New Delhi, March 23: Scientists today presented what they claim is the strongest evidence yet that nuclear fusion — the nuclear reaction that powers stars — can be attained on a tabletop.

The claims announced by US researchers at a meeting of the American Chemical Society at Salt Lake City, Utah, are likely to rekindle a debate over a controversial science that erupted exactly 20 years ago to the day , but was quickly discredited.

Researchers from the US Navy’s Space and Naval Warfare Systems Centre in San Diego, said they have visual evidence for high energy neutrons, a telltale signature of fusion, that had never been seen in tabletop fusion experiments until now .

Their low energy nuclear reaction experiments were aimed at coaxing atoms to fuse with one another in a relatively simple chemical laboratory apparatus. Conventional efforts to tame nuclear fusion use sophisticated furnaces and strong magnetic fields to heat and confine gases at extreme temperatures — from 10 to 100 times hotter than within the solar interior.

The US scientists passed an electric current through a solution of palladium chloride mixed with heavy water and used a special plastic to spot tracks of what they believe are high energy neutrons emitted during nuclear fusion.

“People always asked — where are the neutrons?” said Pamela Mosier-Boss, a chemist at the US naval laboratory. “We now have evidence — there are neutrons — in these low energy nuclear reactions.”

But nuclear scientists cautioned that the so-called cold fusion research has been plagued by lack of reproducibility and the new results would gain acceptance only if they could be reproduced by other groups.

“If this neutron claim is validated, it would be significant. The absence of neutrons appeared to violate laws of physics — you can't have fusion without neutrons," said Amit Roy, director of the Inter University Nuclear Science Centre, New Delhi.

“But it [the neutron observations] has to be reproduced and validated by independent people,” Roy told The Telegraph .

The first claims about cold fusion surfaced 20 years ago on March 23 1989, when two chemists Martin Fleishmann and Stanley Pons claimed they had observed heat attributed to nuclear fusion with college-level chemical apparatus. The claims had triggered speculation that fusion energy could be harnessed for energy applications through a relatively inexpensive and easy route.

But their results could not be reproduced by independent groups and cold fusion science fell into a decline. However, a few isolated groups in the US, Italy, and Japan have continued to study the phenomenon.

Physicist Mahadeva Srinivasan at the Bhabha Atomic Research Centre, Mumbai, had embarked on studies on low energy nuclear reactions in 1989, but abandoned the work a few years later.

“The recent advances in this field are not appreciated in India,” Srinivasan, since retired, told The Telegraph from Salt Lake City, where he presented some of the results of his experiments conducted during the early-1990s.

The field is also hobbled by an absence of theoretical ideas to explain experimental observations. “There’s solid evidence for nuclear reactions, but theoretical work [to support the idea of fusion] is extremely weak and unconvincing,” said Steven Krivit, a speaker at the ACS meeting, who has been tracking low energy nuclear reactions.

“One of the reasons this field is not treated with respect is because there's so much speculation about fusion,” Krivit told The Telegraph. It could be a nuclear reaction with neutrons, but without fusion, he said.

Since 1989, cold fusion studies have generated dozens of research papers, and its proponents believe widespread reproducibility is no longer a problem. But even Mosier-Boss warned that it is too early to predict when low energy nuclear reactions would be ready for real world energy applications.