CERN’s Long Shutdown 2 (LS2) proved an excellent opportunity for the Advanced Wakefield (AWAKE) experiment to consolidate its plans and prepare for its second run following the successful results from Run 1. We met with Giovanni Zevi Della Porta, the run coordinator, to discuss the lessons carried forward from Run 1 and the main goals of the collaboration.
The main principle of AWAKE is the use of a proton bunch driver beam in a plasma, generating wakefields that can accelerate a trailing electron bunch to high energies. Different ways to create plasma wakefield acceleration exist worldwide, including the use of lasers or electrons to “drive” the wake in the plasma, but what makes AWAKE unique is the use of high-energy protons delivered from CERN’s SPS. Protons from the SPS, travelling with an energy of 400 GeV, are injected into the 10 meter long “plasma cell” of AWAKE, which contains Rubidium gas uniformly heated to 180ºC or higher. These protons are accompanied by a laser pulse that ionizes the Rubidium gas creating a plasma. As this drive beam of positively charged protons travels through the plasma, it causes the otherwise-randomly-distributed negatively charged plasma-electrons to oscillate in a wavelike pattern creating wakefields. An electron bunch is then injected into the oscillating plasma at relatively low energies and “rides” the plasma wave to get accelerated. During AWAKE Run 1 (2016 – 2018), AWAKE successfully demonstrated for the first time that a proton beam can be used to drive wakefields in plasma and subsequently showed that externally injected electrons get accelerated from 20 MeV to 2 GeV over a length of 10 meters, i.e. achieving an average accelerating gradient of 200 MV/m – a gradient much stronger than achieved using conventional technologies.
The benefits of the proton-driven plasma is that “due to the higher stored energy of currently available proton beams (10s of kJ) compared to lasers or electrons (10s of J), we can drive the wakefield for tens or even hundreds of meters” says Zevi Della Porta and continues “which consequently means that we can accelerate the electrons to higher energies within a single plasma cell”. In fact, this proves to be one of the key selling points of proton-driven plasma wakefields, since connecting a chain of short plasma cells and keeping a bunch of particles travelling through them is far from trivial.
A key property of proton-driven plasma acceleration is the so-called self modulation of the high energy proton bunch. This has been demonstrated for the first time in AWAKE Run 1 and this will be further studied during AWAKE Run 2. The self-modulation instability is a key effect that enables to consider existing proton beams as drivers for plasma wakefield acceleration. Available high energy proton beams consist of rather long (∼10 cm) bunches and cannot directly excite plasma waves of the required sub-millimeter length, as the beam frequency spectrum has a negligible component at the plasma frequency. “Thanks to the self-modulation instability, the proton bunches are adjusted to the natural frequency of the plasma. The instability takes the long proton bunch, which would be too long to excite the plasma, and modulates it to smaller micro-bunches separated by the right wavelength to provide the desired excitation”. However, when seeded by random noise, self-modulation happens at a different phase for each shot, so it is impossible to know at which position to inject the electrons so that they are accelerated rather than decelerated.
During Run 1, AWAKE first demonstrated that the self-modulation of the proton bunch can be seeded by a laser ionisation front. To that aim, the ionizing laser used to create Rubidium plasma was timed to the middle of the proton bunch, generating a powerful beam-plasma interaction at a time scale much shorter than the plasma wavelength, thus fixing the phase of the self-modulation.
Experimental setup of the first run (Run 1) of the AWAKE experiment.
The second milestone achieved in Run 1 was the acceleration of externally injected electrons in the plasma wakefield driven by the self-modulated proton bunch. However, during Run 1, the electron beam was spreading along several micro-bunches and only a fraction of the electrons were in the right phase for focusing and accelerating through the plasma. This poses one of the challenges for AWAKE’s Run 2.
