SUPERSTRONG FIELDS IN PLASMAS: Third International Conference on Superstrong Fields in Plasmas
827(2006); http://dx.doi.org/10.1063/1.2195192View Description Hide Description
Interaction of an intense, ultrashort laser pulse with a gas‐jet target is investigated through femtosecond optical interferometry to study the dynamics of ionization of the gas. Experimental results are presented in which the propagation of the pulse in the gas and the consequent plasma formation is followed step by step with high temporal and spatial resolution. We demonstrate that, combining the phase shift with the measurable depletion of fringe visibility associated with the transient change of refractive index in the ionizing region and taking into account probe travel time can provide direct information on gas ionization dynamics.
827(2006); http://dx.doi.org/10.1063/1.2195193View Description Hide Description
The leading physical mechanism responsible for collisionless absorption of intense laser radiation in overdense matter (solids, liquids, clusters, aerosols) is addressed and discussed in detail. It is shown that the plasma layers become anharmonic oscillators with variable eigenfrequency under the strong driving field and can therefore enter into resonance with it. Already existing collisionless absorption models are discussed in the light of the new mechanism.
827(2006); http://dx.doi.org/10.1063/1.2195194View Description Hide Description
The flux density time evolution of radiation at the even harmonics frequencies of a test wave is found and its link with the evolution of the current density produced by a constant electric field is established. Even harmonics generation is studied in two regimes: that of free accelerated motion which takes place for small time intervals and that of current saturation corresponding to the time moments much larger than the inverse of the electron‐ion effective collision frequency when a quasi‐stationary electron motion is established. The generation of even harmonics of pump wave in weakly non uniform plasma with electron heat flux is described as well.
827(2006); http://dx.doi.org/10.1063/1.2195195View Description Hide Description
The nonlinear interaction between the electron‐positron pairs produced by an electromagnetic wave in a plasma is investigated for a circularly polarized wave using the relativistic Vlasov equation with a source term based on the Schwinger formula for the pair creation rate. The damping of the wave, the nonlinear up‐shift of its frequency due to the plasma density increase and the effect of the damping on the wave polarization are investigated.
827(2006); http://dx.doi.org/10.1063/1.2195196View Description Hide Description
Fast relativistic particles generated by an ultra strong laser pulse interacting with an overdense plasma, are the source of strong quasi‐static magnetic fields. The physical mechanism underlying this process is known as the current filamentation (Weibel) instability which is an electromagnetic instability driven by the electron momentum anisotropy. Here we investigate the development of this instability in the fluid, relativistic, collisionless regime in the case of beams with a finite transverse size. We show that the development of a spatially resonant mode plays a key role in determining the kind of magnetic structure generated during the evolution of the instability.
827(2006); http://dx.doi.org/10.1063/1.2195197View Description Hide Description
Measurements have been made of the magnetic field generated by the passage of high intensity short laser pulses through underdense plasmas. For a 30 fs, 1 J, 800 nm linearly‐polarised laser pulse, an azimuthal magnetic field is observed at a radial extent of approximately 200 μm. The field is found to exceed 2.8 MG. For a 1 ps, 40 J, 1054 nm circularly‐polarised laser pulse, a solenoidal field is observed that can exceed 7 MG. This solenoidal field is absent with linear polarised light, and hence can be considered as an Inverse Faraday effect. Both types of field are found to decay on the picosecond timescale. For both the azimuthal and solenoidal fields produced by such intense lasers, the production of energetic electrons by the interaction is thought to be vital for magnetic field generation.
827(2006); http://dx.doi.org/10.1063/1.2195198View Description Hide Description
Particle‐in‐cell simulations of relativistically strong laser pulses interacting with overdense plasma targets predict that coherent motion of electrons leads to the efficient generation of strong attosecond electromagnetic pulses and dense attosecond electron bunches. The optimal conditions for these attosecond phenomena are achieved in the λ3 regime, when few‐cycle laser pulses are focused to a wavelength‐limited spot, producing maximal intensity and maximal gradients with a given energy. The natural synchronism of these attosecond phenomena should enable a kind of relativistic attosecond optoelectronics.
Stimulated Raman Cascade into Photon Condensation and Large Amplitude EM Solitons in Laser Plasma Interaction827(2006); http://dx.doi.org/10.1063/1.2195199View Description Hide Description
Stimulated Raman scattering, stimulated Raman cascade and the transition from Raman cascade into photon condensation, induced by a linearly‐polarized intense laser interacting with low‐density uniform plasmas, are studied by particle simulations. It is found that, at appropriate laser amplitude and plasma condition, a large amplitude relativistic EM soliton forms due to the strong photon condensation. The standing, backward‐ and forward‐accelerated ultra‐relativistic electromagnetic solitons induced by intense laser pulses are observed. In addition to the inhomogeneity of the plasma density the acceleration of solitons depends upon, both, the laser intensity and the plasma length. The frequency of solitons is about half of the unperturbed electron plasma frequency, while the transverse electric, magnetic and electrostatic field have half‐, one‐ and one‐cycle spatial structure, respectively.
