Scientists have been chasing the dream of harnessing the reactions that power the Sun since the dawn of the atomic era. Interest, and investment, in the carbon-free energy source is heating up. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For the better part of a century now, astronomers and physicists have known that a process called thermonuclear fusion has kept the Sun and the stars shining for millions or even billions of years. And ever since that discovery, they’ve dreamed of bringing that energy source down to Earth and using it to power the modern world. It’s a dream that’s only become more compelling today, in...