Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 47 (2014)

Reinforcement of magnetic shield for HTc SQUID beam current
monitor at the RIBF†
T. Watanabe,∗1 N. Fukunishi,∗1 M. Kase,∗1 O. Kamigaito,∗1 S. Inamori,∗2 and K. Kon∗2
To measure the DC current of high-energy heavy-ion
beams nondestructively at a high resolution, a highcritical-temperature (HTc) superconducting quantum
interference device (SQUID) beam current monitor
henceforth referred as HTc SQUID monitor has beendeveloped for use in the radioactive isotope beam factory (RIBF) at RIKEN.1) Beginning this year, themagnetic shielding system has been greatly reinforced.
Since the measurement resolution is determined by thesignal to noise ratio, this resolution is improved by attenuating the external magnetic noise and RF background noise. These noises are mainly produced by
the distribution and transmission lines from the highcurrent power supplies and high-power RF cavities of
the cyclotrons.
To reinforce the existing magnetic shield, we developed a hybrid magnetic shielding method based on theproperties of perfect diamagnetic materials and ferromagnetic materials; we were able to realize a highshielding eﬀect despite the compact system. This system consists of two shielding parts: one for the HTc
current sensor and the other for ferromagnetic shielding materials. The HTc current sensor used to produce a shielding current produced by the beam1) also
works as the superconducting shield via the Meissner eﬀect (perfect diamagnetism). The ferromagnetic
shielding materials are composed of high permeability
alloys (Permalloy, Mu-metal, etc.). The HTc SQUID
is installed inside the frame and onto the HTc current
sensor, and the frame is covered by the cap. Consequently, the HTc SQUID is almost completely surrounded by the hybrid magnetic shielding system. A
photograph of the completed hybrid magnetic system
is shown in Fig. 1.
In the acceleration facility, since there exist AC magnetic noises of 50 Hz and higher order and which aremuch stronger than terrestrial magnetism, an active
magnetic ﬁeld canceller system (JEOL Ltd.) was designed and introduced to the HTc SQUID monitor.
This system is comprised of a magnetic ﬁeld control
unit, combined AC/DC magnetic ﬁeld sensors, andcompensation coils. The compensation coils consistof three pairs of coils that are arranged perpendicular
to each other. Each of these pairs forms a so-called“Helmholtz-Coil-Pair,” able to produce a homogenous
magnetic ﬁeld in between the pairs; each pair controls
one direction (along x-, y-, or z- axis). A photograph of†∗1∗2

Condensed from the proceedings in European Conference on
Applied Superconductivity (EUCAS 2013)
RIKEN Nishina Center
TEP Corporation

2 Flame(inside SQUID)

3 Cap

1 HTc CurrentSensor

4 BandBundling BandFig. 1. Photograph of the completed hybrid magnetic system. 1: HTc current sensor with ferromagnetic shielding materials, 2: frame, 3: cap, and 4: band.
-Coil-Cag

Helmholtz

Mgnetic FieldSensors

eHTc SQUID
MonitorFig. 2. HTc SQUID monitor with active magnetic field canceller system.

the active magnetic canceller system is shown in Fig. 2.
To evaluate the performance of the hybrid magnetic
shielding system and the active magnetic canceller system, the output signals of the HTc SQUID were analyzed in the time and frequency domains. The signal
was measured in the room next to where the power
supplies for RIBF were located, where the leakage magnetic ﬁeld of the 50 Hz component was measured bya Gauss meter as 4.5 ×10−4 T. On the other hand,the output signal of the 50 Hz component of the HTc
SQUID monitor was 6 ×10−14 T. Based on these ﬁndings, we consider that the combination of the hybrid
magnetic shielding system and the active magnetic
canceller system can attenuate the external magnetic
noise to 10−10 .
References1) T. Watanabe et al., Proc. 2010 Beam Instrumentation
Workshop (BIW10), Santa Fe, U.S.A (2010) p. 523.

- 153 -