Traveling-wave Thomson scattering: a source of X-ray photons with high per-shot yield and tunable energy and bandwidth

Thomson sources, driven by laser-wakefield accelerated (LWFA) electrons or small linacs are compact in size and can provide ultrashort, hard X-ray pulses of high brilliance [1]. However, in head-on (180°) Thomson setups, the finite Rayleigh length at small interaction diameters ultimately prevents further advances in peak brilliance, since it becomes increasingly difficult to avoid the nonlinear Thomson regime.
Effectively, not the laser or electron properties, but the interaction geometry limits the performance of all future Thomson sources that aim for high single-shot photon yields.
In order to circumvent this bottleneck, we present a novel concept, which allows obtaining centimeter to meter long optical undulators, where interaction length and diameter are independent of each other.
With an ultrashort, high-power laser pulse in an oblique angle scattering geometry using tilted pulse fronts, electrons and laser remain overlapped while both beams travel over distances much longer than the Rayleigh length.
Such a Traveling-wave Thomson scattering (TWTS) design [2] is particularly interesting for future LWFA-driven Thomson sources with photon yields per pulse that can be orders of magnitudes beyond current designs. In addition, TWTS offeres unique advantages with respect to the minimum scattered bandwidth, which here is independent of the ultrashort laser pulse duration. Instead, it is controlled by the width of a cylindrically focused laser beam.
The possibility to freely choose a side-scattering angle -- from 0 to over 120° -- enables tuning of the photon energy without having to change the electron energy. For optimized LWFA electron sources operating within a narrow energy range, this opens up access to large parts of the X-ray spectrum and hence many applications.
Towards experimental realization, we show how a Traveling-wave setup has to be implemented. An emphasis is put on the use of varied-line spacing (VLS) gratings for dispersion precompensation of the laser beam at large interaction angles to achieve the required overlap between laser and electrons within the interaction region.

[1] A. D. Debus et al., Linear and non-linear Thomson-scattering x-ray sources driven by conventionally and laser plasma accelerated electrons, Proc. SPIE, Vol. 7359, 735908, 2009
[2] A. D. Debus et al., Traveling-wave Thomson scattering and optical undulators for high-yield EUV and X-ray sources, Applied Physics B: Lasers and Optics 100(1), 61-76, 2010