Cleaning a fusion reactor mirror with controlled plasma discharge
Cleaning a fusion reactor mirror with controlled plasma discharge lead image
Observing a fusion reactor in progress is a tricky task due to the extreme environment. The detectors needed cannot be directly exposed to the reactor due to strong magnetic fields and particle bombardment, so instead a mirror is placed to give the diagnostics systems an indirect line-of-sight to the fusion plasma. However, eroded material from the reactor chamber walls will deposit onto the mirror surface, degrading the mirror’s optical properties over time.
Sackers et al. studied a possible solution to this problem, where a controlled plasma discharge is used to sputter away the deposited material and restore the mirror to working condition.
“In our study, we simulated coverage using a mirror mock-up made of tungsten to characterize in detail the conditions enabling a successful cleaning,” said author Marc Sackers. “The main requirements are limiting damage to the mirror, homogeneity of the processes, and cleaning speed. Our work illustrates operating scenarios of the discharge that fulfill these.”
The researchers employed a tungsten mirror to simulate the deposition of tungsten, the hardest material in the reactor chamber. They ignited argon plasmas in front of the mirror while varying plasma pressure and power coupled into the plasma to determine the optimal parameters to clean the mirror without damaging it.
Determining if the mirror’s optical properties are sufficiently restored is crucial, so the team is currently studying one such method that relies on backscattered hydrogen atoms from a local plasma.
“This approach makes it possible to provide an end of cleaning indicator,” said Sackers. “The admixture of hydrogen to argon changes the conditions in the plasma to an extent that has to be investigated in the future.”
Source: “Plasma parameters and tungsten sputter rates in a high-frequency CCP,” by M. Sackers, C. Busch, Ts. V. Tsankov, U. Czarnetzki, Ph. Mertens, and O. Marchuk, Physics of Plasmas (2022). The article can be accessed at https://doi.org/10.1063/5.0083613 .
