Sorry, But Fusion Power Result Was Disappointing
This week, the National Ignition Facility (NIF) laser-driven fusion project reported a greater energy output than input. Media revelry ensued. I’m here to rain cold water all over the parade.
Reading the report, published in Nature, I calculated the efficiency of this system. The result is disappointing, to say the least: 28,000 units of energy are drawn from the power grid to create one single unit of fusion energy.
So, what’s going on? Why doesn't this result live up to the hyped reports? To understand the answer, a little knowledge of the underlying physics is necessary.
NIF’s fusion reaction occurs in a 2 mm spherical pellet of deuterium and tritium, inside a tiny gold can called a hohlraum. This cylinder is itself only 6 mm in diameter and 9 mm long. The energy delivered to crush the fuel pellet and trigger fusion comes from imploding the hohlraum with massive laser power.
To achieve this, 192 laser beams are focused on the hohlraum and synchronized within a margin of error of a single nanosecond. (If the lasers are out of sync, the pellet will be crushed unevenly, creating a pancake or cigar shaped blob instead of a smaller sphere. These configurations are less compressed, hindering fusion.)
Though the laser beams fire some 1.8 million joules of energy into the hohlraum, the pellet absorbs a measly 12,000 joules of energy. After fusion occurred, the pellet released 15,000 joules of energy. Success! Energy output was greater than input.
But, hold on. Let’s not forget about all that wasted energy. Remember, the laser beams shot out 1.8 million joules of energy, meaning that more than 99% of the incoming energy was lost en route!
It gets worse. A tremendous quantity of electricity is required to charge up the capacitors that are needed to create the laser blast. The total energy of this stored electrical charge is 422 million joules. The laser only fired 1.9 million joules into the hohlraum, so 99.5 percent of the energy was lost before it was even fired by the laser. Then 99 percent of the laser’s energy was lost remaining before hitting the pellet.
In other words, for every unit of energy extracted from the fuel, 28,000 units had to be put in! That’s neither economical nor practical, and it doesn’t provide much reason for optimism. NIF hoped to be doing 100 times better by a year ago.
Still, this should be considered progress down the unimaginably long and difficult trail to commercial fusion power.