Scaling EUV and X-ray Thomson Sources to Optical Free-Electron Laser Operation with Traveling-Wave Thomson Scattering

Abstract
Traveling-Wave Thomson-Scattering (TWTS) provides optical undulators with hundreds to thousands of undulator periods from high-power, pulse-front tilted lasers pulses. These allow to realize optical free-electron lasers (OFELs) with state-of-the-art technology in electron accelerators and laser systems in TWTS. The talk focuses on experimental realization and the combination of TWTS and laser-wakefield acceleration allowing for ultra-compact, inherently synchronized and highly brilliant light sources.

Summary
Traveling-Wave Thomson-Scattering (TWTS) employs a side-scattering geometry where laser and electron propagation direction of motion enclose the interaction angle ϕ. Tilting the laser pulse front with respect to the wave front by half the interaction angle optimizes electron and laser pulse overlap by compensating the spatial offset between electrons and the laser pulse-front at the beginning of the interaction when the electrons are far from the laser pulse axis. The laser pulse-front tilt ensures continuous overlap over the whole laser pulse width while the electrons cross the laser beam path. TWTS thus allows to control the interaction length by the laser pulse width rather than laser pulse duration. Utilizing wide, petawatt class laser pulses allows to realize thousands of optical undulator periods.

The photon yield of TWTS sources can therefore be orders of magnitude higher than that of classic head-on Thomson sources. TWTS thereby remains compact and provides narrowband and ultra-short ultraviolet to γ-ray radiation pulses just as classic Thomson sources.

Two key features of TWTS allow for the realization of optical free-electron lasers (OFELs). First, it provides optical undulators with lengths required for microbunching and thus coherent radiation amplification. Second, the variability in interaction angle allows to control the electron beam quality requirements for a target radiation wavelength. This is used to reduce the electron beam quality requirements to a level technically feasible today. Small interaction angle scenarios (ϕ∼10∘) typically yield the best trade-off between requirements on electron beam quality, laser power and laser intensity stability.

In the talk we will show that TWTS OFELs emitting extreme ultraviolet radiation are realizable today with existing technology for electron accelerators and laser systems. Especially the ultra-low emittance of laser wakefield accelerated electron beams can be exploited to compensate for their one percent level energy spreads. We discuss an experimental setup to generate the tilted TWTS laser pulses. The method presented provides dispersion compensation, required due to angular dispersion, and is especially relevant when building compact, high-yield hard X-ray TWTS sources in large interaction angle setups.