Generation of ultrashort pulses of electrons, X‐rays and optical pulses by relativistically strong light827(2006); http://dx.doi.org/10.1063/1.2195200View Description Hide Description
We report recent results of experiments in which relativistic optical effects play an important role, at peak laser intensities above 1019 W/cm2. These effects are leading to novel radiation sources, all with femtosecond pulse durations: (1) the generation of optical photons by means of pulse compression via relativistic cross‐phase modulation, (2) ponderomotive deflection of laser accelerated electron beams, and (3) the generation of well‐collimated keV‐energy x‐ray beams by means of either Thomson scattering or betatron oscillations in ion channels.
827(2006); http://dx.doi.org/10.1063/1.2195201View Description Hide Description
The self‐action dynamics of three‐dimensional wave packets whose width is of the order of carrier frequency is studied under rather general assumptions concerning the dispersion properties of the medium. The condition for the wave field collapse is determined. Self‐action regimes in a dispersion‐free medium and in media with predominance of anomalous or normal group velocity dispersions are numerically investigated. It is shown that, for extremely short pulses, nonlinearity leads not only to the self‐compression of the wave field but also to a “turn‐over” of the longitudinal profile. In a dispersionless medium, the formation of a shock front within the pulse leads to the nonlinear dissipation of linearly polarized radiation and to self‐focusing stabilization. For circularly polarized radiation, the wave collapse is accompanied by the formation of an envelope shock wave.
Self‐Gompression and Self‐Focusing Instability of Femtosecond Multiterawatt Laser Pulses in Underdense Plasmas827(2006); http://dx.doi.org/10.1063/1.2195202View Description Hide Description
Self‐compression and self‐focusing instabilities of femtosecond, multiterawatt laser pulses propagating at subrelativistic intensities in strongly undercritical plasmas are studied. Our numerical simulations show that the spatiotemporal self‐compression works effectively in underdense (1% of the critical density) plasmas and relativistically intense, few‐cycle laser pulses can be produced in this way over propagation distances less than 2 mm. We found that pulses with Gaussian radial and time profiles are very robust maintaining their initial shape and proportions during this process, while, in general, the interplay between the self‐compression and the self‐focusing has proved to be very sensitive to the shape of laser pulses.
Explosions of Methane and Deuterated Methane Clusters Irradiated by Intense Femtosecond Laser Pulses827(2006); http://dx.doi.org/10.1063/1.2195203View Description Hide Description
Intense, femtosecond irradiation of atomic and molecular clusters can initiate Coulomb explosions generating particle energies sufficient to drive nuclear fusion. It has been proposed that heteronuclear clusters with a mixture of heavy and light ions will not explode by the simple, equilibrium Coulomb model but that dynamic effects can lead to a boosting of energy of the lighter ejected ions. We present experimental confirmation of this theoretically predicted ion energy enhancement in methane and deuterated methane clusters. We then present data on fusion induced by explosions of dueterated methane clusters examining both fusion yield and neutron angular distribution.
827(2006); http://dx.doi.org/10.1063/1.2195204View Description Hide Description
The non‐equilibrium transformations (crystal‐to‐crystal and melting) induced by sub‐picosecond lasers on space scale of nanometers and time scale of less than picosecond are considered in this presentation. We demonstrate that the fast (during the pulse time) change in the inter‐atomic potential due to the electrons excitation is responsible for the swift atomic displacement. In fact this is a modification of familiar Lindemann criterion for melting in equilibrium to non‐equilibrium conditions, when the electron temperature replaces the equilibrium temperature in the perturbation of potential. We also show that the optical response builds up in picosecond time when a material conversion is performed in a bulk at the distance comparable to the evanescent wave penetration depth.
We present the results of the experimental and theoretical studies on non‐equilibrium melting of Gallium by 150 fs pulses at intensity well below the ablation threshold. Direct measurements of the transient optical properties and theoretical analysis indicate that the melting in its conventional sense either is not completed even when the deposited energy exceeds 3 times the equilibrium enthalpy of melting, or that, most probably, some transient state of matter has been created during the interaction.
In conclusion we address some unresolved problems in understanding of ultra‐fast phase transformations induced by ultra‐short laser pulses in non‐equilibrium conditions.
827(2006); http://dx.doi.org/10.1063/1.2195205View Description Hide Description
The existence of highly efficient ion acceleration regimes in collective laser‐plasma interactions opens up the possibility to develop high‐energy‐physics (HEP) facilities in conjunction with projects for inertial confined nuclear fusion (ICF) and neutron spallation sources.
