Experimental design of optical free-electron lasers in the traveling-wave Thomson-scattering geometry

Traveling-Wave Thomson-Scattering (TWTS) realizes optical free-electron lasers (OFEL) from the extreme ultraviolet to the X-ray range with existing electron accelerators and high-power laser systems. In TWTS ultrashort laser pulses and relativistic electron bunches are utilized in a side-scattering geometry where laser pulse and electron bunch direction of motion enclose an interaction angle. The laser electric field thereby provides the optical undulator field for radiation generation. When the electrons traverse the laser beam cross-section, TWTS provides continuous overlap of electrons and laser pulse by employing a laser pulse-front tilt. Long optical undulators are thus realized in TWTS allowing for microbunching of the electron beam leading to the realization of TWTS OFELs. We present the scaling laws for the electron beam and laser pulse requirements to operate TWTS OFELs and show with example scenarios that TWTS OFELs can be realized with existing radio-frequency electron accelerators such as ELBE at HZDR as well as laser-wakefield accelerated electrons. We detail the necessary equipment in a TWTS OFEL experiment and discuss how current experimental limitations affect the design of TWTS OFEL setups.