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 densityneand temperatureTewere 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 localTewas observed in front of the LH launcher mouth. The localnewas  (7−10)×1016 m−3, consistent values needed for the good coupling. ©1995 American Institute of Physics.