Andreas Döpp’s collaboration with LOA yields important results
Two papers have been authored recently by Andreas, presenting results of collaboration with the group led by Prof. Victor Malka.
In ‘An all-optical Compton source for single-exposure x-ray imaging’ published in Plasma Physics and Controlled Fusion Control the authors prove that laser wakefield based Compton scattering provides unique properties that may be of interest for ultrafast pump-probe experiments, e.g. for initial fusion research. They present results on a single-pulse scheme that uses a plasma mirror to reflect the drive beam of a laser plasma accelerator and to make it collide with the highly-relativistic electrons in its wake. The accelerator is operated in the self-injection regime, producing quasi-monoenergetic electron beams of around 150 MeV peak energy. Scattering with the intense femtosecond laser pulse leads to the emission of a collimated high energy photon beam. Using continuum-attenuation filters the authors measure significant signal content beyond 100 keV and with simulations estimate a peak photon energy of around 500 keV.
The picture shows a single-exposure x-ray image of a clock with a photography as inset.
The source divergence is about 13 mrad and the pointing stability is 7 mrad. It was demonstrated that the photon yield from the source is sufficiently high to illuminate a centimeter-size sample placed 90 centimeters behind the source, thus obtaining radiographs in a single shot. It was therefore shown that the photon flux of a single shot is sufficient for imaging of macroscopic objects. Beyond applications in fundamental research, the source has also promising properties for medical and industrial applications.
‘Energy boost in laser wakefield accelerators using sharp density transitions’ was published in Physics of Plasmas and presents studies of the possibility of increased electron energy gain using density tapering. The energy gain in laser wakefield accelerators is limited by dephasing between the driving laser pulse and the highly relativistic electrons in its wake. Since this phase depends on both the driver and the cavity length, the effects of dephasing can be mitigated with appropriate tailoring of the plasma density along propagation. Preceding studies have discussed the prospects of continuous phase- locking in the linear wakefield regime.
However, most experiments are performed in the highly non- linear regime and rely on self-guiding of the laser pulse. Due to the complexity of the driver evolution in this regime, it is much more difficult to achieve phase locking. As an alternative, the authors study the scenario of rapid rephasing in sharp density transitions, as was recently demonstrated experimentally. Starting from a phenomenological model, they deduce expressions for the electron energy gain in such density profiles. The results are in accordance with particle-in-cell simulations, and gain estimations are presented for single and multiple stages of rephasing.
An all-optical Compton source for single-exposure x-ray imaging, A Döpp, E Guillaume, C Thaury, J Gautier, I Andriyash, A Lifschitz, V Malka, A Rousse and K Ta Phuoc, Plasma Phys. Control. Fusion 1–5 (2016). doi:10.1088/0741-3335/58/3/034005
Energy boost in laser wakefield accelerators using sharp density transitions, A. Döpp, E. Guillaume, C. Thaury, A. Lifschitz, K. Ta Phuoc and V. Malka, Phys. Plasmas 23, 056702 (2016). http://dx.doi.org/10.1063/1.4946018