![]() ![]() With this milestone now achieved, the collaboration is ready to tackle our next challenges, starting with the commissioning of a new plasma source." The acceleration therefore becomes sustainable and controlled, producing an unparalleled overall gradient.Įdda Gschwendtner, the AWAKE project leader at CERN, looks to the future with optimism: "The ultimate success of the wakefield technology developed by AWAKE rests on the feasibility of seeding the proton bunch self-modulation. Injection of the electron bunch whose acceleration the experiment is targeting can then be timed perfectly. By injecting this bunch several hundreds of picoseconds before the protons enter the plasma, the front of the proton beam modulates in sync, creating a regular wakefield whose phase can be precisely measured," explains Livio Verra, a physicist in the Lepton Accelerators and Facilities (ABP-LAF) section in the Beams department and the first author of the paper. ![]() "To preserve the reproducibility of the entire modulated proton beam, and thereby its ability to accelerate electrons, we devised a technique to control exactly when the modulation begins: we seed it with an initial electron bunch, different from the one that is targeted for acceleration. ![]() Luckily, the long proton beam from the SPS automatically breaks up into such small bunches when propagating through the plasma (it "self-modulates"), which is what allowed AWAKE to demonstrate the first acceleration of electrons using this technique in 2018. For the electrons to ride the waves of the plasma efficiently, the length of the proton bunch needs to equal the plasma wavelength. A short electron bunch can then be injected into the proton wake to be accelerated to high energy. The proton beam from the SPS is injected into a vapor source containing rubidium, which is transformed into a plasma (a state of ionized gas) by a laser pulse that precedes the proton bunch. To grasp the concept of seeding, it is necessary to delve into the technology behind AWAKE. In a Physical Review Letters paper published on 6 July, the collaboration showed how such a modulation of the proton beam can be controlled by seeding the process with relativistic electrons-a crucial step towards a workable wakefield-based accelerator. To create the appropriate wakefields in the plasma for efficient electron acceleration, the long proton beam extracted towards AWAKE from the CERN Super Proton Synchrotron (SPS) needs to be broken up into smaller bunches in a process known as modulation. AWAKE, on the other hand, is the first experiment to investigate the use of protons, rather than lasers or electron beams, to drive the plasma. While plasma wakefields have been shown to produce acceleration gradients up to 1000 times superior to those achieved with radiofrequency cavities, their use in high-energy and particle physics experiments has been limited by the impractical nature of current techniques, which require the juxtaposition of several plasma sources to achieve high energies. ![]()
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