Opposite of Cryogenics Cryogenics is related to very low temperatures, so, what is the term when referring to very high temperatures?
 A: Plasma physics. Sufficiently high temperatures will turn any material into a plasma (decomposing any molecules in the process).
A: To give MSalter's answer a "ics" word you say "magnetohydrodynamics".
However, I don't think the parallel is perfect. One can say that you have a cryogenic gas system or a where you simply mean "very cold" or  cyrogenic hydrogen target (if you're a particle physicist, say) where you mean "cold enough to be liquid", but there doesn't seem to be latin word for "very hot": we have "high temperature ceramics".
"Thermogenic" would seem to be the obvious parallel construction, but it's taken.
A: I would say simply "high temperature physics", just as the Kurchatov's institute for High Temperatures (litteral translation) in Russia. 
I would be careful with the "magnetohydrodynamics" term used by dmckee, as this term covers a discipline in which conductive fluids move while undergoeing electro magnetic forces. You can do magnetohydrodynamics (MHD for intimates) with just very salty water, two blades, a 9 V battery and a Neodymium (NdFeB) magnet, at home: put the magnet under a dish filled with salty water, connect the battery to both blades (+, -), and put the blades inside the water, face to face, above the magnet: if your magnet's field reaches the inside of the dish, you should see the water move. This is simply Lorentz force acting on a conductive, liquid medium. Very funny to do. But has nothing to do, at least not in all cases, with high temperature physics: MHD can be used with plasmas and hence high temperature physics, but plasmas are not necessary to observe MHD, as well as a magnetic field (without which there would be no Magneto-hydro-dynamics) is not necessary to have interesting applications with high temperature physics, and plasmas.
Finally, note that high temperature physics is not automatically related to "gaseous" plasmas: the center of the earth, the center of white dwarfs (the stars), are very "hot" media, but are not really comparable to a free, gaseous plasma. This field is more related to the "warm dense matter" (as are some nuclear experiments), an active subject of research nowadays.
