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Relativistic electrons generated by the interaction of petawatt-class short laser pulses with solid targets can be used to generate bright x-rays via bremsstrahlung. The efficiency of laser energy transfer into these electrons depends on multiple parameters including the focused intensity and pre-plasma level. This paper reports experimental results from the interaction of a high intensity petawatt-class glass laser pulses with solid targets at a maximum intensity of 1019 W cm−2. In-situ measurements of specularly reflected light are used to provide an upper bound of laser absorption and to characterize focused laser intensity, the pre-plasma level and the generation mechanism of second harmonic light. The measured spectrum of electrons and bremsstrahlung radiation provide information about the efficiency of laser energy transfer.

The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the α-particle heating expected to trigger a burn wave in the hydrogen pellet and resulting in high thermonuclear gain. However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where theory agrees with the data. Here, we report experimental data at low projectile velocities near the Bragg peak, where the stopping force reaches its maximum. This parameter range features the largest theoretical uncertainties and conclusive data are missing until today. The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to disprove several standard models for the stopping power for beam velocities typically encountered in inertial fusion. On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions., The energy loss of ions in plasma is a challenging issue in inertial confinement fusion and many theoretical models exist on ion-stopping power. Here, the authors use laser-generated plasma probed by accelerator-produced ions in experiments to discriminate various ion stopping models near the Bragg peak.

We apply Fourier-transform spectral interferometry (FTSI) to study the interaction of intense laser pulses with ultrathin targets. Ultrathin submicrometer-thick solid CH targets were shot at the PHELIX laser facility with an intensity in the mid to upper 10^{19} W/cm^{2} range using an innovative double-pulse structure. The transmitted pulse structure was analyzed by FTSI and shows a transition from a relativistic transparency-dominated regime for targets thinner than 500 nm to a hole-boring-dominated laser-plasma interaction for thicker targets. The results also confirm that the inevitable preplasma expansion happening during the rising slope of the pulse, a few picoseconds before the maximum of the pulse is reached, cannot be neglected and plays a dominant role in laser-plasma interaction with ultrathin solid targets.

Population kinetics and spectral synthesis codes have been developed to analyze Kα emission from partially ionized chlorine in hydrocarbon plasmas irradiated with high-intensity ultra-short laser pulses. The population kinetics processes are calculated using a bi-Maxwellian electron temperature distribution composed of fast and cold electrons. The fast electrons dominantly contribute to ionize the K-shell bound electrons. i.e., inner-shell ionization, while the cold electrons produce ionization from the outer-shell electrons. The resultant Kα emission provides a distinct spectral signature for each charge state. Also included in the calculation are the opacity effects in the framework of an escape probability method. It is shown that the Kα emission is saturated at a plasma thickness of more than a few microns. Further, we find that unresolved satellite lines overlap significantly with the corresponding parent Kα lines of the next charge state, and make a large contribution to composite spectrum. Finally, time-dependent properties of the Kα emission are also discussed briefly.

The expansion of a semi-infinite plasma slab into vacuum is analyzed with a hydrodynamic model implying a steplike electron energy distribution function. Analytic expressions for the maximum ion energy and the related ion distribution function are derived and compared with one-dimensional numerical simulations. The choice of the specific non-Maxwellian initial electron energy distribution automatically ensures the conservation of the total energy of the system. The estimated ion energies may differ by an order of magnitude from the values obtained with an adiabatic expansion model supposing a Maxwellian electron distribution. Furthermore, good agreement with data from experiments using laser pulses of ultrashort durations τ(L)

The effective increase of the critical density associated with the interaction of relativistically intense laser pulses with overcritical plasmas, known as self-induced transparency, is revisited for the case of circular polarization. A comparison of particle-in-cell simulations to the predictions of a relativistic cold-fluid model for the transparency threshold demonstrates that kinetic effects, such as electron heating, can lead to a substantial increase of the effective critical density compared to cold-fluid theory. These results are interpreted by a study of separatrices in the single-electron phase space corresponding to dynamics in the stationary fields predicted by the cold-fluid model. It is shown that perturbations due to electron heating exceeding a certain finite threshold can force electrons to escape into the vacuum, leading to laser pulse propagation. The modification of the transparency threshold is linked to the temporal pulse profile, through its effect on electron heating., Comment: 13 pages, 12 figures; fixed some typos and improved discussion of review material

Fast ignition of inertial fusion targets is one of the promising ways for future energy production. Recent studies of ignition with fast electrons have shown only little progress - mainly caused by the unresolved problem of controlled electron transport in dense plasmas. Ion-assisted ignition schemes are less sensitive in this regard and laser energies in the range of 50 - 100 kJ are required for fuel ignition. Picosecond laser pulses with intensities exceeding 10 22 Wcm -2 and a high intensity contrast of better than 10 -10 are needed thereby. The cone-guided target proves to be the most efficient scheme, although targets without a cone are more appropriate for high repetition rate operation.

