Superhot, superfast: Laser heating

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  • Published: Dec 1, 2015
  • Channels: Atomic
thumbnail image: Superhot, superfast: Laser heating

Hot, hot, hot

Lasers could heat materials to temperatures hotter than the centre of the Sun - 10 million Celsius - in only 20 femtoseconds, according to new research from Imperial College London. Graphic by DB

Lasers could heat materials to temperatures hotter than the centre of the Sun - 10 million Celsius - in only 20 femtoseconds, according to new research from Imperial College London.

Theoretical physicists have worked out an extremely rapid heating mechanism, which they suggest could allow certain materials to be heated to extremely high temperatures almost instantaneously. The ability to rapidly create high temperatures may be of interest to those attempting to crack nuclear fusion as a power generation technology. If demonstrated experimentally this rate of heating would be approximately one hundred times faster than the rates currently observed in experimental fusion reactors that use the world's highest-powered laser system at the Lawrence Livermore National Laboratory in California; energetic lasers have been using in fusion energy research for many years. Now, that the scheme has been posited, experimentalists will be working to beat each other to be the hottest the fastest.

Shocking stuff

Fundamentally, energy from a laser impinging on matter excites electrons in the atoms, ionising the material. The energy in the electrons then gets passed to the ions. This can be quite a slow process, relatively speaking. This can be quite a slow process, relatively speaking. Heating ions directly would avoid the energy bottleneck of heating the electrons first. The Imperial researchers - Arthur Turrell, Steven Rose and Mark Sherlock have now discovered that should a high-intensity laser be fired at certain types of material, it can generate an electrostatic shockwave in the material that leads to direct heating of the ions. The team reports details in the journal Nature Communications.

"It's a completely unexpected result," says Turrell. "One of the problems with fusion research has been getting the energy from the laser in the right place at the right time. This method puts energy straight into the ions," he explains.

Bulk response

One might imagine that laser-induced electrostatic shockwaves would simply push ions ahead of them, forcing them to accelerate away but not heating them. However, Turrell and colleagues' computer modelling suggests that some combinations of ions will lead to differential acceleration, which then causes friction, which in turn causes rapid heating. The effect would be most pronounced in solid targets containing two specific types of ion. "That the actual material used as a target mattered so much was a surprise in itself," adds Rose. "In materials with only one ion type, the effect completely disappears."

The heating process is rapid because the shockwave crushes the ions together, raising the local density of the material almost tenfold which has a synergistic effect on generating friction. "Faster temperature changes happen when atoms smash together in accelerators like the Large Hadron Collider, but these collisions are between single pairs of particles," explains Turrell. By contrast, the proposed technique could be explored at many laser facilities around the world and be used to heat relatively large numbers of particles in a bulk material at solid density, rather than pairs of ions.

"Currently the technique is quite inefficient, and only acts over a modest volume within the material," Turrell tolds us. "One immediate goal is to see if we can enhance the effect in any way so that larger volumes are heated." He adds that, "Another is to see if the raw materials for nuclear fusion reactions - special isotopes of hydrogen - could be heated in this manner."

Turrell's short term vision for the work is that he hopes to see experimentalists take up the challenge and demonstrate this effect in the laboratory. "In the longer term, it would be fantastic if it were successfully applied to a scenario which was closer to fusion," he adds. "In some ways the laboratory experiments will be easier than the simulations - we did not have the computing power to do these calculations in three spatial dimensions but in the lab, nature can do the calculations in 3D for us!"

Related Links

Nature Commun 2015, online: "Ultra fast collisional ion heating by electrostatic shocks"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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