Collisionality scaling of the electron heat flux in ETG turbulence

Abstract

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 longtime saturated electron heat flux (the level most relevant to energy transport) decreases as the electron collisionality decreases. At early simulation times, the heat flux "quasisaturates" 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.

Ion-scale turbulence in MAST: anomalous transport, subcritical transitions, and comparison to BES measurements

“…Figure 6(b) shows that the electron heat flux, Q exp e /Q gB , is not fully captured by our nonlinear ion-scale simulations: namely, in our simulations, we observe Q e /Q i ∼ 0.6, whereas from the experiment we expect Q exp e /Q exp i ∼ 7.6 (see figure 3). It is likely that electron- scale turbulence [14,28,21,22,23,68] is present in the real machine, while it cannot be resolved in our simulations, and that it dominates electron heat transport. Thus, given the likely existence of turbulence on both electron and ion scales, a programme of gyrokinetic simulations capturing electron and ion scales simultaneously would ideally be necessary.…”

Impact of plasma parameter on self-organization of electron temperature gradient driven turbulence

“…The energy injection rate is plotted by using its absolute value, and blue and green symbols indicate energy injection (þ) and dissipation (Â), respectively. In (b), the power laws of the inverse energy cascade and the normal enstrophy cascade, (7) and (13), are shown by arrows. FIG.…”

“…11,12 The suppression of electron heat transport by the electron scale zonal flows is also confirmed in the toroidal ETG turbulence. 13 The generation and saturation mechanisms of ETG zonal flows have been discussed based on various secondary and tertiary instabilities such as the modulational instability driven by pump waves, 14,15 and the Kelvin-Helmholtz-like instabilities excited by shear flows in linear streamers 8,16 and in zonal flows. 11 On the other hand, another mechanism to determine ETG zonal flows was discussed based on the selforganization in 2D rotating fluid turbulence.…”

A scale-separated approach for studying coupled ion and electron scale turbulence

“…This approach allows one to simulate multi-scale turbulence using electron scale flux tubes nested within an ion scale flux tube.Scale-Separated Turbulence otherwise questionable. Examples may be found in [11][12][13][14][15][16][17]. Nonetheless, it is known that electron scale turbulence can drive experimentally relevant levels of transport in some cases [13].…”

“…Nonetheless, it is known that electron scale turbulence can drive experimentally relevant levels of transport in some cases [13]. Electron scale transport has been observed on NSTX [18], and is a candidate for anomalous transport on MAST [14,17].Without directly simulating or observing the full multi-scale turbulence, it is difficult to assess to what extent there are cross scale interactions in the turbulence, and whether or not all scales will contribute significantly to the transport. Unfortunately, studying multi-scale turbulence through direct simulation is made very challenging by the size of (m e /m i ) 1/2 for a realistic deuterium plasma, (m e /m i ) 1/2 ∼ 1/60, which determines the separation of ρ th,e /ρ th,i ∼ (m e /m i ) 1/2 and v th,i /v th,e ∼ (m e /m i ) 1/2 .…”