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Demonstration of a 10-metre-long discharge plasma source for plasma wakefield acceleration

This paper reports the successful experimental characterization and beam-based validation of a 10-metre-long pulsed-DC discharge plasma source as a reproducible and uniform alternative to laser-ionised vapour for driving plasma wakefield acceleration in the AWAKE experiment.

Original authors: C. Amoedo, N. Lopes, N. E. Torrado, F. Silva, P. Muggli, L. Verra, M. Turner, G. Zevi Della Porta, M. Bergamaschi, A. Clairembaud, J. Mezger, F. Pannell, N. Z. van Gils, E. Gschwendtner, A. Sublet, th

Published 2026-09-09

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CC BY 4.0

Original authors: C. Amoedo, N. Lopes, N. E. Torrado, F. Silva, P. Muggli, L. Verra, M. Turner, G. Zevi Della Porta, M. Bergamaschi, A. Clairembaud, J. Mezger, F. Pannell, N. Z. van Gils, E. Gschwendtner, A. Sublet, the AWAKE Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

To understand the ambition behind this work, one must first grasp the challenge of building the next generation of particle accelerators. Traditional machines, like the massive rings that circle the Earth, rely on radio waves to push particles forward. While effective, these waves can only provide a certain amount of push before they break down, forcing scientists to build machines that are hundreds of kilometres long to reach the highest energies needed to probe the fundamental secrets of the universe. A more compact alternative involves using plasma, a hot, electrically charged gas, to create wakefields. Imagine a boat moving through water; it leaves a trail of waves behind it. If a particle bunch travels through plasma, it creates a similar wake. If a second bunch of particles rides this wake at just the right moment, it can be accelerated to incredible speeds over a very short distance. The catch is that to reach the energy levels required for high-energy physics, this acceleration needs to happen over a much longer stretch of plasma than has ever been successfully created in a single, uniform tube.

The experiment described in this paper, conducted at CERN in Geneva, tackles this specific hurdle. The researchers, part of the AWAKE collaboration, have successfully tested a new method for creating a ten-metre-long column of plasma that is stable and uniform enough to be used for these high-speed accelerations. For years, the team has relied on a laser to ionise rubidium vapour, turning it into plasma. While this method works well for shorter distances, the laser itself becomes a limiting factor when trying to extend the plasma column to the tens or hundreds of metres required for future machines. The team needed a different approach that could scale up without losing the precise control needed to keep the particles on track. They turned to an electrical discharge, a process similar to how a lightning bolt forms, but carefully controlled and repeated.

The researchers built a ten-metre-long glass tube and filled it with noble gases, specifically helium, argon, and xenon. Instead of using a laser, they applied a high-voltage electrical pulse to the gas. This pulse strips electrons from the gas atoms, instantly transforming the gas into plasma. The key to their success was a "double-pulse" technique. The first pulse acts as a spark to ignite the gas, while a second, slightly longer pulse heats the plasma to a steady state. This two-step process allows the plasma to form reliably and consistently, event after event. The team ran this system thousands of times, testing how well it performed with each of the three different gases. They found that the electrical discharge could consistently produce plasma densities suitable for acceleration, ranging from one hundred trillion to one quadrillion electrons per cubic centimetre.

To verify that the plasma was behaving exactly as needed, the researchers did not just look at the gas; they used the particle beam itself as a measuring tool. They fired high-energy proton bunches from the Super Proton Synchrotron through the plasma. When these protons travel through the plasma, they naturally break up into a train of tiny micro-bunches, a phenomenon known as self-modulation. The speed at which these micro-bunches form is directly tied to the density of the plasma. By measuring the frequency of this pattern, the scientists could calculate the density of the plasma inside the tube. They compared these beam-based measurements with independent laboratory measurements taken using laser interferometry, a technique that measures how light waves shift as they pass through the gas. The two methods agreed remarkably well, confirming that the electrical discharge was creating a plasma with the correct density.

Perhaps the most critical finding was the consistency of the system. For a particle accelerator to work, the plasma cannot fluctuate wildly from one moment to the next; it must be reproducible. The team measured the variation in the plasma density from one electrical discharge to the next and found it to be less than one percent. This level of stability is comparable to the performance of the laser-based system they are replacing, proving that the electrical discharge is a viable alternative. The system worked reliably over a period of three weeks and more than twenty thousand discharge cycles, showing that it can endure the rigours of continuous operation. Furthermore, the researchers demonstrated that by simply adjusting the timing between the electrical pulse and the arrival of the proton beam, they could access different plasma densities within the same tube, offering a flexible way to tune the experiment without changing the hardware.

This work establishes that a ten-metre-long plasma source driven by electrical discharge is not only possible but also robust and reproducible. It opens the door to scaling these sources to even greater lengths, potentially reaching the hundreds of metres needed for future high-energy colliders. While the current design is tailored to the specific needs of the AWAKE experiment, the underlying principle of using electrical discharges to create long, uniform plasma columns could be applied to other plasma-based acceleration projects. The success of this ten-metre prototype suggests that the path toward building much larger, more powerful, and more compact particle accelerators is becoming clearer, moving from theoretical concepts to tangible, working technology.

