The main problem is how to contain something that is 100 million degrees in temperature. That is the temperature required to start a nuclear fusion reaction. Any containment vessel would be vaporized instantly. Scientists are trying magnetic bottles, that is: areas of intensely high magnetic fields that will hold the reacting hydrogen molecules in place. Another alternative is the injection of very tiny exploding pellets in a series of pulses, which are held in place by the pressure wave of the explosions coming one after the other. There has been very limited success in both these methods, but nothing significant yet.