Hollow counter-propagating lasers extend the energy of optically accelerated ions

Beams of relativistic ions have many practical applications in industry, medicine, and science, but controlling the trajectories of the accelerated ions remains a major challenge. One promising acceleration method uses an interference pattern, or beat-wave, between two intense laser beams. The rapid variation of light intensity can create huge ponderomotive forces on ions trapped within the beat-wave. For a practical beat-wave optical accelerator, a major limitation exists: To achieve high energies, accelerated ions must remain trapped within the small spot of two counter-propagating, chirped lasers.

A new scheme reported in Physics of Plasmas shows how to overcome this limitation by controlling the optical angular momentum — the “twist” — of two counter-propagating, chirped lasers. The authors of this theoretical framework present a way to drive ions with ponderomotive acceleration over millimeter-scale distances, significantly longer than the light’s wavelength, by beating two chirped beams. “The beating between these lasers provides a traveling accelerating structure that can be with the ion for longer distances, thereby increasing the final energy of the ion,” said Hossein Saberi, who co-authored the paper with two others. The configuration also offers precise control of the beam’s energy by tuning the laser intensities or chirp.

For this axial acceleration, the group presents the superiority of doughnut-shaped beam profiles over Gaussian beams. The hollow central region within the doughnut-hole of these twisted pulses helps to trap the ions during acceleration. By comparison, a Gaussian profile drives the ion beam in the perpendicular directions, drawing ions away from the axial intensity, limiting acceleration. The donut-shaped mode mitigates much of this effect.

The chiral behaviour the group realized with twisted light came as a surprise. When the two beams were given different orbital angular momenta, the heavy particles picked up this momentum as chiral motion along their accelerated paths.

Source: “Ponderomotive beatwave ion acceleration using twisted light,” by Hossein Saberi, Jorge F. Vieira, and Luis O. Silva, Physics of Plasmas (2017). The article can be accessed at https://doi.org/10.1063/1.5005093 .