“`html

Achieving 2 petawatts, the NSF-funded ZEUS facility at U-M facilitates investigations that could enhance healthcare, national defense, materials research, and beyond

The ZEUS laser facility at the University of Michigan has approximately doubled the peak power of any other laser in the country with its inaugural experiment at 2 petawatts (2 quadrillion watts).

Exceeding 100 times the global electricity output, this immense power occurs only during the fleeting moment of its laser pulse—merely 25 quintillionths of a second in duration.

“This achievement signifies the genesis of experiments venturing into uncharted realms of American high-field science,” remarked Karl Krushelnick, director of the Gérard Mourou Center for Ultrafast Optical Science, which encompasses ZEUS.

Investigation at ZEUS will have applications spanning medicine, national defense, materials research, and astrophysics, alongside plasma science and quantum physics. Funded by the U.S. National Science Foundation, ZEUS operates as a user facility—inviting research teams from across the nation and globally to propose experiments that undergo an independent selection process.

“One impressive aspect of ZEUS is it’s not merely a single enormous laser hammer; it can also divide the light into several beams,” stated Franklin Dollar, professor of physics and astronomy at the University of California, Irvine, whose group is conducting the first user experiment at 2 petawatts. “Possessing a national asset like this, which allocates time to users based on the most promising experimental ideas aimed at pushing scientific boundaries, truly revitalizes high-intensity laser science in the U.S.”

Dollar’s team alongside the ZEUS team aspires to generate electron beams with energies comparable to those produced in particle accelerators several hundred meters long. This would yield 5-10 times greater energy than electron beams previously generated at the ZEUS facility.

“Our objective is to achieve elevated electron energies using two distinct laser beams—one to establish a guiding channel and the other to propel electrons through it,” explained Anatoly Maksimchuk, U-M research scientist specializing in electrical and computer engineering, who guides the development of the experimental zones.

They plan to accomplish this partly through a reconfigured target. They extended the chamber that houses the gas that the laser pulse impacts, helium in this trial. This engagement generates plasma, detaching electrons from the atoms, transforming the gas into a mixture of free electrons and positively charged ions. These electrons are propelled behind the laser pulse like wakesurfers closely following a speedboat, a process known as wakefield acceleration.

Light propagates slower through plasma, allowing electrons to catch up with it. In a less dense, elongated target, electrons spend more time accelerating before catching up to the laser pulse, enabling them to achieve higher maximum speeds.

This demonstration of ZEUS’s capability sets the stage for a critical experiment anticipated later this year, during which the accelerated electrons will collide with laser pulses moving in the opposite direction. From the electrons’ perspective, the 3-petawatt laser pulse will appear a million times more potent—a zettawatt-scale pulse. This is what lends ZEUS its full title: “Zettawatt Equivalent Ultrashort laser pulse System.”

“The foundational research conducted at the NSF ZEUS facility has numerous potential applications, including enhanced imaging techniques for soft tissues and advancements in technologies used to combat cancer and other illnesses,” commented Vyacheslav Lukin, program director in the NSF Division of Physics, which oversees the ZEUS initiative. “Researchers leveraging the unique capacities of ZEUS will broaden the horizons of human understanding in novel manners and generate fresh opportunities for American innovation and economic development.”

The ZEUS facility occupies a space comparable to a school gymnasium. In one corner of the area, a laser generates the initial infrared pulse. Optical instruments known as diffraction gratings extend it temporally so that when the pump lasers add energy to the pulse, it does not become so intense that it begins to tear the air apart. At its most expansive, the pulse measures 12 inches across and several feet in length.

“`

After a series of four rounds of pump lasers supplying energy, the pulse enters the vacuum chambers. A subsequent set of gratings flattens it into a 12-inch disk that measures merely 8 microns in thickness—approximately 10 times thinner than a standard sheet of printer paper. Despite its 12-inch width, its intensity has the potential to transform air into plasma; however, it is then condensed to 0.8 microns in width to maximize intensity for the experiments.

“Operating as a midscale facility, we can maneuver more flexibly than larger institutions such as particle accelerators or the National Ignition Facility,” stated John Nees, U-M research scientist in electrical and computer engineering, who spearheads the ZEUS laser development. “This adaptability encourages fresh ideas from a wider community of scientists.”

The journey to achieving 2 petawatts has been meticulous and gradual. Acquiring the necessary components to construct the system has proven more challenging than anticipated. The most significant obstacle is a sapphire crystal imbued with titanium atoms. Measuring nearly 7 inches in diameter, it is the essential element of the system’s final amplifier, which elevates the laser pulse to its full power.

“The crystal we expect to receive this summer will propel us to 3 petawatts, and it has taken four and a half years to create,” remarked Franko Bayer, project manager for ZEUS. “Given the dimensions of the titanium sapphire crystal we possess, only a handful exist globally.”

In the interim, the transition from the previous HERCULES laser’s 300 terawatt output to merely 1 petawatt on ZEUS led to concerning darkening of the gratings. Initially, they needed to ascertain the cause: Were they irreparably harmed or simply darkened by carbon deposits due to the powerful beam disrupting molecules in the imperfect vacuum chamber?

Upon discovering it was indeed carbon deposits, Nees and the laser team had to calculate how many laser shots could occur safely before needing to clean the gratings. If the gratings became overly dark, they could warp the laser pulses in a manner that would compromise the optics further along the path.

Ultimately, the ZEUS team has already contributed 15 months to user experiments since the grand opening in October 2023, as there remains substantial scientific inquiry possible with a 1 petawatt laser. Thus far, it has hosted 11 distinct experiments involving a total of 58 researchers from 22 institutions, including international collaborators. Over the coming year—between user experiments—the ZEUS team will proceed with enhancing the system towards its full power.

Krushelnick holds the title of Henry J. Gomberg Collegiate Professor of Engineering and serves as a professor in nuclear engineering and radiological sciences, electrical and computer engineering, and physics.