Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 51 (2018)

Maintenance of vacuum equipment of accelerators
S. Watanabe,∗1 Y. Watanabe,∗1 E. Ikezawa,∗1 N. Sakamoto,∗1 N. Yamada,∗1 M. Kase,∗1 K. Oyamada,∗2
M. Nishida,∗2 K. Yadomi,∗2 J. Shibata,∗2 and A. Yusa∗2
This annual report shows the maintenance of vacuum equipment in accelerators in 2017.
First, the maintenance procedure of vacuum equipment in RILAC is described. The worst condition ofvacuum in RILAC was in cavity No.5. The pressure ofthe vacuum in the cavity was 1.0 × 10−3 Pa in February 2017. Vacuum leak points were investigated usinga helium leak detector. However, the location of theleak point was not identified because the behavior of
the helium gas was complicated. In November 2017,an inner cylinder and a drift tube in cavity No.5 wereinvestigated. The drift tube was placed on the innercylinder. The space outside the inner cylinder was vacuum and the space inside the cylinder was atmosphere.
A gap between the drift tube and inner cylinder wassealed using an O-ring. To find the location of vacuum
leak, a dye penetrant inspection material was used.
Accordingly, the vacuum leak point was found in theO-ring between the drift tube and inner cylinder. Although the O-ring should be replaced with a new one,it was a special product and there was no spare. Anorder was thus passed to a supplier (Sumitomo LTD)
for an O-ring.
The maintenance of vacuum in cyclotrons was carried out as follows. At AVF, in January 2017, as acompressor of a cryopump did not work so that a turbo
molecular pump (TMP) was installed in place of thecryopump, and the AVF was pumped down throughthe TMP. In February 2017, a vacuum leak was detected at an insulator of RF No.2, and the corresponding O-ring was replaced with a new one. The pressure of the vacuum was thus improved. In August
2017, a broken angle valve connected with a TMP inDTL was replaced with a new one. The RIKEN ring
cyclotron (RRC) showed leaks at two locations: oneat the bellows connected with RF No.2 and anotherwas due to a pressure increase of 1.2 × 10−5 Pa in Nsector. The leak in the bellows was treated with asealing agent, however, the location of the leak point
was not confirmed. The N-sector was divided into two
spaces separated by a membrane with one part in highvacuum and the other in low vacuum. The pressure in
the high-vacuum which was in the order of 10−5 Pa,
could not be decreased. Although the location of thevacuum leak was checked using a helium leak detector, the leak point was not found. If there was noleak outside the N-sector, the space of high vacuum
would be connected with that of low vacuum. The helium leak detector was connected to the space of low∗1∗2

RIKEN Nishina Center
SHI Accelerator Service LTD- 142 -vacuum. When helium gas was inleted into the spaceof high vacuum, the leak detector reacted. Thus theconnection between the two spaces was confirmed bycreating holes in the membrane. As such gas in thelow vacuum space flows into the high vacuum space
through the holes. However, the condition of the membrane could not be known. To confirm the condition,a Dee of the N-sector was removed in August 2017.
The location of the leak point in the bellows and theconnection between the spaces of high and low vacuum were investigated. When the Dee was removed,a crack on the surface of the bellows was confirmed.
To solve this problem, the bellow should be replacedwith a new one. However a large-scale repair work isneeded to replace the bellows; this requires a considerable amount of time and a huge budget. Therefore,
the vacuum leak at the bellows was tentatively treatedwith a sealing agent. Moreover, the inside of the Deeor N-sector could be observed after the removal of theDee. Two holes were confirmed on the membrane. Gas
in the low vacuum space flowed into the high vacuum
space, and the pressure in the high vacuum space increased. The ion beams were deviated from orbit andhit the membrane, thus creating holes. Therefore, theproblem of high pressure in the space of high vacuum
must be solved. To decrease the pressure in the highvacuum space, the pressure in the low vacuum space
was reduced. This reduction in the high-vacuum spaceoccurs because inlet flow of gas from the space of lowvacuum reduces with pressure reduction. To reduce
the pressure in the space of low vacuum, a TMP witha pumping speed of 50 L/s, connected to this space
was replaced with a TMP of larger pumping speed of
220 L/s. After replacing the TMP, the pressure in
the high-vacuum space was reduced in the order of
4 × 10−6 Pa.