Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 51 (2018)

Progress in the dc-to-pulse converter FRAC
M. Wakasugi,∗1 M. Togasaki,∗1 T. Ohnishi,∗1 K. Kurita,∗2 R. Toba,∗3 M. Watanabe,∗1 and K. Yamada∗2
An appropriate dc-to-pulse converter is required at
the SCRIT electron scattering facility,1,2) where a continuous radioactive ion beam from the ISOL, called
ERIS,3) has to be converted to a pulsed beam witha duration of 300–500 µs to inject target ions intothe SCRIT device. We developed a new type of adc-to-pule converter, named fringing-Rf-ﬁeld activateddc-to-pule Converter (FRAC), by utilizing the fringing RF ﬁeld at both ends of RFQ rods. It is basedon an RFQ linear trap technique and works underan ultra-high-vacuum condition. Ions continuously injected into FRAC are decelerated by the alternating
longitudinal electric ﬁeld produced in the distorted RFﬁeld appearing around both ends of the RFQ rods.
They are accumulated in FRAC for a signiﬁcantly longtime, stacked during the FRAC operation period, andconsequently ejected as a high-intensity pulsed beam.
This edge eﬀect notably appears only when the energy
of incoming ions is less than several eV at the injection
barrier and the energy spread is considerably small.
There are barrier electrodes with an aperture 6 mm
in diameter at both ends of the RFQ rods. A slightly
lower voltage, VBI , compared to the ion beam energy
is applied to the injection barrier electrode to accept
the continuously incoming beam, and the beam is decelerated to a few eV at the injection barrier. The dcvoltage, VRF Q , which is lower than that at the injection
barrier by typically 20 V, is applied to the RFQ rods toform a longitudinal potential well for ion conﬁnement,
and the ejection barrier voltage is set to be higher than
the ion energy. The “basic operation mode” in which
the continuous ion beam is injected into FRAC withthe ﬁxed DC potential structure as mentioned above,successfully contributed to the ﬁrst electron scatteringexperiment for the 132 Xe iosotope.4)
We attempted other operation modes to enhancethe dc-to-pulse conversion eﬃciency toward the experiments for radioactive isotopes. They are two-stepstacking, in which the pre-pulsed beams produced bythe grid switching at the ion source are injected into
FRAC and stacked again in FRAC for 1 s. The prepulsed beam has a duration of 200 µs, repetition rateof 500 Hz, and the duty of 0.1. The injection barrier potential is synchronously switched between open
and close with the pre-pulsed beam arrival. This isa “synchronous pulse injection (SPI) mode” Since wefound the energy spread of the pre-pulsed beam is doubled relative to that of the dc beam owing to the shiftof the most-probable energy with time, we suppressed
∗1
∗2
∗3

RIKEN Nishina Center
Department of Physics, Rikkyo University
Department of Electrical Engineering, Nagaoka Universityof Thchnology- 175 -

the energy-to-time correlation by modulating VBI andVRF Q to trace the energy shift when the beam is injected. This is an advancement of the SPI mode, and
it is called the “modulated injection barrier (MIB)mode.” Another operation mode is a “ramped RFQ
voltage (RRV) mode,” in which VRF Q is ramped down
during a FRAC operation period with a rate of 10 V/s.
This procedure expands the longitudinal phase volume
for the ion stacking, adiabatically moves the formerly
stacked ions to a lower energy region, and provides freespace for the later incoming ions. Although the energy
spread of the output pulsed beam becomes wider, ahigher dc-to-pulse conversion eﬃciency is expected.
The waveforms of the output pulsed beams in these
operation modes are shown in Fig. 1. The output
pulsed beam intensity was enhanced by a factor of 2.5
for the SPI mode, a factor of 4.5 for the MIB mode,
and a factor of 10 for the RRV mode relative to thebasic operation mode. The dc-to-pulse conversion efﬁciencies at an operation frequency of 1 Hz are, forinstance, 0.8% in the basic operation mode, 2.1% in
the SPI mode, 3.4% in the MIB mode, and 5.6% in theRRV mode. While the dc ion beam intensity was practically 4.6 nA, in this measurement, a high-intensity
pulsed beam containing 1.6×109 ions was obtained at1 Hz.

Fig. 1. Waveforms of the output pulsed beam obtained infour operation modes.

References1) M. Wakasugi et al., Nucl. Instrum. Methods B 317, 668(2013).
2) T. Ohnishi et al., Physca Scripta T 166, 014071 (2015).
3) T. Ohnishi et al., Nucl. Instrum. Methods B 317, 357(2013).
4) K. Tsukada et al., Phys. Rev. Lett. 118, 262501 (2017).