Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 51 (2018)

Development of an oﬀ-axis electron beam source for cold highlycharged ion generation in a linear combined ion trap
N. Kimura,∗1,∗2 H. Kato,∗3 K. Okada,∗1 N. Nakamura,∗4 N. Ohmae,5 H. Katori,5,∗6 and M. Wada∗7,∗2power supply output while changing the anode voltage.
The cathode voltage was ﬁxed at 300 V. A typical intensity of >0.1 mA at 300 eV in the trap region with
an eﬃciency of >50% was achieved.
In order to focus the electron beam in the µ-EBIT,a strong magnetic ﬁeld needs to be applied using handwound coils of a superconducting wire.4) As the nextstep, will generate HCI such as Ho14+ in the µ-EBIT,
and try to crystallize the HCI in the linear RF trap bysympathetic cooling with laser-cooled Be+ .

Some particular transitions in highly charged ions(HCI) are sensitive to possible time variation of theﬁne structure constant. High-precision spectroscopyof such transitions can be a new probe for the veriﬁcation of fundamental physics.1,2) To perform suchspectroscopy, ion trapping and cooling of HCI are indispensable. Schmöger et al.3) observed Coulomb crystallization of highly charged Ar in a linear RF trap;
however, no spectroscopy has been performed yet. We
designed and constructed a compact cryogenic setup inwhich a microscopic electron beam ion trap (µ-EBIT)
and a linear RF trap are enclosed.4) In this setup, HCI
are generated in the µ-EBIT, while laser-cooled Be+
ions are stored in the linear RF trap. Since the twotraps are arranged collinearly, the center axis line must
be reserved for the cooling laser path. Additionally,preserving the super-high vacuum necessary for storing
HCI precludes the use of any hot electron source. We
developed a cold cathode electron beam source withan “oﬀ-axis” geometry to fulﬁll such constraints.
Figure 1 shows the geometry of the electron beamsource with electron beam trajectories, which are simulated by SIMION8.0 code. The cold cathode has athrough hole for the laser path, and a Coniferous Carbon Nano Structure (CCNS) is generated on the surface in advance.5) When a high voltage is applied tothe anode, ﬁeld emission electrons are extracted fromthe cathode surface. The emitted electrons ﬂy according to the electron optics, and some fraction of themwill reach the trap region with the energy given by thecathode voltage.
After an aging process was performed by applying
high voltages for a long time under ultra-high vacuum
condition, we obtained an I-V characteristic plot of
the electron beam source, as shown in Fig. 2. The circle plots represent the electron beam intensity reaching
the trap region and the square plots represent the totalemission from the cold cathode. The electric current(I ) and voltage (V ) were measured by monitoring the∗1∗2∗3∗4∗5∗6∗7Fig. 1. Sketch of electron beam source with trajectories
calculated using SIMION 8.0.

Fig. 2. I-V plot of electron beam of 300 eV.

Department of Physics, Sophia University
RIKEN Nishina Center
Research Institute for Measurement and Analytical Instrumentation (RIMA), National Metrology Institute of Japan(NMIJ), National Institute of Advanced Industrial Scienceand Technology (AIST)
Institute for Laser Science, The University of ElectroCommunications
Quantum Metrology Laboratory, RIKEN
Department of Applied Physics, Graduate School of Engineering, The University of Tokyo
Wako Nuclear Science Center (WNSC), Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator
Research Organization (KEK)

References
1) J. C. Berengut et al., Phys. Rev. Lett. 105, 120801(2010).
2) V. A. Dzuba et al., Phys. Rev. A 91, 022119 (2015).
3) L. Schmöger, et al., Science 347, 1233–1236 (2015).
4) N. Kimura et al., RIKEN Accel. Prog. Rep. 50, 202(2016).
5) H. Kato et al., Diamond and Related Materials 55, 41–44 (2015).

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