No abstract

Proton and ion acceleration using high-intensity lasers is a field of rapidly growing interest. For possible applications of proton beams produced in laser-solid interactions, the generation of beams with controllable parameters such as energy spectrum, brightness, and spatial profile is crucial. Hence, the physics underlying the acceleration processes has to be well understood. After the first proof-of-principle experiments [1,2], systematical studies were carried out to examine the influence of target material and thickness [3,4]. To establish the influence of the main laser parameters such as intensity, pulse energy, and duration over a wide range, results from different laser systems have to be compared, since usually each system covers a small parameter range only. Besides these parameters, strength and duration of the prepulse due to amplified spontaneous emission (ASE) play an important role, too [3]. We report on experiments carried out to establish the influence of the laser prepulse due to ASE and the target thickness on the acceleration of protons from thin aluminum foils.The protons originate from water and hydrocarbon contaminations on the foil surfaces. We used the 6-TW ATLAS laser facility at MPQ delivering 150 fs pulses at 790 nm wave length containing up to 900 mJ of energy. The pulses are focused by an f /2.5 off-axis parabolic mirror onto aluminum foils of 0.8 . . .86 µm thickness to intensities in excess of 10 19 W/cm 2 .The duration of the ASE prepulse having a peak intensity of 8 × 10 11 W/cm 2 can be controled by means of an ultra-fast Pockels cell in the laser chain. The shortest prepulse duration is 500 ps and it can be extended to several ns. The protons accelerated from the foils are detected by a Thomson parabola positioned in normal direction of the target rear side. CR 39 plates are used as a detector. The proton pits made visible by etching the CR 39 in NaOH after the shot are counted by an optical microscope equipped with a pattern-recognition software.

Temporal probing of a number of fundamental dynamical processes requires intense pulses at femtosecond or even attosecond (1 as = 10(-18) s) timescales. A frequency 'comb' of extreme-ultraviolet odd harmonics can easily be generated in the interaction of subpicosecond laser pulses with rare gases: if the spectral components within this comb possess an appropriate phase relationship to one another, their Fourier synthesis results in an attosecond pulse train. Laser pulses spanning many optical cycles have been used for the production of such light bunching, but in the limit of few-cycle pulses the same process produces isolated attosecond bursts. If these bursts are intense enough to induce a nonlinear process in a target system, they can be used for subfemtosecond pump-probe studies of ultrafast processes. To date, all methods for the quantitative investigation of attosecond light localization and ultrafast dynamics rely on modelling of the cross-correlation process between the extreme-ultraviolet pulses and the fundamental laser field used in their generation. Here we report the direct determination of the temporal characteristics of pulses in the subfemtosecond regime, by measuring the second-order autocorrelation trace of a train of attosecond pulses. The method exhibits distinct capabilities for the characterization and utilization of attosecond pulses for a host of applications in attoscience.

Collimated jets of carbon and fluorine ions up to 5 MeV/nucleon ( approximately 100 MeV) are observed from the rear surface of thin foils irradiated with laser intensities of up to 5 x 10 (19)W/cm(2). The normally dominant proton acceleration could be surpressed by removing the hydrocarbon contaminants by resistive heating. This inhibits screening effects and permits effective energy transfer and acceleration of other ion species. The acceleration dynamics and the spatiotemporal distributions of the accelerating E fields at the rear surface of the target are inferred from the detailed spectra.