522 research outputs found
Origins of plateau formation in ion energy spectra under target normal sheath acceleration
Target normal sheath acceleration (TNSA) is a method employed in
laser--matter interaction experiments to accelerate light ions (usually
protons). Laser setups with durations of a few 10 fs and relatively low
intensity contrasts observe plateau regions in their ion energy spectra when
shooting on thin foil targets with thicknesses of order 10 m. In
this paper we identify a mechanism which explains this phenomenon using one
dimensional particle-in-cell simulations. Fast electrons generated from the
laser interaction recirculate back and forth through the target, giving rise to
time-oscillating charge and current densities at the target backside. Periodic
decreases in the electron density lead to transient disruptions of the TNSA
sheath field: peaks in the ion spectra form as a result, which are then spread
in energy from a modified potential driven by further electron recirculation.
The ratio between the laser pulse duration and the recirculation period
(dependent on the target thickness, including the portion of the pre-plasma
which is denser than the critical density) determines if a plateau forms in the
energy spectra.Comment: 11 pages, 12 figure
Generation of attosecond electron bunches and X-ray pulses from few-cycle femtosecond laser pulses
Laser-plasma electron accelerators can be used to produce high-intensity
X-rays, as electrons accelerated in wakefields emit radiation due to betatron
oscillations.Such X-ray sources inherit the features of the electron beam;
sub-femtosecond electron bunches produce betatron sources of the same duration,
which in turn allow probing matter on ultrashort time scales. In this paper we
show, via Particle-in-Cell simulations, that attosecond electron bunches can be
obtained using low-energy, ultra-short laser beams both in the self-injection
and the controlled injection regimes at low plasma densities. However, only in
the controlled regime does the electron injection lead to a stable, isolated
attosecond electron bunch. Such ultrashort electron bunches are shown to emit
attosecond X-ray bursts with high brillianceComment: 12 pages, 5 figure
Effects of oblique incidence and colliding pulses on laser-driven proton acceleration from relativistically transparent ultrathin targets
The use of ultrathin solid foils offers optimal conditions for accelerating
protons from laser-matter interactions. When the target is thin enough that
relativistic self-induced transparency (RSIT) sets in, all of the target
electrons get heated to high energies by the laser, which maximizes the
accelerating electric field and therefore the final ion energy. In this work,
we first investigate how ion acceleration by ultraintense femtosecond laser
pulses in transparent CH solid foils is modified when turning from normal
to oblique () incidence. Due to stronger electron heating, we find
that higher proton energies can be obtained at oblique incidence but in thinner
optimum targets. We then show that proton acceleration can be further improved
by splitting the laser pulse into two half-pulses focused at opposite incidence
angles. An increase by in the maximum proton energy and by a
factor of in the high-energy proton charge is reported compared to the
reference case of a single normally incident pulse.Comment: 11 pages, 7 figure
The Apoptotic Regulator Nrz Controls Cytoskeletal Dynamics via the Regulation of Ca2+ Trafficking in the Zebrafish Blastula
SummaryBcl-2 family members are key regulators of apoptosis. Their involvement in other cellular processes has been so far overlooked. We have studied the role of the Bcl-2 homolog Nrz in the developing zebrafish. Nrz was found to be localized to the yolk syncytial layer, a region containing numerous mitochondria and ER membranes. Nrz knockdown resulted in developmental arrest before gastrulation, due to free Ca2+ increase in the yolk cell, activating myosin light chain kinase, which led to premature contraction of actin-myosin cables in the margin and separation of the blastomeres from the yolk cell. In the yolk syncytial layer, Nrz appears to prevent the release of Ca2+ from the endoplasmic reticulum by directly interacting with the IP3R1 Ca2+ channel. Thus, the Bcl-2 family may participate in early development, not only by controlling apoptosis but also by acting on cytoskeletal dynamics and cell movements via Ca2+ fluxes inside the embryo
Generation of attosecond electron bunches and x-ray pulses from few-cycle femtosecond laser pulses
Laser-plasma electron accelerators can be used to produce high-intensity x-rays, as electrons accelerated in wakefields emit radiation due to betatron oscillations. Such x-ray sources inherit the features of the electron beam; sub-femtosecond electron bunches produce betatron sources of the same duration, which in turn allow probing matter on ultrashort time scales. In this paper we show, via Particle-in-Cell simulations, that attosecond electron bunches can be obtained using low-energy, ultra-short laser beams both in the self-injection and the controlled injection regimes at low plasma densities. However, only in the controlled regime does the electron injection lead to a stable, isolated attosecond electron bunch. Such ultrashort electron bunches are shown to emit attosecond x-ray bursts with high brilliance
