66,721 research outputs found
Ideal magnetohydrodynamic simulations of unmagnetized dense plasma jet injection into a hot strongly magnetized plasma
We present results from three-dimensional ideal magnetohydrodynamic
simulations of unmagnetized dense plasma jet injection into a uniform hot
strongly magnetized plasma, with the aim of providing insight into core fueling
of a tokamak with parameters relevant for ITER and NSTX (National Spherical
Torus Experiment). Unmagnetized dense plasma jet injection is similar to
compact toroid injection but with much higher plasma density and total mass,
and consequently lower required injection velocity. Mass deposition of the jet
into the background appears to be facilitated via magnetic reconnection along
the jet's trailing edge. The penetration depth of the plasma jet into the
background plasma is mostly dependent on the jet's initial kinetic energy, and
a key requirement for spatially localized mass deposition is for the jet's
slowing-down time to be less than the time for the perturbed background
magnetic flux to relax due to magnetic reconnection. This work suggests that
more accurate treatment of reconnection is needed to fully model this problem.
Parameters for unmagnetized dense plasma jet injection are identified for
localized core deposition as well as edge localized mode (ELM) pacing
applications in ITER and NSTX-relevant regimes.Comment: 16 pages, 8 figures and 2 tables; accepted by Nuclear Fusion (May 11,
2011
Tendency of spherically imploding plasma liners formed by merging plasma jets to evolve toward spherical symmetry
Three dimensional hydrodynamic simulations have been performed using smoothed
particle hydrodynamics (SPH) in order to study the effects of discrete jets on
the processes of plasma liner formation, implosion on vacuum, and expansion.
The pressure history of the inner portion of the liner was qualitatively and
quantitatively similar from peak compression through the complete stagnation of
the liner among simulation results from two one dimensional
radiationhydrodynamic codes, 3D SPH with a uniform liner, and 3D SPH with 30
discrete plasma jets. Two dimensional slices of the pressure show that the
discrete jet SPH case evolves towards a profile that is almost
indistinguishable from the SPH case with a uniform liner, showing that
non-uniformities due to discrete jets are smeared out by late stages of the
implosion. Liner formation and implosion on vacuum was also shown to be robust
to Rayleigh-Taylor instability growth. Interparticle mixing for a liner
imploding on vacuum was investigated. The mixing rate was very small until
after peak compression for the 30 jet simulation.Comment: 28 pages, 16 figures, submitted to Physics of Plasmas (2012
Non-linear Simulations of MHD Instabilities in Tokamaks Including Eddy Current Effects and Perspectives for the Extension to Halo Currents
The dynamics of large scale plasma instabilities can strongly be influenced
by the mutual interaction with currents flowing in conducting vessel
structures. Especially eddy currents caused by time-varying magnetic
perturbations and halo currents flowing directly from the plasma into the walls
are important. The relevance of a resistive wall model is directly evident for
Resistive Wall Modes (RWMs) or Vertical Displacement Events (VDEs). However,
also the linear and non-linear properties of most other large-scale
instabilities may be influenced significantly by the interaction with currents
in conducting structures near the plasma. The understanding of halo currents
arising during disruptions and VDEs, which are a serious concern for ITER as
they may lead to strong asymmetric forces on vessel structures, could also
benefit strongly from these non-linear modeling capabilities. Modeling the
plasma dynamics and its interaction with wall currents requires solving the
magneto-hydrodynamic (MHD) equations in realistic toroidal X-point geometry
consistently coupled with a model for the vacuum region and the resistive
conducting structures. With this in mind, the non-linear finite element MHD
code JOREK has been coupled with the resistive wall code STARWALL, which allows
to include the effects of eddy currents in 3D conducting structures in
non-linear MHD simulations. This article summarizes the capabilities of the
coupled JOREK-STARWALL system and presents benchmark results as well as first
applications to non-linear simulations of RWMs, VDEs, disruptions triggered by
massive gas injection, and Quiescent H-Mode. As an outlook, the perspectives
for extending the model to halo currents are described.Comment: Proceeding paper for Theory of Fusion Plasmas (Joint Varenna-Lausanne
International Workshop), Varenna, Italy (September 1-5, 2014); accepted for
publication in: to Journal of Physics: Conference Serie
Kinetic Monte Carlo and Cellular Particle Dynamics Simulations of Multicellular Systems
Computer modeling of multicellular systems has been a valuable tool for
interpreting and guiding in vitro experiments relevant to embryonic
morphogenesis, tumor growth, angiogenesis and, lately, structure formation
following the printing of cell aggregates as bioink particles. Computer
