7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Numerical Analysis on Relaxation Process after Electron Injected into
Malmberg-Penning Trap
T. Kikuchi1, H. Ishii2, N. Suzutani1, Y. Park3, Y. Soga2, K. Takahashi1, T. Sasaki1
1
Nagaoka University of Technology,
2
Kanazawa University,
3
National Institute of Technology, Anan College
e-mail (speaker): tkikuchi@vos.nagaokaut.ac.jp
An inertial fusion power generation system using
massive ions such as heavy ions and giant cluster ionshas been proposed [1]. This system requires a kA-classhigh-current particle accelerator. For a pulse
compression process in the final stage, the beam
parameters are affected by the strong space charge effect.
Since it is difficult to construct a full-scale heavy-ion
accelerator, we are studying the beam physics bycompact experimental devices using electrons [2-4].
Using a newly developed kinetic energy measurement
method [5], we are investigating the experimentallyobserved rapid relaxation processes in electrons incident
on the Malmberg-Penning trap [6].
In addition to experimental approach, we are also
investigating the beam dynamics using theoretical andnumerical approaches [7-9]. The relaxation process isseparated by the various time scales. During the timescale of 10 μs after the injection of electrons into theMalmberg-Penning trap, the electron cloud with initial
velocity distribution truncated with fast particles
regenerated the fast particles. The relaxation process wasreproduced by the numerical simulation withmulti-particle tracking [10,11].
During the time scale of 1 μs immediately after theinjection of electrons into the Malmberg-Penning trap,
the electron velocity distribution was drastically decayed[6]. In this paper, we report the results of a numericalanalysis of the experimentally observed extremely fast
relaxation process of electrons incident on theMalmberg-Penning trap.
Figure 1 shows the electron distribution function
history at each applied electric field strength in the axialdirection. According to the experimental condition [6],we carried out the numerical simulation withthree-dimensional multi-particle tracking method [8-11].
The electrons have the offset kinetic energy of 13.7 eV in
the axial direction at the initial condition. The initial
temperature of electrons was 0.023 eV. The electron
bunch length was 160 mm, and the radius was 0.4 mm.
Figure 1(a) indicated the fast electrons over the kinetic
energy of 20 eV were generated. On the other hand, thefast electrons over the kinetic energy of 20 eV were notobserved well as shown in Fig. 1(b). The result impliedthat the relaxation process with the lower strength of theaxial applied electric field is faster than that with thehigher strength of the axial applied electric field.
References[1] K. Takayama, et al., Phys. Lett. A 384, 126692(2020)[2] T. Kikuchi, Y. Soga, K. Horioka, J. Plasma Fusion
Res. 93, 345-352 (2017) (in Japanese)[3] Y. Sakai, et al., Nucl. Instrum. Methods Phys. Res.
A733, 70-74 (2014)[4] Y. Park, et al., NIFS PROC 93, 84-87 (2013)[5] Y. Park, et al., Plasma Fusion Res. 14, 1201046(2019)[6] H. Ishii, et al., NIFS PROC 121, 44-47 (2021)[7] Y. Soga, private communications[8] T. Sato, et al., J. Phys.: Conf. Series 717, 012100(2016)[9] T. Kikuchi, et al., IEEE Trans. Plasmas Sci. 44,
216-220 (2016)[10] N. Suzutani, et al., High Energy Density Phys. 36,
100828 (2020)[11] N. Suzutani, et al., J. Japan Society Simulation Tech.
12, 15-20 (2020)

Figure 1. Electron distribution function history for (a) lower and (b) higher cases for applied electric field in axial
direction.