Thursday, October 03, 2013

Nuclear Fusion and Imploding Porcupines

When the sun makes energy through nuclear fusion, it has the benefit of not having to pay real cash for the energy expended to make more energy. Here on Earth, the effort to make fusion energy affordable and practical has been a lot tougher, though the payout is potentially so great – and the benefits manifest - that much effort has gone into it.

Every now and then we get a peek into the fusion world, which is almost always almost ready to almost produce a plausible reactor. And this will happen – I’m sure of it – someday.

To create fusion reactions, the NIF [National Ignition Facility] scientists fire lasers into a hohlraum, or a hollow cylinder made of gold. The laser pulses, lasting billionths of a second, hit a tiny sphere that is full of deuterium (hydrogen with an extra neutron) and tritium (hydrogen with two extra neutrons).

As the laser beams hit the hohlraum, the gold emits X-rays that are so powerful they vaporize the metal surface of the sphere. That vaporization puts immense pressure on the deuterium and tritium, and induces fusion, smashing the hydrogen atoms into helium, plus one neutron.

This is very basically what the sun does, minus the hohlraum and gold, but featuring hydrogen and helium. But this part isn’t where the economics begin to work. This is:

The problem is that even tiny imperfections in the surface of the sphere will mean the pressure on the deuterium and tritium isn't perfectly even all the way around. Result? "It implodes like a porcupine," Edwards told LiveScience. This uneven "reverse explosion" results in energy waste so that more energy is put into the system than comes out of it.

Well, that’s an unfortunate metaphor. Do porcupines implode? We hope the National Ignition Facility’s John Edwards didn’t find a way to do that in a previous job.

This is the kind of sentence we’re used to in fusion stories:

Right now, the amount of energy coming out of the NIF setup is about 80 percent of what is put in.

That’s like the sun, too, though the sun doesn’t need to worry about break-even. Jesse Emspak’s story goes further to express the perpetual doubt about fusion, but, like so many others, including myself, he hopes for the best:

Still, Edwards is optimistic. "Our goal is to demonstrate that ignition is feasible," he said. "We've made a huge amount of progress, and we're close to achieving what our calculations say should be happening in a regime slightly less demanding than full-up ignition implosions."

“Close to achieving.” Let’s look forward to the day when that phrase and fusion are not logically linked. The whole story’s worth a read.

Here’s the Web site for NIF – it’s part of Lawrence Livermore National Laboratory in California. On its homepage, it points out that Star Trek: Into Darkness was filmed there, so there’s that. Here’s its page on fusion if you’d like to learn more.


jimwg said...

Re: "On its homepage, it points out that Star Trek: Into Darkness was filmed there, so there’s that."

I believe segments of the original "Tron" was filmed there too long before. ("Wow, that's a Big door!...")

James Greenidge
Queens NY

Nick said...

Hi Mark Flanagan,
Great article and well written.
Mark, if the shell isn't evenly coated, here is a suggestion.
In the early 70's our government needed to extremely perfect circular ball bearings.
At the time, there was only one solution. Manufacture these in the weightlessness of space.
The first few tries were a huge improvement, but weren't good enough.
Yet in less than 3 years, the world's most perfect ball bearings were being produced at a very lower cost for their need.
So encapsule and then encoat the burnable shield in the same weightlessness.
And today, it doesn't have to be done in space.
Just a suggestion.
Nick Negoescu