361 research outputs found
Applicability of different geometry approaches to simulations of turbulence in highly sheared magnetic fields
Verification of Gyrokinetic codes: theoretical background and applications
In fusion plasmas the strong magnetic field allows the fast gyro-motion to be
systematically removed from the description of the dynamics, resulting in a
considerable model simplification and gain of computational time. Nowadays, the
gyrokinetic (GK) codes play a major role in the understanding of the
development and the saturation of turbulence and in the prediction of the
subsequent transport. Naturally, these codes require thorough verification and
validation.
Here we present a new and generic theoretical framework and specific
numerical applications to test the faithfulness of the implemented models to
theory and to verify the domain of applicability of existing GK codes. For a
sound verification process, the underlying theoretical GK model and the
numerical scheme must be considered at the same time, which has rarely been
done and therefore makes this approach pioneering. At the analytical level, the
main novelty consists in using advanced mathematical tools such as variational
formulation of dynamics for systematization of basic GK code's equations to
access the limits of their applicability. The verification of numerical scheme
is proposed via the benchmark effort.
In this work, specific examples of code verification are presented for two GK
codes: the multi-species electromagnetic ORB5 (PIC) and the radially global
version of GENE (Eulerian). The proposed methodology can be applied to any
existing GK code. We establish a hierarchy of reduced GK Vlasov-Maxwell
equations implemented in the ORB5 and GENE codes using the Lagrangian
variational formulation. At the computational level, detailed verifications of
global electromagnetic test cases developed from the CYCLONE Base Case are
considered, including a parametric -scan covering the transition from
ITG to KBM and the spectral properties at the nominal value.Comment: 16 pages, 2 Figures, APS DPP 2016 invited pape
Gyrokinetic and kinetic particle-in-cell simulations of guide-field reconnection. I: Macroscopic effects of the electron flows
In this work, we compare gyrokinetic (GK) and fully kinetic Particle-in-Cell
(PIC) simulations of magnetic reconnection in the limit of strong guide field.
In particular, we analyze the limits of applicability of the GK plasma model
compared to a fully kinetic description of force free current sheets for finite
guide fields (). Here we report the first part of an extended comparison,
focusing on the macroscopic effects of the electron flows. For a low beta
plasma (), it is shown that both plasma models develop magnetic
reconnection with similar features in the secondary magnetic islands if a
sufficiently high guide field () is imposed in the kinetic PIC
simulations. Outside of these regions, in the separatrices close to the X
points, the convergence between both plasma descriptions is less restrictive
(). Kinetic PIC simulations using guide fields
reveal secondary magnetic islands with a core magnetic field and less energetic
flows inside of them in comparison to the GK or kinetic PIC runs with stronger
guide fields. We find that these processes are mostly due to an initial shear
flow absent in the GK initialization and negligible in the kinetic PIC high
guide field regime, in addition to fast outflows on the order of the ion
thermal speed that violate the GK ordering. Since secondary magnetic islands
appear after the reconnection peak time, a kinetic PIC/GK comparison is more
accurate in the linear phase of magnetic reconnection. For a high beta plasma
() where reconnection rates and fluctuations levels are reduced,
similar processes happen in the secondary magnetic islands in the fully kinetic
description, but requiring much lower guide fields ().Comment: 18 pages, 13 figures. Revised to match with the published version in
Physics of Plasma
Subproton-scale cascades in solar wind turbulence: driven hybrid-kinetic simulations
A long-lasting debate in space plasma physics concerns the nature of
subproton-scale fluctuations in solar wind (SW) turbulence. Over the past
decade, a series of theoretical and observational studies were presented in
favor of either kinetic Alfv\'en wave (KAW) or whistler turbulence. Here, we
investigate numerically the nature of the subproton-scale turbulent cascade for
typical SW parameters by means of unprecedented high-resolution simulations of
forced hybrid-kinetic turbulence in two real-space and three velocity-space
dimensions. Our analysis suggests that small-scale turbulence in this model is
dominated by KAWs at and by magnetosonic/whistler fluctuations
at lower . The spectral properties of the turbulence appear to be in
good agreement with theoretical predictions. A tentative interpretation of this
result in terms of relative changes in the damping rates of the different waves
is also presented. Overall, the results raise interesting new questions about
the properties and variability of subproton-scale turbulence in the SW,
including its possible dependence on the plasma , and call for detailed
and extensive parametric explorations of driven kinetic turbulence in three
dimensions.Comment: 6 pages, 4 figures, accepted for publication in The Astrophysical
Journal Letter
Multiscale nature of the dissipation range in gyrokinetic simulations of Alfv\'enic turbulence
Nonlinear energy transfer and dissipation in Alfv\'en wave turbulence are
analyzed in the first gyrokinetic simulation spanning all scales from the tail
of the MHD range to the electron gyroradius scale. For typical solar wind
parameters at 1 AU, about 30% of the nonlinear energy transfer close to the
electron gyroradius scale is mediated by modes in the tail of the MHD cascade.
