What is the role of plasma in nuclear fusion energy production? I read that confined plasma in electromagnetic field uses in TOKAMAK nuclear fusion reaction. But I don't know what is the main role of plasma in fusion reaction. Is it use for producing energy required to fuse nuclei or something else?
 A: You may have things a bit mixed up.
Plasma is not something that plays a role in fusion as if it were a tool or an instrument for its achievement. It is instead the only possible medium where nuclear fusion can occur: very basically, high enough temperature for protons to overcome the Coulomb repulsion, and high enough density for increased chances of fusion reactions.
So it is an intimate part of nuclear fusion, rather than an appendix or a component of it. All components of nuclear fusion rather revolve around the plasma: how to heat it, how to contain it, how to shape it, how to control it, etc. 
Whether in nature or in laboratories, these questions are answered by ancillary structures (e.g. large gravitational and magnetic fields, vacuum vessels, magnets, neutral beam injectors, radio-frequency antennas, lasers or solenoids), but the plasma will always be the central and indispensable part of the whole process of producing energy through nuclear fusion.
A: At the temperatures required for fusion reactions, matter can only exist in the plasma state. That's rather inconvenient for designers of fusion reactors, since plasma is hard to control and contain.
And that's why the claims about cold-fusion were so appealing. Cold fusion promised to provide fusion power without the hassle of dealing with high temperature plasma. Unfortunately, those claims turned out to be false.
