K. Kitachi

K. Toi

S. Ohdachi

T-IIU Group JIPP

T. Oike

National Institute for Fusion Science (NIFS-Repository)

§ 13. Measurement of Magnetic Field Fluctua-tions within Last Closed Flux Surface with Movable Magnetic Probe Array Kitachi, K., Oike, T. (Dept. Energy Eng. Science, Nagoya Univ.) Toi, K., Ohdachi, S., JIPP T-IIU Group Magnetic fluctuation may play an important role in plasma transport on high temperature plasmas heated by auxillay heating. However, magnetic fluctuations are usually monitored by so-called Mimov coils. They were mounted far from the last closed flux surface (LCFS), except that "inter-nal" magnetic fluctuations were only measured for ohmically heated low temperature plasmas in small tokamaks. So, we have developed a new type of movable magnetic probe array to detect magnetic fluctuations inside as well as outside LCFS. Figure 1 shows the structure of the movable magnetic probe array. There are 3 probes to detect radial magnetic fluctuations with 21 mm radial spacing between each probe, and 7 probes to detect poloidal (toroidal) magnetic fluctuations with 7 mm radial spacing. These probes were installed inside a SUS304-pipe of 13 mm diameter and 1mm thickness. This probe array can be inserted into a plasma beyond LCFS, where the probe position is scanned in the range of r/a = 1.4-0.8. As shown in Fig.1, the SUS-pipe is covered with a cap of Carbon-Carbon composite with 20% Boron, to protect the pipe from plasma bombard-ment and to reduce impurity efflux from it. Figure 2 shows the effect when the probe is inserted into plasma. Oxygen flux and H a emission also remain unchanged. The radial and poloidal components of the fluctuating magnetic field were measured in NBI heated plasmas of - 350 ms duration. The plasma parameters are Teoz 1500 (eV), Tioz 800 (eV) and nez5x 1013 cm-3 during NBI. Figure 3 shows a radial profile of poloidal magnetic fluctuations in various frequency range. The poloidal mode number m is determined from the radial variation ofB 6 • We assumeB 6 oc r-<m+l)[1-(r/b) 2m] (wall rasius b=320.0 mm) outside LCFS [2]. Coherent 180 component of f=9.0-20.0 kHz has m=2 and incoherent component of 50.0-60.0 kHz m=12. Driving Mechanism (stroll::c 130.0 mm) -----~1 296.0 Center I Poloidalaectioo I Fig. 1. Schematic of movable magnetic probe ~ 0.8 :5 ~ 0.6 b :.0 0.4 ~ ;:::- 0.2 0 Time (rns) Fig. 2. Time evolution of impurity (CII line) in NBI heated plasma when the probe is inserted. 10-9 ..--. ~ 10·10 ..... ... ~ 10·11 N 10-12 Minor radius (mm) Fig. 3. Radial profile of amplitude ofpoloidal magnetic fluctuations. Reference 1) M.Malacarne, P. A. Duperrex, et al., Nucl. Fusion 21.., 2113 ( 1987) 2) Y. J. Kim, K. W. Gentle, Ch. P. Ritz, et al., Phys. Fluids B.-2 ,674 (1991) 

Measurement of Magnetic Field Fluctuations within Last Closed Flux Surface with Movable Magnetic Probe Array

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