49 research outputs found
Driving positron beam acceleration with coherent transition radiation
Positron acceleration in plasma wakefield faces significant challenges since
the positron beam must be pre-generated and precisely coupled into the
wakefield, and most critically, suffers from defocusing issues. Here we propose
a scheme that utilizes laser-driven electrons to produce, inject and accelerate
positrons in a single set-up. The high-charge electron beam from wakefield
acceleration creates copious electron-positron pairs via the Bethe-Heitler
process, followed by enormous coherent transition radiation due to the
electrons' exiting from the metallic foil. Simulation results show that the
coherent transition radiation field reaches up to 10's GV m-1, which captures
and accelerates the positrons to cut-off energy of 1.5 GeV with energy peak of
500 MeV and energy spread is about 24.3%. An external longitudinal magnetic
field of 30 T is also applied to guide the electrons and positrons during the
acceleration process. This proposed method offers a promising way to obtain GeV
fast positron sources
Generation of Intense High-Order Vortex Harmonics
This paper presents the method for the first time to generate intense
high-order optical vortices that carry orbital angular momentum in the extreme
ultraviolet region. In three-dimensional particle-in-cell simulation, both the
reflected and transmitted light beams include high-order harmonics of the
Laguerre-Gaussian (LG) mode when a linearly polarized LG laser pulse impinges
on a solid foil. The mode of the generated LG harmonic scales with its order,
in good agreement with our theoretical analysis. The intensity of the generated
high-order vortex harmonics is close to the relativistic region, and the pulse
duration can be in attosecond scale. The obtained intense vortex beam possesses
the combined properties of fine transversal structure due to the high-order
mode and the fine longitudinal structure due to the short wavelength of the
high-order harmonics. Thus, the obtained intense vortex beam may have
extraordinarily promising applications for high-capacity quantum information
and for high-resolution detection in both spatial and temporal scales because
of the addition of a new degree of freedom
Generation of Ultra-intense Gamma-ray Train by QED Harmonics
When laser intensity exceeds 10^22W/cm^2, photons with energy above MeV can
be generated from high-order harmonics process in the laser-plasma interaction.
We find that under such laser intensity, QED effect plays a dominating role in
the radiation pattern. Contrast to the gas and relativistic HHG processes, both
the occurrence and energy of gamma-ray emission produced by QED harmonics are
random and QED harmonics are usually not coherent, while the property of high
intensity and ultra-short duration is conserved. Our simulation shows that the
period of gamma-ray train is half of the laser period and the peak intensity is
1.4e22W/cm^2. This new harmonic production with QED effects are crucial to
light-matter interaction in strong field and can be verified in experiments by
10PW laser facilities in the near future.Comment: 12 pages, 4 figure
Ultra-bright, ultra-broadband hard x-ray driven by laser-produced energetic electron beams
We propose a new method of obtaining a compact ultra-bright, ultra-broadband hard X-ray source. This X-ray source has a high peak brightness in the order of 1022 photons/(s mm2 mrad2 0.1\%BW), an ultrashort duration (10 fs), and a broadband spectrum (flat distribution from 0.1 MeV to 4 MeV), and thus has wide-ranging potential applications, such as in ultrafast Laue diffraction experiments. In our scheme, laser-plasma accelerators (LPAs) provide driven electron beams. A foil target is placed oblique to the beam direction so that the target normal sheath field (TNSF) is used to provide a bending force. Using this TNSF-kick scheme, we can fully utilize the advantages of current LPAs, including their high charge, high energy, and low emittance
Proton Acceleration in Underdense Plasma by Ultraintense Laguerre-Gaussian Laser Pulse
Three-dimensional particle-in-cell simulation is used to investigate the
witness proton acceleration in underdense plasma with a short intense
Laguerre-Gaussian (LG) laser pulse. Driven by the LG10 laser pulse, a special
bubble with an electron pillar on the axis is formed, in which protons can be
well-confined by the generated transversal focusing field and accelerated by
the longitudinal wakefield. The risk of scattering prior to acceleration with a
Gaussian laser pulse in underdense plasma is avoided, and protons are
accelerated stably to much higher energy. In simulation, a proton beam has been
accelerated to 7 GeV from 1 GeV in underdense tritium plasma driven by a
2.14x1022 W/cm2 LG10 laser pulse
Scheme for proton-driven plasma-wakefield acceleration of positively charged particles in a hollow plasma channel
A new scheme for accelerating positively charged particles in a plasma
wakefield accelerator is proposed. If the proton drive beam propagates in a
hollow plasma channel, and the beam radius is of order of the channel width,
the space charge force of the driver causes charge separation at the channel
wall, which helps to focus the positively charged witness bunch propagating
along the beam axis. In the channel, the acceleration buckets for positively
charged particles are much larger than in the blowout regime of the uniform
plasma, and stable acceleration over long distances is possible. In addition,
phasing of the witness with respect to the wave can be tuned by changing the
radius of the channel to ensure the acceleration is optimal. Two dimensional
simulations suggest that, for proton drivers likely available in future,
positively charged particles can be stably accelerated over 1 km with the
average acceleration gradient of 1.3 GeV/m.Comment: 16 pages, 4 figures, 25 reference
Exploration of sleep function connection and classification strategies based on sub-period sleep stages
BackgroundAs a medium for developing brain-computer interface systems, EEG signals are complex and difficult to identify due to their complexity, weakness, and differences between subjects. At present, most of the current research on sleep EEG signals are single-channel and dual-channel, ignoring the research on the relationship between different brain regions. Brain functional connectivity is considered to be closely related to brain activity and can be used to study the interaction relationship between brain areas.MethodsPhase-locked value (PLV) is used to construct a functional connection network. The connection network is used to analyze the connection mechanism and brain interaction in different sleep stages. Firstly, the entire EEG signal is divided into multiple sub-periods. Secondly, Phase-locked value is used for feature extraction on the sub-periods. Thirdly, the PLV of multiple sub-periods is used for feature fusion. Fourthly, the classification performance optimization strategy is used to discuss the impact of different frequency bands on sleep stage classification performance and to find the optimal frequency band. Finally, the brain function network is constructed by using the average value of the fusion features to analyze the interaction of brain regions in different frequency bands during sleep stages.ResultsThe experimental results have shown that when the number of sub-periods is 30, the α (8–13 Hz) frequency band has the best classification effect, The classification result after 10-fold cross-validation reaches 92.59%.ConclusionThe proposed algorithm has good sleep staging performance, which can effectively promote the development and application of an EEG sleep staging system