Alice Delserieys

Single-shot characterization of independent femtosecond extreme ultraviolet free electron and infrared laser pulses

Two-color above threshold ionization of helium and xenon has been used to analyze the synchronization between individual pulses of the femtosecond extreme ultraviolet (XUV) free electron laser in Hamburg and an independent intense 120fs mode-locked Ti:sapphire laser. Characteristic sidebands appear in the photoelectron spectra when the two pulses overlap spatially and temporally. The cross-correlation curve points to a 250fs rms jitter between the two sources at the experiment. A more precise determination of the temporal fluctuation between the XUV and infrared pulses is obtained through the analysis of the single-shot sideband intensities.

Optical Thomson scatter from a laser-ablated magnesium plume

We have carried out an optical Thomson scatter study of a KrF laser-ablated Mg plume. The evolution of the electron temperature and density at distances 2–5mm from the target surface has been studied. We have observed that the electron density falls more rapidly than the atomic density and believe that this is a result of rapid dielectronic recombination. A comparison of the electron density profile and evolution with simple hydrodynamic modeling indicates that there is a strong absorption of the laser in the plasma vapor above the target, probably due to photoionization. We also conclude that an isothermal model of expansion better fits the data than an isentropic expansion model. Finally, we compared data obtained from Thomson scatter with those obtained by emission spectroscopy under similar conditions. The two sets of data have differences but are broadly consistent.

Optical Thomson scatter from laser-ablated plumes

We have obtained density and temperature informations on an expanding KrF laser-ablated magnesium plume via optical Thomson scatter with a frequency doubled Nd:YAG laser. The electron temperature was found to decay with the expected T{sub e}{proportional_to}t{sup -1} dependence. However, we have found the electron density to have a time dependence n{sub e}{proportional_to}t{sup -4.95} which can be explained by strong recombination processes. We also observed atomic Raman satellites originating from transitions between the different angular momentum levels of the metastable {sup 3}P{sup 0} term in Mg I.

Plasma Opacity and Laser Ablation Measurements Using X-Ray Lasers

The use of x-ray lasers as probes of the opacity of hot dense plasma and rates of laser ablation is considered. It is shown that x-ray lasers are sufficiently bright to overcome plasma emission and enable plasma opacity to be measured. A demonstration experiment is presented where the temporal evolution of the opacity of a thin iron plasma at high temperature (30 – 250 eV) formed from an initially 50 nm thick solid tamped with a plastic overlay after heating by a laser pulse has been measured using the transmission of a nickel-like silver x-ray laser at 13.9 nm. The experimental results are compared to transmission calculations based on the iron opacity evaluated in a post-processor from predictions of the plasma conditions using a fluid and atomic physics code (EHYBRID). In another experiment, it is shown that laser ablation of a solid iron layer that is not tamped can be determined by the change in transmission of a 21.2 nm x-ray laser.

Precursor model and preschool science learning about shadows formation

Abstract Background This work is based on the idea that young children benefit from early introduction of scientific concepts. Few researches describe didactical strategies focusing on physics understanding for young children and analyse their effectiveness in standard classroom environments. Purpose The aim is to identify whether didactical strategies referring to a precursor model can be used to help children construct explanation of physical phenomena. Sample We present results that test children’s capacity (aged 5-6) to build knowledge within a precursor model in order to describe, explain and predict the phenomenon of shadow formation. Design and methods The teaching intervention’s efficiency is tested in a standard classroom setting. Data were collected through individual interviews, using identical tasks before and after the teaching intervention. Results The analysis of childrens ideas shows that the use of a constructivist didactical strategy referring to a precursor model of shadow formation by teachers has a positive effect on children’s understanding and ability to identify shadows as a physical phenomenon. Conclusion Such results support the importance of science teaching in preschools. In particular, the didactical strategy focuses children’s attention to a critical aspect of their understanding and destabilizes their previous representations. It has implications for preschool teacher professional development.

Line shapes and emission properties in colliding plasmas

Single (Al) and double (Al/Mg) foils have been irradiated with high energy laser pulses to study the radiative properties during the interaction of highly ionized plasma jets with gases and solids. X‐ray microscopy with two‐dimensional curved Bragg crystals was shown to provide a detailed topological structure with 15 μm resolution and a 5 cm large field of view. The realization of simultaneously high spectral resolution enabled to identify not only resonance and intercombination lines but even single satellite transitions. Gas pressure dependent emission features for resolved line transitions inside the jets of H‐, He‐ and Li‐like aluminum ions was discovered when the jet moves into the gas. In the case of two‐foils, accumulation features near the non‐irradiated target have been observed. At large distances from irradiated single foils, anomalous high (3 orders of magnitude) intensities of Li‐like intercombination transitions from double excited states (satellite lines) have been identified in the front ...