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

We describe a compact THz-pump and X-ray-probe beamline, based on an electron linac, for ultrafast time-resolved diffraction applications. Two high-energy electron ($\gamma>50$) bunches, $5~$ns apart, impinge upon a single-foil or a multifoil radiator and generate THz radiation and X-rays simultaneously. The THz pulse from the first bunch is synchronized to the X-ray beam of the second bunch by using an adjustable optical delay of THz pulse. The peak power of THz radiation from the multifoil radiator is estimated to be $0.14~$GW for a $200~$pC well-optimized electron bunch. GEANT4 simulations show a carbon foil with thickness of $0.5~-~1.0~$mm has the highest yield of $10~-~20~$keV hard X-rays for a $25~$MeV beam, which is approximately $10^3$ photons/(keV pC-electrons) within a few degrees of the polar angle. A carbon multifoil radiator 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 per pulse. The longitudinal time profile of X-ray pulse ranges from $400~-~600~$fs depending on the acceptance angle. The broadening of the time duration of X-ray pulse 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 the low beam current.