414 research outputs found
Anticorrelation between Ion Acceleration and Nonlinear Coherent Structures from Laser-Underdense Plasma Interaction
In laser-plasma experiments, we observed that ion acceleration from the
Coulomb explosion of the plasma channel bored by the laser, is prevented when
multiple plasma instabilities such as filamentation and hosing, and nonlinear
coherent structures (vortices/post-solitons) appear in the wake of an
ultrashort laser pulse. The tailoring of the longitudinal plasma density ramp
allows us to control the onset of these insabilities. We deduced that the laser
pulse is depleted into these structures in our conditions, when a plasma at
about 10% of the critical density exhibits a gradient on the order of 250
{\mu}m (gaussian fit), thus hindering the acceleration. A promising
experimental setup with a long pulse is demonstrated enabling the excitation of
an isolated coherent structure for polarimetric measurements and, in further
perspectives, parametric studies of ion plasma acceleration efficiency.Comment: 4 pages, 5 figure
Short Intense Laser Pulse Collapse in Near-Critical Plasma
It is observed that the interaction of an intense ultra-short laser pulse
with an overdense gas jet results in the pulse collapse and the deposition of a
significant part of energy in a small and well localized volume in the rising
part of the gas jet, where the electrons are efficiently accelerated and
heated. A collisionless plasma expansion over 150 microns at a sub-relativistic
velocity (~c/3) has been optically monitored in time and space, and attributed
to the quasistatic field ionization of the gas associated to the hot electron
current. Numerical simulations in good agreement with the observations suggest
the acceleration in the collapse region of relativistic electrons, along with
the excitation of a sizeable magnetic dipole that sustains the electron current
over several picoseconds. Perspectives of ion beam generation at high
repetition rate directly from gas jets are discussed
Enhanced hard x-ray emission from microdroplet preplasma
We perform a comparative study of hard x-ray emission from femtosecond laser plasmas in 15 mu m methanol microdroplets and Perspex target. The hard x-ray yield from droplet plasmas is similar or equal to 68 times more than that obtained from solid plasmas at 2x10(15) W cm(-2). A 10 ns prepulse at about 5% of the main pulse appears to be essential for hard x-ray generation from droplets. Hot electron temperature of 36 keV is measured from the droplets at 8x10(14) W cm(-2), whereas a three times higher intensity is needed to obtain similar hot electron temperatures from Perspex plasmas. Particle-in-cell simulations with very long scale-length density profiles support experimental observations. (c) 2006 American Institute of Physics
Polarimetric detection of laser induced ultrashort magnetic pulses in overdense plasma
The interaction of intense ( ~ 1016 Wcm-2), subpicosecond pulses with solid targets can generate highly directional jets of hot electrons. These electrons can propagate in the solid along with the counterpropagating return shielding currents. The spontaneous magnetic field that is generated by these currents, captures in its time evolution, important information about the dynamics of the complex transport processes. By using a two pulse pump-probe polarimetric technique the temporal evolution of multimegagauss magnetic fields is measured for optically polished BK7 glass targets, each coated with a thin layer of either copper or silver. A simple model is then used for explaining the observations and for deducing quantitative information about the transport of hot electrons
Mapping giant magnetic fields around dense solid plasmas by high resolution magneto-optical microscopy
We investigate distribution of magnetic fields around dense solid plasmas
generated by intense p-polarized laser (~10^{16} W.cm^{-2}, 100 fs) irradiation
of magnetic tapes, using high sensitivity magneto optical microscopy. We
present evidence for giant axial magnetic fields and map out for the first time
the spatial distribution of these fields. By using the axial magnetic field
distribution as a diagnostic tool we uncover evidence for angular momentum
associated with the plasma. We believe this study holds significance for
investigating the process under which a magnetic material magnetizes or
demagnetizes under the influence of ultrashort intense laser pulses.Comment: 17 pages of text with 4 figure
Role of prepulses in the interaction of intense, ultrashort lasers with "structured" surfaces
We examine enhanced hard x-ray emission (20 - 200 keV) from plasmas produced on nanoparticles coated optically polished copper surface under different prepulse conditions. We observe that enhancement reduces with increasing prepulse intensity. The dynamics of the process is seen to be in the ps regime. We attribute this to preplasma formation on nanoparticles and subsequent modification/destruction of the nanostructure layer before the arrival of the main pulse. It is suggested that high-contrast ultrashort pulses are essential for nanoparticles to function as yield enhancers
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