RESUMEN
Observations in the ASDEX Upgrade tokamak show a correlation between the gradient of the intrinsic toroidal rotation profile and the logarithmic gradient of the electron density profile. The intrinsic toroidal rotation in the center of the plasma reverses from co- to countercurrent when the logarithmic density gradients are large, and the turbulence is either dominated by trapped electron modes or is at the transition between ion temperature gradient and trapped electron modes. A study based on local gyrokinetic calculations suggests that the dominant trend in the observations can be explained by the combination of residual stresses produced by E × B and profile shearing mechanisms.
RESUMEN
The heavy ion beam probe (HIBP) is an established nonperturbing diagnostic for high spatially and temporary resolved measurements of magnetically confined plasma parameters such as potential, density, and temperature. These quantities can be determined from the change in the ion beam parameters (charge, intensity, and trajectory) passing through a plasma volume due to collisions with electrons and interaction with the confining magnetic field. One of the problems that should be solved during HIBP installation and tuning is the coordinate matching. Conventionally the coordinate mapping of the HIBP measurement point is provided by ray tracing calculations of the ion beam in the magnetic field. However, it is very difficult to include all physical effects and uncertainties in the model. Thus, the result of the calculations may differ from the real probing position. In order to improve the mapping precision of the HIBP installed at the WEGA stellarator an additional measurement of the beam position is provided using a primary beam detector array inside the vacuum vessel. This allows comparing the measured and calculated ion beam positions in order to prove the calculated coordinate precision and include adjustments in the calculation code if necessary. The principle and the results of this calibration, which is not specific to WEGA but could be adapted to other experiments as well, are presented in this work.
RESUMEN
The ordinary-extraordinary-Bernstein-mode conversion process for overdense plasma heating with electron-Bernstein waves is demonstrated in the WEGA stellarator at low magnetic field (approximately 50 mT) at 2.45 GHz. For the first time the conversion from an O wave to an X wave is clearly demonstrated by probe measurements of amplitude and phase of the wave field in the conversion region and supported by two-dimensional full-wave calculations. The propagation and resonant absorption of the Bernstein wave is measured in fast power modulation experiments.