Can fusion arrive soon enough?

 kw: book reviews, nonfiction, science, controlled fusion, renewable energy

Could this power our future?

This is the inside of a Tokamak, a bagel-shaped magnetic "bottle", as the temperature in the plasma (the violet stuff) is ramping up toward 100 million degrees or so, a temperature needed to trigger fusion between deuterium and tritium nuclei.

The Star Builders: Nuclear Fusion and the Race to Power the Planet, by Arthur Turrell, is a hopeful progress report of sorts, on the one near-renewable energy technology that just might secure the future of the human species on earth.

I expect that the technology will soon drop the adjective "nuclear", and just be called "fusion" or "fusion energy", because of intense public antipathy towards all things nuclear, based on hyped-up fears of future disasters such as those at Three Mile Island, Chernobyl, and Fukushima. Maybe we can invent a new adjective to distinguish between "fusion" meaning fusion energy, and other uses of the word, such as "Asian fusion", for eclectic Eastern cuisine, and "The fusion of enlightenment and entertainment", the purview of media host Glenn Beck.

The recipe for sustaining energy-producing fusion reactions is simple. For any type of nucleus, or pair of nuclei, a specific combination of temperature and pressure is needed to initiate the fusing of nuclei into bigger ones, and there is a well known function that relates the amount of fusion energy produced, per kg of "stuff", at any combination of pressure and temperature increase beyond that threshold. As Dr. Turrell describes, the "easiest" reaction is between deuterium (hydrogen with a nucleus containing one proton and one neutron) and tritium (hydrogen with a nucleus containing a proton and two neutrons). In the conditions a Tokamak is aiming for, with a pressure of a few millibars (about 1/100 atmospheric pressure), the threshold temperature is 150 million K (=150 million °C, minus 273, a trifling amount we can ignore; also equal to about 270 million °F).

It might seem trivial to work with a low pressure like that, but the issue is not pressure, but the consequences of searingly hot plasma touching the materials of the "bottle", such as the metal walls of a Tokamak. Consequence 1: the metal boils. Consequence 2: the plasma cools off, quenching any fusion reaction. Result: the device won't make energy, and it may be destroyed.

Thus, there are three ways to keep the plasma "in the bottle": magnetic confinement, inertial confinement, or gravitational confinement. Gravitational confinement is what every star uses to keep its fusion engine working.

A star with a mass of at least 0.075 that of the Sun, or about 25,000 times the mass Earth, has a dense enough core, at a high enough temperature, for about one in a trillion collisions between two hydrogen nuclei (protons, with no neutrons at all), to fuse into a deuteron (a deuterium nucleus), absorbing an electron (or kicking out a positron) in the process to convert one proton to a neutron. Deuterons more readily fuse with more protons to produce tritons (tritium nuclei) and then again to form helium nuclei (also with the absorption of another electron or ejection of a positron). The threshold temperature is several million K, at a pressure of a few million atmospheres.

On Earth, gravitational confinement is out, so magnetic or inertial methods must be used. The Tokamak is one kind of magnetic "bottle", and is currently favored as the most likely to "work." A version called the Spherical Tokamak, shaped more like an apple than a bagel, is described and may wind up working better. Current research is aiming to deal with huge amounts of instability that occur when you try to keep a superhot plasma "in the bottle."

Inertial confinement is based on methods that compress a pellet of fusible material by a factor of a hundred or so (in diameter; a million-fold in volume), which pushes the temperature to millions of degrees, and any fusion that is going to happen is completed in a few billionths of a second. Nothing works that fast except laser light.

The National Ignition Facility (NIF) uses the world's most powerful laser system, 192 lasers that create about 400 megajoules of infrared energy. A megajoule is a million watt-seconds, or 278 watt-hours. A "shot" begins with a pulse, 20 nanoseconds long, in 192 beams, with a total of 53 kilowatt-hours of energy. The light is up-converted twice to become about 2 megajoules of UV light, still compressed into 20 ns, which gets focused into two tiny openings, each a little bigger than a pinhole, at the ends of a barrel-shaped gold capsule called a hohlraum, that contains a fuel pellet (hydrogen isotopes at the center of other layers). This deposit of an incredi-jillion watts per square whatever, on the inside surface of the hohlraum, produces X-rays that compress the fuel pellet. Fusion has to happen in one or two ns, while inertia keeps the pressure in the now-compressed capsule at some unimaginable number of gigabars (billions of atmospheres) of pressure, at millions of degrees. NIF was built for research. A power plant using the technology would have to drop a fuel pellet every few seconds (and that consumes a lot of gold), for years and years, to be a commercial energy-producing reality. After every shot at NIF it takes days to repair damage caused by the immense power of the lasers.  Yeah, I'd say the odds are pretty long…

As of the writing of Star Builders, a Tokamak has achieved 67% of "ignition", and NIF has hit 3%. Commercial power production requires not just ignition, not just break-even (power out = power in), but 10x to 100x of break-even. What if running a 500 MW power plant consumed 400 MW? The remaining 100 MW that could be sold will be costly indeed, and there will be an incredible waste heat problem. A 500 MW fusion plant must consume no more than 1% to 10% of its own energy.

Other than waste heat, other wastes represent an area where fusion energy excels. Current nuclear fission power plants produce radioactive waste, which must be safely stored or disposed of. Nuclear fusion, on the other hand, does produce some radioactive waste, but only in very small amounts, comparatively. Any nuclear process releases lots of neutrons, but a fusion plant can be built of materials that either don't absorb many neutrons or do not become radioactive by doing so.

The big question is this: How soon will any of these schemes be ready for prime time? From that point, how long will it take to build a few thousand power plants, worldwide, to take over electricity generation from coal (still #1 worldwide), natural gas, oil, and all the rest. I'd include windmills in that, because they aren't really renewable; they are an arcane way of turning electricity in one place into steel, aluminum and other materials, and using wind to recover most (but NOT ALL!) of the original energy used to make the windmill, until the windmill must be replaced.

The only energy technology that is (only since about 2010) renewable over its life cycle: Solar-electric panels. But this has big drawbacks, the biggest being, the Sun shines only half the day. Battery storage is still too costly and its energy density is too low, to make much of a dent in the problem.

Dr. Turrell is optimistic. To him, we are dramatically under-invested in fusion research. If throwing money at the problem could speed it up, so a facility somewhere achieves commercially-ready fusion power by 2024, that would be great, but could we then ramp up fast enough to save ourselves from the climate change that is growing around us?

Disclaimer: I know "warming" is in part human-caused. I am not convinced it is an absolute negative. The impact will be negative in some places, and positive in others. Bet on it.

But can we grow fusion power enough to stop making carbon dioxide completely? Can we do it fast enough to avert a climate crisis? Not without making more use of fission energy, while the research is completed and fusion (when it arrives) gets rolled out worldwide. The sideboards on my estimate of the time frame are between 2040 and 2140...or later. A lot is still not known. Interesting times are ahead! This book will be a useful reference in years ahead.