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September 20, 2007

Di-positronium > Gamma Ray Lasers > Laser ignition nuclear fusion

A US team has created thousands of Di-positronium molecules by merging electrons with their antimatter equivalent: positrons. The discovery, reported in the journal Nature, is a key step in the creation of ultra-powerful lasers known as gamma-ray annihilation lasers.

Gamma ray lasers would have a lot of uses. The progress with Bose condensates and with positron traps has proceeding fairly quickly. This could be a fairly rapid technological progression with a lot of other new technological possibilities along the way. It also sounds like it will be fairly compact and lightweight, which would be good for any fusion propulsion system.


The ultra-high vacuum target chamber, where the intense positron pulse is implanted into the porous silica film. The magnet coils carry a current of 1000 amps for a few hundred milliseconds to generate the strong magnetic field needed to compress the positron beam. (Credit: David Cassidy, UC-Riverside)


To make the molecules, Dr Cassidy and his team used a specially designed trap to store millions of the positrons.

A burst of 20 million were then focused and blasted at a porous silica "sponge".

"It's like having a trickle of water filling up a bath and then you empty it out and you get a big flush," said Dr Cassidy.

As the positrons rushed into the voids they were able to capture electrons to form atoms. Where atoms met, they formed molecules.

By measuring the gamma-rays that signalled their annihilation, the team estimated that up to 100,000 of the molecules formed, albeit for just a quarter of a nanosecond (billionth of a second).

Dr Cassidy believes that increasing the density of the positronium in the silicon would create an exotic state of matter known as a Bose-Einstein condensate (BEC).

"At even higher densities, one might expect the material to become a regular, crystalline solid," wrote Professor Clifford Surko, of the University of Californian, San Diego, in an accompanying article.

Taking it one step further, scientists could use the spontaneous annihilation of the BEC, and the subsequent outburst of gamma-rays, to make a powerful laser.

He highlighted an experiment at the National Ignition Facility (NIF) in the US where scientists envisage using 192 lasers to heat a fuel target to try to kick-start nuclear fusion.

"Imagine doing that but you no longer need hundreds of lasers," he said.

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