Creation of electron-positron plasma with superstrong laser field

We present a short review of recent progress in studying QED effects within the interaction of ultra-relativistic laser pulses with vacuum and e ^? e ⁺ plasma. Current development in laser technologies promises very rapid growth of laser intensities in the near future. Two exawatt class facilities (ELI and XCELS, Russia) in Europe are already in the planning stage. Realization of these projects will make available a laser intensity of ～ 10²⁶?W/cm² or even higher. Therefore, discussion of nonlinear optical effects in vacuum are becoming compelling for experimentalists and are currently gaining much attention. We show that, in spite of the fact that the expected field strength is still essentially less than E _S = m ² c ³/e? = 1.32 · 10¹⁶?V/cm, the nonlinear vacuum effects will be accessible for observation at the ELI and XCELS facilities. The most promissory effect for observation is pair creation by a laser pulse in vacuum. It is shown, that at intensities ～ 5 · 10²⁵?W/cm², creation even of a single pair is accompanied by the development of an avalanche QED cascade. There exists a distinctive feature of the laser-induced cascades, as compared with the air showers arising due primarily to cosmic rays entering the atmosphere. In our case the laser field plays not only the role of a target (similar to a nucleus in the case of air showers) but is also responsible for the acceleration of slow particles. It is shown that the effect of pair creation imposes a natural limit for the attainable laser intensity and, apparently, the field strength E ～ E _S is not accessible for a pair-creating electromagnetic field at all.     

Limits for two-photon ande + e − decay widths of positron-electron scattering resonances for√s=1.78 to 1.92 MeV

Motivated by the observation of energy- and momentum-correlatede ⁺ e ^? pairs in heavy ion collisions, a search for resonances ine ⁺ e ^? scattering at the corresponding energies has been made. Thee ⁺ e ^? decay channel is analyzed in a time window from ～ 10^?13 s to ～ 10^?11 s with a set-up optimized for low-background detection of delayede ⁺ e ^? decays. The two-photon decay channel of a hypothetical resonance is investigated by measuring the two-photon annihilation-in-flight excitation function. New upper limits for the partiale ⁺ e ^? decay width Γ_ee of a few meV are derived for total centre-of-mass energies √s between 1.78 and 1.92 MeV, taking into account the dilepton as well as the two-photon decay of a neutral resonance.     

Influence of ultrashort laser pulse duration on the X-ray emission spectrum of plasma produced in cluster target

Line emission spectrum of a laser plasma produced in an argon cluster jet target was measured on the n ¹ P1?1¹ S ₀ (n=5–9) transitions of the helium-like Ar XVII ion for a pulse duration varying from 45 fs to 1.1 ps and a constant fluence of ～10⁵ J/cm². The independent modeling of the relative intensities of the transitions from the n=5,..., 10 levels, as well as of the 2¹ P ₁? 1² S ₀ and 2³ P ₁?1² S ₀ lines and dielectronic satellites indicates that the electron temperature is anomalously low and that the electron density in emitting plasma increases with shortening the laser pulse. The excitation from the ground state by a small fraction of hot electrons is expected to be the main channel of populating the Ar XVII levels.     

On <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> pair production by colliding electromagnetic pulses

Electron-positron pair production from vacuum in an electromagnetic field created by two counterpropagating focused laser pulses interacting with each other is analyzed. The dependence of the number of produced pairs on the intensity of a laser pulse and the focusing parameter is studied with a realistic three-dimensional model of the electromagnetic field of the focused wave, which is an exact solution of the Maxwell equations. It has been shown that e⁺e^? pair production can be experimentally observed when the intensity of each beam is I～10²⁶ W/cm², which is two orders of magnitude lower than that for a single pulse.     

Thomson Scattering Applied to a Noisy Radiative Plasma

Thomson scattering with a 1.5 ms long pulse mode 20 J ruby laser has been applied to a radiative argon plasma with electron densities n_e from 2.5 10¹⁹ m^?3 to 1.5 10²⁰ m^?3 and an electron temperature T_e of about 3 eV. Photon counting techniques have been used. The accuracy of n_e and T_e to be reached is about 5% after 10 shots. The signal to noise ratio S/N has been optimized by the use of optical filters and a special purpose grating. The effects of these elements on S/N have been calculated. The entrance angle, transmission and quantum efficiency have also been optimized. A comparison between 5 possible laser systems, including a normal mode and a Q-switched mode ruby laser, has been carried out.     

