Speaker
Renaud Dejarnac
Description
The access to H-mode is a crucial step for ITER operation but is still not completely understood and the prediction of its L- to H-mode power threshold (PLH) relies on a multi-machine scaling law based on macroscopic parameters such as density, magnetic field and machine size [1]. However, it has been demonstrated that local effects, e.g., divertor geometry or X-point height, can affect strongly but also differently PLH from one machine to another. The reliability of the empirical scaling law is therefore questioned, making the predictions for ITER uncertain.
In the tokamak COMPASS, the PLH strongly increases with the height of the X-point (up to 50% after a fresh boronization). COMPASS has the particularity to access the H-mode in ohmic when the power crossing the separatrix reaches a certain threshold. In this study, the L-H transition is triggered with a plasma current ramp-up and mechanisms at play are investigated for a low and a high X-point configuration using an extensive set of diagnostics. In L-mode just before the L-H transition, a larger radial electric field (Er) is measured close to the separatrix by the horizontal reciprocating probe for the low X-point configuration when PLH is the lowest, thus confirming the role of Er in stabilizing the turbulent transport. Considering the gradient of the electron temperature (dTe/dr) being proportional to Er, a similar result is observed by the high resolution Thomson scattering system around the separatrix. Prior the L-H transition, dTe/dr is two times higher for the low X-point, whilst exhibiting the same value in H-mode for both the high and low configurations. In the divertor, the electron temperature at the outer strike-point measured by Langmuir probes is 50% higher in the low X-point configuration. In the frequency spectra, a well localized mode at 6 kHz is observed only for the high X-point configuration prior the L-H transition and disappears when the H-mode is reached. This mode runs in the ion diamagnetic drift direction, therefore it is not a dithering mode nor a low frequency zonal flow and is seen on electric probe, Da and magnetic coil signals.
[1] Y. R. Martin et al., J. of Physics: Conf. Series 123 (2008) 012033
Primary author
Renaud Dejarnac