Collisional dissipation occurs across the entire kinetic range
. Both mechanisms thus act on multiple coupled scales,
which have to be retained for a comprehensive picture of the dissipation range
in Alfv\'enic turbulence.Comment: Made several improvements to figures and text suggested by referee
Identity, Belonging and Political Activism in The Sri Lankan Communities in Germany
This research examines the dynamic relationship of Sinhalese and Tamils living in Germany in regards to their home and host country, and seeks to better understand the complexities of their political involvement. The research is based on qualitative research methodologies. The author conducted 30
interviews in Berlin during the period 2006–2008 to inform this research.
The research not only provides an overview of the historical contexts of Sri
Lanka and Germany, it also links these histories with the processes of outmigration from Sri Lanka and in-migration to Germany. It then continues to
analyse the construction of belonging in the context of the Sri Lankan diaspora
communities in Berlin. The research examines how far concepts of home,
citizenship, nationalism and identity construction shape the sense of belonging of first and second generation Sinhalese and Tamils in Berlin. Finally, it analyses the ways the members of the Sri Lankan diaspora communities engage in economic, political, social, cultural and virtual activities in the home and host country, and how far these activities shape belonging and are politically motivated. The research also considers and studies the gendered nature of belonging and transnational political practices.
The research uniquely combines the study of the Tamil diaspora with the study
of the Sinhalese diaspora in Berlin. It allows new insights into the complex and
multiple constructions of belonging and identity and into the interplay of gender, ethnicity and generations, and it highlights the importance of political activism in the conceptualization of belonging
Noise Levels and Noisiness of some Power Generators in Federal College of Education Environs, Pankshin, Plateau State Nigeria
This research was on determination of noise levels and noisiness in noys of some power generators and comparing the results to international noise regulatory bodies permissible noise limits. Measurement of sound pressure levels from different generators rated 650 watts to 5.0kilo watts was carried out using sound level meter SL 4010 EUROLAB. The corresponding noisiness was determined using a noy chart 150 507 using the representative frequencies of a complex sound. The measured sound pressure levels ranged from 76 &BA to 97 dBA. The noisiness ranged from 45.20noys to 189.06 noys. The noise levels were higher than the international proposal for noise limit of 45 dBA aquivalent to 5.6 noy daytime and 40dBA equivalent to 3.4noys evening time. The noise levels were above the occupational safety and Health Administration (OSHA) exposure permissible noise limit of 80dBA equivalent to 63noys and also higher than the world Health Organization (WHO) exposure permissible noise limit of 90dBA equivalent to 120.3noys. Keywords: noise level, noisiness, noy, power generators
Collision-dependent power law scalings in 2D gyrokinetic turbulence
Nonlinear gyrokinetics provides a suitable framework to describe
short-wavelength turbulence in magnetized laboratory and astrophysical plasmas.
In the electrostatic limit, this system is known to exhibit a free energy
cascade towards small scales in (perpendicular) real and/or velocity space. The
dissipation of free energy is always due to collisions (no matter how weak the
collisionality), but may be spread out across a wide range of scales. Here, we
focus on freely-decaying 2D electrostatic turbulence on sub-ion-gyroradius
scales. An existing scaling theory for the turbulent cascade in the weakly
collisional limit is generalized to the moderately collisional regime. In this
context, non-universal power law scalings due to multiscale dissipation are
predicted, and this prediction is confirmed by means of direct numerical
simulations.Comment: 7 pages, 5 figures, accepted for publication in Physics of Plasma
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