We have performed numerical simulations with the 2D PIC code CALDER (coll. Erik Lefebvre – CEA-DAM BIII) to understand the temporal evolution of the high intensity ultra-short laser pulse into the plasma. First, we have tested the code by simulating the propagation of the laser in conditions relevant to an experiment performed by collaborators from Pisa University and we succeeded to described and interpret part of their experimental results. We have studied the temporal evolution of the lase pulse into a plasma generated with a laser intensity of 3.5x1018W/cm2 ,and Ne0=1019cm-3 (Nitrogen and Argon plasma) [Giulietti 2013]. We have isolated different dominant contributions of the plasma during the propagation, onto the laser pulse spectrum (blue and red shift of the laser pulse due to self modulation by the plasma wave – blue shifting by the decreasing electronic density gradient).
Finally, we have considered the same input parameters (“bubble regime” accessible with UHI100laser – 100TW -25fs) than the ones corresponding to our study of the betatron emission and we have described the temporal evolution of the laser pulse propagation into the plasma.
Figure 3: Ey component of the field on the propagation axis (a) at t=8ps and (c) t=8.3ps and respectively (b) and (d) the associated electronic density map at the same time.
We have reported, on figure 3, the Ey component of the field on the propagation axis at t=8ps and the corresponding electronic 2D map, compared to the same two figures at t=8.3ps. The oscillations of the ionic cavity (figure 3(b) and d)), which have an influence on the emission properties, are correlated to strong modifications of the laser pulse on the time scale of an optical cycle, emphasing one or the other laser optical cycle (bold – on figure 3 (a) and (c)). This is the result of cumulated effects of the compression and depletion of the pulse itself during its propagation in the plasma.
[Giulietti2013] A. Giulietti, A. André, S. Dobosz Dufrénoy et al., submitted to PoP
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