Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 52 (2019)

Performance test of bulk-niobium cavities for new superconducting
linear accelerators
K. Yamada,∗1 K. Ozeki,∗1 N. Sakamoto,∗1 K. Suda,∗1 and O. Kamigaito∗1
In order to upgrade the beam intensity and energy ofthe RIKEN Linear Accelerator (RILAC),1) an upgrade
project of the RILAC is now ongoing. The beam intensity is increased by the newly built 28 GHz superconducting (SC) ECR ion source,2) and the beam energy
is boosted over 6.5 MeV/nucleon for ions with mass tocharge ratio of 6 by the new SC linear accelerator (SRILAC).3) The SRILAC consists of three cryomodules including ten SC quarter-wavelength resonators (QWRs)
with a resonant frequency of 73 MHz. The SC-QWRs
are made of bulk niobium and kept at 4.2 K by using alarge liquid-helium refrigerator.
In order to test the performance of SC-QWRs in theSC state, a test stand was prepared at RIKEN basedon the test stand at KEK.4) We drilled a hole in thefloor and installed a cryostat with an inner diameter of700 mm and a depth of 3240 mm. A magnetic shield was
put into the cryostat to prevent the penetration of geomagnetism and achieved a value of less than 10 mGauss.
The SC-QWR to be tested is suspended from a flange
and stored in the cryostat as shown in Fig. 1. Liquid
helium is poured into the cryostat until the cavity issubmerged, and the SC-QWR is tested by applying RFpower. X-rays may be generated at high voltage by factors such as field emission of electrons, and the test standis structured to be covered with an iron shield. The validity of the new test stand was confirmed by comparing
the test result of a 73 MHz prototype SC-QWR carriedout in the RIKEN and the KEK.
The performance test of actual SC-QWRs took placefrom June 2018. When the assembly of an SC-QWR wascompleted, the performance test was carried out sequen-

Ironshield

SC- QWR
Magnetic
shieldFig. 2. Q0 vs Eacc plot of the bulk SC-QWRs. The green
star represents the criteria of the SRILAC.

tially and it was confirmed whether or not acceptableperformance was obtained. In the early days, there wasa problem of vacuum leak of the SC-QWR, but it wasstopped by tightening the flange bolts again. All the SCQWRs were fabricated until November 2018 and tested
immediately. The details of the test procedure are thesame as that given in Ref. 4).
Figure 2 shows the quarity factor Q0 plotted against
acceleration voltage Eacc for all SC-QWRs. Althoughmultipacting was observed at 0.9 MV/m for each SCQWR as indicated in the figure, it could be processed
within a few hours. The test results indicate very highvalues of Q0 and Eacc for all SC-QWRs; thus, all theSC-QWRs passed the acceptance test successfully. Themaximum Eacc is significantly higher than the required
value, and the exponential deterioration of Q0 has not
yet been observed for all SC-QWRs. Note that the resonant frequency of all SC-QWRs is in the range of thefrequency tuner as expected.
The SC-QWR with which the performance was confirmed was slowly leaked in an ISO class 1 clean room
immediately, the test coupler and vacuum exhaust pipe
were removed, and blank flanges were attached to sendback to the factory for the post-process of installing thetitanium jacket. Currently the SC-QWRs are being installed in the SRILAC cryomodules. The three cryomodules will be completed and allocated in the beginning ofMarch 2019.
References

CryostatFig. 1. Installation of a test assembly by suspending a SCQWR into the cryostat.
∗1

RIKEN Nishina Center

1) M. Odera et al., Nucl. Instrum. Methods Phys. Res. A
227, 187 (1984).
2) T. Nagatomo et al., in this report.
3) N. Sakamoto et al., Proc. of LINAC18, WE2A03, 620(2018).
4) K. Yamada et al., Proc. of LINAC16, THPLR040, 939(2016).

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