Electron-positron pair production by electromagnetic pulses

Electron-positron pair production in vacuum by a single focused laser pulse and by two counter-propagating colliding focused pulses is analyzed. A focused laser pulse is described using a realistic three-dimensional model based on an exact solution of Maxwell’s equations. In particular, this model reproduces an important property of focused beams, namely, the existence of two types of waves with a transverse electric or magnetic vector (e-or h-polarized wave, respectively). The dependence of the number of produced pairs on the radiation intensity and focusing parameter is studied. It has been shown that the number of pairs produced in the field of a single e-polarized pulse is many orders of magnitude larger than that for an h-polarized pulse. The pulse-intensity dependence of the number of pairs produced by a single pulse is so sharp that the total energy of pairs produced by the e-polarized pulse with intensity near the intensity I _S = 4.65 × 10²⁹ W/cm² characteristic of QED is comparable with the energy of the pulse itself. This circumstance imposes a natural physical bound on the maximum attainable intensity of a laser pulse. For the case of two colliding circularly polarized pulses, it is shown that pair production becomes experimentally observable when the intensity of each beam is I ～ 10²⁶ W/cm², which is one to two orders of magnitude lower than that for a single pulse.     

Effect of the spectral distribution of a chirped pulse on the interaction of the high-power laser radiation with matter

The interaction of high-power picosecond laser radiation with solid targets is experimentally studied for the first time at various spectral distributions of a chirped laser pulse. The interaction of the high-power laser radiation with the target is studied at four regimes of the experimental setup: (i) at a relatively high contrast (10³) in the picosecond (Δt ～ 25 ps) range, (ii) at a relatively low contrast (3 × 10¹) in the picosecond (Δt ～ 25 ps) range, (iii) with spectral distortions of the chirped pulse, and (iv) with a strongly modulated spectrum of the chirped pulse. The results obtained reveal a strong dependence of the atomic and nuclear processes in the laser picosecond plasma on the spectral distribution of the chirped laser pulse. The prospects for the application of the spectral interferometry of chirped pulses for the online control of the parameters of the high-power laser radiation are demonstrated.     

超短超强激光与稀薄等离子体相互作用中后孤立子的观测

开展了超短超强激光与稀薄等离子体相互作用实验,在实验中采用等离子体单色成像法观测等离子体发光图像,捕捉到了近乎对称的环形等离子体发光结构. 在对实验结果进行分析并与理论预言进行比较后确认这是由激光-等离子体相互作用形成的后孤立子云外围的高密度等离子体壳层发光所致. 同时通过对等离子体通道的观测还发现,孤立子的形成对超短超强激光在稀薄等离子体中的传输产生了非常大的影响. 关键词： 超短超强激光 稀薄等离子体 单色成像 后孤立子     

Measurement of the compressibility of a deuterium plasma at a pressure of 1800 GPa

The thermodynamic properties of a highly compressed deuterium plasma have been measured using an explosive spherical experimental chamber. The experiment has been performed with an X-ray diffraction complex consisting of three betatrons and a multichannel optoelectronic system of the detection of X-ray images of the process of the explosive spherical compression of deuterium. The density of the shock-compressed deuterium plasma ρ = (4.3 ± 0.7) g/cm³ at the pressure P = 1830 GPa has been detected at the initial pressure of gaseous deuterium P ₀ = 267 atm and the temperature T ₀ = 10.5°C. Under such conditions, the plasma is strongly nonideal (Γ ～ 450) with the degenerate (nλ _e ³ ～ 280) electron component and with an electron density of about 2.8 × 10²³ cm^?3.     

Anomalous slowdown of relaxation in an ultracold plasma

Recent experiments by T.C. Killian et al. [Phys. Rev. Lett. 83, 4776 (1999)], in which an ultracold plasma (N _e～2×10⁹ cm^?3, T _e～0.1K, and T _i～10 μK) with anomalously long lifetime of ～100 μs was obtained, are explained based on a previously developed theory. The results of computer simulations of the plasma transition into a metastable state and initial heating of electrons up to several K are presented. An expression earlier obtained for the rate of the metastable plasma recombination agrees with the measured anomalously long lifetime. A conclusion is drawn that the previously predicted new physical object—a metastable overcooled plasma—is realized experimentally.     

