Petawatt, picosecond laser pulses offer rich opportunities in generating synchrotron x-rays. This paper\nconcentrates on the regimes accessible with the PETAL laser, which is a part of the Laser Megajoule (LMJ)\nfacility. We explore two physically distinct scenarios through Particle-in-Cell simulations. The first one\nrealizes in a dense plasma, such that the period of electron Langmuir oscillations is much shorter than the pulse\nduration. Hallmarks of this regime are longitudinal breakup (ââ?¬Å?self-modulationââ?¬Â) of the picosecond-scale laser\npulse and excitation of a rapidly evolving broken plasma wake. It is found that electron beams with a charge of\nseveral tens of nCcan be obtained, with a quasi-Maxwellian energy distribution extending to a few-GeVlevel.\nIn the second scenario, at lower plasma densities, the pulse is shorter than the electron plasmaperiod. The pulse\nblows out plasma electrons, creating a single accelerating cavity, while injection on the density downramp\ncreates a nC quasi-monoenergetic electron bunch within the cavity. This bunch accelerates without\ndegradation beyond 1 GeV. The x-ray sources in the self-modulated regime offer a high number of photons\n(âË?¼1012) with the slowly decaying energy spectra extending beyond 60 keV. In turn, quasimonoenergetic\ncharacter of the electron beam in the blowout regime results in the synchrotron-like spectra with the critical\nenergy around 10 MeVand a number of photons> 109.Yet, much smaller source duration and transverse size\nincrease the x-ray brilliance by more than an order of magnitude against the self-modulated case, also favoring\nhigh spatial and temporal resolution in x-ray imaging. In all explored cases, accelerated electrons emit synchrotron x-rays of high brilliance, B > 1020 phot Synchrotron sources\ndriven by picosecond kilojoule lasers may thus find an application in x-ray diagnostics on such facilities such\nas the LMJ or National Ignition Facility (NIF)
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