II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Incidents involving the DMT3 magnet in the beam transport line from
SRC to BigRIPS
K. Kusaka,∗1 K. Yoshida,∗1 M. Ohtake,∗1 and Y. Yanagisawa∗1The DMT3 magnet in the “T-course” beamline is designed as a resistive-type magnet with saddle-shaped
correction coils in addition to main coils. The correction coils are installed in the gap of the magnet andwere originally excited with the main coils in series.
The main coil has 72 turns and consists of 6 double
pancakes, in which a 13.5 × 13.5 mm hollow conductor
is wound 6 times in each layer. The correction coil, on
the other hand, is a 12-turn double pancake. However,the layer isolation of the lower correction coil was damaged in an October 2017 incident.1) Furthermore, theupper correction coil was found to be short-circuitedin November 2019.2) We then investigated the possible
use of the DMT3 magnet without correction coils byincreasing the energizing current.
As the maximum current of the original DMT3
power supply was 650 A, an additional auxiliary DCpower supply was introduced in the DMT3 excitation
circuit in a parallel connection. Water-cooled protection diodes were also used for safety.2) At the end of
the beam time in December 2019, we excited only themain coils in the DMT3 magnet using two power supplies with currents of 150 A and 563 A. We conﬁrmedthat the uranium beam focused on the BigRIPS target
well and the main coils were well cooled with suﬃcientwater ﬂow. However, ramping the DMT3 power supply from 0 to 563 A with the simultaneous use of theauxiliary power supply with a current of 150 A caused
instability; therefore, we were forced to ramp up in a
stepwise manner to avoid instability.
In March 2020, we again tested power supplies forthe operation of the DMT3 magnet without correction
coils. Using the same excitation circuit, we energized
the main coils while monitoring the voltage of eachcoil pancake. We found that the lower main coil wasdamaged.
Figure 1 shows the excitation voltage at each pancake of the DMT3 coils. Firstly, we energized the magnet by using the auxiliary power supply only with acurrent of 150 A. The voltages of all the pancakes coincided. We then further energized the magnet using
the DMT3 power supply with a maximum current of650 A. The total current was 800 A, which was larger
than the current of 710 A used to transport uranium
beams from SRC. The excitation voltages of the second and the fourth pancakes of the lower main coils
decreased with time, while the voltage of other four
pancakes increased because of the temperature rise ofconductors. These unstable behaviors in excitation
voltage indicate damage to the isolation between coil
∗1Fig. 1. Excitation voltage at each pancake of the DMT3lower main coil.

layers. We then decreased the current and re-energizedwith currents of 100 A and 610 A. The excitation voltage of all pancakes coincided in this case. We consider
that the two pancakes are not critically damaged.
Although the lower main coil of DMT3 was damaged, 238 U and 70 Zn beams were successfully transported from SRC to BigRIPS targets in 2020 beamtime. We excited the DMT3 magnet with the maincoils and undamaged lower correction coil, which wasinstalled in 2018.
New main coils for the DMT3 magnet are now being
designed and fabricated. The new main coil is designed
so as to ﬁt in the DMT3 iron pole and yoke. Furthermore, we increased the number of turns from 72 to 84
such that the original DMT3 power supply energizes
the magnet without an auxiliary power supply. New
main coils will be installed in early summer 2021.
References

1) K. Kusaka et al., RIKEN Accel. Prog. Rep. 51, 173(2018).
2) K. Kusaka et al., RIKEN Accel. Prog. Rep. 53, 104(2019).

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