Preformed Plasma Research Articles

Hot electron generation and transport using Kα emission

We have conducted experiments on both the Vulcan and Titan laser facilities to study hot electron generation and transport in the context of fast ignition. Cu wires attached to Al cones were used to investigate the effect on coupling efficiency of plasma surround and the pre-formed plasma inside the cone. We found that with thin cones 15% of laser energy is coupled to the 40μm diameter wire emulating a 40μm fast ignition spot. Thick cone walls, simulating plasma in fast ignition, reduce coupling by x4. An increase of pre-pulse level inside the cone by a factor of 50 reduces coupling by a factor of 3.

Optical guiding of a laser beam in an axially nonuniform plasma channel

AbstractThe guiding of a laser beam in a plasma channel formed by a short laser prepulse is investigated. Due to the self defocusing of an ionizing short laser prepulse, the plasma channel formed is axially nonuniform. When a delayed second laser beam is allowed to propagate through such a preformed plasma channel, convergence and divergence of the beam is observed due to the relative competition of the refraction and diffraction phenomenon. We have solved the wave equation governing the propagation characteristics of an ionizing prepulse and a delayed pulse by the moment theory approach. Results have been compared with the paraxial ray model of Liu and Tripathi (1994). The moment theory predicts the propagation of guided laser beam over several Rayleigh lengths.

Enhanced Isochoric Heating from Fast Electrons Produced by High-Contrast, Relativistic-Intensity Laser Pulses

Thin, mass-limited targets composed of V/Cu/Al layers with diameters ranging from 50 to 300 microm have been isochorically heated by a 300 fs laser pulse delivering up to 10 J at 2x10{19} W/cm{2} irradiance. Detailed spectral analysis of the Cu x-ray emission indicates that the highest temperatures, of the order of 100 eV, have been reached when irradiating the smallest targets with a high-contrast, frequency-doubled pulse despite a reduced laser energy. Collisional particle-in-cell simulations confirm the detrimental influence of the preformed plasma on the bulk target heating.

Limitation on Prepulse Level for Cone-Guided Fast-Ignition Inertial Confinement Fusion

The viability of fast-ignition (FI) inertial confinement fusion hinges on the efficient transfer of laser energy to the compressed fuel via multi-MeV electrons. Preformed plasma due to the laser prepulse strongly influences ultraintense laser plasma interactions and hot electron generation in the hollow cone of an FI target. We induced a prepulse and consequent preplasma in copper cone targets and measured the energy deposition zone of the main pulse by imaging the emitted K_{alpha} radiation. Simulation of the radiation hydrodynamics of the preplasma and particle in cell modeling of the main pulse interaction agree well with the measured deposition zones and provide an insight into the energy deposition mechanism and electron distribution. It was demonstrated that a under these conditions a 100 mJ prepulse eliminates the forward going component of approximately 2-4 MeV electrons.

Prepulse effects on the generation of high energy electrons in fast ignition scheme

The energy distribution of the produced high energy electrons in the interaction of ultraintense picosecond laser pulses with high-Z solid targets is shown to be sensitive to the preformed plasma created by the prepulse and the amplified spontaneous emission pedestal. The created preformed plasmas, which are obtained by radiation hydrodynamic simulations for the present heating laser system at ILE, Osaka University, are seen to extend up to 30–100 μm just before the arrival of the main pulse. The dependences of the coupling efficiency of the laser energy to high energy electrons, and the energy spectra of these accelerated electrons, on this preformed plasma, are studied via a two-dimensional particle-in-cell simulation code. It is found that in a small preformed plasma case, J×B heating is dominant and the produced electron temperature agrees well with Haines’ scaling law [Haines et al., Phys. Rev. Lett., 102, 045008 (2009)]. While in a large preformed plasma case, in addition to J×B heating and/or vacuum heating, other acceleration mechanisms, such as stochastic heating, can accelerate electrons to very high energies, carrying a significant fraction of input laser energy. Even after several picoseconds, the number of high energy electrons (0.5 MeV<E<5 MeV) generated in a small preformed plasma case can be several times larger than that of a large preformed plasma case.

Experimental Evidence of Short Light Pulse Amplification Using Strong-Coupling Stimulated Brillouin Scattering in the Pump Depletion Regime

The energy transfer from a long (3.5 ps) pump pulse to a short (400 fs) seed pulse due to stimulated Brillouin backscattering in the strong-coupling regime is investigated. The two pulses, both at the same wavelength of 1.057 microm are quasicounterpropagating in a preformed underdense plasma. Relative amplification factors for the seed pulse of up to 32 are obtained. The maximum obtained amplified energy is 60 mJ. Simulations are in agreement with the experimental results and suggest paths for further improvement of the amplification scheme.

