In a hubristic imitation of the sun, fusion reactors combine hydrogen atoms at high temperatures. Tri Alpha heated their hydrogen sample up to 10 million degrees Celsius, or 18 million degrees Fahrenheit (the temperature at which it becomes plasma). Then a fusion reactor held that plasma in place, which is usually done with electromagnetic fields. This means that most fusion reactors have high energy demands, though they promise gigantic yields. The reactor made by Tri Alpha used 10 megawatts to heat the gases to just 10 million degrees. But to get to the three billion degrees that is required to provoke a fusion reaction, they'll need a lot more power. The design does seem capable of handling it--the field Tri Alpha created lasted for five milliseconds before their machine ran out of energy to maintain it.