September 17, 2010 9:28 WSPC - Proceedings Trim Size: 9.75in x 6.5in main
As lasers actually are the most powerful sources of electromagnetic energy on earth, we present here analytical estimates and numerical simulations of generation of the HFGW ininteraction of high power laser pulse with a medium in different geometries. First, during the laserplasmainteraction, a strong shock driven by the ablation pressure is generated in the bulk material. In this configuration, material is accelerated in the shock front and in the ablation zone. Because of a short laser pulse duration (ns) GW are generated in the GHz domain. During the laserinteraction with a planar thick foil (more than 100µm thickness) the laser launch a shock with a velocity V s , this shock accelerates the medium along the z axis and produces a
in relativistic laser–plasmainteraction
M. Raynaud 1* , A. Héron 2 & J.‑c. Adam 2
the excitation of surface plasmawaves (SpW) by an intense short laser pulse is a useful tool to enhance the laser absorption and the electron heating in the target. In this work, the influence of the transverse laser profile and the pulse duration used to excited SPW is investigated from Fluid and 2D Particle-in-Cell simulations. We show the existence of a lobe of surface plasma wave modes. Our results highlight surface plasmawaves excitation mechanism and define the laser parameters to optimise the SpW excitation and the kinetic energy of the associated electron trapped in the wave. it opens the door to monitor the spectral mode distribution and temporal shape of the excited surface wavesin the high relativistic regime. The most important result of the study is that—at least in 2D— the charge and the energy of the electron bunches depend essentially on the laser energy rather than on temporal or spatial shape of the laser pulse.
To further detail the effects of electron bunching and ejection, let us regress to a 1D model for a wave normally incident on an abrupt plasma boundary. The ponderomo- tive force from the incident and reflected waves acts on electrons within the skin depth [see Fig. 3(a), I]. For a sufficiently large wave amplitude this leads immediately to breaking of the stimulated Langmuir oscillations . Thus, electrons from the skin layer are thrown toward the bulk of the plasma [Fig. 3(a), II]. At the same time, a counterstream of electrons arises from the bulk plasmain the direction of the skin layer [Fig. 3(a), III] due to the attractive force toward ions that were left behind by the inward-driven electrons and by the repulsive force of those electrons. The velocity of the counterstreaming electrons is relativistic. This behavior is typical of normal incidence effects, and leads to the self-intersection of electron trajectories. Near the points where the flows ‘‘stop,’’ e.g., x $ 0:12, where the counterstreaming elec- tron flow reverses to the inward-driven electron flow as observed in Fig. 3(a), III, the electron density n e becomes very high. In maintaining a finite flux, n e v e , when the electron flow velocity v e approaches zero, n e must spike. For short pulses, for pulses with a sharp tail, or even at each half-cycle of linearly polarized radiation, the elec-
It is well known that electrons trapped in traveling plasmawaves can be accelerated to high energies. 1 Also, the proposal of direct excitation of plasmawaves by lasers has greatly stimulated theoretical and experimental investiga- tions toward the development of plasma-based electron ac- celerators. Among various acceleration schemes, 2,3 particular attention has been focussed on the possibility of creating large amplitude plasmawavesin the wake of a laser pulse. 4 Recently, with the arrival of laser systems capable of deliv- ering light pulses of extremely high intensities 共of the order of 10 21 W cm ⫺2 ) in the femtosecond regime and with high temporal contrast 共better than 10 8 ), 5 much attention has been raised to absorption mechanisms and high energy electron generation involving overdense plasmas with stepwise den- sity profiles resulting from laser–solid interaction. 6 – 8
restrictions imposed by the size of the microwave antennas on the excited SWs wavelength with the standard method of microwave magnetic fields, magnetostatic SWs frequen- cies in such devices did not exceed a few GHz at low field, which is a major limiting factor for ultrafast applications. On the other hand, due to the strong exchange interaction between spins, the exchange SWs mode can have higher frequencies compared to the magnetostatic one. The possibility of triggering such exchange modes requires a nonuniform dynamical field across the magnetic film thickness  , which is very challenging to induce using microwave antennas [13 –15] . Recently, femtosecond laser pulses have been used as an efficient stimulus to excite a coherent spin precession [16,17] . In particular, it was demonstrated that femtosecond laser pulses can excite exchange standing SWs (SSWs) in metallic [18,19] and semiconductor [20,21] ferromagnets via thermal processes. On the other hand, most studies on iron garnets have been dedicated to the excitation and control of the homogenous resonance mode (k ¼ 0, i.e., low frequency) via nonthermal excitation mechanisms [22 –29] . An important question in this context concerns the possibility to take advantage of femtosecond laser pulses for triggering a highfrequency SSW in dielectric thin films of iron garnets.
