Princeton Physicists Say Heating Fusion Fuel Before Compressing It Could Cut Ignition Energy
A rewrite of the 70-year-old Lawson criterion finds that raising temperature before density is cheaper, and that a trace of tungsten from reactor walls can roughly double the pressure required.
Physicists at the U.S. Department of Energy's Princeton Plasma Physics Laboratory say fusion reactors may reach ignition with less energy if they heat the plasma first and add dense fuel afterward, reversing the order most designs follow.
Luis Delgado-Aparicio, Masayuki Ono and Jonathan Menard published the work in Physical Review Letters (DOI 10.1103/mmc9-nzfx). It is a theoretical calculation, not an experiment, and the team says digital experiments will come before any test on a physical machine.
The paper reworks the Lawson criterion, the roughly 70-year-old benchmark that sets how hot, dense and well confined a fusion plasma must be for the reaction to sustain itself. The classic version leaves out several real-world effects. The Princeton team added four of them: the buildup of helium "ash" left over from fusion reactions, light and heavy impurities in the plasma, energy lost as synchrotron radiation, and heat leakage that grows as temperature rises.
With those factors included, the researchers found a more efficient route to ignition. Most approaches raise the plasma's density first and then add heat. The new model suggests heating the plasma first and then increasing density, which takes substantially less total energy to reach the same ignition conditions.
The calculation also carries a warning for reactor designers. Dozens of planned fusion machines use tungsten for their inner walls because it withstands extreme heat. The team found that even trace amounts, as little as one part in 10,000 in the plasma, can roughly double the pressure needed to reach ignition. Tungsten atoms knocked off the walls radiate away energy, which makes a fusion burn harder to sustain.
Helium ash is the helium produced when two hydrogen isotopes fuse. It stays in the plasma, dilutes the fuel and cools the reaction, which is why leaving it out of the old formula flatters the odds. Synchrotron radiation is light shed by electrons as they spiral in the magnetic field, another way energy leaks out.
The result matters because the central challenge for commercial fusion is no longer simply getting any fusion at all. It is doing so efficiently enough to produce more energy than the machine consumes. A lower-energy path to ignition would reduce the size and cost of the heating systems that make up a large part of a power plant's design.
The work sits in the same effort at PPPL to understand the physics of tokamaks, the doughnut-shaped devices used to confine plasma with magnetic fields. Because the findings rest on calculations, they will need confirmation in simulations and then in experiments. Whether real plasmas behave as the model predicts, and whether impurity levels can be kept low enough, will decide how useful the heat-first idea turns out to be for the next generation of reactors.
