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STRUCTURING ULTRA-INTENSE LASER PULSES TO ACCELERATE PARTICLES
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Lucas ROVIGE *, Jérôme TOUGUET, Igor ANDRIYASH and Cédric THAURY Laboratoire d ' Optique Appliquée( LOA), CNRS, École polytechnique, ENSTA, Institut Polytechnique de Paris, Palaiseau, France * lucas. rovige @ ensta. fr
https:// doi. org / 10.1051 / photon / 202614056
Structuring a laser pulse in space and time enables arbitrary control over the velocity of its intensity peak, from subluminal to superluminal regimes. Combining spherical aberration from an axiparabola, which extends the focus into an elongated line, with a radial group delay allows the peak to travel at exactly c in a plasma. Such a flying focus can be deployed at ultrahigh intensity to prevent electron dephasing in a laserwakefield accelerator, locking particles in phase with the accelerating field to reach significantly higher energies.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License( https:// creativecommons. org / licenses / by / 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Particle accelerators are central to fundamental discovery through high-energy colliders and secondary light sources such as free-electron lasers and synchrotrons, but also to a wide range of industrial and medical applications, from non-destructive testing to radiotherapy. But as accelerating fields of conventional radio-frequency accelerators are plateauing, laser-plasma acceleration [ 1 ] offers a promising alternative. When an intense laser pulse propagates through a plasma, it drives a high-amplitude plasma wave in its wake: the ponderomotive force expels electrons from the high-intensity region, and the resulting charge-density modulation sustains a longitudinal wakefield. The fields associated with this wake structure can reach three to four orders of magnitude beyond RF cavities, enabling the design of ultra-compact accelerators.
In practice, the acceleration cannot proceed indefinitely due to three main factors: diffraction limits the laser’ s high-intensity region, depletion transfers its energy into the plasma, and dephasing limits the particle energy gain. Dephasing occurs because the laser travels slightly below c in the plasma allowing accelerated electrons to eventually outrun the accelerating phase of the wake. These three limitations, diffraction, depletion, and dephasing, share a common origin: the evolution of the driving laser is tied to how the light itself propagates through the plasma.
Structured light offers a clear path to overcome these limitations. Over the past decade, it has been proposed and subsequently demonstrated that structuring laser pulses in space and time enables the point of peak intensity to move independently
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