23 research outputs found
Nonlinear excitation of low-n harmonics in reduced magnetohydrodynamic simulations of edge-localized modes
Nonlinear simulations of the early ELM phase based on a typical type-I ELMy
ASDEX Upgrade discharge have been carried out using the reduced MHD code JOREK.
The analysis is focused on the evolution of the toroidal Fourier spectrum. It
is found that during the nonlinear evolution, linearly subdominant low-n
Fourier components, in particular the n = 1, grow to energies comparable with
linearly dominant harmonics. A simple model is developed, based on the idea
that energy is transferred among the toroidal harmonics via second order
nonlinear interaction. The simple model reproduces and explains very well the
early nonlinear evolution of the toroidal spectrum in the JOREK simulations.
Furthermore, it is shown for the n = 1 harmonic, that its spatial structure
changes significantly during the transition from linear to nonlinearly driven
growth. The rigidly growing structure of the linearly barely unstable n = 1
reaches far into the plasma core. In contrast, the nonlinearly driven n = 1 has
a rigidly growing structure localized at the plasma edge, where the dominant
toroidal harmonics driving the n = 1 are maximal and in phase. The presented
quadratic coupling model might explain the recent experimental observation of
strong low-n components in magnetic measurements [Wenninger et al., Non-linear
magnetic perturbations during edge localized modes in TCV dominated by low n
mode components, submitted to Nuclear Fusion]
Testing of the new JOREK stellarator-capable model in the tokamak limit
In preparation for extending the JOREK nonlinear MHD code to stellarators, a
hierarchy of stellarator-capable reduced and full MHD models has been derived
and tested. The derivation was presented at the EFTC 2019 conference.
Continuing this line of work, we have implemented the reduced MHD model
(arXiv:1907.12486) as well as an alternative model which was newly derived
using a different set of projection operators for obtaining the scalar momentum
equations from the full MHD vector momentum equation. With the new operators,
the reduced model matches the standard JOREK reduced models for tokamaks in the
tokamak limit and conserves energy exactly, while momentum conservation is less
accurate than in the original model whenever field-aligned flow is present.Comment: 23 pages, 1 table, 7 figures. Submitted to Journal of Plasma Physic
A three-dimensional reduced MHD model consistent with full MHD
Within the context of a viscoresistive magnetohydrodynamic (MHD) model with
anisotropic heat transport and cross-field mass diffusion, we introduce novel
three-term representations for the magnetic field (background vacuum field,
field line bending and field compression) and velocity (
flow, field-aligned flow and fluid compression), which are amenable to
three-dimensional treatment. Once the representations are inserted into the MHD
equations, appropriate projection operators are applied to Faraday's law and
the Navier-Stokes equation to obtain a system of scalar equations that is
closed by the continuity and energy equations. If the background vacuum field
is sufficiently strong and the is low, MHD waves are approximately
separated by the three terms in the velocity representation, with each term
containing a specific wave. Thus, by setting the appropriate term to zero, we
eliminate fast magnetosonic waves, obtaining a reduced MHD model. We also show
that the other two velocity terms do not compress the magnetic field, which
allows us to set the field compression term to zero within the same reduced
model. Dropping also the field-aligned flow, a further simplified model is
obtained, leading to a fully consistent hierarchy of reduced and full MHD
models for 3D plasma configurations. Finally, we discuss the conservation
properties and derive the conditions under which the reduction approximation is
valid. We also show that by using an ordering approach, reduced MHD equations
similar to what we got from the ansatz approach can be obtained by means of a
physics-based asymptotic expansion.Comment: 18 pages. This article was published in Physics of Plasma
How well can VMEC predict the initial saturation of external kink modes in near circular tokamaks and stellarators?
