Are computer simulations of core turbulence realistic?
Gyrokinetic simulations are the most advanced and physically realistic model of plasma turbulence we have, but how realistic are they exactly? In a new study (https://doi.org/10.1063/1.5018930), the amplitude of the fluctuations, their radial size, and the phase angle between density and temperature fluctuations are compared to simulations which are able to match the experimental heat fluxes.
Turbulence measurements on ASDEX Upgrade
The loss of heat via turbulence in tokamak plasmas is the reason fusion reactors will need to be bigger than was expected when they started out making them in the 50s. The understanding and control of turbulence is therefore very important. For the first time ever, fluctuations which give rise to electron heat loss from tokamaks have been detected in at the ASDEX Upgrade tokamak. The first measurements took place in Helium plasmas and were subsequently detected in the more common Deuterium plasmas. These measurements allow a direct test of very sophisticated computer codes, capable of predicting the turbulent heat loss and lead to a better understanding of the drive and behaviour behind it.
You can find out more of the details here [S.J. Freethy, RSI (2016), http://dx.doi.org/10.1063/1.4958908]. This project is part of a growing basis of fluctuation measurements on ASDEX Upgrade, with new diagnostics under development to probe this important question even deeper.
Fast electron studies during ELMs on MAST
The Edge Localised Mode, or ELM, is a solar flare like instability which takes place at the edge of a fusion plasma. We’re not too worried about them on current machines, but future machines could be damaged by such events. Microwave measurements made on the MAST tokamak , showed that intense bursts of microwave radiation had to come from fast moving electrons accelerated by a strong parallel electric field. This was supported by JOREK modelling (shown above).
For more information, check out the paper [S.J. Freethy, PRL (2015), http://dx.doi.org/10.1103/PhysRevLett.114.125004 ]
Lensless imaging for Fusion reactors
Phased arrays and Synthetic Aperture imaging techniques have a long history of development, but their first application to tokamaks was in 2012 by the SAMI (Synthetic Aperture Microwave Imaging) team [V.F. Shevchenko, J. Inst. (2012), http://dx.doi.org/10.1088/1748-0221/7/10/P10016]. The SAMI device is able to simultaneously get a full field 2D image of passive thermal emission, and image radiation reflected from the plasma [S.J. Freethy, PPCF (2013), http://dx.doi.org/10.1088/0741-3335/55/12/124010], making it an advanced and flexible diagnostic for fusion plasmas.
In its inception, it was designed to answer questions about Bernstein wave mode conversion on the MAST tokamak, it has grown from there to use Doppler Reflectometry imaging to determine magnetic field pitch angles [D.A. Thomas, Nucl. Fusion (2016), http://dx.doi.org/10.1088/0029-5515/56/2/026013]