Speaker
Ulrike Plank
(Max-Planck-Insitut für Plasmaphysik)
Description
The characteristic feature of the H-mode in a tokamak plasma is the edge transport barrier (ETB). It is believed to be caused by the edge radial electric field $E_r,$ which leads via the $\mathbf{E} \times \mathbf{B}$ velocity shear to the suppression of turbulent transport and thus, to the formation of an ETB. Due to its important role in H-mode physics, the edge $E_r$ is considered to be a key player also in the transition from L- to H-mode. On the macroscopic scale it is seen that the heating power has to exceed a certain threshold $P_{\rm thr}$ to transit from L- to H-mode. According to an inter-machine scaling, $P_{\rm thr}$ scales with the line-averaged density $\langle n_\mathrm{e} \rangle $, toroidal magnetic field $B_\phi$ and plasma surface $S$ [1]. The scaling is, however, only valid for high densities, at which the electron and ion channels are coupled [2] and for magnetic configurations with the ion $\nabla B$ drift pointing towards the X-point. Furthermore, it does not include other dependencies, such as the divertor condition or the wall material. Recent experimental studies from ASDEX Upgrade indicate that a critical ${\mathbf{E} \times \mathbf{B}}$ shear is needed to enter H-mode [3]. They could explain the $B_\phi$-dependence of $P_\mathrm{thr},$ as the $E_r$ shear is observed to increase with increasing magnetic field. For a more complete understanding of the L-H transition, the exact location of the $E_r$ shear is crucial. Findings from several machines suggest that the scrape-off layer (SOL) physics influences $P_\mathrm{thr}.$ These are supported by modelling results showing that the divertor and wall conditions change $E_r$ in the SOL [4]. This in turn would impact the ${\mathbf{E} \times \mathbf{B}}$ shear across the separatrix and, therefore, could modify the condition for the L-H transition. To experimentally address these relations, a new gas puff-based charge exchange recombination spectroscopy diagnostics was installed at ASDEX Upgrade in order to measure $E_r$ across the separatrix with high radial resolution at the outer mid-plane.
**References**
[1] Y. R. Martin et al., *J. Phys.: Conf. Ser.* **123** 012033 (2008).
[2] F. Ryter et al., *Nucl. Fusion* **54** 083003 (2014).
[3] M. Cavedon et al., *EPS Conf. Proc.*, 2017.
[4] A.V. Chankin et al., *Plasma Phys. Control. Fusion* **59** 045012 (2017).
Primary author
Ulrike Plank
(Max-Planck-Insitut für Plasmaphysik)
Co-authors
Dr
Marco Cavedon
(Max Planck Insitute for Plasma Physics Garching, Germany)
Dr
Thomas Pütterich
(Max Planck Institute for Plasma Physics, Garching, Germany)
Prof.
Ulrich Stroth
(IPP Garching)
