A fire Thursday night at the National Institute for Fusion Science in Toki, Gifu Prefecture, damaged 5 million yen worth of research equipment.
The blaze was put out in about 30 minutes and no one was injured, officials of the government facility said.
The fire erupted inside the large helical device laboratory at around 9:40 p.m. and damaged a power protection unit, including its steel case and other objects nearby, the officials said.
The institute, administered by the education ministry, is involved in experiments to produce fusion plasma through the helical device, the largest in the world.
The experiments involve heating hydrogen with high frequency electromagnetic waves. The institute does not handle radioactive materials, the officials said.
On Friday, police began probing the site and questioning researchers on how the equipment works to better determine the cause of the fire.
The power protection unit, which costs 5 million yen, was housed in a steel case 3.5 meters high and 3 meters wide.
The unit protects the power source by releasing electricity that builds up inside the high frequency transmitter during experiments.
Officials said researchers switched off the heating unit's power at around 9:20 p.m. before ending Thursday's experiments.
On hearing the fire alarm, about 30 administrative workers and researchers inside the institute rushed to the scene and put out the blaze with fire extinguishers, the officials said. No one was inside the lab when the fire broke out.
The government founded the institute in May 1989 to promote fusion plasma research.
In October 2001, institute researchers succeeded in producing fusion plasma with an electron temperature of 100 million, which is believed to be one condition necessary for nuclear fusion.
The institute is used by universities across the country.
