Producing high-brightness and high-charge(>100 pC) electron bunches at blowout regime requires ultrashort laser pulses with high fluence. The effects of laser pulse heating of the copper photocathode are analyzed in this paper. The electron and lattice temperature is calculated using an improved two-temperature model, and an extended Dowell-Schmerge model is employed to calculate the thermal emittance and quantum efficiency. A timedependent growth of the thermal emittance and the quantum efficiency is observed. For a fixed amount of charge,the projected thermal emittance increases with decreasing laser radius, and this effect should be taken into account in laser optimization at blowout regime. Moreover, laser damage threshold fluence is simulated, showing that the maximum local fluence should be less than 40 mJ/cm^2 to prevent damage to the cathode.
Increasing the peak brightness is beneficial to various applications of the Thomson scattering X-ray source.A higher peak brightness of the scattered X-ray pulse demands a shorter scattering electron beam realized by beam compression in the electron beam-line.In this article,we study the possibility of compressing the 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 the peak brightness of the scattered x-ray pulse.Numerical optimization by ASTRA demonstrates that the peak current can increase from 50 A to>300 A for a 500 pC,10ps FWHM electron pulse,while normalized transverse RMS emittance and RMS energy spread increases very little.Correspondingly,the peak brightness of the Thomson scattering X-ray source is estimated to increase about three times.
