222 research outputs found
Fast ignition driven by quasi-monoenergetic ions: Optimal ion type and reduction of ignition energies with an ion beam array
Fast ignition of inertial fusion targets driven by quasi-monoenergetic ion
beams is investigated by means of numerical simulations. Light and intermediate
ions such as lithium, carbon, aluminium and vanadium have been considered.
Simulations show that the minimum ignition energies of an ideal configuration
of compressed Deuterium-Tritium are almost independent on the ion atomic
number. However, they are obtained for increasing ion energies, which scale,
approximately, as Z^2, where Z is the ion atomic number. Assuming that the ion
beam can be focused into 10 {\mu}m spots, a new irradiation scheme is proposed
to reduce the ignition energies. The combination of intermediate Z ions, such
as 5.5 GeV vanadium, and the new irradiation scheme allows a reduction of the
number of ions required for ignition by, roughly, three orders of magnitude
when compared with the standard proton fast ignition scheme
Fast ignition of inertial fusion targets by laser-driven carbon beams
Two-dimensional simulations of ion beam driven fast ignition are presented.
Ignition energies of protons with Maxwellian spectrum and carbon ions with
quasimonoenergetic and Maxwellian energy distributions are evaluated. The
effect of the coronal plasma surrounding the compressed deuterium-tritium is
studied for three different fuel density distributions. It is found that quasi-
monoenergetic ions have better coupling with the compressed deuterium-tritium
and substantially lower ignition energies. Comparison of quasimonoenergetic
carbon ions and relativistic electrons as ignitor beams shows similar laser
energy requirements, provided that a laser to quasimonoenergetic carbon ion
conversion efficiency around 10% can be achieved.Comment: 8 pages, 10 figures, published in Physics of Plasma
Laser-like X-ray Sources Based on Optical Reflection from Relativistic Electron Mirror
A novel scheme is proposed to generate uniform relativistic electron layers
for coherent Thomson backscattering. A few-cycle laser pulse is used to produce
the electron layer from an ultra-thin solid foil. The key element of the new
scheme is an additional foil that reflects the drive laser pulse, but lets the
electrons pass almost unperturbed. It is shown by analytic theory and by 2D-PIC
simulation that the electrons, after interacting with both drive and reflected
laser pulse, form a very uniform flyer freely cruising with high relativistic
gamma-factor exactly in drive laser direction (no transverse momentum). It
backscatters probe light with a full Doppler shift factor of 4*gamma^2. The
reflectivity and its decay due to layer expansion is discussed.Comment: 5 pages, 3 figures, submitted, invited talk on the workshop of
Frontiers in Intense Laser-Matter Interaction Theory, MPQ, March 1-3, 2010
En-route to the fission-fusion reaction mechanism: a status update on laser-driven heavy ion acceleration
The fission-fusion reaction mechanism was proposed in order to generate
extremely neutron-rich nuclei close to the waiting point N = 126 of the rapid
neutron capture nucleosynthesis process (r-process). The production of such
isotopes and the measurement of their nuclear properties would fundamentally
help to increase the understanding of the nucleosynthesis of the heaviest
elements in the universe. Major prerequisite for the realization of this new
reaction scheme is the development of laser-based acceleration of ultra-dense
heavy ion bunches in the mass range of A = 200 and above. In this paper, we
review the status of laser-driven heavy ion acceleration in the light of the
fission-fusion reaction mechanism. We present results from our latest
experiment on heavy ion acceleration, including a new milestone with
laser-accelerated heavy ion energies exceeding 5 MeV/u
Photon and neutron production as in-situ diagnostics of proton-boron fusion
Short-pulse, ultra high-intensity lasers have opened new regimes for studying
fusion plasmas and creating novel ultra-short ion beams and neutron sources.
