Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 51 (2018)

Improvements on the racetrack microtron at SCRIT
A. Enokizono,∗1 T. Hori,∗1 M. Wakasugi,∗1 and M. Watanabe∗11.2e-41.0e-4Pressure (Pa)The SCRIT group has successfully performed its
ﬁrst physics experiment using stable 132 Xe target1) andopened a new window to the research of unstable nuclei by means of electron scattering. An upgrade of
the SCRIT facility is currently in progress, aiming at
the world’s ﬁrst electron scattering experiment using
short-lived RI targets.
For the SCRIT experiment, a racetrack microtron(RTM; Fig. 1) was used to inject the electron beam
not only into a storage ring but also into the Electronbeam-driven RI separator for SCRIT (ERIS) to irradiate a uranium target to generate RIs.2) A typical
output power of the electron beam is currently ∼20 W(with 20 Hz operation), but 1 kW is required to generate a suﬃcient number of 132 Sn to achieve the luminosity of more than 1026 cm−2 s−1 . As such a highpower RTM needs massive modiﬁcations, e.g., a newmodulator power supply, we decided to improve the20 W beam power up to 50 W by using the current
RTM conﬁguration with several minor upgrades before the future 1 kW upgrade. For example, a studyof a gun-grid-pulsing device and its upgrade plan wasreported by Watanabe et al.3) In addition to the upgrade, the cause of electron-beam instabilities must bedetermined out and ﬁxed. The RTM device is already20 years old, and some of its components are becoming
the source of the instabilities. We found that a surface
of a ceramic RF coupler was carbonized and blackened,as shown in Fig. 1 (inset). As such a carbonized coupler is known to be easily discharged at a high power
input, we carefully polished the surface. Further, wefound that the charge-up of a ceramic duct coupler for
the CT monitor located at the exit of the electron gun
causes beam instability, and the shape of the beam

0.8e-40.6e-40.4e-40.2e-4
2012
01/01

2013
01/01

2014
01/01

2015
01/01

Date

2016
01/01

2017
01/01

2018
01/01

Fig. 2. Trend of the vacuum at RTM cavity in the past sixyears. TMPs were upgraded in August 2017.

proﬁle monitored by a camera installed after the CT
monitor was observed to change. After installing anadditional SUS pipe to hide the ceramic part from theelectron beam, the beam proﬁle stabilized. We are also
attempting to stabilize the RF power input, e.g., theklystron module and the power variator, in the waveguide.
The RTM operation is also frequently disrupted byarc detections especially at higher RF power inputs,
and it unfortunately reduces the duty factor of the experiment time. This issue is considered to be caused
partly by the discharge inside a cavity chamber, and insuch a case, the vacuum improvement of the chamber
could be helpful. A better vacuum is also important
for a stable operation and longer lifetime of RF components. The two (400 L + 300 L) TMPs attachedto the chamber were replaced with two larger (520 L+ 520 L) TMPs. A typical vacuum range during theRTM operation was 2–8×10−5 Pa, as shown in Fig. 2,and with the TMP upgrade, the vacuum was improvedby more than 20%. Note that the seasonal dependence,
which presumably originates from the temperature in
the RTM room, results in a diﬀerence in the vacuum
by a factor of 3–4. Therefore, it is also important tocontrol the room temperature or develop a new coolingsystem for the cavity chamber.
In summary, the improvements of the RTM atSCRIT are in progress to achieve a stable operation
at ∼50 W. This work must be successfully completed
before the future RTM upgrade to 1 kW operation.
References
1) K. Tsukada et al., Phys. Rev. Lett. 118, 262501 (2017).
2) T. Ohnishi et al., PoS (INPC2016) 088.
3) M. Watanabe et al., in this report.

Fig. 1. Overview of SCRIT RTM device.
∗1

RIKEN Nishina Center- 176 -