The AWAKE experiment at CERN is advancing plasma wakefield acceleration using a proton beam to generate wakefields for the acceleration of electrons. Edda Gschwendtner, project leader of AWAKE, discusses this innovative approach.
For the past century, conventional acceleration technology has remained mostly unchanged, utilising radio frequency (RF) cavities in a vacuum to accelerate charged particles via oscillating electromagnetic fields. Since its invention, this method has been integral to accelerators, but it has limitations, especially in linear colliders.
The accelerating gradient, which measures how much particles can be accelerated per metre, is a crucial parameter. At CERN, conventional technologies achieve gradients around 5 MeV/m in the Large Hadron Collider (LHC), with the Compact Linear Collider (CLIC) reaching about 100 MeV/m. Exceeding these gradients risks breakdowns in the structures, limiting further advances.
Plasma wakefield acceleration offers a revolutionary alternative. Plasma, the fourth state of matter, consists of ionised gas, which means it is already broken down.
The acceleration process in plasma wakefield acceleration works differently from conventional methods. An analogy that captures this concept well is that of a lake (the plasma) and a boat (the drive beam). As the boat moves through the lake, it generates wakefields behind it. Surfers (the witness beam) can then ‘ride’ these wakefields and be accelerated. Essentially, in this setup, one beam produces the wakefields while another beam is accelerated by them. In contrast, conventional acceleration typically involves only an electric field acting on a single beam.
The AWAKE experiment
The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) is an advanced wakefield acceleration experiment, evolving from a proof-of-concept study to exploring applications of this acceleration technology for particle physics experiments. Our experiment at CERN uses a proton beam as the ‘boat’ to drive these wakefields, a unique approach worldwide. This method enables us to accelerate an electron beam using the wakefields created by the proton beam.
Using protons as a drive beam presents several advantages over electron or laser beams, particularly for particle physics applications. Our ultimate aim is to accelerate electrons to extremely high energies at the tera-electronvolt (TeV) level. It’s crucial to effectively transfer energy from the driving source – our proton beam – to the electron beam to achieve this.
Utilising an electron or laser beam as a drive beam presents challenges, as they possess much lower energy levels, necessitating multiple stages of plasma sources for achieving desirable energies. This complexity is bypassed when using a proton beam from CERN’s accelerator complex, e.g. the Super Proton Synchrotron (SPS), which already offers high energy. With this setup, we can efficiently transfer energy using a single long plasma source, significantly simplifying the acceleration process. Overall, leveraging a proton beam simplifies our experimental work and helps to overcome the intricate challenges of staging, which have only been realised on a proof-of-concept level with laser drivers.
Anticipated outcomes
The AWAKE experiment follows a comprehensive programme that has evolved over nearly a decade. The initial phase of the experiment, conducted between 2016 and 2018, successfully established proof of concept. The first milestone demonstrated the capability of using a proton beam as a drive beam to generate the wakefields. Building upon this, the second milestone involved the injection of an electron beam into these wakefields, achieving significant acceleration to 2 GeV over a distance of 10m, which was consistent with our expectations.
The project is now in its second phase, AWAKE Run 2, where our goals include demonstrating that the accelerated electron beams can achieve high energies while maintaining quality control. Additionally, we aim to show that by extending the length of the plasma sources, we can accelerate particles to even higher energies.
This second phase was initiated in 2021 and is scheduled to extend until 2034 in accordance with CERN’s timeline. During the forthcoming shutdown periods in 2027 and 2028, we will undertake necessary upgrades to our experimental apparatus.
Our objectives for this second phase are twofold: first, to achieve an acceleration between 4 and 10 GeV over 10m, and second, to control the beam emittance, targeting normalised emittance values between 2 and 30 millimeter-milliradian. Successfully attaining these goals will signify a comprehensive understanding of the acceleration process.
Additionally, we intend to implement a scalable plasma source that will facilitate the attainment of higher energies, aiming for over 10 GeV over extended distances, potentially reaching 20m. By the conclusion of this phase in 2034, we aspire to propose initial particle physics experiments leveraging high-energy electron beams, enabling fixed-target experiments and advancing the field of particle physics.
Innovative technologies and experimental setup
The plasma source is the heart of the experiment, serving as the medium for the acceleration process. Our facility features the longest plasma source in the world, measuring 10m in comparison to the typical plasma sources that range from a few millimetres to a meter.
This proton beam originates from the CERN Super Proton Synchrotron (SPS), which acts as the pre-accelerator for the LHC. The beam is extracted from the SPS and travels along a 750-meter beamline to reach the AWAKE experiment.
Additionally, we require an electron beam that will be accelerated in the plasma. Again, we greatly benefited from CERN’s expertise and technology. Our electron source employs a cathode that emits electrons when hit by a laser. This system is similar to the technology developed for CERN’s linear collider project, CLIC, with which we collaborated closely.
The electron source system generates electrons at a low energy of around 20 MeV. These electrons are injected into the plasma source for acceleration.
Additionally, we incorporate a laser beam that, while not directly involved in acceleration, is crucial for forming the plasma. The plasma source is a rubidium vapour source. To convert this vapour into plasma, we inject this laser beam to ionise it.
Coordinating these three beams – proton, electron, and laser – temporal and spatial alignment is necessary to ensure they arrive simultaneously, which presents significant challenges.
Following the beams’ passage through the plasma, we utilise extensive diagnostics to assess our results. For energy measurement of accelerated electrons, we employ an electron spectrometer featuring quadrupoles and magnets. This spectrometer’s bending degree in a dipole component is inversely related to particle energy, allowing us to determine their energies accurately.
