Filamentation observed in Wakefield acceleration

Researchers at the AWAKE experiment at CERN in Switzerland are studying a potential way to accelerate electrons by burning a group of protons in a plasma. However, one challenge facing this so-called wake-field acceleration is that instabilities can form in the proton beam, degrading the beam structure and reducing its acceleration quality and length. To investigate this behavior, Livio Verra and his colleagues in the AWAKE Collaboration injected a long bunch of protons into a plasma and observed the fragmentation of the bunch into narrow, dense threads (1). The measurements indicate the conditions under which this so-called filamentation instability originates.

The researchers began their experiments by generating a plasma in a long, thin glass tube filled with argon gas. At the end of the tube, they injected a stream of high-intensity proton beams, delivered by CERN’s Super Proton Synchrotron, in which each proton had an energy of 400 GeV. They placed metal screens in the beam path before and after the plasma tube and used them to capture snapshots of the proton beams. From these cross-sectional profiles, the team was able to identify the structural changes within the cluster caused by the interaction with the plasma.

Verra and his colleagues noticed narrow points appearing near the center of the bundle, indicating that it had separated into strands. They determined that this filamentation occurred for proton bundles that had a radius 1.5 times the plasma skin depth—a parameter that describes how deep radiation can penetrate plasmas. The results indicate that instability can be avoided by narrowing the cluster radius below this threshold value.

– Rachel Berkowitz

Rachel Berkowitz is a corresponding editor for Physics Magazine based in Vancouver, Canada.

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