Acceleration of electrons and protons using ultra-short pulse, table-top laser systems
Final Report Abstract
The acceleration and the transport of electrons in solid targets with ultra-short, intense laser pulses were investigated. First experiments were carried out with planar Aluminum foils with different thicknesses. The electron temperature was derived from the bremsstrahlung spectrum generated by the fast electrons in the targets. An optical diagnostics was used to image the light emitted from the target rear side. The spectrum and the intensity of the measured signal are explained assuming that the emitted light is coherent transition radiation (CTR). Following this interpretation, resonance absorption is identified as the predominant mechanism of electron acceleration. The emission of CTR from the target was investigated with a ballistic 1d transport model. An electron temperature of 290 eV was derived which is close to the value of 340 eV obtained from the bremsstrahlung spectrum. Motivated by these results, the electron transport was investigated in micro structured targets. Wedge, pyramid and cone shaped targets were used for these experiments. The images of the optical emission from the micro structured targets were significantly different from the planar foils. Assuming that the emission is generated by fast electrons at the target rear side (e.g. CTR) this result suggests that the electron acceleration and transport are affected by the target geometry. There are different processes as e.g. guiding of the electrons by self generated quasi static magnetic fields and guiding of the laser pulses inside the targets which might contribute to this observation. Due to the more complicated geometry, an analysis with a simple 1d model is not possible. Computer simulations of the electron transport and the emission of light from the target rear are currently und progress. Although some questions are still open, these results are very promising as they demonstrate the power of the optical diagnostics for the investigation of the electron transport in solid targets. In addition experiments on the equation of state of warm dense plasmas were carried out. A beam of fast protons was generated by target normal sheath acceleration with an ultra-intense laser pulse focussed onto a thin metal foil (source foil). The proton beam was incident onto a thin Aluminum foil (sample foil) located at a distance of several 100 μm from the source foil. The sample foil is heated to a temperature in the order of 10 eV on a picosecond time scale. The expansion and the temperature of the Aluminum plasma were measured time resolved with a frequency domain interferometer and a streaked optical pyrometer. The measured expansion history was compared with computer simulations using different equation of state models. The best fit was obtained for the sesame equation of state in combination with a Saha ionization model.
Publications
- "Cone surface roughness and angle dependence in high intensity laser-plasma interaction", APS DPP conference, Orlando, FL, 12.11.2007
N. Le Galloudec, B. I. Cho, E. D'Humieres, J. Osterholz, T. Ditmire
- "Study of Hot Electron Transportation in Foils, Wedges and Cone Targets Irradiated with Ultrashort Laser Pulses", CLEO Conference, Baltimore, MD, 10.05.2007
B. I. Cho, J. Osterholz, G. Dyer, S. Kneip, D. Symes, A. Bernstein, T. Ditmire
