Fast reciprocating probe system for local scrape-off layer measurements in front of the lower hybrid launcher on JT-60U

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Abstract
A fast reciprocating probe system with a long drive shaft was incorporated into a multi‐junction lower hybrid (LH) wave launcher on JT‐60U in order to investigate an improved coupling mechanism of the radio frequency wave to the core plasma. The system has been operated reliably over a horizontal scan of 25 cm in 1.5 s using a compact pneumatic cylinder drive and springs. A double probe measurement provided the scrape‐off layer plasma profile between the last closed flux surface and the first wall with the spatial resolution of 1−2 mm measured with a laser displacement gauge. The profiles of the electron density ne and temperature Te were in good agreement with those obtained with a triple probe method. During the LH wave injection with good coupling to the core plasma, an increase in the local Te was observed in front of the LH launcher mouth. The local ne was (7−10)×1016 m−3, consistent values needed for the good coupling.

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