Microturbulence measurements during the divertor biasing

The application of a bias voltage to a neutralization plate of the upper divertor with respect to the vacuum chamber in the Tokamak de Varennes (TdeV) influences the plasma well inside the separatrix. In particular, the unbiased Ohmic poloidal rotation edge velocity measured by visible spectroscopy is found to be in the electron diamagnetic drift direction (2-3 km/s) and increases by a factor of two for Vbias = 100 V. This coincides with a major reduction of the microturbulence signal at low frequencies (50 kHz < f < 1 MHz) and for all wave numbers probed (20 cm-1 < k < 60 cm-1), as determined from coherent laser scattering measurements. One possible explanation is that the turbulence signal is simply Doppler shifted to frequencies outside the accessible range. This scenario is, however, difficult to reconcile with some observations. Another explanation invokes a reduction of the turbulence level. The variation of the turbulence signal as a function of the applied bias voltage can indeed be reproduced with a theoretical model based on radial and poloidal decorrelation mechanisms, the latter corresponding to poloidal velocity shear stabilization. This model also explains the steepening of the k-spectrum decay during biasing. Biasing also modifies the electron density profile inside the separatrix. These changes of ∇ne cannot explain the behaviour of microturbulence in terms of their effect on the free energy term driving the instability. On the contrary, the observed turbulence behavior, when explained in terms of stabilization, would agree with the plasma maintaining a steeper electron density gradient