Advancing Science: ZEUS Laser's Electron Acceleration

The University of Michigan's ZEUS laser facility has conducted its first official experiment at 2 petawatts. This nearly doubles the peak power of any other laser currently operating in the United States.

The University of Michigan's ZEUS laser facility has conducted its first official experiment at 2 petawatts. This nearly doubles the peak power of any other laser currently operating in the United States.

This milestone marks the beginning of experiments that move into unexplored territory for American high-field science.

Karl Krushelnick, Director, Gérard Mourou Center for Ultrafast Optical Science

ZEUS research has applications in medicine, national security, materials science, astrophysics, plasma physics, and quantum physics. As a user facility funded by the National Science Foundation, ZEUS allows research teams from across the U.S. and worldwide to submit experiment proposals, which are evaluated through an independent review process.

One of the great things about ZEUS is it is not just one big laser hammer, but you can split the light into multiple beams. Having a national resource like this, which awards time to users whose experimental concepts are most promising for advancing scientific priorities, is really bringing high-intensity laser science back to the U.S.

Franklin Dollar, Professor, Physics and Astronomy, University of California, Irvine

The goal of Professor Alec Dollar’s team and the ZEUS researchers is to generate electron beams hundreds of meters long with energy levels comparable to those produced by large-scale particle accelerators. These beams would have five to ten times the energy of any previously generated at the ZEUS facility.

We aim to reach higher electron energies using two separate laser beams, one to form a guiding channel and the other to accelerate electrons through it.

Anatoly Maksimchuk, Research Scientist, Electrical and Computer Engineering, University of Michigan

The researchers aim to achieve this in part by modifying the target design. In this experiment, they extended the cell containing helium gas, the medium into which the laser pulse is directed. The interaction between the laser and the gas forms plasma by stripping electrons from the atoms, creating a mixture of free electrons and positively charged ions. These electrons are accelerated by the laser pulse in a process known as wakefield acceleration, where they follow closely behind the pulse like wakesurfers behind a speedboat.

Because light travels more slowly through plasma, the electrons can catch up to the laser pulse. A longer, less dense target gives the electrons more time to accelerate before reaching the pulse, allowing them to achieve higher energies.

This experiment demonstrates ZEUS’s power and supports the upcoming signature experiment planned for later this year, in which accelerated electrons will collide with laser pulses traveling in the opposite direction.

In the electrons' frame of reference, the 3-petawatt laser pulse will appear as a zettawatt-scale pulse—effectively a million times more powerful. As a result, ZEUS is officially named the “Zettawatt Equivalent Ultrashort laser pulse System.”

Vyacheslav Lukin, program director in the NSF Division of Physics, which oversees the ZEUS project, added, “The fundamental research done at the NSF ZEUS facility has many possible applications, including better imaging methods for soft tissues and advancing the technology used to treat cancer and other diseases. Scientists using the unique capabilities of ZEUS will expand the frontiers of human knowledge in new ways and provide new opportunities for American innovation and economic growth.”

The ZEUS facility is about the size of a school gymnasium. A laser in one corner of the room generates the initial infrared pulse. Diffraction gratings spread the pulse in time to prevent it from becoming so intense during amplification that it ionizes the surrounding air. At its largest, the pulse measures 12 inches wide and several feet long.

After passing through four rounds of pump lasers that increase its energy, the pulse enters vacuum chambers. A second set of gratings compresses it into a 12-inch-wide disk that is only 8 microns thick—about ten times thinner than a sheet of printer paper. Although the pulse is already intense enough at this size to ionize air, it is further focused to a diameter of 0.8 microns to maximize intensity for experiments.

John Nees, a research scientist in electrical and computer engineering at the University of Michigan who leads ZEUS laser construction, noted: “As a midscale-sized facility, we can operate more nimbly than large-scale facilities like particle accelerators or the National Ignition Facility. This openness attracts new ideas from a broader community of scientists.”

Progress to 2 petawatts has been gradual. Acquiring key components for the system has been more difficult than expected. The most significant challenge has been sourcing a sapphire crystal doped with titanium, a critical part of the final amplifier that brings the laser pulse to full strength. This crystal measures nearly 7 inches in diameter.

Franko Bayer, Project Manager for ZEUS, stated, “The crystal that we are going to get in the summer will get us to 3 petawatts, and it took four and a half years to manufacture. The size of the titanium sapphire crystal we have, there are only a few in the world.”

Switching from the 300-terawatt output of the previous HERCULES laser to just 1 petawatt on ZEUS led to an unexpected darkening of the diffraction gratings. The team first needed to determine the cause: were the gratings permanently damaged, or simply coated with carbon deposits from the high-intensity beam breaking apart molecules in a faulty vacuum chamber?

Once they confirmed the deposits were carbon, Nees and the laser team worked to identify how many laser shots could be safely fired between cleanings. If the gratings became too dark, they could distort the laser pulses, potentially damaging downstream optical components.

Despite operating at 1 petawatt, ZEUS remains a valuable scientific tool. Since its official launch in October 2023, the team has spent 15 months conducting user experiments. So far, the facility has supported 11 distinct research projects involving 58 experimenters from 22 universities, including international participants.

The ZEUS team will continue refining the system over the next year while supporting additional user experiments, aiming to reach its full operating capacity.

Krushelnick also holds the title of Henry J. Gomberg Collegiate Professor of Engineering and teaches in the departments of nuclear engineering and radiological sciences, electrical and computer engineering, and physics.

Video Credit: University of Michigan