Nuclear Fusion is the process that powers stars, and could provide almost limitless, clean energy if harnessed. The problem is that Fusion requires such high-temperatures that any known container would vapourize immediately to contain the reaction.
Physicists have successfully demonstrated a “Z-pinch” to contain the nuclear reaction. Although the reaction was small, yielding only about 4mJ, the theory works and hopefully it could be scaled up to a usable size.