Among the various parameters involved in laser-plasma acceleration mechanisms, the electrons themselves can be collected and studied. But, as they are collected at the end of the acceleration process, it is not trivial to get information about the “electron ‘s life”, from the trapping process up to the end of the acceleration one. During the acceleration phase, the electrons acquire relativistic energies and oscillate around the laser axis. This leads to the generation of up to g-ray emission [Cipiccia2011] called betatron radiation. Several experimental and numerical studies have shown that there is a strong link between the electron properties and the betatron emission characteristics [Rousse2007, Ta Phuoc2006] .
We have performed numerical studies on the properties of the betatron emission from relativistic electrons [Andre2012] in conditions closed to what can be achieved on UHI100 laser (CEA-SLIC – 100TW -25fs).
From the 2D PIC simulations that we have been performed with PICLS (2D PIC code from collaboration with E. d’Humières - CELIA-Université Bordeaux1) coupled to a home-made program developed by Arnaud André (PhD student) to calculate the emission from accelerated electrons, we have shown that the betatron radiation properties are strongly linked to the ones of the electron beam. From slight variations of the input conditions, as reported in figure 1, with the electronic density ranging from 6 to 7x1018cm-3, we are able to link the properties of the electron bunch at the end of the acceleration process (oscillation phase, energy, number of electron in each bunch if several trapped electron bunches) to the shapes of emitted radiation and the spectral extent of the angular distribution on the laser propagation axis [André2012].
Figure 1: (a) emission profile along the propagation axis (qz=0) by unit of solid angle for Ne0=7x1018cm-3 (right axis) and Ne=6x1018cm-3 (left axis) and a0=2.5. Angular distribution of spectral intensity of the emitted radiation of electrons, on qz=0 at Ne0=6x1018cm-3 (b) and Ne0=7x1018cm-3 (c).
Varying the laser intensity (from a0=2.5 to a0=3.5), we have shown evidence that the deformation of the ionic cavity play a significant role on the shape of the angular distribution of the spectral intensity of the emitted radiation (maximal extent out of the propagation axis as reported on figure 2 (right)) and on the radiated emission profile (not centered on laser axis). We report, on figure 2, the trajectories of accelerated electrons (left side) and the associated spectral extension of the angular distribution (right side).
Figure 2: Electron trajectories at the end of the acceleration process for a0=3.5, Ne0=6x1018cm-3, (left) and angular distribution of spectral intensity of the emitted radiation in qz=0 (the white dashed line symbolizes the propagation axis)
[Cippicia2011] S. Cippicia et al., Nat. Phys. (2011) 7 867
[Rousse2007] A. Rousse et al., Eur. Phys. Journal (2007) 45 391
[Ta Phuoc2006] K. Ta Phuoc, Phys. Rev. Lett. (2006) 97 225002
[André2012] PhD thesis manuscript, 2012, Univ. Paris Sud