Hot electrons, keV X-rays and fast ions from femtosecond laser produced micro plasma

Summary form only given. The scaling of hot electron generation in the intermediate intensity range of 10/sup 15/ to 10/sup 17/ W/cm/sup 2/ is very important for the generation of keV X-rays and fast ions. Such intensities can be achieved with sub-millijoule 120 fs Ti:Sapphire laser pulses focused to spots with diameters of a few microns producing microplasma sources of hot particles and X-rays. Traditionally absorption in this intensity range has been ascribed to a combination of collisional and resonant absorption depending on the angle of incidence, scale length of the plasma and polarization of the incident laser pulses. However, it is reported that vacuum heating should overtake resonant absorption for I/spl lambda//sup 2/>4/spl times/10/sup 16/ Wcm/sup -2/ /spl mu/m/sup 2/ and thus becomes important for steep density profiles while Raman heating at the quarter critical density surface becomes important for extended plasma profiles. Recently there have been reports of electron jets formed at these intermediate intensities in the target normal, specular and the E-field directions depending on the polarization and prepulse levels. A clear picture of how the various generation mechanisms contribute to these observations is still lacking. We report here a study to characterize the electron, keV X-ray and fast ion generation in this intensity regime for 800 nm, 120 fs laser pulses focused to small spot diameters on the surfaces of solid targets. The goal is to better characterize the various mechanisms contributing to the hot electron generation, their propagation outwards from the small focal spot region and the subsequent generation of keV X-rays and fast ions. Directional emission of electrons is observed which is dependent on the laser polarization and target geometry. Modeling of the interaction process is carried out with 1D, 2D and 3D PIC codes in order to better understand the generation mechanisms and explain the features observed in the experiments. Exper- mental and modeling results will be presented and compared.