We review a number of instruments employed in a high-intensity J-KAREN-P laser-solid interaction experiment and discuss the applicability of the diagnostics to the best target position determination with a ~10 μm accuracy, while the focal spot size was ~1 μm and peak intensity was up to 7×1021 W/cm2. We discuss both front- and back-side diagnostics, some of them operated in the infrared, visible and ultraviolet ranges, while others in the extreme ultraviolet, soft X-ray and gamma-ray ranges. We found that the applicability of some of the instruments to the best at-focus target position determination depends on the thickness of the target.
As an alternative to Compton backscattering and bremsstrahlung, the process of colliding highenergy electron beams with strong laser fields can more efficiently provide both cleaner and brighter
source of photons in the multi-GeV range for fundamental studies in nuclear and quark-gluon physics.
In order to favor the emission of high-energy quanta and minimize their decay into electron-positron
pairs the fields must not only be sufficiently strong, but also well localized. We here examine
these aspects and develop the concept of a laser-particle collider tailored for high-energy photon
generation. We show that the use of multiple colliding laser pulses with 0.4 PW of total power is
capable of converting more than 18 % of the initial multi-GeV electron beam energy into photons,
each of which carries more than half of the electron energy
A novel regime of high frequency radiation generation via reflection at the electron density spikes in under- dense plasma is proposed. Intense driver laser pulse propagating in underdense plasma forms dense electron singularities near the front part of the bow waves, moving at relativistic velocity. By irradiating a source pulse counterpropagating to the electron density singularities, it is reflected and compressed, producing ultrashort coherent high order harmonics with frequency upshift.
We report on the J-KAREN-P laser facility at QST, which can provide PW peak power at 0.1 Hz on target. The system
can deliver short pulses with an energy of 30 J and pulse duration of 30 fs after compression with a contrast level of
better than 1012. Such performance in high field science will give rise to the birth of new applications and breakthroughs,
which include relativistic particle acceleration, bright x-ray source generation, and nuclear activation. The current
achieved laser intensity on target is up to > 9x1021 Wcm-2 with an energy of ~9 J on target. The interaction with a 3 to 5-
μm stainless steel tape target provides us electrons with a typical temperature of more than 10 MeV and energetic proton
beams with typical maximum energies of > 40 MeV with good reproducibility. The protons are accelerated in the Target
Normal Sheath Acceleration regime, which is suitable for many applications including as an injector into a beamline for
medical use, which is one of our objectives.
We present an overview of our systematic studies of the surface modifications resulting from the interactions of both single and multiple picosecond soft x-ray laser (SXRL) pulses with materials, such as gold (Au), copper (Cu), aluminum (Al), and lithium fluoride (LiF). We show experimentally the possibility of the precise nanometer size structures (~10–40 nm) formation on their surfaces by ultra-low (~10–30 mJ/cm2 ) fluencies of single picosecond SXRL pulse. Comparison experimental results with the atomistic model of ablation, which was developed for the single SXRL shot interaction with dielectrics and metals, is provided. Theoretical description of surface nanostructures is considered and is shown that such structures are formed after laser illumination in a process of mechanical spallation of ultrathin surface layer of molten metal. Spallation is accompanied by a strong foaming of melt, breaking of foam, and freezing of foam remnants. Those remnants form chaotic nanostructures, which are observed in experiments. Our measurements show that electron temperature of matter under irradiation of SXRL was lower than 1 eV. The model calculation also predicts that the ablation induced by the SXRL can create the significant low electron temperature. Our results demonstrate that tensile stress created in LiF and metals by short SXRL pulse can produce spallative ablation of target even for drastically small fluencies, which open new opportunities for material nano processing.
Following three different types of high power lasers at Kansai Photon Science Institute are overviewed and controlling
the laser damages in these laser systems are described: (1) PW-class Ti:sapphire laser for high field science, (2) zig-zag
slab Nd:glass laser for x-ray laser pumping, and (3) high-repetition Yb:YAG thin-slab laser for THz generation. Also
reported is the use of plasma mirror for characterization of short-wavelength ultrashort laser pulses. This new method
will be useful to study evolution of plasma formation which leads to laser damages.