and energy of 60 µJ in Gaussian state and 50 µJ in vortex state. Our Gaussian SH pulse was shaped into an optical vortex by the method described in [ 30 ], which ensures more than one-octave spectral bandwidth of vortex generation: First SH was circu- larly polarized by a quarter wave plate (QWP) and converted to an optical vortex (OV) beam by an S-waveplate (RPC-405-06- 557, Workshop of Photonics) (SWP). Subsequent polarization filtering by a second quarter wave plate (QWP) and a polarizer (POL) ensured generation of a linearly polarized vortex in the SH beam profile over the whole spectral bandwidth of the SH pulse. A 0.5 mm thick Si wafer and various commercial THz filters were used to remove the highfrequency part of the pump and transmit only THz radiation, which was then collimated and shrank by parabolic mirrors in a telescope configuration to match the detector aperture. Imaging of the generated THz beam was performed with a thermal camera detector (Vario- CAM head HiRes 640, InfraTec GmbH), sensitive in the range 0.1 – 40 THz (3000 – 7.5 µm). Spectra of THz radiation were ob- tained from Fourier transformed interferometric measurements using pyroelectric detector (TPR-A-65 THz, Spectrum Detector Inc.), sensitive in the range 0.1 - 300 THz (3000 - 1 µm) with a flat response function from ∼ 3 to ∼ 100 THz. The laser-to-THz conversion efficiency for the regular Gaussian pulses was about 10 −4 , but dropped to ∼ 10 −5 in the case of the SH vortex pump, which we attribute to the differences in the spatial intensity distribution of the Gaussian FH and vortex SH beam.
waves at oblique propagation angles, and Langmuir waves and electron-acoustic waves propagating parallel to the am- bient magnetic field (Roth and Hudson, 1986). For strong electron beams the electron-acoustic waves can start to dom- inate resulting in spectrum that extends only up to f pe (Gary and Tokar, 1985). Typically, electron-acoustic waves are ob- served at frequencies that are near the plasmafrequency of the cold (non drifting) population of electrons. The electron- acoustic waves can evolve into electron holes (Berthomier et al., 2000), leading to broad-band spectra. In the case of a Buneman instability, where electrons move with respect to ions, lower frequencywaves of the broad-band charac- ter are generated. Electron or ion-holes can also be formed in this case. The missing electron distribution function data does not allow us to observationally identify the free energy source; however, for us it is important that, in principle, all highfrequency wave emissions are local electrostatic insta- bilities that do not propagate far from the generation region; only slight changes in the electron distribution or electron beam strength can create different type of spectra that are seen in our observations.
Polyurethane Water confinement
A B S T R A C T
Laser stripping is a process which typically includes different forms of ablation phenomena. The presented work investigates a mechanical stripping process using high pressure laser-induced shock wavesin a water confined regime. Power density is studied as a parameter for selective laser stripping on painted specimens and for adhesion relations with single layer epoxy targets. A flashlamp-pumped Nd:YAG laser with fixed spot size (4 mm) is shot on single layer epoxy and several layers of polymeric paint applied on a AA 2024-T3 (Aluminium) substrate. After laser treatment, samples are investigated with optical microscopy, profilometer and chemical analysis (FTIR & TGA). The results show that selective laser stripping is possible between different layers of external aircraft coatings and without any visual damage on the substrate material. In parallel to the experi- mental work, a numerical model has been developed to explain the background of the physical mechanisms and to qualitatively evaluate the detailed stress analysis and interfacial failure simulation for a single layer of epoxy on an aluminium substrate. The predicted failure patterns agree with the surfaces of the tested specimens observed by a microscope.
generation and x-ray emission due to breaking of the resonant plasma wave. @S1063-651X~99!06904-4#
PACS number ~s!: 52.40.Nk, 52.50.Jm
During the past decade, with the development of intense laser sources of ultrashort ~femtosecond! pulse duration, laser-plasmainteraction experiments have been able to study a new kind of physical situations that can only exist on such ultrashort time scales: laser-produced hot high-density plas- mas, characterized by very steep density gradients @1,2#, where the hydrodynamic expansion does not play a dominant role. Much attention has been drawn to possible applications of these unusual plasmas to generate ultrafast pulses of x radiation in the keV range @2#.
