In a plasma accelerator, particles are accelerated like surfers riding a wave created in the plasma. If the leading bunch and the trailing bunch are not aligned, an instability arises that shakes the trailing bunch. It's as if a surfer on a wave starts wobbling from side to side: their trajectory gets messed up, and they might fall off the wave.
Scientists at SLAC have for the first time observed signs of this instability in experiment E302 at the FACET-II facility. They directed a beam through the plasma and looked at screens after a spectrometer magnet, which separates particles by energy. The screens showed characteristic oscillations of the trailing bunch. To verify the observations, they ran computer simulations that reproduced the same oscillations.
Plasma accelerators promise to make particle accelerators thousands of times more compact. But to use them for precision experiments, the beam must be kept stable. The new diagnostic method will help monitor the instability and search for ways to mitigate it.
A plasma accelerator just a few centimeters long can accelerate electrons to energies that would require hundreds of meters in a conventional accelerator.