Following the successful Run 1, the goal for the AWAKE team for Run 2 is to accelerate the electrons to with a high energy gradienties (1GV/m) while preserving the beam quality. Run 2 also aims to show that the process is scalable so that by the end of this program the AWAKE-scheme technology could be used for first particle physics applications. As Zevi Della Porta explains: “The goal of the second run is to move beyond measuring the achieved higher acceleration, but and also to understand control the charge, energy spread and emittance of the accelerated beam as well as demonstrate the reproducibility and scalability of our experiment”; as a good quality of the beam would pave the way for future application of this technique.
In order to achieve electron acceleration to even higher energies with good beam quality and high capture rate, the AWAKE experiment will be modified to include two plasma cells and two electron beams. The first plasma cell and electron beam are used to have the proton beam self-modulated along its entire bunch length. The second plasma cell is used to accelerate externally injected electrons from the second electron beam system in the plasma wakefields. Zevi Della Porta explains that “to reach the Run 2 physics goals, and keeping in mind possible applications of our beams in fixed-target experiments, we have defined a sequence of milestones”.
The first stage of Run 2, which is being pursued in the ongoing proton run, is to demonstrate the seeded self-modulation of the entire proton bunch. This is achieved by injecting the laser well ahead of the proton bunch so that the entire proton bunch travels in plasma, and injecting the electron bunch just in front of the proton bunch. The electron bunch generates a seed wakefield strong enough to determine the starting point of the proton bunch self-modulation, i.e. its phase.
Design of the foreseen experimental setup for the second run (Run 2) of the AWAKE experiment.
The second stage of Run 2 is to demonstrate the stabilization of the micro-bunches with a density step in the first plasma cell. “For proton-driven wakefields to work on larger scales, we should ensure that the protons create powerful wakefields during their entire trip through the plasma and not only for the first few meters. Keeping the wakefield high for the full distance would allow thinking of extending to longer distances and thus energies”. Simulations have already shown the role of accurately choosing the plasma density profile in order to ensure that high wakefields will be maintained throughout the medium. A new plasma cell system with density steps has been prototyped in EHN1 and first tests in the lab show reliable performance and control.
The third stage of Run 2, relying on the completion of the first two, is to ensure low emittance and good beam quality after electron acceleration in the 2nd plasma cell. “The goal is to preserve electron beam quality throughout the acceleration process, so that the final emittance is at the 10 mm mrad level”. To achieve that, the electron bunch that will be injected in the 2nd plasma source should be short enough to fit into half of a plasma wavelength so that all electrons are captured in the focusing and accelerating phase, and intense enough to cause beam-loading of the wakefield resulting in uniform acceleration. To achieve these properties, a higher energy (150 MeV) compared to Run 1 (20 MeV) will be used. A new 150 MeV electron source and a new transfer line will be needed to meet the requirements of the third stage of Run 2. “The design of the electron system is not trivial and it gets more tricky when you want to match the electron beam transverse and longitudinal properties to the plasma properties”.
While the first two stages can be completed in the existing AWAKE facility, adding a second plasma cell and a new electron beam system for AWAKE poses a demand for more space. Currently AWAKE uses only the very upstream area of the old CERN’s Neutrinos to Gran Sasso (CNGS) target area and has placed a request for using the rest of it for installing the second plasma cell and the electron beam systems as an integrated part of their experimental programme. Dismantling the CNGS area is a timely process that could take up to 18 months, with a proposed start at the end of 2024 profiting from CERN’s Long Shutdown 3 and would require removing the old CNGS target and a few thousand concrete blocks. “During the dismantling works the commission of the new electron source, tests of our vapor sources as well as the other individual components can be performed before being installed. In fact, for the third stage of Run 2, most of AWAKE will be dismantled and rebuilt completely”.