(Received 26 October 2018; published 12 March 2019)
Plasma wakefield acceleration (PWFA) is a novel acceleration technique with promising prospects for both particle colliders and light sources. However, PWFA research has so far been limited to a few large- scale accelerator facilities worldwide. Here, we present first results on plasma wakefield generation using electron beams accelerated with a 100-TW-class Ti:sapphire laser. Because of their ultrashort duration and high charge density, the laser-accelerated electron bunches are suitable to drive plasmawaves at electron densities in the order of 10 19 cm −3 . We capture the beam-induced plasma dynamics with femtosecond resolution using few-cycle optical probing and, in addition to the plasma wave itself, we observe a distinctive transverse ion motion in its trail. This previously unobserved phenomenon can be explained by the ponderomotive force of the plasma wave acting on the ions, resulting in a modulation of the plasma density over many picoseconds. Because of the scaling laws of plasma wakefield generation, results obtained at highplasma density using high-current laser-accelerated electron beams can be readily scaled to low-density systems. Laser-driven PWFA experiments can thus act as miniature models for their larger, conventional counterparts. Furthermore, our results pave the way towards a novel generation of laser-driven PWFA, which can potentially provide ultralow emittance beams within a compact setup.
As a consequence of SPW excitation, we observe an enhancement of the electron heating: the number and energy of the hot electron is increased compared to the case of a flat target where no SPW is excited. In Fig. 1 , we observe two different populations of electrons: “cold” electrons, which have gained less energy during the interaction and “hot” electrons, which have been efficiently heated and have gained a considerable amount of energy. Part of the energy absorbed from the laser when the SPW is excited is con- verted to “hot” electrons flowing parallel to the front surface of the plasma 17 and does not participate to the bulk heating. However, the majority of the “hot” electrons crosses the tar- get toward the rear surface. The two populations, “hot” and “cold” electrons, are associated to two quasi-exponential slopes observed in the spectra, in the low and high energy range. Thus, assuming 4 , 30 that the electrons have a distribu- tion function of the form n e ðEÞ expðE=k B T e Þ, we can derive the electron temperature for the two populations and define T cold and T hot . In the following, we will use these two
7 1 1. INTRODUCTION
Since its invention over a decade ago [1, 2], HCF compressors have provided tremendous thrust to the field of ultrafast optics . In a HCF filled with a rare gas at constant pressure, the spectrum of amplified, CEPlocked femtosecond laser pulses can be nonlin early broadened to support pulse durations approach ing the laser period. Adequate dispersive mirror tech nology can then be used to temporally compress the pulses, forming CEPcontrolled, high peakpower, fewopticalcycle waveforms that can drive extreme nonlinear interactions with matter . Such wave forms are now routinely used to generate attosecond pulses in the extreme ultraviolet region via highhar monic upconversion in gases . When focused down to wavelengthlimited spot sizes, fewcycle pulses could even be used to drive highly efficient attosecond pulse generation from relativistic laserplasma inter actions . Reaching such a regime, however, demands that the ultrashort pulses have multimJ energy and very high spatial beam quality. Extended mode confinement of the laser beam inside the HCF ensures spatially homogeneous spectral broadening, excellent beam pointing stability and neardiffraction limited spatial beam quality, features that other non guided techniques, such as filamentation, cannot afford [7, 8]. However, problems arise when trying to scale standard HCF compressors beyond the mJ energy level: selffocusing and multiphoton ioniza tion prevent efficient coupling of energy into the fiber and excite highorder modes of the waveguide, thereby limiting the power density and spatial resolu tion attainable with the beam transmitted through the fiber. In practice, a standard HCF compressor with a
In this Letter, we present an experiment where high- order harmonics of the laserfrequency are generated through the interaction of plasma mirrors [19,20] with UHI laser vortices, produced by simply inserting a spiral phase plate in a high-power femtosecond laser beam just before focusing. By studying the amplitude and phase properties of these extreme ultraviolet (XUV) beams, we obtain clear evidence for the helical wavefronts of the laser beam on target at full power. From a fundamental point of view, we verify in an unambiguous way that the topological charge of the nth harmonic beam is n times that of the driving laser beam, and thus, provide the first experimental validation  of the OAM conservation rule at such ultrahigh intensities. Furthermore, we analyze the key physical effects that determine the mode content of these beams, using a simple yet general approach that applies to any harmonic generation process driven by LG laser beams. This is, to the best of our knowledge, the first experiment demonstrating the controlled exchange of OAM between laser light and matter at relativistic intensities, opening the way of the exploitation of LG beams in UHI physics.
