The AWAKE collaboration manages to control the instabilities of a group of protons in plasma

The Advanced WAKEfield Experiment (AWAKE) is a large experiment conducted at CERN that investigates the acceleration of the plasma wake field. It is the first research effort in this field to use a pool of relativistic protons as a driver of plasma wake fields to accelerate witness electrons to high energies.

the use of a proton heap has numerous advantages for plasma acceleration experiments. In particular, it allows researchers to maintain a large acceleration gradient over long distances, without having to divide the accelerator into several different sections.

The AWAKE collaboration, the group of researchers involved in the AWAKE experiment, includes more than 100 engineers and physicists from 23 different institutes around the world. In a recent article published in Physical Review Lettersthis great team of scientists shows that the self-modulation of a group of protons can be controlled by seeding instability.

“The available proton pools are much longer than the typical plasma wavelength,” Livio Verra, one of the researchers who carried out the study, told Phys.org. “To generate high-amplitude wakefields, we rely on the self-modulation instability of the cluster in the plasma. This process transforms the long cluster into a train of microclusters, spaced by the period of the wakefields, that drive high-amplitude wakefields.”

To ensure that the proton cluster self-modulation process is reproducible and can be controlled with high levels of precision, it is necessary to “seed” the cluster instability. In their previous studies, the researchers achieved this by igniting the plasma within the proton pool using a laser pulse.

Despite their promising results, they found that this method had the important limitation of modulating only a fraction of the proton group.

“In our new paper, we show that self-modulation can be seeded using the wake fields driven by a previous electron pool,” Verra explained. “In this case, the entire group of protons self-modulates in a controlled and reproducible way, which is an important milestone for the future of the experiment.”

In the context of proton-driven plasma wake field accelerators, the self-modulation process is essentially an instability, where the amplitude of the wake fields in the plasma grows along the proton pool and across the plasma. The growth of this self-modulation is determined by two key parameters, namely the amplitude of the seed wakefields, which defines the initial value of the fields, and the growth ratewhich defines how fast the instability grows.

“By seeding self-modulation with the electron pool above, we disentangled these two parameters, which other seeding methods always correlate with,” Verra said. “This means that the seed electron pool parameters define the amplitude of the seed wake fields and the proton pool parameters define the instability growth rate.”

Using the approach presented in their paper, Verra and colleagues were able to independently control the self-modulation growth of a proton cluster in CERN’s plasma particle accelerator using two different “knobs.” These are essentially the two key parameters that define the growth of self-modulation.

Recent work by this team of researchers shows that the entire pool of protons in their plasma particle accelerator self-modulates in a reproducible manner. This crucial finding could pave the way for a new experimental design in protons. acceleration of the plasma trail fieldwhich are based on two separate plasmas.

One of these plasmas would be specifically involved in the self-modulation process, while the other in the acceleration of electrons. These two plasmas will be separated by a separation region, where the injection of the group of control electrons takes place.

“Since the second plasma will form beforehand, the entire proton pool must self-modulate,” Verra said. “Furthermore, showing control of an instability is an important independent physics result, which could be extended to other particular topics in plasma physics.”

Since early 2022, the AWAKE collaboration has been conducting several studies focused on seeding self-modulating instability in plasma using an electron cluster. Currently, they are specifically exploring the tolerances of their method in terms of spatial and temporal alignment between beams.

“The questions we’re trying to address are: how far apart in a transverse position can electron and proton beams be injected, without destructive instabilities occurring?” Vera added. “And: how far should the electron pool be injected from the proton pool to seed effectively? In 2023-2024, we’re going to study the effect of a plasma density step on self-modulation and on the amplitude of the wakefields, and then we will modify the experiment to accommodate the second plasma for the acceleration experiment.

The team’s ultimate goal will be to deliver high-quality, high-energy electron clusters into particle physics experiments. His next studies will take new steps in this direction.

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