Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 53

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∗1
After the October 2017 incident, wherein the lower
correction coil in the DMT3 magnet in the “T-course”
beam line was damaged,1) a new lower correction coil wasfabricated at Toshiba and installed in DMT3 in March2018. In addition to repairing the correction coil, we renewed its water-cooling channels such that the coolingwater lines of correction-coil pancakes are independent
from those of the main-coil pancakes. The DMT2 magnet, which is a “2-Tesla” resistive-type dipole magnet
identical to DMT3, was also improved in the watercooling design. A 18 O beam at 230 MeV/nucleon and238
U beam at 345 MeV/nucleon were safely transported
to the BigRIPS target via the “T-course” beam line withthe DMT2 and DMT3 magnets in 2018.
When we improved the cooling-water piping, weﬂushed all the cooling channels in DMT2 and DMT3coils. Every pancake of the main and the correction coils
was ﬂushed with compressed air and water. We found aconsiderable amount of black powdery impurities in thewater that rinsed all the cooling channels. The composition of the impurities was analyzed using an electronprobe micro analyzer (EPMA), and we found that theimpurity was mostly copper-oxide crystals. The amountof the impurities we extracted from the non-damaged
upper correction coil of the DMT3 magnet was 0.8 g.
We investigated the cooling-water channel in the damaged correction coil by cutting the coil conductor. We
cut the damaged coil into 11 pieces. Figure 1 shows,as an example, a cross section of the correction coil cut
at a saddle-shaped coil end. Although we found no notable evidence of blockage by impurities, it is likely thata shortage of heat removal due to poor cooling-water ﬂow
in the correction coil damaged the epoxy isolation.
A 78 Kr beam and 124 Xe beam at 345 MeV/nucleon
were transported and RI beam experiments were successfully performed in the early summer 2019. However,at the beginning of the uranium beam time in November 2019, we again faced a similar incident, wherein theupper correction coil in the DMT3 magnet was damaged. The DMT2 and DMT3 magnets were excited withcurrents of 596 A and 594 A, respectively. However,the total excitation voltage of the DMT3 magnet wasFig. 1. Cross section of the damaged lower correction coil atthe saddle-shaped coil end.
∗1RIKEN Nishina CenterFig. 2. Damaged upper correction coil of the DMT3 magnet.

131.6 V, which was about 10 V lower than the voltageof the DMT2 magnet (141.2 V). Furthermore, the temperatures of the cooling-water outlet of the upper andlower correction coils of the DMT3 magnet were 20.5◦ Cand 40.1◦ C, respectively. We concluded that the uppercorrection coil in the DMT3 magnet was short-circuited.
In order to complete the beam service successfully, weisolated the upper correction coil from the excitation circuit, as we had done in 2017. The DMT3 magnet wasre-excited while monitoring the voltage of each coil pancake.
Furthermore, we introduced an additional auxiliary
DC power supply in the DMT3 excitation circuit in aparallel connection. Water-cooled protection diodes werealso used for safety. We excited the DMT3 magnet using two power supplies with currents of 110 A and 536 A
so that the uranium beam was safely transported to theBigRIPS target from SRC. After 2 days of beam-timesuspension, scheduled experiments were performed till
December 6. At the end of the beam time, we tested thetransportation of the uranium beam using the DMT3magnet without correction coils. We excited only themain coils of DMT3 magnet using two power supplieswith currents of 110 A and 563 A. We conﬁrmed thatthe uranium beam focused on the BigRIPS target welland the main coils were well cooled with a suﬃcient water ﬂow.
We investigated the upper correction coil by inserting a ﬁberscope CCD camera into the space between themain coils of the DMT3 magnet after the beam time.
Damage was found at the position of the second coil support from the beam exit side. Figure 2 shows the damageon the outer circumference of the upper correction coil.
The origin of the incident is under investigation.
Reference1) K. Kusaka et al., RIKEN Accel. Prog. Rep. 51, 173 (2018).
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