II-8. Accelerator

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Improvement of the transmission eﬃciency of RILAC
T. Nishi,∗1 T. Nagatomo,∗1 O. Kamigaito,∗1 N. Sakamoto,∗1 K. Yamada,∗1 and Y. Watanabe∗1
The performance of the RIKEN heavy-ion linac(RILAC) has been upgraded1) with a new superconducting electron cyclotron resonance ion source (SCECRIS)2) and a superconducting linac booster (SRILAC).3) Since July 2020, we have been successfully
providing a high-intensity V13+ beam with an energyof 6 MeV/nucleon to the GARIS-III4) experiment.
The RILAC facility leaves room for further improvement of the transmission eﬃciency to provide
higher-intensity beams. The beam from SC-ECRIS
is transported through the low-energy beam transport(LEBT) system, which is equipped with a slit tripletthat controls the beam emittance in RILAC. The beamis then bunched and accelerated by a radio frequency
quadrupole (RFQ) and injected to the RILAC cavities,
booster cavities, and SRILAC. The transmission fromthe entrance of the RFQ to the entrance of SRILAC
was inadequate at about 50%.
One of the causes of the low transmission eﬃciency was the mismatch of beam optics between theRFQ and the ﬁrst acceleration cavity of RILAC. Thebeam line consists of two quadrupole doublets (QDL15and QDL16) and a re-buncher, as shown in Fig. 1.
The beam envelope should be narrowed inside the rebuncher because the duct radius in the re-buncher is20 mm, which is smaller than the 32 mm duct radiusin the rest of the beam line. The problem was thatthe QDL15 magnets were relatively far from an edgeof the vane of RFQ (515 mm) and close to the rebuncher. The beam spreading in the long drift space
after the RFQ should be suppressed in short drift spaceby a strong magnetic ﬁeld for it to pass through there-buncher. In contrast, the magnetic ﬁeld of QDL15a,
which is a horizontally focussing magnet, was limited
to suppress the enhancement of the beam size in thevertical direction. Consequently, the optics lost ﬂexibility and could not realize the emittance matching
and good transmission eﬃciency simultaneously.

Therefore, we rearranged the QDL15 quadrupole
magnets together with the other devices (a steerer, agate valve, and a proﬁle monitor) in September 2021.
The cooling pipes of the RFQ were also relocated toavoid interference with the magnets. The improved
transmission eﬃciency and the current applied to themagnets are summarized in Table 1. These values weremeasured when the beam was delivered to GARIS-III
after the transmission optimization by RILAC operators. The transmission eﬃciency was measured by
Faraday cups in the beam lines. L31, 014, 121, and6A1 are the names of the locations where the Faraday cups are installed, representing the entrance tothe RFQ, the entrance of the ﬁrst cavity of RILAC,
the middle point of 1st and 2nd cavities, and downstream of the last cavity of RILAC, respectively. As
presented in Table 1, a larger current can be applied
to QDL15a as expected. The transmission eﬃciency isimproved by 5% upto 014 and 121 and by 10% upto
6A1. The simultaneous improvement in transmission
from L31 to 014 and L31 to 121 is expected to be theresult of increased ﬂexibility of the optics downstreamof the RFQ. For further increase in the transmission
eﬃciency, we plan to measure the phase ellipse withthe proﬁle monitors installed between the RFQ andRILAC and to optimize the optics by realizing emittance matching to the acceptance of the ﬁrst cavity ofRILAC.
Table 1. Quadrupole magnet currents and transmission of
RILAC after optimization.
QDL15a (H)QDL15b (V)Current [A]
QDL16a (H)QDL16b (V)Transmission
014
eﬃciency from L31 [%]121
6A1

Jul. 2021 Dec. 2021
38.0
46.8
36.8
40.8
21.5
29.2
13.0
30.1
80–85
9065
70
50–55
60–65

In conclusion, the beam transmission from the RFQ
was successfully improved. The upgrade of the aged
beam line upstream of RILAC is essential to improve
the transmission further.
References
1) RIKEN Accel. Prog. Rep. 54, S1 (2021).
2) T. Nagatomo et al., Rev. Sci. Instrum. 91, 023318(2020).
3) N. Sakamoto et al., Proc. SRF2019, (Dresden, Germany,2019), pp. 750–757.
4) H. Haba for RIKEN SHE Collaboration, Proc. EXON2018, (Petrozavodsk, Russia, 2020) pp. 192–199.

Fig. 1. Top view of the beam line from the RFQ to the 1stcavity of RILAC before the rearrangement.
∗1

RIKEN Nishina Center- 62 -1