The possibility of achieving limitless clean energy through nuclear fusion has edged a step closer after scientists worked out how to "see" where energy is delivered during fast ignition. The team, from the University of California, San Diego and General Atomics, said being able to see the energy flow will help them test ways of improving energy deliver to fuel targets.

Fast ignition is one approach in the quest for nuclear fusion – energy generated from fusing atoms (the same way the Sun generates its energy). It involves reactions from a high-intensity laser. First, hundreds of lasers compress the fusion fuel to high density. The high-intensity laser then delivers energy to heat the compressed fuel very quickly.

However, for fast ignition to succeed, researchers must work out how to direct energy from the high-intensity laser into the densest region of the fuel. Publishing their findings in the journal Nature Physics, the team has worked out a way to see where the energy travels when the laser hits the fuel target.

This technique was developed with the use of copper traces inside the fuel capsule. When the beam is directed at the target, it creates high-energy electrons that hit the copper and make them emit X-rays the scientists can see.

Study co-author Christopher McGuffey said: "Before we developed this technique, it was as if we were looking in the dark. Now, we can better understand where energy is being deposited so we can investigate new experimental designs to improve delivery of energy to the fuel."

Using their technique, scientists were then able to achieve a record high efficiency of energy delivery from the laser to the fuel. Reaching 7% efficiency, this is four times better than previous fast ignition experiments. "We hope this work opens the door to future attempts to improve fast ignition," said Farhat Beg, director of the Centre for Energy Research at UC San Diego.