Burning Plasma

In August I received an urgent email from my brother with the title “Fusion”. The National Ignition Facility (NIF) had created a burning plasma — a star on Earth — a major milestone on the long road to controlled nuclear fusion.

A plasma is an ionized gas, but in this context “burning” does not mean chemically combining with oxygen. Rather it refers to the nuclear reaction

{}^2_1 \mathrm{H}^{+} + {}^3_1 \mathrm{H}^{+} \rightarrow {}^4_2 \mathrm{He}^{++} + {}^1_0 \mathrm{n} + 18~\text{MeV},

where hydrogen isotopes deuterium and tritium combine to form helium. Nuclear reactions produce millions of times the energy of chemical reactions, but they are hard to initiate as hydrogen ions repel one another electromagnetically unless they are close enough to allow the short-ranged nuclear force to convert mass energy into kinetic energy.

NIF uses the world’s most powerful laser, capable (briefly) of almost 1000 times the power output of the United States’ electrical grid. The ultraviolet laser pulse heats a small gold cavity or hohlraum, which converts the incident radiation into x-rays, which causes an enclosed deuterium-tritium pellet to implode, which increases the pellet’s temperature and density to initiate fusion.

After missing its 2012 ignition goal, and steadfastly fine-tuning the experimental design since then, the August shot generated much more energy than was delivered to the pellet (but slightly less energy than was delivered to the hohlraum). NIF director Mark Herrmann reported that, “Everyone has a spring in their step”. To me, controlled nuclear fusion, like humans on Mars, has always seemed 20 years away; with progress at NIF (and elsewhere), today it seems closer.


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