209 research outputs found
Constrained Hyperbolic Divergence Cleaning for Smoothed Particle Magnetohydrodynamics
We present a constrained formulation of Dedner et al's hyperbolic/parabolic
divergence cleaning scheme for enforcing the \nabla\dot B = 0 constraint in
Smoothed Particle Magnetohydrodynamics (SPMHD) simulations. The constraint we
impose is that energy removed must either be conserved or dissipated, such that
the scheme is guaranteed to decrease the overall magnetic energy. This is shown
to require use of conjugate numerical operators for evaluating \nabla\dot B and
\nabla{\psi} in the SPMHD cleaning equations. The resulting scheme is shown to
be stable at density jumps and free boundaries, in contrast to an earlier
implementation by Price & Monaghan (2005). Optimal values of the damping
parameter are found to be {\sigma} = 0.2-0.3 in 2D and {\sigma} = 0.8-1.2 in
3D. With these parameters, our constrained Hamiltonian formulation is found to
provide an effective means of enforcing the divergence constraint in SPMHD,
typically maintaining average values of h |\nabla\dot B| / |B| to 0.1-1%, up to
an order of magnitude better than artificial resistivity without the associated
dissipation in the physical field. Furthermore, when applied to realistic, 3D
simulations we find an improvement of up to two orders of magnitude in momentum
conservation with a corresponding improvement in numerical stability at
essentially zero additional computational expense.Comment: 28 pages, 25 figures, accepted to J. Comput. Phys. Movies at
http://www.youtube.com/playlist?list=PL215D649FD0BDA466 v2: fixed inverted
figs 1,4,6, and several color bar
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Experience with the Quality Assurance of the Superconducting Electrical Circuits of the LHC Machine
The coherence between the powering reference database for the LHC and the Electrical Quality Assurance (ELQA) is guaranteed on the procedural level. However, a challenge remains the coherence between the database, the magnet test and assembly procedures, and the connection of all superconducting circuits in the LHC machine. In this paper, the methods, tooling, and procedures for the ELQA during the assembly phase of the LHC will be presented in view of the practical experience gained in the LHC tunnel. Some examples of detected polarity errors and electrical non-conformities will be presented. The parameters measured at ambient temperature, such as the dielectric insulation of circuits, will be discussed
Automatic System for the D.C. High Voltage Qualification of the Superconducting Electrical Circuits of the LHC Machine
A d.c. high voltage test system has been developed to verify automatically the insulation resistance of the powering circuits of the LHC. In the most complex case, up to 72 circuits share the same volume inside cryogenic lines. Each circuit can have an insulation fault versus any other circuit or versus ground. The system is able to connect up to 80 circuits and apply a voltage up to 2 kV D.C. The leakage current flowing through each circuit is measured within a range of 1 nA to 1.6 mA. The matrix of measurements allows characterizing the paths taken by the currents and locating weak points of the insulation between circuits. The system is composed of a D.C. voltage source and a data acquisition card. The card is able to measure with precision currents and voltages and to drive up to 5 high voltage switching modules offering 16 channels each. A LabVIEW application controls the system for an automatic and safe operation. This paper describes the hardware and software design, the testing methodology and the results obtained during the qualification of the LHC superconducting circuits
Protostellar outflows with Smoothed Particle Magnetohydrodynamics (SPMHD)
The protostellar collapse of a molecular cloud core is usually accompanied by
outflow phenomena. The latter are thought to be driven by magnetorotational
processes from the central parts of the protostellar disc. While several 3D
AMR/nested grid studies of outflow phenomena in collapsing magnetically
supercritical dense cores have been reported in the literature, so far no such
simulation has been performed using the Smoothed Particle Hydrodynamics (SPH)
method. This is mainly due to intrinsic numerical difficulties in handling
magnetohydrodynamics within SPH, which only recently were partly resolved. In
this work, we use an approach where we evolve the magnetic field via the
induction equation, augmented with stability correction and divergence cleaning
schemes. We consider the collapse of a rotating core of one solar mass,
threaded by a weak magnetic field initially parallel to the rotation axis so
that the core is magnetically supercritical. We show, that Smoothed Particle
Magnetohydrodynamics (SPMHD) is able to handle the magnetorotational processes
connected with outflow phenomena, and to produce meaningful results which are
in good agreement with findings reported in the literature. Especially, our
numerical scheme allows for a quantitative analysis of the evolution of the
ratio of the toroidal to the poloidal magnetic field, which we performed in
this work.Comment: 5 pages, 4 figures. Accepted to MNRAS Letter
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