Controlled Betatron X-Ray Radiation from Tunable Optically Injected Electrons

Abstract

The features of Betatron x-ray emission produced in a laser-plasma accelerator are closely linked to the properties of the relativistic electrons which are at the origin of the radiation. While in interaction regimes explored previously the source was by nature unstable, following the fluctuations of the electron beam, we demonstrate in this Letter the possibility to generate x-ray Betatron radiation with controlled and reproducible features, allowing fine studies of its properties. To do so, Betatron radiation is produced using monoenergetic electrons with tunable energies from a laser-plasma accelerator with colliding pulse injection [J. Faure et al., Nature (London) 444, 737 (2006)]. The presented study provides evidence of the correlations between electrons and x-rays, and the obtained results open significant perspectives toward the production of a stable and controlled femtosecond Betatron x-ray source in the keV range.The continuous progress made over the past decade in the production of ultrashort x-ray radiation has opened novel research horizons with countless applications. The most advanced short pulse x-ray source to date is the free electron laser (FEL), producing x-ray pulse orders of magnitude brighter than any other source [1]. However, at such large facilities, beam time access is inevitably limited and there is therefore an interest in producing compact sources delivering x-ray pulses, even if less intense than in a FEL, sufficiently bright to satisfy the need of many applications. For this reason, the research on complementary femtosecond x-ray sources remains dynamic and novel sources are developed in both synchrotron and laser-plasma interaction communities. Produced in relativistic laser-plasma interaction, the Betatron radiation presents promising features to become a bright and compact femtosecond x-ray source. Demonstrated in 2004, this scheme allowed us to produce for the first time broadband low divergence femtosecond x-ray beams in the keV energy range from laser-plasma interaction [2]. Since then the source has been developed and widely characterized. It can now generate radiation with divergence down to below 10 mrad and in the 10 keV range [3]. However, since its first demonstration, the Betatron radiation has always been produced by self-injected electrons in the bubble or blowout regime [2][3][4][5][6][7]. In that case, with present laser technologies, the laser pulse has to be shrunk in time and space to generate an appropriate bubble structure. These effects, that result from the relativistic self-focusing and self-shortening, are strongly nonlinear and by nature unstable; small fluctuations of any of the experimental parameters will likely lead to important fluctuations of the Betatron radiation properties. Self-injected laser-plasma accelerators do not allow us to easily control the electron and radiation properties. In addition, correlations between the electron beam energy and x-ray properties are difficult to observe because the relevant parameters can not be controlled i...

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“…This result is confirmed by test-particle simulations based on the ideal ion cavity model [20]. With this simplified approach we have identified the basic parameters which fit the Betatron radiation features [21]. In Fig.…”

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“…The the electron spectrum is broad while in Ref. [23] quasi-mono-energetic electron beams were accelerated. In Fig.…”