827(2006); http://dx.doi.org/10.1063/1.2195206View Description Hide Description
This talk will describe ongoing and planned fast ignition theoretical and experimental work using the Z‐machine in concert with the upgraded Z‐Beamlet/PW laser system. Z can produce x‐ray powers of 100–250TW, x‐ray energies 1–1.8MJ, and > 200eV radiation temperatures and thus represents an interesting platform for compressing large quantities of matter for use as fast ignition targets. Soon Z will undergo refurbishment, and the resulting parameters for Z‐R are even more interesting. The Z‐Beamlet system is currently undergoing short pulse conversion with the plan that Z‐Beamlet/PW will deliver 0.5–2 kJ in 0.5–10 psec in FY07 when Z‐R will be available. Numerical simulations of laser/plasma interaction, electron transport, and ion generation at relevant densities are being performed using LSP, a 3D implicit hybrid PIC code. LASNEX simulations of the compression of deuterium/tritium fuel in various reentrant cone geometries are being performed. Analytic and numerical modeling has been performed to determine the conditions required for fast ignition breakeven scaling. These theoretical results indicate that to achieve fusion output equal to the energy deposited by fast particles will require about 5% of the laser energy needed for ignition and might be an achievable goal (within a factor of 2) with Z‐Beamlet/PW.
827(2006); http://dx.doi.org/10.1063/1.2195207View Description Hide Description
A new amplifying laser concept based on Coherent Amplification Network (CAN) is proposed to solve the high‐peak high‐average‐power quandary. The amplification network is based on identical telecommunication diode‐pumped fiber lasers. The philosophy behind the approach is to build the amplifying system from numerous small but identical parts as opposed to larger but non‐identical components like in the laser Mégajoule in France or NIF in the USA. The basic amplification scheme is in‐fiber Chirped Pulse Amplification. Besides the possibility to simultaneously provide high peak and high average power, the technique gives independent control of the output beam spatial and temporal coherence, as well as the pupilary distribution. In addition to being rugged, CAN offers the additional benefit of being inexpensive and low maintenance. A conceptual design based on CAN is presented that offers an alternative to the next CERN Linear Collider (CLIC).
Design and performance of a Petawatt subpicosecond N2O‐laser pumped by HF — chemical laser radiation827(2006); http://dx.doi.org/10.1063/1.2195208View Description Hide Description
Progress on developing a petawatt laser source in 10μm region is described. Analysis of optical pumping N2O containing active media by pulsed multifrequency HF laser has been performed. It is shown that amplification of ultrashort pulses should be carried out in the gain band centered at 930cm−1. Amplification of seed ultrashort (∼1ps) pulses in atmospheric and high pressure N2O (up to 5atm) amplifiers pumped by powerful pulsed HF chemical lasers is theoretically studied. It is shown that N2O atmospheric pressure amplifiers can be effectively used for production output energy of 1kJ.
827(2006); http://dx.doi.org/10.1063/1.2195209View Description Hide Description
The Project of petawatt excimer laser was started at Lebedev Physical Institute (LPI) in collaboration with High‐Current Electronics Institute (HCEI). It utilizes Ti:Sa front‐end to generate 1mJ, 50 fs pulses, frequency conversion into 2ω or 3ω and direct amplification of short pulses in a chain of excimer amplifiers with e‐beam or photochemical pumping. Multi‐terawatt output in 20‐J, subpicosecond pulses at KrF (λ = 248 nm) is projected being combined with 4‐kJ, nanosecond pulses. Petawatt femtosecond pulses at Kr2F (400 nm) might be expected, if transient absorption could be reduced in e‐beam‐pumped gain medium. For photochemically‐pumped XeF amplifiers (480 nm) multi‐petawatt level is attainable with a high temporal contrast.
Measurements of Femtosecond Pulse Duration by means of Michelson Interferometer without Nonlinear Elements827(2006); http://dx.doi.org/10.1063/1.2195210View Description Hide Description
Autocorrelators based on Michelson interferometer using a nonlinear element are commonly used for measuring ultra‐short laser pulses. In principle it is possible to use any nonlinear interaction, for example wave mixing (second harmonic generation), photodiodes with two‐photon absorption, etc. But each of them has its disadvantages. The speed of nonlinear interactions might restrict their application for measurement of ultra‐short pulses. Another disadvantage of most autocorrelators with nonlinear elements is the need for relatively high laser power. Also various factors such as laser instability, quality of nonlinear crystal, and in the first place geometrical factors influence the nonlinear interaction: a little change of the alignment in a scheme with wave mixing can bring to considerable variation in two waves interaction.
In this work we consider an autocorrelator without nonlinear elements. The mirror in one arm of a Michelson interferometer was moved along the optical axis, and the change of interference maxima and minima was registered with a slow IR photodiode. Signal from photodiode (correlation function) was observed by digital oscilloscope. The sech2 laser pulse temporal shape was selected being in a good agreement with the obtained correlation function. The experiments were performed at the University of Milano‐Bicocca using the two laser front ends of the multi‐stage Nd:glass “ATILLA” laser system. The first one was a Quantronix Nd:YLF master oscillator with pulse duration of ∼ 100 ps, the second one a Timebandwidth Nd:glass femtosecond oscillator with pulse duration of ∼ 200 fs.
In conclusion, this technique is very simple, not limited by speed of nonlinear interaction, and in principle it could be used for measuring of pulses with any duration and with very low energy. The quality of optics, mechanics and other elements of the scheme is not essential, but results of measurements are quite precise.