Chapter 5. Laser ion acceleration and high energy radiation generation from near-critical gas jets
10 13 W/cm 2 for a one micron wavelength laser. Ion acceleration produced from the interaction of a very high intensity laser with underdense targets has been studied with Particle-In-Cell (PIC) simulations [68-70], but the simulated targets did not reproduce the gas targets available for experiments. The first experiments of ion acceleration via laserinteraction with underdense targets showed radial acceleration . Recent experiments proved that using high intensity lasers one can obtain strong longitudinal proton acceleration , [72,73]. In this case the energy of the longitudinally accelerated protons is greater than the one of the transversally accelerated protons. The accelerating mechanism that causes the energetic accelerated ions seen in , [72,73] has not yet been described in detail and therefore is the center of an ongoing debate [63, 64]. In the case of near-critical gas targets, the density gradient is not easy to achieve experimentally (however, such experimental endeavors have been achieved in the past ), thus, the dependence of the particle acceleration on the specific plasma density is important in order to optimize such experimental efforts.
width than CWE, as opposed to what is observed exper- imentally.
The explanation for this apparent contradiction lies in the phase properties of the harmonics. As we have seen in Sec. IV F 3, the phase of CWE harmonics has a fairly strong dependence on the laser intensity. On the oppo- site, we observed in Sec. V H that the phase of ROM harmonics is almost independent of this intensity in the moderately relativistic interaction regime. Since the laser intensity varies in time in an actual experiment, accord- ing to the intensity envelop of the driving laser pulse, CWE harmonics have a large temporal phase -in other words, they are far from their FTL duration- while ROM harmonics do not. This temporal phase leads to a spec- tral broadening of the harmonics, which accounts for the difference observed between CWE and ROM harmonics. This interpretation of the spectral width of CWE har- monics is corroborated by several experimental observa- tions, which all support the theoretical and numerical analysis of Sec. IV F. Experimental evidence of the neg- ative intrinsic chirp of CWE harmonics was first obtained in Ref. , in which this chirp, and the associated spec- tral broadening, were partially compensated by introduc- ing a positive chirp on the driving laser pulse. In addi- tion, a direct measurement of the intensity dependence of the emission time of CWE attosecond pulses using an interferometric technique agrees well with numerical re- sults . Finally, the analysis of Sec. IV F showed that the phase variations of CWE harmonics only depends on the laser pulse temporal intensity profile, but not on its peak intensity. This is confirmed experimentally by the fact that the spectral width of CWE harmonics is observed to be independent of the peak laser intensity (Fig. 43(a)).
width than CWE, as opposed to what is observed exper- imentally.
The explanation for this apparent contradiction lies in the phase properties of the harmonics. As we have seen in Sec. IV F 3, the phase of CWE harmonics has a fairly strong dependence on the laser intensity. On the oppo- site, we observed in Sec. V H that the phase of ROM harmonics is almost independent of this intensity in the moderately relativistic interaction regime. Since the laser intensity varies in time in an actual experiment, accord- ing to the intensity envelop of the driving laser pulse, CWE harmonics have a large temporal phase -in other words, they are far from their FTL duration- while ROM harmonics do not. This temporal phase leads to a spec- tral broadening of the harmonics, which accounts for the diﬀerence observed between CWE and ROM harmonics. This interpretation of the spectral width of CWE har- monics is corroborated by several experimental observa- tions, which all support the theoretical and numerical analysis of Sec. IV F. Experimental evidence of the neg- ative intrinsic chirp of CWE harmonics was ﬁrst obtained in Ref. , in which this chirp, and the associated spec- tral broadening, were partially compensated by introduc- ing a positive chirp on the driving laser pulse. In addi- tion, a direct measurement of the intensity dependence of the emission time of CWE attosecond pulses using an interferometric technique agrees well with numerical re- sults . Finally, the analysis of Sec. IV F showed that the phase variations of CWE harmonics only depends on the laser pulse temporal intensity proﬁle, but not on its peak intensity. This is conﬁrmed experimentally by the fact that the spectral width of CWE harmonics is observed to be independent of the peak laser intensity (Fig. 43(a)).
