The effect of electron cyclotron heating on density fluctuations at ion and electron scales in ITER baseline scenario discharges on the DIII-D tokamak

Abstract

Experiments simulating the ITER baseline scenario on the DIII-D tokamak show that torque-free pure electron heating, when coupled to plasmas subject to a net co-current beam torque, affects density fluctuations at electron scales on a sub-confinement time scale, whereas fluctuations at ion scales change only after profiles have evolved to a new stationary state. Modifications to the density fluctuations measured by the phase contrast imaging diagnostic (PCI) are assessed by analyzing the time evolution following the switch-off of electron cyclotron heating (ECH), thus going from mixed beam/ECH to pure neutral beam heating at fixed . Within 20 ms after turning off ECH, the intensity of fluctuations is observed to increase at frequencies higher than 200 kHz; in contrast, fluctuations at lower frequency are seen to decrease in intensity on a longer time scale, after other equilibrium quantities have evolved. Non-linear gyro-kinetic modeling at ion and electron scales scales suggest that, while the low frequency response of the diagnostic is consistent with the dominant ITG modes being weakened by the slow-time increase in flow shear, the high frequency response is due to prompt changes to the electron temperature profile that enhance electron modes and generate a larger heat flux and an inward particle pinch. These results suggest that electron heated regimes in ITER will feature multi-scale fluctuations that might affect fusion performance via modifications to profiles.

Experimental investigation and gyrokinetic simulations of multi-scale electron heat transport in JET, AUG, TCV

“…In particular, the turbulence generated by the NL saturation of TEM is a source of 'electron temperature stiffness' for sufficiently large values of the TEM drive, resulting in an upper boundary for the normalised T e logarithmic gradient R/L Te = −R∇T e • r/T e , with R the plasma major radius and r the radial unit vector. However, it has been shown that electron temperature gradient (ETG) modes [1], which can be destabilized at electron-scales by increasing R/L Te , can also impact the electron heat transport, both by directly producing a turbulent q e and by exchanging energy with lower-k θ ITG-TEM turbulence through multi-scale coupling [2][3][4][5][6][7][8][9]. Therefore, the relative role of TEM and ETG in setting an upper boundary for the electron temperature peaking has to be determined depending on plasma parameters.…”

A combined Phase Contrast Imaging and heterodyne interferometer for multiscale fluctuation measurements in tokamak plasmas

“…The MIT Phase Contrast Imaging (PCI) group has established PCI diagnostics on DIII-D [2], Alcator C-Mod [3], and W7-X [4]. PCI has been used in the study of electrostatic turbulence and transport, with studies of including L-H transitions [5], ELMs [6], QH-mode [7], ITG/TEM instabilities and comparisons with modeling [8][9][10][11][12], I-mode [13], and the effects of ECH heating [14]. PCI has also been used in the study of MHD instabilities [15][16][17].…”