1,993 research outputs found
Recent ASDEX Upgrade research in support of ITER and DEMO
Recent experiments on the ASDEX Upgrade tokamak aim at improving the physics base for ITER and DEMO to aid the machine
design and prepare efficient operation. Type I edge localized mode (ELM) mitigation using resonant magnetic perturbations
(RMPs) has been shown at low pedestal collisionality (
ν
∗
ped
<
0
.
4
)
. In contrast to the previous high
ν
∗
regime, suppression only
occurs in a narrow RMP spectral window, indicating a resonant process, and a concomitant confinement drop is observed due
to a reduction of pedestal top density and electron temperature. Strong evidence is found for the ion heat flux to be the decisive
element for the L–H power threshold. A physics based scaling of the density at which the minimum
P
LH
occurs indicates that
ITER could take advantage of it to initiate H-mode at lower density than that of the final
Q
=
10 operational point. Core density
fluctuation measurements resolved in radius and wave number show that an increase of
R/L
T
e
introduced by off-axis electron
cyclotron resonance heating (ECRH) mainly increases the large scale fluctuations. The radial variation of the fluctuation level
is in agreement with simulations using the GENE code. Fast particles are shown to undergo classical slowing down in the
absence of large scale magnetohydrodynamic (MHD) events and for low heating power, but show signs of anomalous radial
redistribution at large heating power, consistent with a broadened off-axis neutral beam current drive current profile under these
conditions. Neoclassical tearing mode (NTM) suppression experiments using electron cyclotron current drive (ECCD) with
feedback controlled deposition have allowed to test several control strategies for ITER, including automated control of (3,2) and
(2,1) NTMs during a single discharge. Disruption mitigation studies using massive gas injection (MGI) can show an increased
fuelling efficiency with high field side injection, but a saturation of the fuelling efficiency is observed at high injected mass as
needed for runaway electron suppression. Large locked modes can significantly decrease the fuelling efficiency and increase
the asymmetry of radiated power during MGI mitigation. Concerning power exhaust, the partially detached ITER divertor
scenario has been demonstrated at
P
sep
/R
=
10 MW m
−
1
in ASDEX Upgrade, with a peak time averaged target load around
5MWm
−
2
, well consistent with the component limits for ITER. Developing this towards DEMO, full detachment was achieved
at
P
sep
/R
=
7MWm
−
1
and stationary discharges with core radiation fraction of the order of DEMO requirements (70% instead
of the 30% needed for ITER) were demonstrated. Finally, it remains difficult to establish the standard ITER
Q
=
10 scenario at
low
q
95
=
3 in the all-tungsten (all-W) ASDEX Upgrade due to the observed poor confinement at low
β
N
. This is mainly due to
a degraded pedestal performance and hence investigations at shifting the operational point to higher
β
N
by lowering the current
have been started. At higher
q
95
, pedestal performance can be recovered by seeding N
2
as well as CD
4
, which is interpreted as
improved pedestal stability due to the decrease of bootstrap current with increasing
Z
eff
. Concerning advanced scenarios, the
upgrade of ECRH power has allowed experiments with central ctr-ECCD to modify the
q
-profile in improved H-mode scenarios,
showing an increase in confinement at still good MHD stability with flat elevated
q
-profiles at values between 1.5 and 2.European Commission (EUROfusion 633053
Overview of ASDEX Upgrade results
The ASDEX Upgrade (AUG) programme is directed towards physics input to critical
elements of the ITER design and the preparation of ITER operation, as well as addressing
physics issues for a future DEMO design. Since 2015, AUG is equipped with a new pair of
3-strap ICRF antennas, which were designed for a reduction of tungsten release during ICRF
operation. As predicted, a factor two reduction on the ICRF-induced W plasma content could
be achieved by the reduction of the sheath voltage at the antenna limiters via the compensation
of the image currents of the central and side straps in the antenna frame. There are two main
operational scenario lines in AUG. Experiments with low collisionality, which comprise
current drive, ELM mitigation/suppression and fast ion physics, are mainly done with freshly
boronized walls to reduce the tungsten influx at these high edge temperature conditions. Full
ELM suppression and non-inductive operation up to a plasma current of Ip = 0.8 MA could
be obtained at low plasma density. Plasma exhaust is studied under conditions of high neutral
divertor pressure and separatrix electron density, where a fresh boronization is not required.
Substantial progress could be achieved for the understanding of the confinement degradation
by strong D puffing and the improvement with nitrogen or carbon seeding. Inward/outward
shifts of the electron density profile relative to the temperature profile effect the edge stability
via the pressure profile changes and lead to improved/decreased pedestal performance.
