Supercomputers Fire Lasers to Shoot Gamma Ray Beam

Estimated reading time: 9 minutes

Ever play with a magnifying lens as a kid? Imagine a lens as big as the Earth. Now focus sunlight down to a pencil tip. That still wouldn't be good enough for what some Texas scientists have in mind. They want to make light even 500 times more intense. And they say it could open the door to the most powerful radiation in the universe: gamma rays.

Comic book readers might know about gamma rays. The Incredible Hulk was transformed from mild scientist into wild superhero by gamma rays from a nuclear explosion. The real gamma rays form in nature from radioactive decay of the atomic nucleus. Besides hazardous materials, you'd have to look in exotic places like near a black hole or closer to home at lightning in the upper atmosphere to find natural forces capable of making gamma rays.

Scientists have found that gamma rays, like the Hulk, can do heroic things too -- if they can be controlled. Hospitals now eradicate cancer tumors using a 'gamma ray knife' with surgical precision. The rays can also image brain activity. And gamma rays are used to quickly scan cargo containers for terrorist materials.

But it's near impossible to make gamma rays with non-radioactive materials. To do that today one needs a colossal atom smasher like at CERN or SLAC. No one has been able to make a gamma ray beam from lasers. But it can be done, say scientists at The University of Texas (UT) at Austin.

Supercomputers might have helped unlock a new way to make controlled beams of gamma rays from a laser that fits on a table-top, according to research physicist Alex Arefiev, who has a dual appointment at the Institute for Fusion Studies and at the Center for High Energy Density Science at UT Austin. Arefiev co-authored the study, "Enhanced multi-MeV photon emission by a laser-driven electron beam in a self-generated magnetic field," published May 2016 in the journal Physical Review Letters.

"One of the key results that we found is that a laser pulse can be efficiently converted into a beam of very energetic photons," Arefiev said. "They are more than one million times more energetic than the photons in the laser pulse. Until recently, there hasn't been a method for producing a beam of such energetic photons. So the proposed regime can be groundbreaking for a number of applications and also for fundamental science studies."

Arefiev and colleagues want to fire up the Texas Petawatt Laser, one of the most powerful lasers in the world. They'll target a piece of solid plastic with a tiny chamber drilled through that's filled with plastic foam. Simulations run on the Lonestar and Stampede supercomputers of the Texas Advanced Computing Center (TACC) show that the laser goes through the target chamber without making a hole, like sunlight through a pane of glass. Along the way it energizes the electrons of the foam. This plasma of high-energy electron particles then release a controlled beam of ultra-energized photons, the gamma rays.

Study lead David Stark said, "It's exciting to be able to work in collaboration with people at the Texas Petawatt Laser," which is also at UT Austin. "That was one of the benefits to doing this study, being able to combine plasma physics with the optical capabilities that are just in the basement of our building." Stark was then a graduate student of the physics department at UT Austin, and has since completed his PhD and moved on to an appointment at Los Alamos National Laboratory.

The scientists found even more than just radiation, said study co-author Toma Toncian. "In a nutshell, we have discovered using numerical simulations a physical regime where we would generate the highest magnetic fields ever generated on Earth. A side benefit is that we would also generate one of the most intense gamma ray sources." Toncian is the assistant director of the Center for High Energy Density Science at UT Austin.

The ultra-high magnetic fields induced by the laser strike are key to what the scientists describe as 'relativistic transparency' of the target. For instance, if you aim your normal laser pointer at a blackboard, some light is reflected but mainly it's absorbed at the surface. The electrons in the material follow the oscillation of the laser field and short circuit it so it cannot propagate inside the board.

"In our case," Toncian explained, "the electrons are getting heavier and heavier because we are accelerating them very close to the light speed. They become immobile. They cannot respond anymore to the high oscillating light of the laser. Suddenly, the laser can propagate inside the target because the electrons cannot short circuit the laser light."

Besides relativity, the scales of the experiment can boggle the mind. They're working with some of the world's most powerful laser light, amplified to a petawatt -- a billion million watts. The light burst dwarfs by several several hundred times the power from all of the world's electric plants combined. But it only lasts only a few hundred femtoseconds -- a millionth of one billionth of a second. That's about as long as it takes for the laser light to go through the target, which is only 1/100 as thick as a human hair.

"On that timescale, we need to be able to resolve the dynamics," Stark said. "Because that's how we understand the physics of what's going on. We needed to use in our kinetic simulation very high resolution on both space and time."

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