High energy ion beams (> MeV) generated by intense laser pulses promise to be viable alternatives to conventional ion beam sources due to their unique properties such as high charge [1, 2], low emittance [3,4], compactness and ease of beam delivery [5]. Typically the acceleration is due to the rapid expansion of a laser heated solid foil, but this usually leads to ion beams with large energy spread. Until now, control of the energy spread has only been achieved at the expense of reduced charge and increased complexity [6,7,8]. Radiation pressure acceleration (RPA) provides an alternative route to producing laser-driven monoenergetic ion beams [9,10]. In this paper, we show the interaction of an intense infrared laser with a gaseous hydrogen target can produce proton spectra of small energy spread (σ ∼ 4%), and low background. The scaling of proton energy with the ratio of intensity over density (I/n) indicates that the acceleration is due to the shock generated by radiation-pressure driven hole-boring of the critical surface [11,12]. These are the first high contrast mononenergetic beams that have been theorised from RPA [9,10,13,14,15], and makes them highly desirable for numerous ion beam applications.

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Using a high-pressure carbon-dioxide laser amplifier enriched with the oxygen-18 isotope, we produced a 5-ps, 10-µm pulse of the 1 TW peak power without splitting, which otherwise occurs due to spectral modulation by the rotation structure of the CO(2) amplification band.

Inverse Compton scattering of laser photons by ultrarelativistic electron beam provides polarized x-to γ-ray pulses due to the Doppler blueshifting. Nonlinear electrodynamics in the relativistically intense linearly polarized laser field changes the radiation kinetics established during the Compton interaction. These are due to the induced figure-8 motion, which introduces an overall redshift in the radiation spectrum, with the concomitant emission of higher order harmonics. To experimentally analyze the strong field physics associated with the nonlinear electron-laser interaction, clear modifications to the angular and wavelength distributions of x rays are observed. The relativistic photon wave field is provided by the ps CO 2 laser of peak normalized vector potential of 0.5 < a L < 0.7, which due to the quadratic dependence of the strength of nonlinear phenomena on a L permits sufficient effects not observed in past 2nd harmonic study with a L ≈ 0.3 laser [M. Babzien et al., Phys. Rev. Lett. 96, 054802 (2006)]. The angular spectral characteristics are revealed using K-, L-edge, and high energy attenuation filters. The observation indicates existence of the electrons' longitudinal motion through frequency redshifting understood as the mass shift effect. Thus, the 3rd harmonic radiation has been observed containing on-axis x-ray component that is directly associated with the induced figure-8 motion. These are further supported by an initial evidence of off-axis 2nd harmonic radiation produced in a circularly polarized laser wave field. Total x-ray photon number per pulse, scattered by 65 MeV electron beam of 0.3 nC, at the interaction point is measured to be approximately 10 9 .

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