Burning plasma fusion reactions, thought to be crucial for building working fusion reactors, are producing more high-energy particles than researchers expected. Solving the mystery of why could be key to making fusion viable.
When a highly compressed and hot gas called a plasma begins to heat itself up from internal reactions, it is said to be a burning plasma, a state of matter only recently created at Lawrence Livermore National Laboratory鈥檚 National Ignition Facility (NIF) in California. We expect the charged particles, or ions, in the plasma to have an even distribution of energies because they collide so frequently that they will quickly transfer their energy between one another.
But when at NIF and his colleagues calculated the ion energies produced from a burning plasma, they found an uneven distribution.
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NIF uses a laser that creates heat and pressure to turn a deuterium and tritium fuel into a burning plasma in which fusion occurs. Each time the team fired the laser into the fuel, called a shot, the ion energy pattern became clearer. 鈥淚t was a very subtle statistical effect, but then as we got more shots, and more analysis, and did more shots after the ones that we published, we saw that this was a very robust trend in the data,鈥� says Hartouni.
They measured the energies of neutrons expelled by the burning plasma along five 20-metre paths, which allowed them to calculate the energies of the ions that produced them. They found that there was a very slight uneven spread in the neutrons鈥� energies, which implied the same for the ions. 鈥淭hese results show that the deuterium and tritium ions, when they come together to fuse, have more energy than we were expecting,鈥� says team member .
Because the shifts in neutron energies are so comparatively tiny, 鈥渢he experimental measurements here are extremely difficult鈥�, says at Imperial College London. But if they are borne out in future experiments, understanding how the plasma heats up and the ions move will be essential for harnessing fusion as a power source.
It isn鈥檛 clear why the ions are heating in this way, but one possible reason could be energy imparted from the alpha particles, which are helium nuclei, that are produced in the fusion reaction. 鈥淚t鈥檚 kind of challenging our understanding of how the fast ions moving through the plasma are actually heating it, and where they鈥檙e ending up,鈥� says Chittenden.
Nature Physics