Finally, there are plans to install scalable plasma sources that would allow acceleration over longer distances. This is often referred to as the fourth step of Run 2 though Zevi Della Porta explains that it is a parallel ongoing effort as these source technologies are currently already in the development stage. The technique AWAKE currently uses for creating the plasma, based on shining a laser through the vapor to ionize it uniformly, is not efficient over longer distances and cannot support O(100) meters plasma cells. The two alternative options currently explored are a “Helicon” source, where ionization is driven by low-frequency EM waves generated by RF antennas, and a “discharge” source, where ionization is driven by a very high-current arc. Coming up with a reliable approach for scalable plasma up to 100m is another key for the long-term applications of AWAKE.
Coming back to the 2021 proton run, in addition to electron seeding AWAKE also plans to study other effects such as the hosing instability. This could lead to the growth of transverse perturbations on the beam due to the nonlinear coupling of the beam electrons to the plasma electrons at the edge of the plasma channel through which the beam propagates. “Simulations predict that this phenomenon/instability emerges but they don’t tell us the exact point at which this happens. It could be after 1, 10 or 100 meters. Run 1 of AWAKE taught us that this instability doesn’t rise unless it is artificially induced at the beginning of the plasma”. This brings some confidence to the team that the hosing instability is not a showstopper for the length and plasma density on which they are working. Another effect AWAKE plans to study in the 2021 run is the interaction between the front and back of a proton bunch when they are self-modulated with different phases. This process could take place in a future second plasma cell, when only part of the bunch is modulated in the first cell. Zevi Della Porta explains that when experimenting on a novel acceleration technique “For every effect that we observe we want to be able to turn it on and off as this is an essential part of the experimental process”.
Run 1 offered some of the necessary and critical steps to move to a larger system. These experimental efforts are constantly supported by detailed simulations developed by the AWAKE Simulation Coordination group. Exploring these new frontiers in accelerator technology, calls for a continuous interplay between our simulations and experimental results in order to account for all tiny effects that appear in laboratory conditions.
LS2 has also been a very fruitful period for the AWAKE collaboration, in preparation for the 2021 proton run, allowing to test and tune a number of parameters in their experimental setup including the electron and laser sources. A number of small laser experiments, allowed to study how to most effectively ionize the medium over the distance of 10 meters, the optimum intensity as well as the choice between a broad or narrow wavelength in the laser. A lot of effort was also put in working with the electron beam, handling it more reliably and optimizing its characterization (including its use as a diagnostic tool during Run 2). Interestingly, a number of collaborators also worked in developing machine-learning techniques for controlling the beam optics and trajectory that prove to be critical in achieving long-term high-quality acceleration of electron bunches.
After the preparation and experiments of LS2, the proton run started in July and already achieved excellent results in the first few weeks of proton data. Zevi Della Porta explains: "The experiment worked incredibly well, and within the first few days of the summer run we had re-established Run 1-like self-modulation. Most importantly, by the middle of the second week we were able to clearly observe electron-seeding of proton bunch self-modulation. We knew this should happen in principle, but we were not completely sure that our existing electron beam would be intense enough to perturb the proton bunch significantly. And, due to Covid restrictions, the installation of a dedicated beam screen at the beginning of the plasma cell had been delayed, so aligning electrons and protons was particularly challenging. This is preliminary, but in fact we found that, once the electron/proton alignment was properly set, even our lowest-charge beam was able to generate electron-seeding. So we are now in a situation where we can fully study this process, and also move to other goals of the 2021 run, which will use electron seeding to generate hosing and to seed with different phases the front and back of the proton bunch".
“AWAKE has a very well defined physics program and a well-defined short, mid- and long-term plan. After the successful demonstration of Run 2, the AWAKE technology could be used already in the mid-term future for first fixed target experiments searching for new physics beyond the Standard Model, using e.g. SPS protons as drivers”. A long-term step could be to couple AWAKE with a proton collider and develop an electron-proton collider programme while collisions between electrons and positrons would be the ultimate step. The success of Run 1 and first steps in Run 2 showcase that the AWAKE scheme offers great potential for future applications and the recent update of the European Strategy for Particle Physics raised support for further development of this technology at CERN in collaboration with its global partners.