Two co-existing turbulent regimes discovered in magnetically confined plasma
DOI: 10.1063/10.0003799
Two co-existing turbulent regimes discovered in magnetically confined plasma lead image
One major challenge in using tokamaks for reliable fusion power is magnetically confined plasma is prone to short, intense bursts of heat and particles, leading to disabling energy loss and reactor wall damage.
To learn more about how these bursts form and how to control them, researchers developed the helimak, a simplified version of the tokamak, enabling turbulence control by varying the radial electric field for experimentation. The geometry of the magnetic field lines in the helimak contained in the plasma are open and helical, making them easier to study than the tokamak’s closed toroidal-shaped field lines.
In their Texas Helimak study, when Pereira et al. applied a negative electrostatic bias with the intent of reducing turbulence, they found a transition region where the turbulence presents two alternating turbulent regimes. One consists of gaussian, or normal, fluctuations, in which the bursts are suppressed. The other, a burst-dominating one, is characterized by a train of short-duration pulses and bursts with random amplitudes.
Analyzing the signals from electrostatic probes at different radial positions of the machine, the researchers discovered the turbulence regime changes in the vicinity of the biasing plate, going from bursty turbulence to gaussian distribution.
“While the region close to where the bias is applied changes the turbulence regime, the far low field side region is almost unaffected by the biasing, keeping a burst-dominated turbulence,” author Felipe Augusto Cardoso Pereira said.
Meanwhile, an alternating turbulent regime was found in between, with a fraction of time in the bursty regime and fraction in the suppressed regime.
The researchers hope to develop a reliable method and model to identify and explain such transitions between the turbulence regimes.
Source: “Coexistence of turbulence regimes in the texas helimak,” by F. A. C. Pereira, D. L. Toufen, Z. O. Guimarães-Filho, I. L. Caldas, R. L. Viana, and K. W. Gentle, Physics of Plasmas (2021). The article can be accessed at https://doi.org/10.1063/5.0033381 .
