Measurement of magnetic field fluctuations within last closed flux surface with movable magnetic probe array in the JIPP T-IIU tokamak

Poloidal and radial magnetic field fluctuations less than 100 kHz are measured with a newly developed movable magnetic probe array in the JIPP T-IIU tokamak. The probe array protected by a carbon-carbon composite with 20% boron is inserted beyond the last closed flux surface (LFCS) up to r/a 0.77 without deleterious effect on ohmically and neutra beam heated plasmas. From the radial variation in the fluctuation amplitude just outside the LCFS the poloidal mode number m is estimated to be 4 or less for the low frequency coherent part (f > 30 kHz) and about 10 for the relatively high frequency incoherent part (f > 50 kHz).