Collisionality scaling of the electron heat flux in ETG turbulence
Plasma Physics and Controlled Fusion
Reason for embargo
In electrostatic simulations of MAST plasma at electron-gyroradius scales, using the local flux-tube gyrokinetic code GS2 with adiabatic ions, we find that the long-time saturated electron heat flux (the level most relevant to energy transport) decreases as the electron collisionality decreases. At early simulation times, the heat flux 'quasi-saturates' without any strong dependence on collisionality, and with the turbulence dominated by streamer-like radially elongated structures. However, the zonal fluctuation component continues to grow slowly until much later times, eventually leading to a new saturated state dominated by zonal modes and with the heat flux proportional to the collision rate, in approximate agreement with the experimentally observed collisionality scaling of the energy confinement in MAST. We outline an explanation of this effect based on a model of ETG turbulence dominated by zonal-nonzonal interactions and on an analytically derived scaling of the zonal-mode damping rate with the electron-ion collisionality. Improved energy confinement with decreasing collisionality is favourable towards the performance of future, hotter devices.
This work was supported in part by EPSRC under grant numbers EP/H002081/1, EP/I501045/1, EP/L000237/1 and EP/M022463/1; and by the EC under grant agreement number 633053. The work of AAS was also supported in part by grants from the UK STFC and EPSRC. YcG was supported by the National R&D Program through the National Research Foundation of Korea (grant number NRF-2014M1A7A1A01029835), and the KUSTAR-KAIST Institute. WD was supported by the US DOE grants DE-FC02-08ER54964 and DE-FG02-93ER54197. Computations were performed at the UK's HECToR and ARCHER services, on EFDA's HPC-FF facility, and on the Helios system at IFERC-CSC.
This is the author accepted manuscript. The final version is available from IOP Publishing via the DOI in this record.
Vol. 59 (5), article 055002