Decay and recondensation of electron-hole liquid in germanium under CO2 laser pulse irradiation: Investigation by microwave conductivity measurements

In microwave conductivity investigations of photoexcited germanium at low temperatures under CO₂ laser pulse irradiation the evaporation of EHL and e-h plasma formation have been observed. This plasma irreversibly vanishes at high CO₂ laser intensities I_CO2 >4 × 10⁵ W cm^?2 but recondenses at low intensities. It was found that complete and irreversible disappearance of EHL is due to the e-h plasma throw out to the crystal boundaries by phonon wind, generated in 10.6 μm radiation absorption whereas at I_CO2 > 10⁶ W cm^?2 it is connected with the crystal lattice heating over the condensation critical temperature. A theoretical analysis of the CO₂ laser produced phonon wind interaction with e-h plasma is briefly presented. By comparing with experimental data on recondensation process the phonon wind efficiency is estimated.     

Die Erzeugung von Seriengrenzkontinua mit Hilfe von Gleitfunken zur Absolutmessung im Vakuum-Ultraviolett

A lithium plasma is produced by discharging a 40 kV, 0,3 μF capacitor through a lithium-hydride-capillary (diameter 2 mm, length 20 mm) in vacuum (p～10^?4 Torr). During the first half-cycle (0,6 μs) Bremsstrahlung of Li III is observed in the visible and infrared, and the Lyman series of Li III together with the recombination continuum in the vacuum UV (100 Å). The high members of the Lyman series are broadened by Stark effect giving an electron density of about 6 · 10¹⁸ cm^?3. In the infrared the radiation is emitted from an optically thick plasma at a time when the free-free continuum in the visible is emitted from an optically thin plasma. Temporal development of electron temperatureT _e and electron density N_e has been measured from the absolute intensity in these spectral regions. Typical values ofT _e=230 000° K andN _e 5 · 10¹⁸ cm^?3 e.g. have been obtained. For these values the relaxation time for an ionisation equilibrium is short compared to the observation time. The complete ionisation of Li III has been checked by absorption measurements near 100 A. Thus the absolute intensity of the recombination continuum could be calculated. An experimental arrangement was built to measure simultaneously the time history of the intensity in the vacuum ultraviolet, in the visible, and infrared spectral regions.     

Ar plasma waveguide produced by a low-intensity femtosecond laser

Using the interaction of a low-intensity femtosecond laser pulse (30 fs, 6 × 10¹⁵ Wcm^? 2) with argon cluster jet produced from a slit nozzle, we experimentally probe the formation of a uniform plasma waveguide by the interferogram analysis. The results about evolution of plasma channel demonstrate that it is feasible to produce the plasma waveguide for an fs laser pulse of low-intensity. It takes tens of nanoseconds to form a plasma waveguide. The simulation by one-dimensional Gaussian plasma hydrodynamic expansion model indicates that the temperature of plasma channel is not high under this condition. Thus it takes tens of nanoseconds to form a plasma waveguide.     

Emission characteristics and parameters of tin and CuSbSe2 laser plumes

The emission characteristics and parameters of laser plumes of tin and CuSbSe₂ compound are studied at distances of 1 and 7 mm from the target. The recombination times of singly and doubly charged tin ions are, respectively, 116 and 27 ns at a distance of 1 mm from the target and 148 and 64 ns at a distance of 7 mm. In the case of the CuSbSe₂ compound, the recombination times of antimony and copper ions are determined to be, respectively, 60 and 75 ns at a distance of 1 mm and 707 and 976 ns at a distance of 7 mm. The time-averaged temperatures and concentrations of electrons of the tin laser plasma are determined at a distance of 7 mm from the target (T _e = 0.42 eV and n _e = 2.9 × 10¹⁵ cm^?3), and the same parameters for the laser plasma based on the CuSbSe₂ compound are determined at distances of 1 and 7 mm from the target (T _e = 0.62 eV, n _e = 1.4 × 10¹⁶ cm^?3 and T _e = 0.86 eV, n _e = 8.4 × 10¹⁵ cm^?3).     

