The AWAKE collaboration achieves control over the instabilities of a proton bunch in plasma

Ingrid Fadelli is a writer for the website Phys.org.

The Advanced Wakefield Experiment is a large experiment that investigates wakefield acceleration. It is the first research effort in this field to use a bunch of protons as a wakefield driver.

There are many advantages to using a bunch of protons. It allows researchers to maintain a large acceleration over long distances without having to split the accelerator into different parts.

More than 100 engineers and physicists are involved in the AWAKE collaboration. A large team of scientists shows in a paper that seeding the instability can be used to control the self-modulation of a bunch of protons.

Livio Verra is one of the researchers who carried out the study. We rely on the self-modulation instability of the bunch to drive large wakefields. The long bunch is transformed into a train of micro-bunches that drive large wakefields.

The instability of the bunch needs to beseeded so that it can be controlled with high levels of precision. The researchers used a laser pulse to turn on the protons.

Despite their promising results, they found that the method had limitations.

Verra said that the paper shows that the self-modulation can beseeded using wakefields. This is an important milestone for the future of the experiment, as the entire protons self-modulate in a controlled and reproducible way.

The self-modulation process is an instability where the wakefields grow along the protons and along the plasm. The growth of this self-modulation is determined by two factors, the beginning value of the fields and the growth rate.

Verra said that seeding the self-modulation with the preceding electron bunch disentangled the two parameters. The parameters of the seed electron bunch define the wakefields of the instability.

Verra and his colleagues were able to control the growth of the self-modulation of a proton bunch using two different methods. The self-modulation's growth is defined by these two parameters.

A recent work done by a team of researchers shows that the whole bunch of protons in their particle accelerator can self-modulate. This finding could pave the way for a new design in wakefield acceleration, which relies on two separate Plasmas.

One of these plasmas is involved in the self-modulation process. There is a gap region where the witness electron bunch is injected.

The entire group of protons needs to be self-modulated since the second one will be pre- formed. The control of an instability is an important physics result that could be applied to other subjects in the field.

Several studies have been conducted by the AWAKE collaboration on the seeding of the self-modulation instability. They are currently looking at their method's tolerances in regards to spatial and timing alignment between beams.

We are trying to figure out how far from each other in the same position the electron and protons beams can be injected. Verra said something else. How far ahead the electron bunch needs to be injected for seeding to be effective. We will modify the experiment after we study the effect of a density step on the self-modulation and wakefields.

The ultimate goal of the team is to deliver high-quality and high-energy electron bunches. Further steps will be taken in this direction.

More information: Controlled growth of the self-modulation of a relativistic proton bunch in plasma. Physical Review Letters(2022). DOI: 10.1103/PhysRevLett.129.024802. Journal information: Physical Review Letters

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