Popping a Picture of Plasma Bubbles at Low Density
Popping a Picture of Plasma Bubbles at Low Density lead image
Laser-driven plasma accelerators are useful tabletop sources of extremely short and powerful X-ray pulses. They provide a way to accelerate electrons to high energies and are smaller and less expensive than conventional synchrotrons.
As the laser pulse driving such an accelerator propagates through plasma, it forms an electron-voided sphere in its wake. These spheres, or plasma bubbles, are full of positively charged ions that create enormous electric fields, thereby trapping and accelerating electrons to GeV energy.
To examine plasma bubbles with high resolution and at low densities, as well as improve laser-driven plasma accelerators, Chang et al. observed their Faraday rotation patterns.
“We imaged plasma bubbles by backlighting them with an extremely short, polarized light flash,” said author Michael Downer. “The magnetic field from the electron bunch trapped inside the bubble caused the polarization of the portions of the backlighting flash, that passed through the bubble’s densest regions, to rotate.”
While the Faraday rotation pattern technique is not new, the researchers used it in plasma 40 times less dense than previous work. In less dense plasma, they could accelerate electrons to higher energies, over longer distances, and create stronger magnetic fields.
Faraday rotation is generally weaker in less dense plasma, but because the accelerating electrons were at higher energies and more numerous, their stronger magnetic fields created Faraday rotations comparable to those observed in more dense plasma.
The team said the setup can be easily implemented at any existing GeV accelerator. They are currently working on taking Faraday rotation movies, which will capture consecutive images of plasma bubbles and show how they evolve.
Source: “Faraday rotation study of plasma bubbles in GeV wakefield accelerators,” by Yen-Yu Chang, Xiantao Cheng, Andrea Hannasch, Maxwell LaBerge, Joseph M. Shaw, Kathleen Weichman, James Welch, Aaron Bernstein, Watson Henderson, Rafal Zgadzaj, and Michael C. Downer. Physics of Plasmas (2021). The article can be accessed at https://doi.org/10.1063/5.0072262 .