Generation of fast ions in femto-and picosecond laser plasmas at low intensities of the heating radiation

X-ray spectra from Teflon targets irradiated by laser pulses with a duration of 60 fs to 1 ps have been investigated experimentally. It is shown that, when the contrast of the laser pulse is sufficiently low, the effect of self-focusing of the main laser pulse in the plasma produced by the prepulse can significantly enhance the generation efficiency of fast particles. In this case, ions with energies as high as ～1 MeV are observed at relatively low laser intensities, q _las ≈ (4–6) × 10¹⁶ W/cm².     

Steady state and transient self-interaction of a laser beam in a strongly ionized moving plasma

The steady state and transient self-interaction of a laser beam with a strongly ionized plasma flowing transverse to the direction of propagation have been investigated by a phenomenological approach using perturbation theory, WKB and paraxial ray approximations. The effect of the transverse motion of the plasma has been included by a convection term in the energy balance equation and is found to result in the non-symmetrical heating of electrons. As a result the beam is shifted towards the direction of transverse flow of the plasma by an amount that increases with the flow velocity. The extent of asymmetry in self-focusing along the transverse directions is, however very small. In a typical case of 7.6×10⁵ watt laser of ω=10⁴ GHz and initial beam widthr ₀=0.05 cm the transverse shiftx _p=0.1r ₀ is predicted in a distance of propagationz=0.34 cm in a strongly ionized plasma of electron densityN _e=10¹⁶ cm^?3 and transverse flow velocityW ₀=10⁷ cm/sec.     

Determination of the electron concentration and temperature,as well as the reduced electric field strength,in the plasma of a high-voltage nanosecond discharge initiated in atmospheric-pressure nitrogen by a runaway electron beam

We report on the results of spectroscopic measurements of electron concentration N _e and temperature T _e , as well as the reduced electric field strength E/N in the plasma of a high-voltage nanosecond discharge in the gap with a strongly nonuniform electric field distribution, which is filled with nitrogen under the atmospheric pressure. The possibility of using the method for determining T _e and E/N, which is based on the determination of the ratio of the peak intensities of the ionic N ₂ ⁺ (λ = 391.4 nm) and molecular N₂ (λ = 394 nm) nitrogen bands, is proved. We detected the mean values of quantities N _e , T _e , and E/N amounting to ～2 × 10¹⁴ cm^?3, ～2 eV, and ～240 Td, respectively. In addition, the dynamics of these quantities is determined.     

X-ray spectroscopy diagnostics of a recombining plasma in laboratory astrophysics studies

The investigation of a recombining laser plasma is topical primarily because it can be used to simulate the interaction between plasma jets in astrophysical objects. It has been shown that the relative intensities of transitions of a resonance series of He-like multicharged ions can be used for the diagnostics of the recombining plasma. It has been found that the intensities of the indicated transitions for ions with the nuclear charge number Z _n ~ 10 are sensitive to the plasma density in the range N _e ~ 10¹⁶–10²⁰ cm^–3 at temperatures of 10–100 eV. The calculations performed for the F VIII ion have determined the parameters of plasma jets created at the ELFIE nanosecond laser facility (Ecole Polytechnique, France) in order to simulate astrophysical phenomena. The resulting universal calculation dependences can be used to diagnose different recombining plasmas containing helium-like fluorine ions.     

Laser pulses designed in time by adaptive hydrodynamic modeling for optimizing ultra-fast laser–metal interactions

Determining optimal temporal pulse shapes is an essential aspect for controlling the nature and the energetic characteristics of the ablation products following laser irradiation of materials on ultra-fast scales. In this respect, adaptive feedback loops based on temporal pulse manipulation have been inserted into a hydrodynamic code. The procedure enables us to reach the theoretical maximal temperature at a certain energy input. Several regimes have been considered with fluences ranging from the ablation threshold (F _th=0.34 J/cm²) up to 10 J/cm², proposing an optimal coupling for laser–solid and laser–plasma interactions in these fluence regimes. We determine shapes of optimal pulses on ultra-short and short scales (up to 42 ps) and forecast optimized interaction scenarios with fundamental control factors difficult to access experimentally. Simulations performed on aluminum reveal that ultra-short pulses are the natural better solution for localizing energy in space and time for F～F _th. For higher fluences, pulses spread over tens of picoseconds and ended by a final peak enable a better impulsive coupling with the nascent plasma, optimizing its maximal temperature.