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Simulations with more fundamental physics did not help

The success of tokamak fusion reactors like ITER depends on the burning plasma having a very hot edge with good confinement. Current tokamaks can easily get into this high performance operation (H-mode). The hot edge lifts the core to the high temperatures. However if not enough heating power is supplied to the tokamak plasma, it falls into low performance operation (L-mode) with a high-level of turbulent transport and a cold edge. A long-standing problem for theory based simulations:  What quantitatively accounts for this high-level of turbulent transport in the L-mode cold edge?
Standard “gyrokinetic” simulations and models average over the fast “gyro-orbit” motion. They account very well for the low turbulence transport in the hot tokamak core. However they significantly underpredict the observed cold L-mode edge transport.  Is some process or mechanism being left out of the standard gyrokinetic simulations?  Or is there a breakdown in the standard gyrokinetic approximation of averaging over the fast “gyro-orbit” motion when the turbulence level is high?
We started to do computer simulations with more fundamental and exact physics to definitively answer the last questions. The new and more expensive “cyclokinetic” simulations follow the fast ion gyro-orbit motion without any averaging approximation.  As required, the less fundamental gyrokinetic simulations recover the more fundamental cyclokinetic simulations when the gyro-orbit motion is fast enough compared to the slow turbulent motion. This is very much like Newton’s theory of gravity recovers Einstein’s when the gravitational forces are weak enough. However when the turbulence level is very high, as in the tokamak cold edge, and the gyro-orbit is not fast enough, the cyclokinetic transport is found to be lower than gyrokinetic transport. The mystery remains:  what is being left out of standard gyrokinetic simulations that would account for the missing L-mode near edge transport?

For more detailed information, please refer to the article “Cyclokinetic models and simulations for high-frequency turbulence in fusion plasmas” by Zhao Deng, R. E. Waltz, and Xiaogang Wang, Front. Phys. 11(5), 115203 (2016). [Photo credits: Zhao Deng]