High-repetition-rate, all-reflective optical guiding and electron acceleration in helium using an off-axis axicon

To understand the work described here, one must first grasp the basic challenge of modern particle physics: how to make tiny particles move incredibly fast. For decades, scientists have used massive machines, often miles long, to push electrons to high speeds. These machines use radio waves to give the particles a kick, but the force they can apply is limited. If you try to push too hard in a short space, the machine breaks or becomes inefficient. This has kept the size and cost of these accelerators enormous, limiting who can use them and what they can do.

A different approach, known as laser wakefield acceleration, offers a way to shrink these machines down to the size of a room. Instead of radio waves, this method uses an intense pulse of laser light fired into a gas. The laser is so powerful that it rips electrons away from the gas atoms, creating a plasma. As the laser pulse moves through this plasma, it pushes the electrons aside, creating a wake behind it, much like the wake of a boat moving through water. This wake creates a powerful electric field that can catch other electrons and accelerate them to high speeds over a very short distance. The key to making this work well is keeping the laser beam focused and strong as it travels through the gas. If the beam spreads out, the acceleration stops. For a long time, keeping such a powerful beam focused over a long distance was difficult and unstable, often requiring complex setups that were hard to maintain or that could be damaged by the laser itself.

Researchers at the ELI Beamlines facility in the Czech Republic, working with colleagues from the University of Maryland, have now demonstrated a new way to solve this focusing problem. They built an experimental setup that uses only mirrors to guide the laser, avoiding any glass or lenses that could be damaged or distorted by the heat of the powerful beam. The core of their innovation is a special reflective cone, called an off-axis axicon, which helps shape the laser beam before it enters the gas. This setup allowed them to create a stable channel in helium gas, a safe and easy-to-handle alternative to the hydrogen often used in similar experiments.

In their experiments, the team fired a laser pulse with a duration of just 30 femtoseconds—a timescale so short it is hard to imagine—carrying 13 joules of energy. They split this pulse into two parts. One part, the main driver, was focused to accelerate the electrons. The other part was shaped into a special ring-like pattern to create a pre-formed channel in the helium gas. This channel acted like a hollow pipe for the main laser pulse, keeping it tightly focused as it traveled through the gas. The researchers tested this system at two different speeds. At a slower rate of 0.2 shots per second, they successfully accelerated electrons to energies of about 5 billion electron volts. This is a significant achievement, showing that the method can produce high-energy beams in a single stage.

The team also pushed the system to operate at a much faster rate of 3.3 shots per second. While they did not measure electron acceleration at this higher speed in the same way, they proved that the laser beam could be guided stably through the plasma channel for long periods without the system degrading. This is a crucial step toward making these accelerators practical for everyday use, as higher repetition rates mean more data and more usable beams. The use of helium was a deliberate choice; unlike hydrogen, which is flammable and requires careful handling, helium is inert and safer to work with in a facility that operates continuously.

The results show that this all-reflective approach is robust. By using mirrors instead of lenses, the researchers avoided the thermal distortions that often plague high-power laser systems. They found that the electron beams produced were stable in direction and had low spread, meaning the particles stayed together in a tight bunch. The energy of the electrons reached up to 5 billion electron volts, which is consistent with what theory predicts for these conditions. The team noted that the process relied on a specific timing between the two laser pulses to create the channel just before the main pulse arrived, a delicate balance that their system managed to maintain shot after shot.

This work does not claim to have solved every problem in particle acceleration, but it provides a clear path forward. The method is simple enough that it does not require major changes to existing laser facilities, making it accessible for other research centers to adopt. By proving that stable, high-energy acceleration is possible with a safe gas and a durable optical setup, the researchers have removed several barriers that have held back the technology. The findings suggest that future accelerators could be built to be smaller, safer, and more reliable, potentially opening the door to new applications in medicine and materials science, as well as further discoveries in fundamental physics. The success of this experiment lies not in a single dramatic breakthrough, but in the steady demonstration that a difficult problem can be solved with a straightforward, durable design.