Study of high-order harmonic generation from nanoparticles

An experimental study on high-order harmonic generation from the interaction of 45 fs Ti:sapphire laser pulses with preformed plasma plumes of metal nanoparticles was carried out. Highly efficient harmonic generation in the range of 9th order to 19th order was observed for Ag nanoparticles. The stability of harmonic generation was enhanced by utilizing special target fabrication techniques and through optimizing the conditions of plasma plume formation. Broadband harmonic generation was observed through the optimization of femtosecond laser intensity and through the use of spectrally broadened laser pulses. The harmonic generation was compared for various target materials (nano and bulk) and for Ag nanoparticle targets prepared from different fabrication techniques. Efficient generation of even- and odd-order harmonics was observed through the use of two-colour pulses. The observations can be explained qualitatively from symmetry breaking of high-order harmonic generation through the introduction of second harmonic pulses. The spectral broadening and shift of harmonic radiation can be understood from the self-modulation of the laser and harmonic radiation in the plasma.

Seeding of a soft-x-ray laser in a plasma waveguide by high harmonic generation

A strongly saturated waveguide-based optical-field-ionization soft-x-ray laser seeded by high harmonic generation was demonstrated for Ni-like Kr lasing at 32.8 nm. Compared with the same laser seeded only with spontaneous emission, seeding with high harmonics yields much smaller divergence, enhanced spatial coherence, and controlled polarization. The integration of high harmonic seeding, optically preformed plasma waveguide, and optical-field-ionization pumping forms one of the optimal archetypes of an ultrashort-pulse soft-x-ray laser.

Experimental and computational characterization of hydrodynamic expansion of a preformed plasma from thin-foil target for laser-driven proton acceleration

AbstractWe characterize the electron density distributions of preformed plasma for laser-accelerated proton generation. The preformed plasma of a titanium target 3 μm thick is generated by prepulse and amplified spontaneous emission (ASE) of a high-intensity Ti:sapphire laser and is measured with an interferometer using a second harmonic probe beam. High-energy protons are obtained by reducing the size of the preformed plasma by changing the ASE duration before main pulse at the front side (laser incidence side) of the target. Simulation results with two-dimensional radiation hydrodynamic code are close to the experimental results for low-density region ~4 × 1019 cm−3 at the front side. In the high-density region near to the target surface, the interferometry underestimates the density due to the substantial refraction. The characterization of hydrodynamic expansion with the interferometer and simulation is a useful tool for investigation of high-energy proton generation.

Characterization of the preformed plasma for high-intensity laser-plasma interaction

The interaction of a very intense, very short laser pulse is modified by the presence of a preformed plasma prior to the main short pulse. The preformed plasma is created by a small prepulse interacting with the target prior to the main pulse. The prepulse has been monitored using a water-cell-protected fast photodiode allowing on every shot a high dynamic measurement of the pulse profile. Simultaneously we have used time-resolved interferometry to look at the preformed plasma on a 300 TW, 700 fs laser. The two-dimensional density maps obtained have been compared with two-dimensional hydrodynamic simulations.

Effect of a nanometer scale plasma on laser-accelerated ion beams

Energies of laser-accelerated ions from thin foils in the so-called ‘ultra-high-contrast’ regime have been measured for various preformed plasma sizes on the non-irradiated foil surface. Whereas energies of protons accelerated in the laser counter-propagating direction remain almost constant for plasma scale length up to 300 nm, we found that plasmas as short as a few tens of nanometers reduce the maximum energy of ions accelerated in the laser direction. These experimental measurements are numerically confirmed with two-dimensional particle-in-cell simulations coupled to hydrodynamic calculation. Moreover, our experimental results, supported by simulations, provide evidence for the occurrence of ion wave breaking, and demonstrate its ability to mitigate the ion energy reduction due to the plasma gradient. This wave breaking is observed and characterized for both proton and carbon ion components.

Generation control of fast electron beam by low-density foam for FIREX-I

Integrated simulations for fast ignition with a cone-guided target and a 10 ps heating laser have been performed to investigate core heating properties in FIREX-I experiments. It is found that the fast electron beam intensity scales well as the inverse square root of the front electron density, and the averaged core temperature reaches a higher value when the preformed plasma on an inner surface of the cone tip has a longer scale length. As the scale length of the preformed plasma is not easily controllable, a low-density foam coated target is proposed to prevent the reduction in the fast electron beam intensity during the laser irradiation. Optimum foam density and thickness are estimated by averaged core electron temperatures calculated by the integrated simulations.

Hot-electron energy coupling in ultraintense laser-matter interaction

We investigate the hydrodynamic response of plasma gradients during the interaction with ultraintense energetic laser pulses using kinetic particle simulations. Energetic laser pulses are capable of compressing preformed plasma gradients over short times, while accelerating low-density plasma backward. As light is absorbed on a steepened interface, hot-electron temperature and coupling efficiency drop below the ponderomotive scaling and we are left with an absorption mechanism that strongly relies on the electrostatic potential caused by low-density preformed plasma. We describe this process, discuss properties of the resulting electron spectra and identify the parameter regime where strong compression occurs. Finally, we discuss implications for fast ignition and other applications.

