Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 48 (2015)

Modification of septum electrode for RRC-EDC
K. Yamada,∗1 S. Ishikawa,∗2 T. Nakamura,∗2 M. Nishida,∗2 and H. Okuno∗1
Electric deﬂection channel (EDC) is an importantdevice to extract the circling ion beam in a cyclotron
by using high-static electric ﬁeld up to 10 MV/m. The
circling beam and the extracting beam in the cyclotron
are separated by a septum electrode, which is a thinground electrode of the EDC and only the extracting
beam is aﬀected by the electric ﬁeld and deﬂected. Because major beam loss in the cyclotron occurs on theseptum electrode, the heat load restricts the maximum
beam intensity accelerated by the cyclotron. Especially for the uranium beam acceleration at the RIBF,
turn separation of the circling beam in the RIKEN ring
cyclotron (RRC) is about 7 mm due to the low acceleration voltage and the short stopping range of ions ina material causes melting of the septum electrode, as
happened in ﬁscal year 2012. Therefore, a new septum
electrode for the RRC-EDC was introduced in order toimprove the heat-load durability of the beam loss.
We fabricated the septum electrode with a V-cut
entrance as shown in Fig. 1. The septum electrode
was made of oxygen-free copper and divided into twopieces, the entrance side and the remainder, to facilitate the replacement of the damaged entrance side.
Thickness of the beam-pass area on the septum electrode was 0.8 mm for the entrance side, 1.6 mm forthe central part, and 5 mm for the exit. The coolingwater pipes between the two pieces were connected bya VCR1) tube ﬁtting. Ten points of E-type thermocouple devices were mounted on the septum electrodefor thermal interlock.
Figure 2 shows the results of the 3D heat transfer calculation for the original and the new septum electrode assuming turbulent forced convective
heat transfer by cooling water. The range of bothplots is from 0 ◦ C to 1100 ◦ C. For the original
septum electrode, which is a simple ﬂat plate, themaximum temperature was about 850 ◦ C assuming

V-cut

4.0 mm(height)×0.8 mm(thickness)×0.05 mm(range)
uniform distribution of 300 W heat load at the entrance, and assuming the heat transfer coeﬃcient of20000 W/(m2 ·K) on the inside of cooling water pipe.
On the other hand, the maximum temperature was reduced to 670 ◦ C for the new septum electrode assuming 500 W uniform heat load for the same dimensions
and same heat transfer coeﬃcient. For both cases, theperipheral temperature was set to 25 ◦ C. The original
septum electrode was replaced by the new one, andthe old driving motors for the RRC-EDC were alsochanged to new stepping motors. The modiﬁed RRCEDC was used for the machine time from the winter of2013, including the RIBF experiment. No failure wasfound on the new septum electrode with about 4.5 kWof 238 U35+ beam and 1.6 kW of 48 Ca beam.
In order to extend the capability for further intensity growth, we have started to develop a mass-lessseptum electrode by arranging numerous thin tungsten ribbons. A test piece of a 50-µm-thick tungsten ribbon was irradiated by varying the intensity of
238 35+Ubeam with an energy of 10.7 MeV/u provided
by the RRC in November 2014 to investigate the melting boundary. The tungsten ribbon melted at an intensity of about 50µA. The result will be comparedwith the heat transfer calculation and the prototype
septum electrode will be fabricated.

Thermocouples

Beam
Fig. 2. Results of the 3D heat transfer calculation aroundthe beam entrance side for the original (upper panel)and the new (lower panel) septum electrode.

Cooling water pipe
Fig. 1. New septum electrode with a V-cut entrance.
∗1∗2References1) Product of Swagelok Co.

RIKEN Nishina Center
SHI Accelerator Service Ltd.

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