Increasing peak brightness is beneficial to various applications of Thomson scattering x-ray source. Higher peak brightness of scattered x-ray pulse demands shorter scattering electron beam realized by beam compression in electron beam-line. In this article, we study the possibility to compress electron beam in a typical S-band normal conducting photo-injector via ballistic bunching, through just adding a short RF linac section right behind the RF gun, so as to improve peak brightness of scattered x-ray pulse. Numerical optimization by ASTRA demonstrations that peak current can increase from ~50 A to >300 A for a 500 pC, 10 ps FWHM electron pulse, while normalized transverse RMS emittance and RMS energy spread increases very little. Correspondingly, the peak brightness of Thomson scattering x-ray source is estimated to increase about 3 times.

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