Seeding and D gas puffing are found to effect the core fueling via changes in a region of high
density on the high field side (HFSHD).
The integration of all above mentioned operational scenarios will be feasible and
naturally obtained in a large device where the edge is more opaque for neutrals and higher
plasma temperatures provide a lower collisionality. The combination of exhaust control
with pellet fueling has been successfully demonstrated. High divertor enrichment values of
nitrogen EN 10 have been obtained during pellet injection, which is a prerequisite for the
simultaneous achievement of good core plasma purity and high divertor radiation levels.
Impurity accumulation observed in the all-metal AUG device caused by the strong neoclassical
inward transport of tungsten in the pedestal is expected to be relieved by the higher
neoclassical temperature screening in larger devices.European Commission (EUROfusion 633053
Real-time evolution of a large-scale relativistic jet
Context. Astrophysical jets are ubiquitous in the Universe on all scales, but
their large-scale dynamics and evolution in time are hard to observe since they
usually develop at a very slow pace.
Aims. We aim to obtain the first observational proof of the expected
large-scale evolution and interaction with the environment in an astrophysical
jet. Only jets from microquasars offer a chance to witness the real-time,
full-jet evolution within a human lifetime, since they combine a 'short', few
parsec length with relativistic velocities.
Methods. The methodology of this work is based on a systematic recalibraton
of interferometric radio observations of microquasars available in public
archives. In particular, radio observations of the microquasar GRS 1758-258
over less than two decades have provided the most striking results.
Results. Significant morphological variations in the extended jet structure
of GRS 1758-258 are reported here that were previously missed. Its northern
radio lobe underwent a major morphological variation that rendered the hotspot
undetectable in 2001 and reappeared again in the following years. The reported
changes confirm the Galactic nature of the source. We tentatively interpret
them in terms of the growth of instabilities in the jet flow. There is also
evidence of surrounding cocoon. These results can provide a testbed for models
accounting for the evolution of jets and their interaction with the
environment.Comment: 5 pages, 3 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysics Letter
Identification of the optical and near-infrared counterpart of GRS 1758-258
Context. Understood to be a microquasar in the Galactic center region, GRS
1758-258 has not yet been unambiguously identified to have an
optical/near-infrared counterpart, mainly because of the high absorption and
the historic lack of suitable astrometric stars, which led to the use of
secondary astrometric solutions. Although it is considered with 1E 1740.7-2942
as the prototypical microquasar in the Galactic center region, the Galactic
origin of both sources has not yet been confirmed. Aims. We attempt to improve
previous astrometry to identify a candidate counterpart to GRS 1758-258. We
present observations with the Gran Telescopio de Canarias (GTC), in which we
try to detect any powerful emission lines that would infer an extragalactic
origin of this source. Methods. We use modern star catalogues to reanalyze
archival images of the GRS 1758-258 field in the optical and near-infrared
wavelengths, and compute a new astrometric solution. We also reanalyzed
archival radio data of GRS 1758-258 to determine a new and more accurate radio
position. Results. Our improved astrometric solution for the GRS 1758-258 field
represents a significant advancement on previous works and allows us to
identify a single optical/near-infrared source, which we propose as the
counterpart of GRS 1758-258. The GTC spectrum of this source is however of low
signal-to-noise ratio and does not rule out a Galactic origin. Hence, new
spectral observations are required to confirm or discard a Galactic nature.Comment: 4 pages, 3 figures, accepted by Astronomy and Astrophysic
Extending the VEF traces framework to model data center network workloads
Producción CientíficaData centers are a fundamental infrastructure in the Big-Data era, where applications and services demand a high amount of data and minimum response times. The interconnection network is an essential subsystem in the data center, as it must guarantee high communication bandwidth and low latency to the communication operations of applications, otherwise becoming the system bottleneck. Simulation is widely used to model the network functionality and to evaluate its performance under specific workloads. Apart from the network modeling, it is essential to characterize the end-nodes communication pattern, which will help identify bottlenecks and flaws in the network architecture. In previous works, we proposed the VEF traces framework: a set of tools to capture communication traffic of MPI-based applications and generate traffic traces used to feed network simulator tools. In this paper, we extend the VEF traces framework with new communication workloads such as deep-learning training applications and online data-intensive workloads.Ministerio de Ciencia e Innovación y Agencia Estatal de Investigación (MCIN/AEI/10.13039/501100011033) R &D Project Grant (PID2019-109001RA-I00)Publicación en abierto financiada por el Consorcio de Bibliotecas Universitarias de Castilla y León (BUCLE), con cargo al Programa Operativo 2014ES16RFOP009 FEDER 2014-2020 DE CASTILLA Y LEÓN, Actuación:20007-CL - Apoyo Consorcio BUCL
