Photograph courtesy Sam Vinko, University of Oxford

Read Caption
The chamber in which aluminum was converted to "hot dense matter" via x-ray bombardment.

Photograph courtesy Sam Vinko, University of Oxford

Hottest Thing on Earth: X-Rays Heat Metal to 3.6 Million Degrees

Laser experiments shedding light on ultradense plasma.

By zapping a piece of aluminum with the world's most powerful x-ray laser, physicists have heated matter to 3.6 million degrees Fahrenheit (2 million degrees Celsius)—making it briefly the hottest thing on Earth.

Only locations such as the heart of the sun or the center of a nuclear explosion are hotter.

To the researchers, though, the cosmic-level heat wasn't a goal but a side effect.

The team is helping to complete a road map for studying the universe's dizzying range of plasmas—gases with equal numbers of electrons and positive ions that, unlike other gases, conduct electricity and respond to magnetic fields.

"We already had data on one extreme of plasma, the tenuous plasma"—material so diffuse it would go almost unnoticed in a vacuum, said physicist Richard Lee, who helped conduct the tests at the SLAC National Accelerator Laboratory in San Mateo, California.

"So we did this to study the other extreme," such as the "hot dense matter" they created, Lee added. (Also see "Densest Matter Created in Big Bang Machine.")

"Once you've got those data, you can improve your computer models to hint at what's happening with plasma on the spectrum in between," said Lee, who co-authored a study of the hot dense matter in this week's issue of the journal Nature.

X-Rayed Matter Blew Out Like a Volcano

To create the hot dense matter, Lee and his colleagues used SLAC's Linac Coherent Light Source, or LCLS.

The device can rapidly fire the most powerful x-ray pulses in the world—their strength deriving from the fact that the machine can focus the laser pulses to a point three times smaller than the width of a single blood cell.

The pulses don't heat the tops of samples, but rather uniformly vaporize them from the inside out.

"Your target blows out like a volcano, crater and all," Lee said.

The extraordinarily intense, rapid, and uniform laser pulses turned small sections of a postage stamp-size piece of aluminum foil into the plasma. By repeating the experiment over and over, the researchers gathered mountains of data about the poorly understood plasma's behavior.

The team is now using the data to make predictions about plasma types between the two extremes, which would be considered warm dense matter. (Related: "'Warm Plasma Cloak' Discovered Enveloping Earth.")

In time, the work could help astrophysicists better understand the processes roiling deep inside of the sun or perhaps within the cores of gas giant planets like Jupiter.

The findings could also help researchers trying to generate controlled fusion power, energy released when two light atomic nuclei merge—long a holy grail of clean energy advocates.

The National Ignition Facility in Livermore, California, for example, is trying to use a laser to zap a target into plasma to spark nuclear fusion. (See "Nuclear Fusion Power a Step Closer After Giant Laser Blast.")

To do that, though, the team must first figure out how to "tune" their laser to control warm dense matter that may appear in the device's planned fusion reactions.

The SLAC results may provide the Livermore team with just the right kind of data they need to improve their machine.

And by setting an upper limit for the kind of x-ray laser energies that will destroy a sample, the new findings could also help some researchers use x-ray lasers like a sort of microscope for peering deep into precious artifacts and other objects.

"We destroyed our target, but most people want to use [x-ray lasers] to see through theirs," Lee said.

"It's like Superman. He's good at using his x-ray vision without burning things up."