Technical Summary: Demonstration of a 10-metre-long discharge plasma source for plasma wakefield acceleration

Problem Statement
Scaling plasma-based wakefield accelerators to energies relevant for high-energy physics remains a central challenge. While plasma gradients are orders of magnitude higher than conventional radio-frequency structures, achieving multi-TeV energies typically requires staging multiple acceleration modules. This staging presents significant experimental hurdles regarding beam quality preservation and alignment. An alternative approach involves using high-energy driver beams (such as the 400 GeV proton bunches from CERN's Super Proton Synchrotron) capable of sustaining accelerating interactions over much longer distances within a single stage. However, this shifts the scalability challenge to the development of plasma sources that can extend to tens of metres while maintaining strict longitudinal density uniformity (variations below 0.25%) and high event-to-event reproducibility. The current AWAKE experiment utilizes a laser-ionised rubidium vapour plasma source (VPS), but scaling this concept to lengths significantly beyond tens of metres is limited by the delivery and constraints of the ionising laser pulse. Consequently, alternative plasma-generation concepts capable of greater length scalability are required.

Methodology
This work presents the first experimental characterisation and beam-based validation of a 10-metre-long pulsed-DC discharge plasma source (DPS) as an alternative to the VPS. The DPS employs a double-pulse DC discharge concept, originally proposed by Instituto Superior Técnico and Imperial College London, which utilizes a high-voltage ignition stage followed by a capacitor discharge heater stage to generate reproducible plasma with nanosecond-level timing jitter.

The experimental campaign involved two phases:

Laboratory Characterisation: A 10-metre prototype (26 mm inner diameter glass tube) was characterised in a surface laboratory at CERN. It was operated with three noble gases—Xenon (Xe), Argon (Ar), and Helium (He)—at pressures of 16, 24, and 45 Pa, respectively. Plasma density evolution was measured using a time-resolved Michelson interferometer with a probe beam traversing the plasma twice (20 m total averaging length).
Beam-Based Validation: The source was installed in the AWAKE experiment. High-energy proton bunches (containing $\sim 3 \times 10^{11}$ protons) were injected into the plasma. The study focused on the unseeded self-modulation instability (SMI) of the proton bunch, where the initially long bunch transforms into a train of microbunches resonant with the plasma wavelength. The modulation frequency ( $f_{mod}$ ) of the resulting microbunch train, measured via optical transition radiation (OTR) and a streak camera, served as a beam-based diagnostic for the line-integrated plasma electron density ( $n_{pe}$ ), based on the relation $f_{mod} \approx f_{pe} \propto \sqrt{n_{pe}}$ .

Key Contributions and Results

Successful Integration and Operation: The 10-metre DPS was successfully integrated into the AWAKE facility and operated reliably over a three-week proton-beam run. It consistently delivered plasma densities in the range of $10^{14}$ to $10^{15} \text{ cm}^{-3}$ across all three tested noble gases.
Density Characterisation: Laboratory interferometry revealed that for a fixed 500 A peak discharge current, Xenon achieved the highest peak density ( $1.78 \times 10^{15} \text{ cm}^{-3}$ ), followed by Argon ( $0.87 \times 10^{15} \text{ cm}^{-3}$ ) and Helium ( $0.36 \times 10^{15} \text{ cm}^{-3}$ ). The density evolution showed rapid increases followed by gradual decay due to recombination, with Xe and Ar exhibiting density oscillations around the peak, likely associated with radial plasma dynamics.
Beam-Based Diagnostic Validation: Self-modulation was observed in all three gases. The modulation frequency extracted from the proton bunch profiles showed good agreement with the independent interferometry measurements. Specifically, for Argon discharges, the beam-based measurements reproduced the oscillatory features observed by interferometry.
Reproducibility: The event-to-event variation in the modulation frequency was measured to be 0.63% for the DPS under fixed conditions. This level of reproducibility is comparable to that of the existing VPS under both seeded (0.51%) and unseeded (0.72%) self-modulation conditions.
Long-Term Stability: Measurements repeated after more than 20,000 additional discharge cycles confirmed that the relationship between discharge timing and plasma density remained consistent within experimental uncertainties, demonstrating the source's reliability over extended operation.
Ion-Mass Effects: The ability to operate with different gases allowed for the study of ion-mass effects on self-modulation, confirming that lighter gases (like Helium) exhibit stronger ion motion effects, while heavier gases (like Argon and Xenon) are preferred for maintaining wakefield amplitude in acceleration scenarios.

Significance
The authors conclude that these results establish the 10-metre DPS as a valid alternative plasma source for beam-driven wakefield experiments. The source meets the critical requirements of providing plasma densities in the necessary range ( $10^{14}–10^{15} \text{ cm}^{-3}$ ) with high reproducibility and tunability via discharge-beam timing.

The paper emphasizes that while the current design is tailored to AWAKE requirements, the underlying discharge concept offers a route to longer, reproducible plasma sources. By joining multiple adjacent plasma modules using shared cathodes and anodes, the DPS architecture can be scaled beyond the current 10-metre prototype to the tens or hundreds of metres required for future high-energy collider applications. Furthermore, the modular nature of the design could allow for controlled longitudinal density steps between sections. The successful demonstration of the DPS supports its further development as a scalable solution for future plasma wakefield acceleration experiments, addressing the limitations of laser-ionised sources in terms of length scalability.

Technical Summary: Demonstration of a 10-metre-long discharge plasma source for plasma wakefield acceleration

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