The motion of a single planar 180° domain wall was studied in single-crystalline gadolinium molybdate, Gd2(MoO4)3. The switching current and the instantaneous wall positions were recorded using polarized light in an optical microscope and subsequent image processing. A pronounced deaging (wake-up) effect is observed represented by an increase of domain-wall shift during cyclic switching at constant voltage amplitude. The experimental data are compared to computer simulations taking into account the kinetic imprint effect under ac cycling. The latter is the change of the spatial distribution of the internal bias field during cycling. It is shown that deaging (wake-up) arises for aged (screened) initial states. After long enough cycling, the spatial distribution of the internal bias field becomes steady state and the wall motion becomes reproducible in all its details. The final distribution of the internal bias field does not depend on the initial state of the sample. The activation energies for deaging and aging are equal within the experimental uncertainty and thus very likely stem from the same micromechanism.

High-order harmonics generated by weakly relativistic femtosecond laser pulses interacting with solid and thin foil targets were studied. It was found that the conversion efficiency is one or two orders of magnitude larger than that of gas-harmonics and thus due to the high laser intensity also the harmonics are very intense. In addition, anomalies and complex interference structures have been observed in the harmonic spectra of the solid targets. They are explained by simulations. Furthermore, for the first time intense harmonics were observed from thin foil targets and, in particular, from the target rear side. This type of studies provides a promising tool for obtaining information about the laser plasma interaction itself, e.g., on the presence of large fields and oscillations inside the foil up to a penetration depth comparable to the laser wavelength.

Isochoric heating of matter by intense heavy ion beams promises to become a fruitful approach to warm dense matter studies. For heating times that are long on the hydrodynamic time scale of the target response a tamped target is essential. The proposed dynamic confinement provides homogeneous target heating by a low-Z tamper, which allows one to apply powerful x-ray scattering diagnostics. To demonstrate the potential of the method, heating of a hydrogen sample with the SIS-18 beam at GSI Darmstadt is investigated numerically. The intense x-ray bursts for diagnostics can be provided by the PHELIX laser currently installed at GSI. In the optimized heating regime, density variations can be reduced to a level of 15% from the initial density value.

We have studied the influence of the target properties on laser-accelerated proton and ion beams generated by the LULI multi-terawatt laser. A strong dependence of the ion emission on the surface conditions, conductivity, shape and material of the thin foil targets were observed. We have performed a full characterization of the ion beam using magnetic spectrometers, Thompson parabolas, radiochromic film and nuclear activation techniques. The strong dependence of the ion beam acceleration on the conditions on the target back surface was found in agreement with theoretical predictions based on the target normal sheath acceleration (TNSA) mechanism. Proton kinetic energies up to 25 MeV have been observed.

It is shown that the hydrodynamic model of a one-dimensional collisionless plasma expansion is contained in the kinetic description as a special case. This belongs to a specific choice for the electron distribution function. Moreover, the consequences of the use of the hydrodynamic approach regarding the temporal evolution of the electron phase space density are investigated. It turns out that only the case of a hydrodynamic description with the adiabatic constant κ=3 is physically self-consistent. Numerical simulations confirm this argumentation. The analysis for the case κ=3 is extended to the kinetics of a relativistic electron gas.

The propagation of a relativistic electron with initial energy ≳100 MeV in a number of simple one-dimensional laser field configurations with circular polarization is studied by solving the relativistic equation of motion in the Landau–Lifschitz approach to account for the radiation friction force. The radiation back-reaction on the electron dynamics becomes visible at dimensionless field amplitudes a ≳ 10 at these high particle energies. Analytical expressions are derived for the energy and the longitudinal momentum of the electron, the frequency shift of the light scattered by the electron and the particle trajectories. These findings are compared with the numerical solutions of the basic equations. A strong radiation damping effect results in reduced light scattering, forming at the same time a broad quasi-continuous spectrum. In addition, the electron dynamics in the strong field of a quasistationary laser piston is investigated. Analytical solutions for the electron trajectories in this complex field pattern are obtained and compared with the numerical solutions. The radiation friction force may stop a relativistic electron after propagation over several laser wavelengths at high laser field strengths, which supports the formation of a stable piston.

Laser energy requirements for the creation of active plasma media with Ne- and Ni-like ions with a `long' subnanosecond prepulse (100 - 1000 ps) and a `short' picosecond main pulse (0.1 - 10 ps) have been determined for foil and low density targets (foams, gases, vapors, etc.). RADEX hydrodynamics and atomic kinetics calculations show that X-ray laser action in the 200 - 400 A can be obtained with 1 - 2 J in 100 - 1000 ps prepulse followed of an approximately 1 ps pulse of similar energy. Transient gains of approximately 100 cm-1 level have been calculated. For the wavelengths 38 - 46 A (`red' edge of `water window' spectral band) a short pulse of only approximately 5 - 10 J is required.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.