From Science Alert:
While nuclear fission requires things to be heated to just a few hundred degrees Celsius, nuclear fusion machines have to recreate conditions on the Sun, so we’re talking several million degrees here.
And because nuclear fusion machines are basically starting their reactions from scratch, we first need to achieve temperatures far hotter than those estimated to exist in the centre of the Sun – at least 100 million degrees Celsius.
So far, the closest anyone’s gotten to the dream of limitless energy is a team of physicists at the Wendelstein 7-X stellarator in Greifswald, Germany, and researchers at China's Experimental Advanced Superconducting Tokamak (EAST) – both of which have been trying to hold onto the super-heated plasma that results from the fusion reaction.
“During the process of nuclear fusion, atoms' electrons are separated from their nuclei, thereby creating a super-hot cloud of electrons and ions (the nuclei minus their electrons) known as plasma,” Daniel Oberhaus explains for Motherboard.
“The problem with this energy-rich plasma is figuring out how to contain it, since it exists at extremely high temperatures (up to 150 million degrees Celsius, or 10 times the temperature at the Sun’s core). Any material you can find on Earth isn’t going to make a very good jar.”