THz-Pump and X-Ray-Probe Sources Based on an Electron Linac

摘要

We describe a compact THz-pump and X-ray-probe beamline, based on an electronlinac, for ultrafast time-resolved diffraction applications. Two high-energyelectron ($\gamma>50$) bunches, $5~$ns apart, impinge upon a single-foil or amultifoil radiator and generate THz radiation and X-rays simultaneously. TheTHz pulse from the first bunch is synchronized to the X-ray beam of the secondbunch by using an adjustable optical delay of THz pulse. The peak power of THzradiation from the multifoil radiator is estimated to be $0.14~$GW for a$200~$pC well-optimized electron bunch. GEANT4 simulations show a carbon foilwith thickness of $0.5~-~1.0~$mm has the highest yield of $10~-~20~$keV hardX-rays for a $25~$MeV beam, which is approximately $10^3$ photons/(keVpC-electrons) within a few degrees of the polar angle. A carbon multifoilradiator with $35$ foils ($25~$$\mu m~$thick each) can generate close to $10^3$hard X-rays/(keV pC-electrons) within a $2^\circ$ acceptance angle. With$200~$pC charge and $100~$Hz repetition rate, we can generate $10^7$ X-rays per$1~$keV energy bin per second or $10^5$ X-rays per $1~$keV energy bin perpulse. The longitudinal time profile of X-ray pulse ranges from $400~-~600~$fsdepending on the acceptance angle. The broadening of the time duration of X-raypulse is observed owing to its diverging effect. A double-crystal monochromator(DCM) will be used to select and transport the desired X-rays to the sample.The heating of the radiators by an electron beam is negligible because of thelow beam current.