Enhancement of betatron x rays through asymmetric laser wakefield generated in transverse density gradients
Laser wakefield acceleration of electrons usually offers an axisymmetry around the laser propagation axis. Thus, the accelerating electrons that are focused on axis often execute small transverse oscillations. In this article, we propose a simple scheme to break this symmetry, which enhances the transverse wiggling of electrons and boosts the betatron radiation emission. Through 3D particle-in-cell simulations, we show that sending the laser with a small angle of incidence on a transverse plasma density gradient generates an asymmetric wakefield. It first provokes injection and then increases the wiggling of the electrons through the transverse shifting of the wakefield axis which occurs when the laser pulse leaves the gradient. Consequently, we show that the radiated energy per unit of charge can increase by a factor >20 when using this scheme, and that the critical energy of the radiation quintuples compared with a reference case without the transverse density gradient
Laser-driven proton acceleration from ultrathin foils with nanoholes
Structured solid targets are widely investigated to increase the energy absorption of high-power laser pulses so as to achieve efficient ion acceleration. Here we report the first experimental study of the maximum energy of proton beams accelerated from sub-micrometric foils perforated with holes of nanometric size. By showing the lack of energy enhancement in comparison to standard flat foils, our results suggest that the high contrast routinely achieved with a double plasma mirror does not prevent damaging of the nanostructures prior to the main interaction. Particle-in-cell simulations support that even a short scale length plasma, formed in the last hundreds of femtoseconds before the peak of an ultrashort laser pulse, fills the holes and hinders enhanced electron heating. Our findings reinforce the need for improved laser contrast, as well as for accurate control and diagnostics of on-target plasma formation
Radiation emission in laser-wakefields driven by structured laser pulses with orbital angular momentum
High-intensity X-ray sources are invaluable tools, enabling experiments at the forefront of our understanding of materials science, chemistry, biology, and physics. Laser-plasma electron accelerators are sources of high-intensity X-rays, as electrons accelerated in wakefields emit short-wavelength radiation due to betatron oscillations. While applications such as phasecontrast imaging with these betatron sources have already been demonstrated, others would require higher photon number and would benefit from increased tunability. In this paper we demonstrate, through detailed 3D simulations, a novel configuration for a laser-wakefield betatron source that increases the energy of the X-ray emission and also provides increased flexibility in the tuning of the X-ray photon energy. This is made by combining two Laguerre-Gaussian pulses with non-zero net orbital angular momentum, leading to a rotation of the intensity pattern, and hence, of the driven wakefields. The helical motion driven by the laser rotation is found to dominate the radiation emission, rather than the betatron oscillations. Moreover, the radius of this helical motion can be controlled through the laser spot size and orbital angular momentum indexes, meaning that the radiation can be tuned fully independently of the plasma parameters
Proton acceleration by a pair of successive ultraintense femtosecond laser pulses
We investigate the target normal sheath acceleration of protons in thin
aluminum targets irradiated at relativistic intensity by two time-separated
ultrashort (35 fs) laser pulses. For identical laser pulses and target
thicknesses of 3 and 6 m, we observe experimentally that the second pulse
boosts the maximum energy and charge of the proton beam produced by the first
pulse for time delays below ps. By using two-dimensional
particle-in-cell simulations we examine the variation of the proton energy
spectra with respect to the time-delay between the two pulses. We demonstrate
that the expansion of the target front surface caused by the first pulse
significantly enhances the hot-electron generation by the second pulse arriving
after a few hundreds of fs time delay. This enhancement, however, does not
suffice to further accelerate the fastest protons driven by the first pulse
once three-dimensional quenching effects have set in. This implies a limit to
the maximum time delay that leads to proton energy enhancement, which we
theoretically determine.Comment: 14 pages, 11 figure
Common denominator of Cu/Zn superoxide dismutase mutants associated with amyotrophic lateral sclerosis: Decreased stability of the apo state
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