The laser group velocity plays a crucial role in laser driven acceleration of electrons and ions. In particular, a highly efficient mechanism of laser driven ion acceleration, Radiation Pressure Acceleration, has a fundamental limit on the maximum attainable ion energy, which is determined by the group velocity of the laser. However there is another limiting factor that may shed the group velocity effects. It is due to the transverse expansion of the target, which happens in the course of a tightly focused laser pulse interaction with a thin foil. Transversely expanding targets become increasingly transparent for radiation thus terminating the acceleration. Utilization of an external guiding structure for the accelerating laser pulse may provide a way of compensating for the group velocity and transverse expansion effects.
High-order harmonic generation of high intensity ultra-short laser pulses by means of laser produced plasmas are discussed. Since with plasma targets there is no limitation on applicable laser intensity the generated harmonics can be substantially intense. Recent results of experiments and computer simulations on the high-order harmonic generation are briefly reviewed. Main attention is paid to the analysis of basic mechanisms of high-order harmonic generation from overdense and underdense plasma targets irradiated by relativistically intense laser pulses.
Radiation reaction radically influences the electron motion in an electromagnetic standing wave formed by two super-intense colliding laser pulses. Depending on the laser intensity and wavelength, the quantum corrections to the electron motion and the radiation reaction force can be independently small or large, thus dividing the parameter space into 4 regions. When radiation reaction dominates, the electron motion evolves to limit cycles and strange attractors. This creates a new framework for high energy physics experiments on the interaction of energetic charged particle beams and colliding super-intense laser pulses.
Experimental demonstration of multi-charged heavy ion acceleration from the interaction between the ultra-intense short pulse laser system and the metal target is presented. The laser pulse of <10 J laser energy, 36 fs pulse width, and the contrast level of ~1010 from 200 TW class Ti:sapphire J-KAREN laser system at JAEA is used in the experiment. Almost fully stripped Fe ions accelerated up to 0.9 GeV are demonstrated. This is achieved by the high intensity laser field of ∼ 1021Wcm−2 interacting with the solid density target. The demonstrated iron ions with high charge to mass ratio (Q/M) is difficult to be achieved by the conventional heavy ion source technique in the accelerators.
The stability of accleration of ions in the RPDA regime against transversal shift of the cluster target relative to gaussian and supergaussian laser pulses is considered. It is shown that the maximum energy of ions decreases while the shift increases, as the target escapes the acceleration domain. The effect of self-focusing for the supergaussian pulse profile is found and interpreted. An analytical approach based on the relativistic mirror model is developed. We also conduct PIC simulations that prove our theoretical estimations. The results obtained can be applied to the optimization of ion acceleration by the laser radiation pressure with mass-limited targets.
We discuss the key important regimes of electromagnetic field interaction with charged particles. Main attention is paid to the nonlinear Thomson/Compton scattering regime with the radiation friction and quantum electrodynamics effects taken into account. This process opens a channel of high efficiency electromagnetic energy conversion into hard electromagnetic radiation in the form of ultra short high power gamma ray flashes.
In this paper we discuss the spectra of the electrons produced in the laser-plasma acceleration experiment at
FLAME. Here a <30 fs laser pulse is focused via an f/10 parabola in a focal spot of 10 μm diameter into a 1.2
mm by 10 mm rectangular Helium gas-jets at a backing pressure ranging from 5 to 15 bar. The intensity achieved
exceeds 1019 Wcm −2. In our experiment the laser is set to propagate in the gas-jet along the longitudinal axis to use the 10 mm gas-jet length and to evaluate the role of density gradients. The propagation of the laser pulse in the gas is monitored by means of a Thomson scattering optical imaging. Accelerated electrons are set to
propagate for 47,5 cm before being detected by a scintillating screen to evaluate bunch divergence and pointing.