The design of the plasma source is intricate, operating at very high temperatures to maintain the necessary density along the length of 10m. The system has a continuous rubidium flow, so we must ensure that rubidium does not escape the plasma source and flows through the vacuum tubes to other equipment. Implementing a simple window is not feasible, as the incoming laser would damage it by drilling a hole.
Therefore, we have developed a complex system at both ends of the 10-meter-long plasma source, referred to as expansion volumes. These volumes are kept very cold, enabling the rubidium vapour to condense into a liquid state and adhere to the walls, thus preventing any vapour from escaping.
Addressing the challenges of plasma wakefield acceleration
Several key parameters must be demonstrated before constructing linear colliders based on this technology. One of the most critical is the quality of the beam produced after traversing the plasma, specifically aiming for low emittance, which is essential for collider applications. High repetition rates and luminosity are crucial in collider physics. Achieving luminosity hinges on having beams with small emittance while maintaining a high repetition rate to maximise collision events, regardless of the driving technology employed, be it lasers, electrons, or protons.
Stability and reproducibility of the acceleration process are also vital considerations. Recent laser-driven plasma wakefield experiments at DESY, Hamburg, Germany, showcased excellent stability, with continuous operation over 24 hours enabled by a feedback system. Ongoing efforts are necessary to ensure that experimental setups or colliders maintain consistent beam quality over long periods.
A notable challenge in plasma wakefield acceleration is the acceleration of positrons, particularly for electron-positron colliders like Higgs factories. While experiments at FACET, SLAC, USA have shown promise in this area, the emittance for positrons remains a concern that requires further exploration.
Seeded self-modulation
An additional challenge in the AWAKE experiment was in using proton drive beams from the SPS, which are at the order of 6cm long. However, to drive strong wakefields, the length of the drive beam should be at the order of the plasma wavelength, which is about 1mm in AWAKE. This discrepancy makes it difficult to produce strong wakefield amplitudes.
To tackle this issue, we profit from the ‘self-modulation instability’, where the long proton beam entering the plasma source modulates into small micro-bunches. The distance between the micro-bunches corresponds to the plasma wavelength, and these micro-bunches resonantly drive strong wakefields.
A significant achievement in the AWAKE experiment was demonstrating that this self-modulation effectively allows the proton beam to act as a drive beam, generating strong wakefields essential for accelerating particles to higher energies.
Furthermore, we have developed a method of controlling this self-modulation, termed ‘seeded self-modulation.’ This allows us to synchronise the electron beam injection with the generated micro-bunches, optimising acceleration conditions.
In the ongoing and future phases of our experimental work, we will continually rely on the principles of self-modulation while taking advantage of the enhanced control that we have achieved. Moreover, we have developed various advanced technologies aimed at improving our ability to manage and characterise the seeded self-modulation process more effectively.
An international effort
AWAKE is an international collaboration involving 19 partnering institutes from across the globe. Significant contributions come from Germany and the United Kingdom, alongside institutions from Portugal, Sweden, Wisconsin (USA), and various institutes in Europe. CERN acts as the host organisation, providing the infrastructure, experimental area, and beams and actively participating in the experiment itself.
AWAKE typically involves around 100 authors in the publications, which fosters strong relationships among team members. A core team remains at CERN, while collaborators work from their respective institutes, with some focusing on simulations and others dedicated to developing equipment that is later transported to CERN for installation.
Within CERN, the entire accelerator sector contributes to the project. Expertise in vacuum technologies, RF techniques, computing, cooling and ventilation, beam instrumentation and many more is crucial, making it an extremely cooperative effort. Clear definitions of interfaces between different institutes are maintained to ensure smooth transitions during handovers of responsibilities or installations of components.
CERN’s beam instrumentation group exemplifies how these partnerships enhance research quality. Developing specialised beam instrumentation for the AWAKE experiment involves not only CERN’s internal efforts but also significant contributions from collaborating institutes, particularly those in the UK.
This collaboration is mutually beneficial, as institutes bring in specialised expertise. By working alongside the diverse CERN team, they gain valuable insights, enriching the research environment. This exchange of knowledge fosters an enriching learning experience, supporting the collective goals of all parties involved in the AWAKE initiative.
The potential for AWAKE
Plasma wakefield acceleration offers a significant advantage in achieving high acceleration gradients, which allows particles to be accelerated much stronger over a certain distance. This capability is particularly attractive for linear colliders, where particles move only once through an accelerating element. To reach higher particle energies, linear accelerators could either be lengthened or could use plasma wakefield acceleration technologies providing strong accelerating gradients.
The first application of the AWAKE technology could, in principle, follow AWAKE’s second phase, when the scalability and the acceleration to high energies while controlling the beam quality will have been demonstrated. Simulations suggest that electron energies up to 200 GeV can be achieved.
One promising proposal includes utilising the electron beam produced by the AWAKE technology to investigate dark photons, which are crucial for understanding dark matter.
Another exciting idea involves colliding the electron beam with a high-intensity laser beam. This would allow for exploration in the realm of strong field quantum electrodynamics (QED) and pave the way for investigations into new energy regions, an area currently devoid of competitors.
In the long term, the development of a Higgs factory using plasma wakefield acceleration is under consideration, with the aim of producing an electron and positron collider at a 250 GeV centre-of-mass.
Overall, as research progresses, the applicability of plasma wakefield technology may extend beyond high-energy physics, potentially benefiting fixed-target experiments and free electron lasers (FELs) that utilise accelerated electron beams to generate X-rays.
Please note, this article will also appear in the 21st edition of our quarterly publication.
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