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On the time response and radial propagation of turbulence during ECRH power modulation in the TJ-II stellarator

Significant progress has been reported regarding the physics understanding of empirical actuators like Electron Cyclotron Resonance Heating (ECRH) to avoid impurity accumulation, including a reduction of the background density gradient, an increase of turbulent diffusion, amplification of core temperature screening and the development of core plasma potential flux surface asymmetries. Furthermore, it has been shown that radial electric fields does not only affects the radial turbulence correlation length but it is also capable of reducing the propagation of turbulence. Determination of timescales and propagation involved in the influence of ECRH on transport and fluctuations can provide an additional critical test for model validation. We have investigated the influence of ECRH on plasma profiles and edge and core turbulence in the TJ-II stellarator. The machine is equipped with two ECRH beam lines (ECRH1 and ECRH2), delivering 300 kW each at a frequency of 53.2 GHz (X mode). In the present experiments, the ECRH1 power was kept constant whereas the ECRH2 power was modulated on/off. The level of broadband plasma fluctuations is significantly increased from the deep core (rho ≈ 0.3) up to edge (rho ≈ 0.9) as ECRH power increases [Fig.]. The evolution of plasma potential and fluctuations shows two times scales during on/off ECRH modulation experiments: a fast time scale in the order of 100 μs and a longer time transport scale in the range of milliseconds. Cross correlation studies between edge probes and core HIBP signals have shown radial propagation velocities in the order of 104 m/s. The influence of ECRH on the probability distribution function of ExB turbulence transport and radial propagation of plasma turbulence will be addressed [1]. Electron temperature driven instabilities [2] as well as ECRH induced kinetic effects [3] are candidates to explain the observed influence of ECRH on TJ-II fluctuation levels.

ECRH power modulation, radial propagation, TJ-II stellarator

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