Scientists at the US Department of Energy's Princeton Plasma Physics Laboratory (PPPL) claim to have discovered a technique to control disruptions within fusion plasmas.
If nuclear fusion can be cracked, it should provide a cleaner, more abundant and more efficient source of energy than nuclear fission. In a fusion reaction, two atoms join together to form one, while releasing vast amounts of energy in the process.
Theoretically, harnessing nuclear fusion in a reactor appears to be a very simple activity. But practically, scientists have found it extremely difficult to come up with a controllable, non-destructive way of doing it.
Some of the major challenges faced by scientists while designing a fusion reactor include plasma heating, exhaust of energy and particles, alpha particle heating, reactor safety and environmental compatibility. Disruption in fusion plasmas is another major issue, which hinders development of fusion reactor technology.
In the current study, scientists focused on tearing modes - the instabilities that generate magnetic islands in the plasma. Magnetic islands are bubble-like structures which can grow in size and act as the key source of plasma disruptions. They initiate disruptive events that stop fusion reactions and damage the facilities where these reactions are carried out.
In 1980s, scientists found for the first time that tearing modes can be stabilised by driving current in the plasma using radio-frequency (RF) waves.
PPPL scientists have now found that stabilisation of tearing modes could further be improved with small changes in plasma temperature, once a key threshold in power is exceeded.
The scientists said that the perturbations in plasma temperature affect the strength of the current drive as well as the amount of RF power deposited in the magnetic islands. This results in a nonlinear feedback mechanism between the temperature perturbations and their impact on the deposition of power.
The level of stabilisation improves when the feedback combines with the sensitivity of the current drive to temperature perturbations. Scientists have named the mechanism as 'RF current condensation'.
"When the power deposition in the island exceeds a threshold level, there is a jump in the temperature that greatly strengthens the stabilising effect. This allows the stabilisation of larger islands than previously thought possible," said Allan Reiman, a theoretical physicist at PPPL and lead author of the paper.
The findings of the study are published in journal Physical Review Letters.
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