Alternatively, electrons are set to propagate in the field of a magnetic dipole before reaching the scintillating screen in order to evaluate their energy spectrum. Our experimental data show highly collimated bunches (<1 mrad) with a relatively stable pointing direction (<10 mrad). Typical bunch electron energy ranges between 50 and 200 MeV with occasional exceptional events of higher energy up to 1GeV.
We demonstrate a new ion diagnosis method for high energy ions by utilizing a combination of a single CR-39 detector and plastic plates, which enables to detect high energy ions beyond the detection threshold limit of the CR-39. This detection method coupled with a magnetic spectrometer is applied to identify high energy ions of 50 MeV per nucleon in laser-driven ion acceleration experiments using cluster-gas targets.
Review of results, obtained by using recently proposed new imaging detector, based on formation of color centers in LiF
crystal and LiF film, for in situ high performance measurements of near-field and far-field properties of soft X-ray lasers
(SXRL) beams is presented. Experiments have been carried out with laser-driven transient-collision plasma SXRL and
free electron SXRL beams. It was demonstrated that due to favorable combination of high spatial resolution, high
dynamic range and wide field of view this technique allows measuring not only intensity distribution across the full
beam and in local areas, but also permits to evaluate coherence and spectral distribution of radiation across the beam.
Experimental diffraction patterns in the images of periodical structures are analyzed by comparison with the modeled
ones in the last case. The estimated accuracy of measurements is between 10-20%.
We present experimental results, theory, and simulations demonstrating two novel sources of coherent X-ray radiation
generated in the relativistic laser (>1018W/cm2) interaction with easily accessible, repetitive, and debris-free gas jet
targets. The first source is based on a relativistic mirror reflecting a counter-propagating laser pulse. A strongly nonlinear
breaking wake wave driven by an intense laser pulse can act as a semi-transparent relativistic flying mirror. Such a
mirror directly converts counter-propagating laser light into a high-frequency (XUV or X-ray) ultrashort pulse due to the
double Doppler effect. In the experimental demonstration with the 9 TW J-KAREN laser, the flying mirror generated in
a He gas jet partially reflected a 1 TW pulse, providing up to ~1010 photons, 60 nJ (~1012 photons/sr) in the XUV range
(12.8-22 nm). The second source is demonstrated with the laser power ranging from 9 to 170 TW in experiments with
the J-KAREN and Astra Gemini lasers. The odd and even order harmonics generated by linearly as well as circularly
polarized pulses are emitted forward out of the gas jet. The 120 TW laser pulses produce harmonics with ~3×1013photons/sr (~600 μJ/sr) in the 120±5 eV spectral range. The observed harmonics cannot be explained by previously
known mechanisms (atomic harmonics, betatron radiation, nonlinear Thomson scattering, etc.). We introduce a novel
mechanism of harmonic generation based on the relativistic laser-plasma phenomena (self-focusing, cavity evacuation,
bow wave generation), mathematical catastrophe theory which explains the formation of structurally stable electron
density singularities, spikes, and collective radiation of a compact charge driven by a relativistic laser.