Key words. gravitationalwaves – stars: pulsars: general
The coming years will be marked by the beginning of oper- ations for the major gravitational wave interferometric anten- nas LIGO, VIRGO, GEO and TAMA. Starting with a sensitiv- ity of the order of h ≈ 10 −22 , they are expected to evolve in a few years into second generation experiments with a sensitivity improved by one or two orders of magnitude depending on the frequency. Ground based detectors will scan the sky search- ing for gravitationalwaves with frequencies between tens of hertz up to tens of kilohertz. Potential sources fall roughly into three classes: bursts, stochastic background, continuous, involving di ﬀerent search techniques as match filtering for co- alescing binary systems and cross-correlation between detec- tors for the stochastic background. The detection of pulsars (and other nearly monochromatic continuous sources) can be achieved by integrating the signal during times of about 10 7 s and searching for statistically significant peaks at fixed fre- quencies in the power spectrum. This method becomes quite complicated as soon as one consider that: 1) pulsars are not
17 W cm 2 lm 2 Þ
IR laser pulse 1 and the plasma mirror technique, 2 which per- mits high pulse contrast (>10 11 ), allows the creation of a sharp-edged overdense plasma before any eventual smooth- ing of the density gradient by hydrodynamic expansion in thick plasmas (L x c=x pe , where L x indicates the plasma thickness, x pe is the plasmafrequency, and c/x pe is the skin depth). In such sharp-edged overdense plasmas, the electro- magnetic energy is weakly absorbed in an optical skin depth by collisional processes and through collisionless mecha- nisms 3 such as sheath inverse bremsstrahlung, 4 ~ J ~ B heat- ing, 5 vacuum heating, 6 and anomalous skin-layer heating. 7 Owing to the ultra-short laser pulse duration and the steep density profile of the plasma, the laser reflection is very high and can easily exceed 80% (Ref. 8 ), thus limiting the produc- tion of high energetic particles. This point is a drastic limita- tion in all applications related to particle acceleration and for the fast ignitor scheme in the framework of the inertial con- finement fusion. 9 , 10
noise at a sufficiently low level and to resolve subtle kinetic effects.) Although these intensities are higher than expected in the standard ICF scenarios, they can be easily achieved in speckles and are also of interest for advanced schemes such as shock ignition [ 42 ]. While weak nonlinearities such as sec- ondary LDIs, generation of higher harmonics of ion acoustic waves and particle trapping operate in near threshold condi- tions, we found quite different scenario at higher intensities. SBS was evolving in strong bursts ended up with formation of a series of cavities with electromagnetic waves trapped inside [ 63 ]. That process resulted in suppression of the scattered field, electron heating and acceleration. Figure 6 (a) shows the formation of density cavities in a simple one-dimensional case of excitation SBS in a homogeneous plasma layer [ 63 ]. This first observation was confirmed in an extended two-dimen- sional SBS simulations [ 64 ] and later the similar scenario was found in SRS and TPD modeling for the shock ignition condi- tions [ 43 , 65 ]. In particular, the plasma cavitation near quarter critical density leads to quenching of the TPD instability at a relatively low level. These cavities shown in figure 6 (b) are not yet observed experimentally because of their micrometric size, but the TPD suppression has been reported in many experiments at highlaser intensities.
For these reasons, phase properties of high-order har- monics generated from atoms or molecules have been extensively studied over the past 15 years [ 1 ]. In this case, the relative phases of the harmonics are directly related to the sub-laser-cycle dynamics of the continuum electron wave-packet responsible for the generation [ 3 ]. These phases are well defined and relatively weak: har- monics are thus synchronized over a broad frequency range, and their superposition produces trains of attosec- ond pulses. Because these dynamics depend on laserin- tensity, the temporal and spatial intensity envelopes of the driving laser-pulse lead to variations of the attosecond pulses properties both in time and space [ 4 ]. These varia- tions result in nontrivial temporal and spatial intrinsic phases of individual harmonics, which in turn affect the properties of the harmonic beam.