively similar to, but
simpler than, the avalanche-like bursts seen in earlier turbulent simulations
and provides an insight into how turbulence is sustained in subcritical plasma
systems. For large flow shearing rate, the system is only convectively
unstable, and given a localised initial perturbation, will eventually return to
a laminar state at a fixed spatial location
Overview of JET results for optimising ITER operation
The JET 2019ā2020 scientific and technological programme exploited the results of years of concerted scientific and engineering work, including the ITER-like wall (ILW: Be wall and W divertor) installed in 2010, improved diagnostic capabilities now fully available, a major neutral beam injection upgrade providing record power in 2019ā2020, and tested the technical and procedural preparation for safe operation with tritium. Research along three complementary axes yielded a wealth of new results. Firstly, the JET plasma programme delivered scenarios suitable for high fusion power and alpha particle (Ī±) physics in the coming DāT campaign (DTE2), with record sustained neutron rates, as well as plasmas for clarifying the impact of isotope mass on plasma core, edge and plasma-wall interactions, and for ITER pre-fusion power operation. The efficacy of the newly installed shattered pellet injector for mitigating disruption forces and runaway electrons was demonstrated. Secondly, research on the consequences of long-term exposure to JET-ILW plasma was completed, with emphasis on wall damage and fuel retention, and with analyses of wall materials and dust particles that will help validate assumptions and codes for design and operation of ITER and DEMO. Thirdly, the nuclear technology programme aiming to deliver maximum technological return from operations in D, T and DāT benefited from the highest DāD neutron yield in years, securing results for validating radiation transport and activation codes, and nuclear data for ITER
New distances to RAVE stars
Probability density functions are determined from new stellar parameters for
the distance moduli of stars for which the RAdial Velocity Experiment (RAVE)
has obtained spectra with S/N>=10. Single-Gaussian fits to the pdf in distance
modulus suffice for roughly half the stars, with most of the other half having
satisfactory two-Gaussian representations. As expected, early-type stars rarely
require more than one Gaussian. The expectation value of distance is larger
than the distance implied by the expectation of distance modulus; the latter is
itself larger than the distance implied by the expectation value of the
parallax. Our parallaxes of Hipparcos stars agree well with the values measured
by Hipparcos, so the expectation of parallax is the most reliable distance
indicator. The latter are improved by taking extinction into account. The
effective temperature absolute-magnitude diagram of our stars is significantly
improved when these pdfs are used to make the diagram. We use the method of
kinematic corrections devised by Schoenrich, Binney & Asplund to check for
systematic errors for general stars and confirm that the most reliable distance
indicator is the expectation of parallax. For cool dwarfs and low-gravity
giants tends to be larger than the true distance by up to 30 percent. The
most satisfactory distances are for dwarfs hotter than 5500 K. We compare our
distances to stars in 13 open clusters with cluster distances from the
literature and find excellent agreement for the dwarfs and indications that we
are over-estimating distances to giants, especially in young clusters.Comment: 20 pages accepted by MNRAS. Minor changes to the submitted versio
Triangularity effects on global flux-driven gyrokinetic simulations
On the road to fusion energy production, many alternative scenarios have been investigated in order to address certain well-known problems of tokamak devices; among which, anomalous transport, ELMs and disruptions. The studies on plasma shaping fall into this effort.
In particular, it has been experimentally observed that when operating in L mode, negative triangularity (NT) features better confinement properties than positive triangularity (PT). However, even though the trend is quite clear, a complete and satisfying theoretical explanation for this experimental findings is still lacking.
With the aim of understanding and describing these improvements starting from first principles, we present the first comparison between PT and NT with global flux-driven gyrokinetic simulations performed with the ORB5 code.
The numerical setup includes: electrostatic turbulence, kinetic trapped electrons, non-linear collisional operator, ECRH source, limiter and wall as boundary conditions. The simulations have been performed on ideal MHD equilibria and kinetic profiles inspired by TCV experiments, in a mixed ITG-TEM regime.
First analysis reveal a strong reduction of transport in NT; while at the edge PT shows superdiffusivity, NT does not. The limiter plays an important role that has to be further clarified
Improving BDD Based Symbolic Model Checking with Isomorphism Exploiting Transition Relations
Symbolic model checking by using BDDs has greatly improved the applicability
of model checking. Nevertheless, BDD based symbolic model checking can still be
very memory and time consuming. One main reason is the complex transition
relation of systems. Sometimes, it is even not possible to generate the
transition relation, due to its exhaustive memory requirements. To diminish
this problem, the use of partitioned transition relations has been proposed.
However, there are still systems which can not be verified at all. Furthermore,
if the granularity of the partitions is too fine, the time required for
verification may increase. In this paper we target the symbolic verification of
asynchronous concurrent systems. For such systems we present an approach which
uses similarities in the transition relation to get further memory reductions
and runtime improvements. By applying our approach, even the verification of
systems with an previously intractable transition relation becomes feasible.Comment: In Proceedings GandALF 2011, arXiv:1106.081
Hydrocarbon generation and migration from Barremian ā Aptian source rocks, Northern Orange Basin, offshore Western South Africa: A 3d numerical modelling study
A 3D numerical modelling workflow was applied to the BarremianāAptian source rock interval in a shelfal to lower slope area of the northern Orange Basin, offshore western South Africa. The main objective was to investigate the timing of hydrocarbon generation and migration. Hydrocarbon migration has previously been investigated in the south of the basin by relating gas escape features with structural elements as seen on seismic sections, but migration pathways are still poorly understood. The modelling study was based on data from three exploration wells (AO-1, AE-1 and AF-1) together with 42 2D seismic sections totalling 3537 km in length, and a 3D seismic cube covering an area of 750 sq. km. Modelled formation temperatures increase from north to south in the study area and were consistent with downhole temperatures at well locations. However, there is variation between measured and modelled values of vitrinite reflectance (VR), especially in the Turonian and Cenomanian intervals. The measured VR is lower than the modelled VR within the Turonian section in the north of the study area, suggesting that erosion has affected the thermal maturity of the sediments. However, in the Cenomanian interval, the measured VR is higher than the modelled VR. Uplift, increased erosion in the hinterland and sediment transport to the coastal areas resulted in Cenomanian progradation of the Orange Basin fill
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