The critical electric field of quantum electrodynamics, called also the Schwinger field, is so strong that it produces
electron-positron pairs from vacuum, converting the energy of light into matter. Since the dawn of quantum
electrodynamics, there has been a dream on how to reach it on Earth. With the rise of laser technology this field has
become feasible through the construction of extremely high power lasers or/and with the sophisticated use of nonlinear
processes in relativistic plasmas. This is one of the most attractive motivations for extremely high power laser
development, i.e. producing matter from vacuum by pure light in fundamental process of quantum electrodynamics in
the nonperturbative regime. Recently it has been realized that a laser with intensity well below the Schwinger limit can
create an avalanche of electron-positron pairs similar to a discharge before attaining the Schwinger field. It has also been
realized that the Schwinger limit can be reached using an appropriate configuration of laser beams. In experiments on
the collision of laser light and high intensity electromagnetic pulses generated by relativistic flying mirrors, with
electron bunches produced by a conventional accelerator and with laser wake field accelerated electrons the studying of
extreme field limits in the nonlinear interaction of electromagnetic waves is proposed. The regimes of dominant
radiation reaction, which completely changes the electromagnetic wave-matter interaction, will be revealed. This will
result in a new powerful source of high brightness gamma-rays. A possibility of the demonstration of the electronpositron
pair creation in vacuum via multi-photon processes can be realized. This will allow modeling under terrestrial
laboratory conditions neutron star magnetospheres, cosmological gamma ray bursts and the Leptonic Era of the
Universe.
The paper is devoted to experimental and theoretical studies of ablation of condensed matter by optical (OL),
extreme ultraviolet (EUV) and X-ray lasers (XRL). Results obtained at two different XRL are compared. The
first XRL is collision Ag-plasma laser with pulse duration τL = 7 ps and energy of quanta hv=89.3 eV, while
the second one is EUV free electron laser (EUV-FEL) and has parameters τL = 0.3 ps and energy of quanta 20.2
eV. It is shown that ablation thresholds for these XRL at LiF dielectric are approximately the same. A theory is
presented which explains slow growth of ablated mass with fluence in case of XRL as a result of transition from
spallative ablation near threshold to evaporative ablation at high fluencies. It is found that the metal irradiated
by short pulse of OL remains in elastic state even in high shear stresses. Material strength of aluminum at very
high deformation rates V/V ~ 109 s-1 is defined.
Electromagnetic wave generation in the extreme ultraviolet (XUV) and infrared (IR) wavelength range occurs
during the interaction of intense short laser pulses with underdense plasmas. XUV pulses are generated through
laser light reflection from relativistically moving electron dense shells (flying mirrors). A proof-of-principle and
an advanced experiment on flying mirrors are presented. Both of the experiments demonstrated light reflection
and frequency upshift to the XUV wavelength range (14-20 nm). The advanced experiment with a head-on
collision of two laser pulses exhibited the high reflected photon number. IR radiation, which is observed in the
forward direction, has the wavelength of 5 μm and dominantly the same polarization as the driving laser. The
source of the IR radiation is attributed to emission from relativistic solitons formed in the underdense plasma.
An ultra-bright high-power X-ray and γ-ray source is proposed. A relativistic flying mirror reflects a counterpropagating
electromagnetic radiation causing its frequency multiplication and intensification, while the role of
the mirror is played by a solid-density thin plasma slab accelerating in the radiation pressure dominant regime.
Frequencies of high harmonics generated at the flying mirror by a relativistically strong counter-propagating
radiation undergo multiplication with the same factor as the fundamental frequency of the reflected radiation,
approximately equal to the quadruple of the square of the mirror Lorentz factor. The theory of the reflectivity
of a moving thin plasma slab is presented.
The results of the theoretical consideration and two dimensional Particle in Cell simulation of electron acceleration with
a short-pulse intense laser propagating through a finite length underdense plasma layer are presented. The fast electron
energy spectrum and emittance are analyzed for moderate to high intensity and for different plasma density. It is shown
that for laser pulse lengths above the plasma wake wavelength the wake field accelerated electrons are further
accelerated by the electromagnetic wave.
Short pulsed X-rays have been experimentally generated by 90 degree Thomson scattering of 2 TW, 90 fs laser pulses by 17 MeV electron beams. A few 100 fs X-ray pulses have been generated via backward Thomson scattering from a few 100 fs electron bunches made by a bunch compression chicane. 100 TW laser and microtron as a 150 MeV electron beam source will be prepared, and the laser and the electron beam will be interacted as a hard X-ray source. Soft X-ray may be generated via laser-plasma nonlinear Thomson scatterings as a source of X-ray microscope.
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