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Deep radial electric field and Reynolds stress profiles measured with Langmuir and ball-pen probes on COMPASS

10 Jul 2018, 14:55

20m

Invited talk New experiments and diagnostic developments

Speaker

Ondřej Grover (Institute of Plasma Physics, The Czech Academy of Sciences, Prague, Czech Republic)

Description

The Reynolds stress and is gradient is a key quantity for the experimental validation of theories of turbulence suppression by zonal flows and other sheared flows. A common diagnostic for its measurement is an array of appropriately positioned Langmuir probes. However, simultaneous Reynolds stress profile measurements with Langmuir and ball-pen probes show a strong influence of electron temperature fluctuations on the final measured value. The diagnostic used on COMPASS is a complex probe head mounted on a horizontal reciprocating manipulator which allows measurements deep in the edge plasma. This enables a direct measurement of $E_r$ as well as the radial-poloidal component of the Reynolds stress tensor $R_{rp}$. While the Reynolds stress gradients calculated from either probe type are found to be comparable in the close vicinity of the LCFS, they are significantly different even in terms of polarity deeper inside the LCFS. Specifically, the $R_{rp}$ calculated from Langmuir probes is found to be lower than from ball-pen probes due to high-frequency $T_e$ fluctuations. The radial profile of the differences in the measured values is examined in detail and a correction for the Langmuir probe measurement in terms of local plasma parameters measurable by Langmuir probes is proposed.