Analogous Saturation Mechanisms of the Ion and Electron Temperature Gradient Drift Wave Turbulence

New experimental results and theoretical arguments indicate that a novel saturation mechanism of the electron temperature gradient modes is related to its coupling to a damped ion acoustic mode. The experimental bicoherence data show multimode coupling between two high frequency radial harmonics of electron temperature gradient in the vicinity of ($\sim 2\mathrm{MHz}$) and one low frequency ion acoustic ($\sim 45\mathrm{kHz}$) mode. A unique feedback diagnostic also verifies this coupling. It is pointed out that a near identical mechanism is responsible for ITG mode saturation [V. Sokolov, and A. K. Sen, Phys. Rev. Lett. 92, 165002 (2004)], indicating its plausible generic nature.

Figure 1

Scheme of the CLM and diagnostic setup.

Figure 2

Power spectra of potential fluctuation.

Figure 3

The bicoherence of ETG mode coupling, frequency units are in kHz. (a) Bicoherence and corresponding power spectrum. (b) A close-up of the bicoherence.

Figure 4

Radial profiles of electron temperature and inverse scale length $1/L_{T_e}=-d(\ln T_e)/dr$.

Figure 5

Measurement of the parallel phase velocity of the low frequency mode. The dashed line shows the power spectrum, the solid line is the phase shift between two axial probes.

Figure 6

The power spectra of potential fluctuations vs different levels of feedback gain. The “Strong” level corresponds to $\sim 0.1k^{*}{T}_e/e$ and the “Weak” level is about $0.03k^{*}{T}_e/e$. (a) Low frequency part (IA mode). (b) High frequency part (ETG mode).