Propagation of chirped laser pulses in a plasma channel

Propagation of an initially chirped, Gaussian laser pulse in a preformed parabolic plasma channel is analyzed. A variational technique is used to obtain equations describing the evolution of the phase shift and laser spot size. The effect of initial chirp on the laser pulse length and intensity of a matched laser beam propagating in a plasma channel has been analyzed. The effective pulse length and chirp parameter of the laser pulse due to its interaction with plasma have been obtained and graphically depicted. The resultant variation in laser frequency across the laser pulse is discussed.

Experimental Evidence of Predominantly Transverse Electron Plasma Waves Driven by Stimulated Raman Scattering of Picosecond Laser Pulses

We report on highly time- and space-resolved measurements of the evolution of electron plasma waves driven by stimulated Raman scattering of a picosecond, single laser speckle propagating through a preformed underdense plasma. Two-dimensional Thomson scatter spectra indicate that the dominant waves have significant transverse components. These results are supported by particle-in-cell simulations which pinpoint the dominant role of the wave front bowing and of secondary nonlinear electrostatic instabilities in the evolution of the plasma waves.

Vacuum electron acceleration by tightly focused laser pulses with nanoscale targets

Electron acceleration using a tightly focused relativistic short laser pulse interacting with a spherical nanocluster, ultrathin foil or preformed mid-dense plasmas is studied by using three-dimensional particle-in-cell simulations with the Stratton–Chu integrals as the boundary conditions for the incident laser fields. The investigation is performed in the regime where the focal spot size is comparable with the laser wavelength. Generation of high-energy electron multibunch jets with quasimonoenergetic or waterbaglike spectra has been demonstrated. The physical process of acceleration and bunching of the electrons is discussed in detail, as well as particles energy and angular distributions for different laser intensities, focusing optics, target parameters, and laser incidence angles.

Study of ultraintense laser propagation in overdense plasmas for fast ignition

Laser plasma interactions in a relativistic regime relevant to the fast ignition in inertial confinement fusion have been investigated. Ultraintense laser propagation in preformed plasmas and hot electron generation are studied. The experiments are performed using a 100 TW 0.6 ps laser and a 20 TW 0.6 ps laser synchronized by a long pulse laser. In the study, a self-focused ultraintense laser beam propagates along its axis into an overdense plasma with peak density 1022/cm3. Channel formation in the plasma is observed. The laser transmission in the overdense plasma depends on the position of its focus and can take place in plasmas with peak densities as high as 5×1022/cm3. The hot electron beams produced by the laser-plasma interaction have a divergence angle of ∼30°, which is smaller than that from laser-solid interactions. For deeper penetration of the laser light into the plasma, the use of multiple short pulse lasers is proposed. The latter scheme is investigated using particle-in-cell simulation. It is found that when the pulse duration and the interval between the pulses are appropriate, the laser pulse train can channel into the plasma deeper than a single longer pulse laser of similar peak intensity and total energy.

Control over the radiation spectrum of a microwave plasma relativistic oscillator

General features of the operation of microwave oscillators based on the Cherenkov resonance interaction of a high-current relativistic electron beam with a preformed plasma are considered. Emphasis is placed on the presence of longitudinal modes of the plasma-beam resonator that make it possible to tune the radiation frequency. Methods by which the radiation frequency can be varied severalfold continuously or in discrete controlled steps and the width of the spectrum of simultaneously generated frequencies can be changed substantially are described. The results of numerical simulations are compared with available experimental data.

Theory and simulation of ion acceleration with circularly polarized laser pulses

Abstract Ion acceleration driven by the radiation pressure of circularly polarized pulses is investigated via analytical modeling and particle-in-cell simulations. Both thick and thin targets, i.e. the “hole boring” and “light sail” regimes are considered. Parametric studies in one spatial dimension are used to determine the optimal thickness of thin targets and to address the effects of preformed plasma profiles and laser pulse ellipticity in thick targets. Three-dimensional (3D) simulations show that “flat-top” radial profiles of the intensity are required to prevent early laser pulse breakthrough in thin targets. The 3D simulations are also used to address the issue of the conservation of the angular momentum of the laser pulse and its absorption in the plasma. To cite this article: A. Macchi et al., C. R. Physique 10 (2009).

Observation of a plasma waveguide in a preformed plasma pumped by double-pulse laser irradiation for the efficient soft x-ray amplification

A molybdenum slab target was irradiated by double pulses of a line-focused chirped-pulse-amplification Nd:glass laser. A probe beam was injected in the plasma in the longitudinal direction, and we found that the probe beam was guided in the plasma with the plasma length of up to 4.3 mm at the time of 0.8-1.5 ns after the peak of the second pulse. Comparison with hydrodynamics code implied that the electron-density dip or flat region was formed, which was favorable for the efficient soft x-ray lasing under the coupling of the seed x-ray and the x-ray amplifier.