Berlin 2014 – scientific programme

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P: Fachverband Plasmaphysik

P 12: Poster Session - Magnetic Confinement

P 12.3: Poster

Tuesday, March 18, 2014, 16:30–18:30, SPA Foyer

Coherent mode generation during EBW heating in TJ-K — •Rennan Bianchetti Morales, Alf Köhn, and Mirko Ramisch — Institut für Grenzflächenverfahrenstechnik und Plasmatechnologie, Universität Stuttgart

Electron Bernstein waves (EBWs) can be used to heat overdense plasmas when the plasma cut-off frequency is higher than the frequency of the injected microwaves. EBWs are electrostatic waves, which cannot propagate in vacuum and, therefore, need to be generated by mode conversion processes. The generation of EBWs is possible when the microwave heating power is high enough to increase the plasma density beyond the cut-off density. At this stage, the EBW mode conversion takes place and heating at the electron cyclotron resonance frequency (ECRF) and its harmonics is achieved. This heating scheme is successfully used in the stellarator TJ-K to heat overdense plasmas in low magnetic fields at ECRF harmonics.

Recent discharges using this heating scenario showed a quasi-coherent mode in density and potential fluctuations. This mode at approximately 4 kHz is dominant in the power spectrum and is evident from the center to the edge of the plasma, peaking at the separatrix region. In the presence of the coherent mode, the broadband turbulent fluctuations appear to be suppressed. This feature is more pronounced during discharges with the lower neutral gas pressures. In this contribution, the generation of this mode and its impact on the ambient turbulence is studied by means of Langmuir probe measurements.