Rapid laser heating simulates fusion processes

19 Nov 2015

An Imperial College research project uses lasers to make materials hotter than the center of the Sun, faster than ever before.

Theoretical physicists from Imperial College London, UK, have developed an extremely rapid heating mechanism based on lasers, which they believe could heat certain materials to 10 million degrees Celsius in much less than one trillionth (10⁻¹²) of a second. They liken the effects to those of a fusion reaction, such as creates the sun’s energy – but say the new technique overcomes a longstanding limitation that has held back its development.

Researcher Dr Arthur Turrell commented, “It’s a completely unexpected result. One of the problems with fusion research has been getting the energy from the laser in the right place at the right time. Our new method puts energy straight into the ions.” The work has just been reported in Nature Communications.

Dr Turrell added that the method, proposed at Imperial for the first time, could be relevant to new avenues of research in thermonuclear fusion energy, where scientists are seeking to replicate the Sun’s ability to produce clean energy.

The heating would be about 100 times faster than rates currently seen in fusion experiments using the world’s most energetic laser system at the Lawrence Livermore National Laboratory in California. The race is now on for fellow scientists to put the team’s method into practice.

Fast friction

Normally, laser-induced electrostatic shockwaves push ions ahead of them, causing them to accelerate away from the shockwave but not heat up. However, using sophisticated supercomputer modelling, the team discovered that if a material contains particular combinations of ions, they will be accelerated by the shockwave at different speeds.

This causes friction, which in turn causes them to rapidly heat. They found that the effect would be strongest in solids with two ion types, such as plastics. “The two types of ions act like matches and a box; you need both,” said study co-author Dr Mark Sherlock from the Department of Physics at Imperial. “Matches will never light on their own - you need the friction caused by striking them against the box to activate the material.”

“That the actual material used as a target mattered so much was a surprise in itself,” added study co-author Professor Steven Rose. “In materials with only one ion type, the effect completely disappears.”

The heating is so fast in part because the material targeted is so dense. The ions are squeezed together to almost ten times the usual density of a solid material as the electrostatic shockwave passes, causing the frictional effect to be much stronger than it would be in a less-dense material, such as a gas.

Further potential

The technique, if proven experimentally, could be the fastest heating rate ever demonstrated in a lab for a significant number of particles. “Faster temperature changes happen when atoms smash together in accelerators like the Large Hadron Collider, but these collisions are between single pairs of particles,” said Dr Turrell. “In contrast the proposed technique could be explored at many laser facilities around the world, and would heat material at solid density.”

The Nature Communications article explains how the experiment has been designed using a different application of high power lasers: “High-intensity lasers can be used to generate shockwaves, which have found applications in nuclear fusion, proton imaging, cancer therapies and materials science. So-called collisionless electrostatic shocks (CES) are one type of shockwave widely studied for applications involving ion acceleration. We have shown a novel mechanism for CESs to heat small amounts of solid density matter to temperatures of ~keV in tens of femtoseconds.”

”Unusually, electrons play no direct role in the heating and it is the ions that determine the heating rate. Ions are heated due to an interplay between the electric field of the shock, the local density increase during the passage of the shock and collisions between different species of ion. In simulations, these factors combine to produce rapid, localized heating of the lighter ion species. Although the heated volume is modest, this would be one of the fastest heating mechanisms discovered if demonstrated in the laboratory.”

About the Author

Matthew Peach is a contributing editor to optics.org.