120 research outputs found
Determination of plasma screening effects for thermonuclear reactions in laser-generated plasmas
Due to screening effects, nuclear reactions in astrophysical plasmas may
behave differently than in the laboratory. The possibility to determine the
magnitude of these screening effects in colliding laser-generated plasmas is
investigated theoretically, having as a starting point a proposed experimental
setup with two laser beams at the Extreme Light Infrastructure facility. A
laser pulse interacting with a solid target produces a plasma through the
Target Normal Sheath Acceleration scheme, and this rapidly streaming plasma
(ion flow) impacts on a secondary plasma created by the interaction of a second
laser pulse on a gas jet target. We model this scenario here and calculate the
reaction events for the astrophysically relevant reaction C(He,
)O. We find that it should be experimentally possible to determine
the plasma screening enhancement factor for fusion reactions by detecting the
difference in reaction events between two scenarios of ion flow interacting
with the plasma target and a simple gas target. This provides a way to evaluate
nuclear reaction cross-sections in stellar environments and can significantly
advance the field of nuclear astrophysics.Comment: 9 pages, 4 figures, 4 tables; minor changes made, accepted by The
Astrophysical Journa
Neutron production from thermonuclear reactions in laser-generated plasmas
The production of intense neutron beams via thermonuclear reactions in
laser-generated plasmas is investigated theoretically. So far, state-of-the-art
neutron beams are produced via laser-induced particle acceleration leading to
high-energy particle beams that subsequently interact with a secondary target.
Here we show that neutron beams of two orders of magnitude narrower bandwidth
can be obtained from thermonuclear reactions in plasmas generated by
Petawatt-class lasers. The intensity of such neutron beams is about one or two
orders of magnitude lower than the one of the state-of-the-art laser-driven
neutron beams. We study to this end the reaction H(, )He in
plasmas generated by Petawatt-class lasers interacting with D gas jet
targets and CD solid-state targets. The results also shows the possibility
of direct measurements of reaction rates at low temperatures of astrophysical
interests. In addition, the use of CD solid-state targets can also lead to
great enhancements on the plasma screening compared to the case of D gas
jet targets, opening new possibilities to study this so far unsolved issue in
the field of astrophysics.Comment: 8 pages, 4 figures; accepted for publication in Physics of Plasma
Mo isomer depletion via beam-based nuclear excitation by electron capture
A recent nuclear physics experiment [C. J. Chiara {\it et al.}, Nature
(London) {\bf 554}, 216 (2018)] reports the first direct observation of nuclear
excitation by electron capture (NEEC) in the depletion of the Mo
isomer. The experiment used a beam-based setup in which Mo highly charged ions
with nuclei in the isomeric state Mo at 2.4 MeV excitation energy were
slowed down in a solid-state target. In this process, nuclear excitation to a
higher triggering level led to isomer depletion. The reported excitation
probability was solely attributed to the so-far
unobserved process of NEEC in lack of a different known channel of comparable
efficiency. In this work, we investigate theoretically the beam-based setup and
calculate excitation rates via NEEC using state-of-the-art atomic structure and
ion stopping power models. For all scenarios, our results disagree with the
experimental data by approximately nine orders of magnitude. This stands in
conflict with the conclusion that NEEC was the excitation mechanism behind the
observed depletion rate.Comment: 6 pages, 3 figures; minor modifications made; accepted for
publication in Physical Review Letter
Nuclear excitation by electron capture in optical-laser-generated plasmas
The process of nuclear excitation by electron capture in plasma environments
generated by the interaction of ultra-strong optical lasers with solid-state
samples is investigated theoretically. With the help of a plasma model we
perform a comprehensive study of the optimal parameters for most efficient
nuclear excitation and determine the corresponding laser setup requirements. We
discern between the low-density plasma regime, modeled by scaling laws, and the
high-density regime, for which we perform particle-in-cell calculations. As
nuclear transition case study we consider the 4.85 keV nuclear excitation
starting from the long-lived Mo isomer. Our results show that
the optimal plasma and laser parameters are sensitive to the chosen observable
and that measurable rates of nuclear excitation and isomer depletion of
Mo should be already achievable at laser facilities existing
today.Comment: 19 pages, 16 figures; minor modifications made; accepted for
publication in Physical Review
Tailoring laser-generated plasmas for efficient nuclear excitation by electron capture
The optimal parameters for nuclear excitation by electron capture in plasma
environments generated by the interaction of ultra-strong optical lasers with
solid matter are investigated theoretically. As a case study we consider a 4.85
keV nuclear transition starting from the long-lived Mo isomer
that can lead to the release of the stored 2.4 MeV excitation energy. We find
that due to the complex plasma dynamics, the nuclear excitation rate and the
actual number of excited nuclei do not reach their maximum at the same laser
parameters. The nuclear excitation achievable with a high-power optical laser
is up to twelve and up to six orders of magnitude larger than the values
predicted for direct resonant and secondary plasma-mediated excitation at the
x-ray free electron laser, respectively. Our results show that the experimental
observation of the nuclear excitation of Mo and the subsequent
release of stored energy should be possible at laser facilities available
today.Comment: 6 pages, 3 figures, 1 table; minor modifications made; accepted for
publication in Physical Review Letter
Quantum effects on plasma screening for thermonuclear reactions in laser-generated plasmas
A quantum plasma screening model based on the density matrix formalism is
used to investigate theoretically the thermonuclear reactions
C(, )O and H(, )He in laser-generated
plasmas over a large range of densities and temperatures. We find that for cold
and dense (solid-state density) plasmas, the quantum model predicts plasma
screening enhancement factors up to one order of magnitude larger than the ones
from classical plasma models. Our results indicate that quantum effects can
enhance the plasma screening for thermonuclear reactions, with potential also
for industrial fusion energy gain. We put forward a possible experimental test
of the screening theory in laser-generated plasmas which could also confirm
predictions from nuclear astrophysics.Comment: 6 pages, 2 figures, 1 tabl
Generating CCG Categories
Previous CCG supertaggers usually predict categories using multi-class
classification. Despite their simplicity, internal structures of categories are
usually ignored. The rich semantics inside these structures may help us to
better handle relations among categories and bring more robustness into
existing supertaggers. In this work, we propose to generate categories rather
than classify them: each category is decomposed into a sequence of smaller
atomic tags, and the tagger aims to generate the correct sequence. We show that
with this finer view on categories, annotations of different categories could
be shared and interactions with sentence contexts could be enhanced. The
proposed category generator is able to achieve state-of-the-art tagging (95.5%
accuracy) and parsing (89.8% labeled F1) performances on the standard CCGBank.
Furthermore, its performances on infrequent (even unseen) categories,
out-of-domain texts and low resource language give promising results on
introducing generation models to the general CCG analyses.Comment: Accepted by AAAI 202
- …