Visualization of energy flow in fast ignition experiments is made possible by the use of copper tracers and a high-tech X-ray imaging system. High Energy Density Physics Group, UC San Diego

A new technique to monitor a process called 'fast ignition' has been developed, in what could be a critical step towards a viable method of creating controlled nuclear fusion.

Fusion ignition, the point at which a nuclear reaction becomes self-sustaining, is one of the great hopes for a new generation of clean, cheap energy generation. But while the reactions have been seen in the cores of thermonuclear weapons, it has yet to be achieved in a controlled manner in a reactor.


One of the possibilities for developing the tech for real is known as the fast ignition process. This two-stage laser process first uses hundred of lasers to compress the fusion fuel -- a mixture of deuterium and tritium in a spherical plastic capsule -- and then uses a high-intensity laser to rapidly heat the compressed fuel.

It's by far the lowest-energy method of potentially creating nuclear fusion, but in order for it succeed, energy from the high-intensity laser must be directed straight at the densest region of the compressed fuel. Previously it wasn't known how to do this, but now a research team led by scientists and engineers at the University of California, San Diego and General Atomics has found a way.

The new monitoring technique finally provides a way of identifying exactly where energy from the laser travels when it hits the fuel.

To do this, copper tracers are added to the fuel capsule. When the high-intensity laser is directed at the target, it generates high-energy electrons that hit the tracers and in turn cause them to emit X-rays. These can be imaged to show where the energy from the laser is going once it's hit the target fuel cell.


With the information provided by the X-ray monitoring system, the team has been able to see which fuel target designs and laser configurations produced the greatest efficiency when it came to delivering energy to the densest part of the target, achieving a record high of 7 percent efficiency: a fourfold improvement on previous fast ignition experiments.

Study co-author Christopher McGuffey, of the High Energy Density Physics Group at the UC San Diego Jacobs School of Engineering said that "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."

The paper notes that "our findings lay the groundwork for further improving efficiency, with 15 percent energy coupling predicted in FI experiments using an existing megajoule-scale laser driver," although this has yet to be tested experimentally. Team lead Professor Farhat Beg said that "we hope this work opens the door to future attempts to improve fast ignition".