By Andrea Hargraves 
Lasers have been used in astronomy for many years
- author, Charlotte Lytton
- scroll, From BBC News
Inside a University of Michigan research laboratory, a bright green light fills a giant vacuum chamber. It's the size of two tennis courts. The walls are protected by 60cm of concrete to prevent radiation leakage, and staff wear masks and hairnets to ensure sensitive electronics are not affected.
This is Zeus, soon to be the most powerful laser in the United States of America – and now appearing in his first official experiments.
Unlike continuous lasers that scan your barcodes in stores, Zeus is a pulsed laser, firing bursts of up to a few billionths of a second. Each pulse will be able to reach a maximum power of three petawatts, which is equivalent to a thousand times the entire world's electricity consumption. A laser capable of producing such highly compressed energy would help researchers study the quantum laws that underpin reality, for example, or recreate extreme astrophysical conditions in space.
But Zeus isn't the only massive laser that could unlock new discoveries in the coming years: a number of other high-energy lasers at facilities from Europe to Asia are hot on its heels.
The field as a whole is “really growing,” says Karl Krushelnik, director of the Gérard Moreau Center for Ultrafast Optical Science at the University of Michigan. “People are pushing the boundaries of technology.”
In the UK, a laser called Vulcan 20-20 will be the most powerful in the world when it is completed in 2029. It will produce a beam billions of times brighter than the most intense sunlight. This single pulse will produce six times more energy than is produced worldwide, but it will last less than a trillionth of a second, with its target being only a few micrometers (or 0.001 of a millimeter) across. Like Zeus, Vulcan 20-20 will welcome scientists from around the world to conduct experiments that could expand our understanding of the universe, nuclear fusion, and even create previously unknown matter.
The 20-petawatt Vulcan 20-20 engine is an £85 million ($106 million) upgrade to the existing Vulcan engine at the Central Laser Facility (CLF) in Harwell, UK – which is being dismantled.
Currently the size of two Olympic swimming pools, its mirrors are one meter wide and each weighs 1.5 tons. Thick white wires coil out of the laser opening as the device curves around the room. Considered state-of-the-art when it was first built at Rutherford Appleton's laboratory in 1997, the new laser will be 100 times brighter.
“The impressive thing is not just the power of the laser, but its intensity,” says Rob Clark, leader of the Experimental Science Group at CLF. To understand this density, imagine 500 million million standard 40-watt light bulbs.
“Now, compress that light into something about a tenth the size of a human hair,” he says. “The result is a very intense light source, and that's what creates all the fun stuff of plasmas, like huge electric and magnetic fields, and particle acceleration.”
Vulcan 20-20 will allow scientists to conduct astrophysical research in the laboratory, recreating the conditions of distant galaxies to analyze the inner workings of stars or gas clouds, or how matter might behave when exposed to specific temperatures and densities.
Alex Robinson, principal theoretical plasma physicist at CLF, explains that the field of study is driven by the desire to explore the universe. He says that astrophysical research is generally “observational.” “You point at some kind of telescope and you see a lot of things. But the question arises as to what's really going on.” The hope is that experiments with lasers this powerful will allow, for the first time, “really rigorous tests of whether certain theories can work or not.”
Among the mysteries expected to be investigated at Oxford are the origins of magnetic fields, which surround most of the fundamental objects in the universe, such as stars and planets. “Why are these magnetic fields there? It's not entirely clear,” says Robinson, and no observations can go back and test why they first appeared.
One testing method could involve combining matter to create shock waves and adding manufactured disturbances, such as those caused by molecular clouds, planets and dust, to see if this “could give rise to magnetic fields.”
Other experiments will explore the origins of cosmic rays (high-energy particles that can travel at nearly the speed of light), how jets (sprays of particles released from high-energy collisions) form, and the structure inside giant planets.
The researchers will also use the Vulcan 20-20 laser to study the formation of new materials. A form of boron nitride, a material harder than diamond, has been found to be metastable – created under very high pressure and density conditions made in the laboratory, which can later survive ambient temperatures.
“Then the question is, what other materials can you make in the same way?” Robinson says. “Do they have cool electronic or optical properties? I don't know. But at least there's a piece that tells us there's something worth exploring.”
Achieving fusion
Nuclear fusion is also on the list of areas where high-energy lasers are used. In July, researchers at the National Ignition Facility at Lawrence Livermore National Laboratory in California used lasers for a net energy gain for the second time.
Following the center's original milestone last December, this year's experiment produced a higher energy yield than the first, once again raising hopes that clean energy could replace our current energy sources. (Fusion reactions do not release greenhouse gases or radioactive waste.)
Fusion was also one of the main areas of study at the Extreme Light Infrastructure for Nuclear Physics (ELI-NP) Center in Mogurile, Romania – which at 10 petawatts holds the title of the world's most powerful laser (MORO). Its director and the University of Michigan facility of the same name said its creation was “equivalent to landing on the moon, where failure is not an option.”
Last year, a Romanian laser operator began partnering with private companies to develop technology that could power the world's first commercial fusion plants. Using the “chirping pulse amplification” technique for which Murrow and Donna Strickland won the 2018 Nobel Prize in Physics, laser pulses will be stretched, reducing their maximum power, before being amplified and compressed again.
“This completely changed the development of lasers,” Clark says, allowing much higher intensities to be achieved at lower power.
Their research on the physical processes of this reaction is expected to be published within three years, before the first commercial fusion plants are built in the 2030s.
Is bigger better?
Physicists are keen to emphasize the collaborative nature of this field, but the size remains a point worth bragging about. According to Chang Hee-nam, director of the Center for Relativistic Laser Science (CoReLS) in South Korea and a professor at the Gwangju Institute of Science and Technology, the center's laser currently “holds the record for the highest laser intensity” in the world, reaching 10^23 watts/cm2 — or intensity As powerful as all the light on Earth is focused to just over a micrometre, or less than one-fifth the diameter of a human hair.
South Korean scientists are using this technology to explore, among other things, proton therapy, a cancer treatment that targets positively charged rays in patients' tumors.
Research that could lead to new medical applications, as well as testing centuries-old ideas about the state of the universe, has been well explored on the four-petawatt CoReLS machine – but the team isn't stopping there. “We are now seeking a higher petawatt laser, and are preparing some proposals for a 25-petawatt laser,” Nam says. If it is ordered within the next six years, as he hopes, it will outperform the Vulcan 20-20 that has not yet been built.
However, Vulcan's Clark says power and density aren't everything. The most important metric now is, “What can you do with it? What science are you pursuing? What are you going to get out of it?”
He says that these lasers, and the researchers who work on them, care about one thing above all else. “It's about building it right and using it right.”