3 March 2004

Nuclear Fusion From Bubbles Blasted With Sound

by Kate Melville

Physical Review E has announced the publication of an article by a team of researchers from Rensselaer Polytechnic Institute (RPI), Purdue University, Oak Ridge National Laboratory (ORNL) and the Russian Academy of Science (RAS) stating that they have replicated and extended previous experimental results that indicated the occurrence of nuclear fusion using a novel approach for plasma confinement.

The approach - called bubble fusion - and the experimental results are being published in an article which is scheduled for publication in the Physical Review journal this month.

The research team used a standing ultrasonic wave to help form and then implode the cavitation bubbles of deuterated acetone vapor. The oscillating sound waves caused the bubbles to expand and then violently collapse, creating strong compression shock waves around and inside the bubbles. Moving at about the speed of sound, the internal shock waves impacted at the center of the bubbles causing very high compression and accompanying temperatures of about 100 million Kelvin.

These new data were taken with an upgraded instrumentation system that allowed data acquisition over a much longer time than was possible in the team's previous bubble fusion experiments. According to the new data, the observed neutron emission was several orders of magnitude greater than background and had extremely high statistical accuracy. Tritium, which also is produced during the fusion reactions, was measured and the amount produced was found to be consistent with the observed neutron production rate.

Earlier test data indicated that nuclear fusion had occurred, but these data were questioned because they were taken with less precise instrumentation.

"These extensive new experiments have replicated and extended our earlier results and hopefully answer all of the previous questions surrounding our discovery," said Richard T. Lahey Jr. at Rensselaer and the director of the analytical part of the joint research project.

Other fusion techniques, such as those that use strong magnetic fields or lasers to contain the plasma, cannot easily achieve the necessary compression, Lahey said. In the approach to be published in Physical Review E, spherical compression of the plasma was achieved due to the inertia of the liquid surrounding the imploding bubbles.

Professor Lahey also explained that, unlike fission reactors, fusion does not produce a significant amount of radioactive waste products or decay heat. Tritium gas, a radioactive by-product of deuterium-deuterium bubble fusion, is actually a part of the fuel, which can be consumed in deuterium-tritium fusion reactions.

Researchers Rusi Taleyarkhan, Colin West, and Jae-Seon Cho conducted the bubble fusion experiments at ORNL. At Rensselaer and in Russia, Professors Lahey and Robert I. Nigmatulin performed the theoretical analysis of the bubble dynamics and predicted the shock-induced pressures, temperatures, and densities in the imploding vapor bubbles. Robert Block, professor emeritus of nuclear engineering at Rensselaer, helped to design, set up, and calibrate a state-of-the-art neutron and gamma ray detection system for the new experiments.

In the first experiments, with the less sophisticated equipment, the team was only able to collect data during a small portion of the five-millisecond intervals between neutron pulses. The new equipment enabled the researchers to see what was happening over the entire course of the experiment.

The data clearly show surges in neutrons emitted in precise timing with the light flashes, meaning the neutron emissions are produced by the collapsing bubbles responsible for the flashes of light, Taleyarkhan said.

"We see neutrons being emitted each time the bubble is imploding with sufficient violence," Taleyarkhan said.

Fusion of deuterium atoms emits neutrons that fall within a specific energy range of 2.5 mega-electron volts or below, which was the level of energy seen in neutrons produced in the experiment. The production of tritium also can only be attributed to fusion, and it was never observed in any of the control experiments in which normal acetone was used, he said.

Whereas data from the previous experiment had roughly a one in 100 chance of being attributed to some phenomena other than nuclear fusion, the new, more precise results represent more like a one in a trillion chance of being wrong, Taleyarkhan said.

"There is only one way to produce tritium - through nuclear processes," he said.

The results also agree with mathematical theory and modeling.

Future work will focus on studying ways to scale up the device, which is needed before it could be used in practical applications, and creating portable devices that operate without the need for the expensive equipment now used to bombard the canister with pulses of neutrons.