356 research outputs found
Beam-Ion Acceleration during Edge Localized Modes in the ASDEX Upgrade Tokamak
The acceleration of beam ions during edge localized modes (ELMs) in a tokamak is observed for the first
time through direct measurements of fast-ion losses in low collisionality plasmas. The accelerated beamion
population exhibits well-localized velocity-space structures which are revealed by means of tomographic
inversion of the measurement, showing energy gains of the order of tens of keV. This suggests that
the ion acceleration results from a resonant interaction between the beam ions and parallel electric fields
arising during the ELM. Orbit simulations are carried out to identify the mode-particle resonances
responsible for the energy gain in the particle phase space. The observation motivates the incorporation of a
kinetic description of fast particles in ELM models and may contribute to a better understanding of the
mechanisms responsible for particle acceleration, ubiquitous in astrophysical and space plasmas.H2020 Marie- Sklodowska Curie programme (Grant No. 708257)Ministerio de Economía y Competitividad. FIS2015-69362-
Three-dimensional non-linear magnetohydrodynamic modeling of massive gas injection triggered disruptions in JET
JOREK 3D non-linear MHD simulations of a D2 Massive Gas Injection (MGI) triggered disruption
in JET are presented and compared in detail to experimental data. The MGI creates an overdensity
that rapidly expands in the direction parallel to the magnetic field. It also causes the growth of
magnetic islands (m=n ¼ 2=1 and 3/2 mainly) and seeds the 1/1 internal kink mode. O-points of
all island chains (including 1/1) are located in front of the MGI, consistently with experimental
observations. A burst of MHD activity and a peak in plasma current take place at the same time
as in the experiment. However, the magnitude of these two effects is much smaller than in the
experiment. The simulated radiation is also much below the experimental level. As a consequence, the thermal quench is not fully reproduced. Directions for progress are identified. Radiation from impurities is a good candidate.EURATOM 63305
Unconventional magnetism in the 4d based () honeycomb system AgLiRuO
We have investigated the thermodynamic and local magnetic properties of the
Mott insulating system AgLiRuO containing Ru
(4) for novel magnetism. The material crystallizes in a monoclinic
structure with RuO octahedra forming an edge-shared
two-dimensional honeycomb lattice with limited stacking order along the
-direction. The large negative Curie-Weiss temperature ( = -57
K) suggests antiferromagnetic interactions among Ru ions though magnetic
susceptibility and heat capacity show no indication of magnetic long-range
order down to 1.8 K and 0.4 K, respectively. Li nuclear magnetic
resonance (NMR) shift follows the bulk susceptibility between 120-300 K and
levels off below 120 K. Together with a power-law behavior in the temperature
dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic
spin correlations with gapless excitations. Electronic structure calculations
suggest an description of the Ru-moments and the possible importance of
further neighbour interactions as also bi-quadratic and ring-exchange terms in
determining the magnetic properties. Analysis of our SR data indicates
spin freezing below 5 K but the spins remain on the borderline between static
and dynamic magnetism even at 20 mK.Comment: 10 pages, 11 figures. accepted in Phys. Rev.
Superconducting order parameter of the nodal-line semimetal NaAlSi
Nodal-line semimetals are topologically non-trivial states of matter
featuring band crossings along a closed curve, i.e. nodal-line, in momentum
space. Through a detailed analysis of the electronic structure, we show for the
first time that the normal state of the superconductor NaAlSi, with a critical
temperature of 7 K, is a nodal-line semimetal, where the
complex nodal-line structure is protected by non-symmorphic mirror crystal
symmetries. We further report on muon spin rotation experiments revealing that
the superconductivity in NaAlSi is truly of bulk nature, featuring a fully
gapped Fermi-surface. The temperature-dependent magnetic penetration depth can
be well described by a two-gap model consisting of two -wave symmetric gaps
with 0.6(2) meV and 1.39(1) meV. The zero-field muon
experiment indicates that time-reversal symmetry is preserved in the
superconducting state. Our observations suggest that notwithstanding its
topologically non-trivial band structure, NaAlSi may be suitably interpreted as
a conventional London superconductor, while more exotic superconducting gap
symmetries cannot be excluded. The intertwining of topological electronic
states and superconductivity renders NaAlSi a prototypical platform to search
for unprecedented topological quantum phases
Canted antiferromagnetic order in the kagome material Sr-vesignieite
We report 51 V NMR, muon spin rotation, and zero-applied-field 63 , 65 Cu NMR measurements on powder samples of Sr-vesignieite, SrCu 3 V 2 O 8 ( OH ) 2 , a S = 1 / 2 nearly kagome Heisenberg antiferromagnet. Our results demonstrate that the ground state is a q = 0 magnetic structure with spins canting either in or out of the kagome plane, giving rise to weak ferromagnetism. We determine the size of ordered moments and the angle of canting for different possible q = 0 structures and orbital scenarios, thereby providing insight into the role of the Dzyaloshinskii-Moriya interaction in this material
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