simulations based on Metropolis Monte Carlo (MMC) algorithms were successful in
explaining and predicting the resulting stationary structures (corresponding to
the lowest adhesion energy state). Here we present two alternatives to the MMC
approach for modeling cellular motion and self-assembly: (1) a kinetic Monte
Carlo (KMC), and (2) a cellular particle dynamics (CPD) method. Unlike MMC,
both KMC and CPD methods are capable of simulating the dynamics of the cellular
system in real time. In the KMC approach a transition rate is associated with
possible rearrangements of the cellular system, and the corresponding time
evolution is expressed in terms of these rates. In the CPD approach cells are
modeled as interacting cellular particles (CPs) and the time evolution of the
multicellular system is determined by integrating the equations of motion of
all CPs. The KMC and CPD methods are tested and compared by simulating two
experimentally well known phenomena: (1) cell-sorting within an aggregate
formed by two types of cells with different adhesivities, and (2) fusion of two
spherical aggregates of living cells.Comment: 11 pages, 7 figures; submitted to Phys Rev
Plasma Edge Kinetic-MHD Modeling in Tokamaks Using Kepler Workflow for Code Coupling, Data Management and Visualization
A new predictive computer simulation tool targeting the development of the H-mode pedestal at the plasma edge in tokamaks and the triggering and dynamics of edge localized modes (ELMs) is presented in this report. This tool brings together, in a coordinated and effective manner, several first-principles physics simulation codes, stability analysis packages, and data processing and visualization tools. A Kepler workflow is used in order to carry out an edge plasma simulation that loosely couples the kinetic code, XGC0, with an ideal MHD linear stability analysis code, ELITE, and an extended MHD initial value code such as M3D or NIMROD. XGC0 includes the neoclassical ion-electron-neutral dynamics needed to simulate pedestal growth near the separatrix. The Kepler workflow processes the XGC0 simulation results into simple images that can be selected and displayed via the Dashboard, a monitoring tool implemented in AJAX allowing the scientist to track computational resources, examine running and archived jobs, and view key physics data, all within a standard Web browser. The XGC0 simulation is monitored for the conditions needed to trigger an ELM crash by periodically assessing the edge plasma pressure and current density profiles using the ELITE code. If an ELM crash is triggered, the Kepler workflow launches the M3D code on a moderate-size Opteron cluster to simulate the nonlinear ELM crash and to compute the relaxation of plasma profiles after the crash. This process is monitored through periodic outputs of plasma fluid quantities that are automatically visualized with AVS/Express and may be displayed on the Dashboard. Finally, the Kepler workflow archives all data outputs and processed images using HPSS, as well as provenance information about the software and hardware used to create the simulation. The complete process of preparing, executing and monitoring a coupled-code simulation of the edge pressure pedestal buildup and the ELM cycle using the Kepler scientific workflow system is described in this paper
Radio frequency induced and neoclassical asymmetries and their effects on turbulent impurity transport in a tokamak
Poloidal asymmetries in the impurity density can be generated by radio
frequency heating in the core and by neoclassical effects in the edge of
tokamak plasmas. In a pedestal case study, using global neoclassical
simulations we find that finite orbit width effects can generate significant
poloidal variation in the electrostatic potential, which varies on a small
radial scale. Gyrokinetic modeling shows that these poloidal asymmetries can be
strong enough to significantly modify turbulent impurity peaking. In the
pedestal the ExB drift in the radial electric field can give a larger
contribution to the poloidal motion of impurities than that of their parallel
streaming. Under such circumstances we find that up-down asymmetries can also
affect impurity peaking.Comment: Paper for 14th International Workshop on Plasma Edge Theory in Fusion
Devices, revised version, submitte
Conceptual design study for heat exhaust management in the ARC fusion pilot plant
The ARC pilot plant conceptual design study has been extended beyond its
initial scope [B. N. Sorbom et al., FED 100 (2015) 378] to explore options for
managing ~525 MW of fusion power generated in a compact, high field (B_0 = 9.2
T) tokamak that is approximately the size of JET (R_0 = 3.3 m). Taking
advantage of ARC's novel design - demountable high temperature superconductor
toroidal field (TF) magnets, poloidal magnetic field coils located inside the
TF, and vacuum vessel (VV) immersed in molten salt FLiBe blanket - this
follow-on study has identified innovative and potentially robust power exhaust
management solutions.Comment: Accepted by Fusion Engineering and Desig
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