Scientists in Germany said Thursday they had reached a milestone in a quest to derive energy from nuclear fusion, billed as a potentially limitless, safe and cheap source. [...] After spending a billion euros and nine years' construction work, physicists working on a German project called the "stellarator" said they had briefly generated a superheated helium plasma inside a vessel -- a key point in the experimental process. [...] The German experiment, using a machine called Wendelstein 7-X, was aimed at seeing whether it was possible to heat helium atoms with a microwave laser and to briefly contain the plasma...