Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 53

Non-thermal equilibrium eﬀect on plasma window with large diameter
N. Ikoma,∗1,∗2 Y. Miyake,∗1 M. Takahashi,∗1 H. Okuno,∗1 S. Namba,∗3 K. Takahashi,∗2 T. Sasaki,∗2 andT. Kikuchi∗2The disposal of high-level radioactive waste (HLW)
is a serious social problem. To address it, a transmutation scheme of long-lived ﬁssion products (LLFP)
employing a high-power accelerator, which provides
deuteron beams of 400 MW, was proposed in the ImPACT Fujita program.1) In such a system, a beam window, which separates the accelerator and target region,is considered one of the key issues. A plasma window (PW),2) which exploits a large pressure gradient
generated by an arc plasma ﬁlling the discharge channel, is considered a promising candidate. However, theﬁrst PW invented by Hershcovitch2) had an apertureof 2.36 mm, while the beam-spot size required in ourcase is 100 mm or larger.
Therefore, we developed a PW with a maximum diameter of 20 mm, as shown in Fig. 1, and demonstrated
conductance reduction by the PW.3) Furthermore, acomparison between the achieved cell pressure Pcell
and the pressure estimated from Eq. (1)4,5) suggested
that the gas (or heavy-particle) temperature Th waslower at larger diameters due to the low-input powerdensity.3)√√
16ηkTh QLPcell ≃,
(1)
πmhr2

Fig. 2. Te vs. PW diameter.

In this paper, the typical electron temperature Te
was estimated based on the report that the axial electron temperature variation in cascade arc devices issmall.6,7) The Te is described by the Spitzer formula:()2/3
ln Λσ,(2)Te ≈2 × 104

where σ is the average electrical conductivity of theplasma obtained from discharge current, voltage, andgeometry of the channel, assuming a uniform cylinder.
The Coulomb logarithm ln Λ was assumed as 10.
In contrast with the tendency of the heavy-particle
temperature,3) the estimated Te shown in Fig. 2 wasalmost constant regardless of the diameters. This may
indicate that the temperatures of electrons and heavy
particles attain a non-equilibrium state as the diameterincreases. Because Th is the most important factor in
the performance of the PW as described by Eq. (1),the non-equilibrium eﬀect will be discussed with theconductance-reduction eﬀect of plasma ignition.
This work was supported by RIKEN Junior Research Associate Program. Furthermore, this work wasfunded by ImPACT Program of Council for Science,
Technology and Innovation (Cabinet Oﬃce, Government of Japan).

where η, k, mh , Q, r, and L are the viscosity of the gas,
Boltzmann constant, mass of gas particles, mass ﬂow
rate, channel radius, and channel length, respectively.

References

1) H. Okuno et al., Proc. Jpn. Acad. Ser. B 95, 430 (2019).
2) A. Hershcovitch, J. Appl. Phys. 78, 5283 (1995).
3) N. Ikoma et al., Plasma Fusion Res. 14, 1206148 (2019).
4) W. A. J. Vijvers et al., Phys. Plasmas 15, 093507(2008).
5) S. Huang et al., Phys. Plasmas 21, 123511 (2014).
6) J. J. Beulens et al., Phys. Fluids B 3, 2548 (1991).
7) J. J. Beulens et al., Plasma Sources Sci. Technol. 2, 180(1993).

Fig. 1. Structure of the PW developed in this study.
∗1
∗2
∗3

RIKEN Nishina CenterGraduate School of Engineering, Nagaoka University ofTechnology
Graduate School of Engineering, Hiroshima University- 92 -