Thursday, June 20, 2013

Fusion Footnote

Although the theoretical background of fusion as a source of energy has been solidly established, experiments on a smaller scale have not yet yielded a positive energy balance (less total energy needed to keep the fusion process going than comes out as a net result).
     Fusion Energy Foundation (link)
Having mentioned fusion power elsewhere, I got curious about the subject again. In a sci-fi novel I once projected a future in which tokomaks (I called them takamaks, a variant spelling) were quietly supplying the world’s energy until the ocean seemed to resist the continued extraction of deuterium from its waters. A fictional speculation, to be sure. That was ten years or more ago. So now I got curious. I wondered what had transpired since. My interest, this time, was more conventional. I wanted to know the energy balance of this technology.

I was curious for a reason. A couple of years ago, in a free-lance assignment, I looked at ethanol production in this context and discovered there that, if all energy used in making ethanol is actually counted, ethanol production has a negative balance, meaning that inputs exceed the energy actually recovered. In the fusion game, this balance is referred to as Q. The output divided by the input is  Q. A Q=1 therefore means that energy expended is equal to the energy harvested. Anything higher than 1 is a gain, meaning that the technology pays for itself in an “energy economy.” If lower than 1, the technology isn’t justified in a purely physical sense. I put that phrase in quotes because energy balance is not the same as economic balance. Ethanol is cost effective, especially since it’s subsidized; from an energy point of vantage it is a loser.

The leading fusion process attempts to fuse a deuterium atom with a tritium atom. These are forms (isotopes) of hydrogen. Hydrogen has a single proton and a lone electron. The overwhelming majority of hydrogen atoms (99.98%) are of this simple type. Deuterium has a proton and a neutron both; a minute amount of hydrogen in the ocean (0.015%) takes the form deuterium. Tritium has a proton and two neutrons. For all practical purposes it does not appear in nature, at least not for long. It is produced by cosmic rays. It can also be produced by man using lithium as a starting medium. Anyway….

When you force a D and a T close enough together, they do everything in their power to resist you. Why? They both carry positive charges and repel. You have to exert energy of 0.01 million electron volts (MeV) to overcome this resistance. When you succeed, you produce an atom of helium, a single neutron that flies off as radiation, and 17.59 MeV of extra energy. This entire reaction therefore requires the 0.01 MeV at the very last stage—by the time you’ve really squeezed the atoms together—to yield the output, thus Q=1759! That’s major, as it were.

You might wonder how much energy an MeV actually carries. Thanks to Wikipedia, I can report that 4 MeV is the stupendous force with which a single snowflake crashes down on a trembling concrete drive. Obviously, in the case of fusion, we need a whole lot of Ds and Ts colliding and fusing before we can talk about real power.

Now we know the maximum potential of this technology. It’s a whole lot more attractive than that of petroleum which, as I remember, produces something like a Q=45. And that ratio has built the modern world. So what has fusion produced thus far in its roughly 60-year history. Developments began in 1950.

The best performance on record to date was delivered by the JET program (Joint European Torus, based in Culham, Great Britain). It delivered Q=0.64 for a few seconds, the short duration being characteristic of these experimental machines. The output was 16 Megawatts, produced by the expenditure of 25 MW. The record!

The ITER program, the biggest yet and still in the building stage, expects to achieve Q=10 in short bursts and Q=5 in more sustained production runs. ITER’s original name was International Thermonuclear Experimental Reactor, but that name worried people what with scare-words like thermonuclear and experimental being so close together, so the long phrase is no longer used. ITER’s fusion reaction is being raised now in Cadarache, in France.

Now the projected Q is a long ways of 1759, of course, but you have to account for the fact that long before we have the D and T close enough together to give them that last, tiny, 0.01 MeV nudge, we have get them into hailing distance first. That requires creating enormously hot plasmas inside magnetic containers that require cryogenic cooling in the -273 Celsius range. The cooling alone is very energy-intensive.

As for timing, the facility won’t be built for another eight years or so—if all goes well. And the distance between 0.64 and 10 is great.

To wind this up, the conditions ideal for bumping the heads of Ds and Ts is at the core of the sun where gravity does most of this work, where D and T have nowhere to go, and very sensitive disturbances can’t simply cause that great plasma up there simply to fizz out—like the light in our tokomaks routinely does.

Re-post Note: This post appeared first on April 9, 2009 in the Wordpress version of LaMarotte. I have since deleted that entire blog to avoid having my work used by advertisers. A selection of such posts will be reproduced here over time.

2 comments:

  1. I'm glad you re-posted this since I missed it back in 2009 and it explains things quite nicely. Thanks. It's a post that addresses—one directly and the other indirectly—two things we touched upon in our conversation yesterday, the energy future and the age of free content in the digital economy.

    I am thinking a lot about the whole issue of how unclear, how uncertain the paths to an end seem in this time of structural change. No doubt we will keep on mulling on this subject. More conversation on the same to follow, I'm sure.

    ReplyDelete
  2. As always, successful fusion energy production is just ten years away.

    However, when it finally does happen (and it will! it will!) it will be a genuine game-changer. I just hope it makes it in time.

    ReplyDelete