Diagnosing the plasma in these experiments is important for optimizing the
fusion yield but difficult due to the picosecond time scales, 10s of
micron-cubed volumes and high densities. We propose to use the yields of
photons and neutrons produced by parallel reactions involving the same
reactants to diagnose the plasma conditions and predict the yields of specific
reactions of interest. In this work, we focus on verifying the yield of the
high-interest aneutronic proton-boron fusion reaction
, which is difficult to measure directly due to
the short stopping range of the produced s in most materials. We
identify promising photon-producing reactions for this purpose and compute the
ratios of the photon yield to the yield as a function of plasma
parameters. In beam fusion experiments, the yield is an
easily-measurable observable to verify the yield. In light of our
results, improving and extending measurements of the cross sections for these
parallel reactions are important steps to gaining greater control over these
laser-driven fusion plasmas.Comment: 23 pages, 7 figures, revtex forma
Dynamics of Nanometer-Scale Foil Targets Irradiated with Relativistically Intense Laser Pulses
In this letter we report on an experimental study of high harmonic radiation
generated in nanometer-scale foil targets irradiated under normal incidence.
The experiments constitute the first unambiguous observation of odd-numbered
relativistic harmonics generated by the component of the
Lorentz force verifying a long predicted property of solid target harmonics.
Simultaneously the observed harmonic spectra allow in-situ extraction of the
target density in an experimental scenario which is of utmost interest for
applications such as ion acceleration by the radiation pressure of an
ultraintense laser.Comment: 5 pages, 4 figure
Dancing to the Partisan Beat: A First Analysis of Political Communication on TikTok
TikTok is a video-sharing social networking service, whose popularity is
increasing rapidly. It was the world's second-most downloaded app in 2019.
Although the platform is known for having users posting videos of themselves
dancing, lip-syncing, or showcasing other talents, user-videos expressing
political views have seen a recent spurt. This study aims to perform a primary
evaluation of political communication on TikTok. We collect a set of US
partisan Republican and Democratic videos to investigate how users communicated
with each other about political issues. With the help of computer vision,
natural language processing, and statistical tools, we illustrate that
political communication on TikTok is much more interactive in comparison to
other social media platforms, with users combining multiple information
channels to spread their messages. We show that political communication takes
place in the form of communication trees since users generate branches of
responses to existing content. In terms of user demographics, we find that
users belonging to both the US parties are young and behave similarly on the
platform. However, Republican users generated more political content and their
videos received more responses; on the other hand, Democratic users engaged
significantly more in cross-partisan discussions.Comment: Accepted as a full paper at the 12th International ACM Web Science
Conference (WebSci 2020). Please cite the WebSci version; Second version
includes corrected typo
Proton acceleration by irradiation of isolated spheres with an intense laser pulse
We report on experiments irradiating isolated plastic spheres with a peak laser intensity of 2-3 x 10(20) W cm(-2). With a laser focal spot size of 10 mu m full width half maximum (FWHM) the sphere diameter was varied between 520 nm and 19.3 mu m. Maximum proton energies of similar to 25 MeV are achieved for targets matching the focal spot size of 10 mu m in diameter or being slightly smaller. For smaller spheres the kinetic energy distributions of protons become nonmonotonic, indicating a change in the accelerating mechanism from ambipolar expansion towards a regime dominated by effects caused by Coulomb repulsion of ions. The energy conversion efficiency from laser energy to proton kinetic energy is optimized when the target diameter matches the laser focal spot size with efficiencies reaching the percent level. The change of proton acceleration efficiency with target size can be attributed to the reduced cross-sectional overlap of subfocus targets with the laser. Reported experimental observations are in line with 3D3V particle in cell simulations. They make use of well-defined targets and point out pathways for future applications and experiments.DFG via the Cluster of Excellence Munich-Centre for Advanced Photonics (MAP) Transregio SFB TR18NNSA DE-NA0002008Super-MUC pr48meIvo CermakCGC Instruments in design and realization of the Paul trap systemIMPRS-APSLMUexcellent Junior Research FundDAAD|ToIFEEuropean Union's Horizon research and innovation programme 633053Physic
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