Again, we're talking about the thermal degree at which it occurs without the use of chemical reaction or other forms of energy transfer. Water boils at 212 degrees Fahrenheit and yet ice sublimates to a very small degree, so water can "become gas" at a range of temperature far below it's boiling...
Let's just say for the sake of this thread that Plasma state is the point at which a gaseous composition begins to break down and ionize. As into say, when there is enough energy to break the molecular bonds in water and it becomes ionized hydrogen and oxygen, excited to a point beyond the...
http://www.madsci.org/posts/archives/2001-02/983243860.Ph.r.html
This person gave a decent answer.
FYI 12000 K = 21140.33 °F (http://www.easysurf.cc/cnver14.htm#kf2)
"Though too bad that the water quickly recombines back. Does the water undergo this process continuously till it's leaking in the high voltage circuit? Why doesn't it continue to stay in its plasma state? "
Any matter would radiate energy until at equilibrium with it's environment.
If you...