Explaining a mysterious barrier to fusion known as the 'density limit' November 10, 2015 Please Register or Log in to view the hidden image! Visualization of a magnetic island growing in a tokamak plasma. Credit: Qian Teng For more than 50 years physicists have puzzled over a daunting mystery: Why do tokamak plasmas spiral apart when reaching a certain maximum density and halt fusion reactions? This "density limit" serves as a barrier that prevents tokamaks from operating at peak efficiency, and understanding what sets this maximum density would speed the development of fusion as a safe, clean and abundant energy source. Recently, researchers at the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL) have revisited an old idea: bubbles called magnetic islands that form in the confining magnetic field produce the observed density limit. The PPPL team, led by David Gates with Roscoe White, Luis Delgado-Aparicio and Dylan Brennan, found new physics overturning decades of thought on the growth of these bubbles. Read more at: http://phys.org/news/2015-11-mysterious-barrier-fusion-density-limit.html#jCp
Tokamaks use plasma mirrors: https://en.wikipedia.org/wiki/Magnetic_mirror A big issue with a typical plasma mirror is that eddy currents are continuously produced and dissipate in any metal pieces that surround the plasma within range of the magnetic field of the plasma mirrors. Anyone can see, this will not work. In order to eliminate the magnetic field losses due to eddy currents, the containing magnets must be superconductors, which at the time of this writing must be maintained at low temperatures. The lossy mirrors will work less and less like a plasma mirror is supposed to the lossier they are. Low temperatures are just something that are going to be difficult to maintain anywhere near a working fusion reactor. Does that help clear up the issue? I'm not saying that a fusion reactor might not become possible in some other configuration (like fusion reactor versions of fuel pellets), only that the chief design flaw and weakest link of the Tokamak has always been its plasma containment mirrors. The record highest magnetic field strength achievable on Earth is on the order of 25 Tesla, and fields approaching that are utilized in the dipole steering and quadrupole focusing magnets of the LHC. The dual beams of protons circulating in the LHC are a plasma of sorts, and there seems to be little difficulty maintaining beam integrity for periods of about 10 hours. But it takes a lot of liquid helium in order to maintain that, and the density of the plasma is not very high, although at 10 TeV, it is quite energetic. Perhaps a circulating plasma design would be a better way to go. You could simply use intermittent beam dumps to remove and use the excess energy stored in the reactor. The idea needs some considerable work to make it a power plant, but at least the means of magnetic containment is not in doubt.