The equilibrium code, VMEC, is used to study external kinks in low
tokamaks and stellarators. The applicability of the code when modelling
nonlinear MHD effects is explored in an attempt to understand and predict how
the initial saturation of the MHD mode depends on the external rotational
transform. It is shown that helicity preserving, free boundary VMEC
computations do not converge to a single perturbed solution with increasing
spectral resolution. Additional constraints are therefore applied to narrow
down the numerical resolution parameters appropriate for physical scans. The
dependence of the modelled (4, 1) kink mode on the external rotational
transform and field periodicity is then studied. While saturated states can be
identified which decrease in amplitude with increasing external rotational
transform, bifurcated states are found that contradict this trend. It was
therefore not possible to use VMEC alone to identify the physical dependency of
the nonlinear mode amplitude on the magnetic geometry. The accuracy of the VMEC
solutions is nevertheless demonstrated by showing that the expected toroidal
mode coupling is captured in the magnetic energy spectrum for stellarator
cases. Comparing with the initial value code, JOREK, the predicted
redistribution of poloidal magnetic energy from the vacuum to plasma region in
VMEC is shown to be physical. This work is a first step towards using VMEC to
study MHD modes in stellarator geometry.Comment: Submitted to Physics of Plasmas. The submission has been modified
according to reviewer comment
Numerical study of tearing mode seeding in tokamak X-point plasma
A detailed understanding of island seeding is crucial to avoid (N)TMs and
their negative consequences like confinement degradation and disruptions. In
the present work, we investigate the growth of 2/1 islands in response to
magnetic perturbations. Although we use externally applied perturbations
produced by resonant magnetic perturbation (RMP) coils for this study, results
are directly transferable to island seeding by other MHD instabilities creating
a resonant magnetic field component at the rational surface. Experimental
results for 2/1 island penetration from ASDEX Upgrade are presented extending
previous studies. Simulations are based on an ASDEX Upgrade L-mode discharge
with low collisionality and active RMP coils. Our numerical studies are
performed with the 3D, two fluid, non-linear MHD code JOREK. All three phases
of mode seeding observed in the experiment are also seen in the simulations:
first a weak response phase characterized by large perpendicular electron flow
velocities followed by a fast growth of the magnetic island size accompanied by
a reduction of the perpendicular electron velocity, and finally the saturation
to a fully formed island state with perpendicular electron velocity close to
zero. Thresholds for mode penetration are observed in the plasma rotation as
well as in the RMP coil current. A hysteresis of the island size and electron
perpendicular velocity is observed between the ramping up and down of the RMP
amplitude consistent with an analytically predicted bifurcation. The transition
from dominant kink/bending to tearing parity during the penetration is
investigated
MHD simulations of formation, sustainment and loss of Quiescent H-mode in the all-tungsten ASDEX Upgrade
Periodic edge localized modes (ELMs) are the non-linear consequences of
pressure-gradient-driven ballooning modes and current-driven peeling modes
becoming unstable in the pedestal region of high confinement fusion plasmas. In
future tokamaks like ITER, large ELMs are foreseen to severely affect the
lifetime of wall components as they transiently deposit large amounts of heat
onto a narrow region at the divertor targets. Several strategies exist for
avoidance, suppression, or mitigation of these instabilities, such as the
naturally ELM-free quiescent H-mode (QH-mode). In the present article, an ASDEX
Upgrade equilibrium that features a QH-mode is investigated through non-linear
extended MHD simulations covering the dynamics over tens of milliseconds. The
equilibrium is close to the ideal peeling limit and non-linearly develops
saturated modes at the edge of the plasma. A dominant toroidal mode number of
is found, for which the characteristic features of the edge harmonic
oscillation are recovered. The saturated modes contribute to heat and particle
transport preventing pedestal build-up to the ELM triggering threshold. The
non-linear dynamics of the mode, in particular its interaction with the
evolution of the edge safety factor is studied, which suggest a possible new
saturation mechanism for the QH-mode. The simulations show good qualitative and
quantitative agreement to experiments in AUG. In particular, the processes
leading to the termination of QH-mode above a density threshold is studied,
which results in the transition into an ELM regime. In the vicinity of this
threshold, limit cycle oscillations are observed.Comment: Revised version with modifications from review process include
Semen CD4+ T cells and macrophages are productively infected at all stages of SIV infection in macaques.
International audienceThe mucosal events of HIV transmission have been extensively studied, but the role of infected cells present in the genital and rectal secretions, and in the semen, in particular, remains a matter of debate. As a prerequisite to a thorough in vivo investigation of the early transmission events through infected cells, we characterized in detail by multi-parameter flow cytometry the changes in macaque seminal leukocytes during SIVmac251 infection, focusing on T cells, macrophages and dendritic cells. Using immunocytofluorescence targeting SIV proteins and real-time quantitative PCR targeting SIV DNA, we investigated the nature of the infected cells on sorted semen leukocytes from macaques at different stages of infection. Finally, we cocultured semen CD4(+) T cells and macrophages with a cell line permissive to SIV infection to assess their infectivity in vitro. We found that primary infection induced strong local inflammation, which was associated with an increase in the number of leukocytes in semen, both factors having the potential to favor cell-associated virus transmission. Semen CD4(+) T cells and macrophages were productively infected at all stages of infection and were infectious in vitro. Lymphocytes had a mucosal phenotype and expressed activation (CD69 & HLA-DR) and migration (CCR5, CXCR4, LFA-1) markers. CD69 expression was increased in semen T cells by SIV infection, at all stages of infection. Macrophages predominated at all stages and expressed CD4, CCR5, MAC-1 and LFA-1. Altogether, we demonstrated that semen contains the two major SIV-target cells (CD4+ T cells and macrophages). Both cell types can be productively infected at all stages of SIV infection and are endowed with markers that may facilitate transmission of infection during sexual exposure
Poorly processed reusable surface disinfection tissue dispensers may be a source of infection
Poorly processed reusable surface disinfection tissue dispensers may be a source of infection
Abstract Background: Reusable surface disinfectant tissue dispensers are used in hospitals in many countries because they allow immediate access to pre-soaked tissues for targeted surface decontamination. On the other hand disinfectant solutions with some active ingredients may get contaminated and cause outbreaks. We determined the frequency of contaminated surface disinfectant solutions in reusable dispensers and the ability of isolates to multiply in different formulations