Graph states in phase space
The phase space for a system of qubits is a discrete grid of points, whose axes are labeled in terms of the elements of the
finite field \Gal{2^n} to endow it with proper geometrical properties. We
analyze the representation of graph states in that phase space, showing that
these states can be identified with a class of non-singular curves. We provide
an algebraic representation of the most relevant quantum operations acting on
these states and discuss the advantages of this approach.Comment: 14 pages. 2 figures. Published in Journal of Physics
Overview of progress in European medium sized tokamaks towards an integrated plasma-edge/wall solution
Integrating the plasma core performance with an edge and scrape-off layer (SOL) that leads
to tolerable heat and particle loads on the wall is a major challenge. The new European
medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade
(AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide
parameter range, is instrumental to progress in the field, as ITER and DEMO core/pedestal
and SOL parameters are not achievable simultaneously in present day devices. A two prong
approach is adopted. On the one hand, scenarios with tolerable transient heat and particle
loads, including active edge localised mode (ELM) control are developed. On the other hand,
divertor solutions including advanced magnetic configurations are studied. Considerable
progress has been made on both approaches, in particular in the fields of: ELM control with
resonant magnetic perturbations (RMP), small ELM regimes, detachment onset and control,
as well as filamentary scrape-off-layer transport. For example full ELM suppression has now
been achieved on AUG at low collisionality with n = 2 RMP maintaining good confinement
HH(98,y2) 0.95. Advances have been made with respect to detachment onset and control.
Studies in advanced divertor configurations (Snowflake, Super-X and X-point target divertor)
shed new light on SOL physics. Cross field filamentary transport has been characterised in a
wide parameter regime on AUG, MAST and TCV progressing the theoretical and experimental
understanding crucial for predicting first wall loads in ITER and DEMO. Conditions in the
SOL also play a crucial role for ELM stability and access to small ELM regimes.European Commission (EUROfusion 633053
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-
The colored Hanbury Brown--Twiss effect
The Hanbury Brown--Twiss effect is one of the celebrated phenomenologies of
modern physics that accommodates equally well classical (interferences of
waves) and quantum (correlations between indistinguishable particles)
interpretations. The effect was discovered in the late thirties with a basic
observation of Hanbury Brown that radio-pulses from two distinct antennas
generate signals on the oscilloscope that wiggle similarly to the naked eye.
When Hanbury Brown and his mathematician colleague Twiss took the obvious step
to propose bringing the effect in the optical range, they met with considerable
opposition as single-photon interferences were deemed impossible. The Hanbury
Brown--Twiss effect is nowadays universally accepted and, being so fundamental,
embodies many subtleties of our understanding of the wave/particle dual nature
of light. Thanks to a novel experimental technique, we report here a
generalized version of the Hanbury Brown--Twiss effect to include the frequency
of the detected light, or, from the particle point of view, the energy of the
detected photons. In addition to the known tendencies of indistinguishable
photons to arrive together on the detector, we find that photons of different
colors present the opposite characteristic of avoiding each others. We
postulate that fermions can be similarly brought to exhibit positive
(boson-like) correlations by frequency filtering.Comment: 18 pages, includes supplementary material of the derivation
Search for Blue Compact Dwarf Galaxies During Quiescence
Blue Compact Dwarf (BCD) galaxies are metal poor systems going through a
major starburst that cannot last for long. We have identified galaxies which
may be BCDs during quiescence (QBCD), i.e., before the characteristic starburst
sets in or when it has faded away. These QBCD galaxies are assumed to be like
the BCD host galaxies. The SDSS/DR6 database provides ~21500 QBCD candidates.
We also select from SDSS/DR6 a complete sample of BCD galaxies to serve as
reference. The properties of these two galaxy sets have been computed and
compared. The QBCD candidates are thirty times more abundant than the BCDs,
with their luminosity functions being very similar except for the scaling
factor, and the expected luminosity dimming associated with the end of the
starburst. QBCDs are redder than BCDs, and they have larger HII region based
oxygen abundance. QBCDs also have lower surface brightness. The BCD candidates
turn out to be the QBCD candidates with the largest specific star formation
rate (actually, with the largest H_alpha equivalent width). One out of each
three dwarf galaxies in the local universe may be a QBCD. The properties of the
selected BCDs and QBCDs are consistent with a single sequence in galactic
evolution, with the quiescent phase lasting thirty times longer than the
starburst phase. The resulting time-averaged star formation rate is low enough
to allow this cadence of BCD -- QBCD phases during the Hubble time.Comment: Accepted for publication in ApJ. 17 pages. 13 Fig
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