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

Development of large channel diameter plasma windowusing indirectly heated hollow cathodeK. Yamasaki1, K. Okuda1, J. Kono1, A. Saito1, D. Mori1, Y Sunada1, O. Yanagi1,Y. Pan2, K. Tomita2, N. Tamura3, H. Okuno4, and S Namba1
1
Graduate School of Advanced Science and Engineering, Hiroshima University
2
Graduate School of Engineering, Hokkaido University3
National Institute for Fusion Science, 4RIKEN
e-mail: kotaro-yamasaki@hiroshima-u.ac.jp
The plasma window (PW) is a plasma application technique that separates a high-pressure (10-100 kPa) and vacuum (~1 Pa) environment without using solid materials
such as glass and stainless steel. The plasma inside thechannel of the PW heats neutral gas flowing from thehigh-pressure environment, increasing the temperature of
the gas and reducing the conductance. Due to the decreasein conductance, the mass flow inside the channel is suppressed, and the pressure difference between the entrance
and exit of the PW is kept substantially high [1]. Thesefeatures enable the PW to transmit the soft-X rays, electron and ion beams, which cannot pass through the glassand stainless steel, to the high-pressure region [2]. This
prominent feature of PW is expected to pave the way forthe new application of quantum science.
One of the applications of the PW under consideration
is a differential pumping of the gas stripper for the heavy
ion beams produced by accelerators [3]. The productionof heavy ion beams, such as gold and uranium ions, hasbeen extensively studied for rare isotope research. ThePW can realize the gas stripper with less consumption ofthe helium gas and with reduced device size. For realizing
this application, the PW with large channel diameter(more than 6 mm) is required to transmit the heavy ion
beams to the gas cell.
We developed a PW, whose inner diameter was 8 mm,with an indirectly heated hollow cathode to achieve sufficiently high pressure separation capability and long operation duration. Schematic diagram of the cathode and thePW apparatus are shown in Figure 1 (a) and (b), respectively. The hollow cathode made of LaB6 was heated by
the C/C composite heater surrounding the hollow cathodeto increase the thermionic electron emission. The PW developed successfully separated 2.4 kPa and 16 Pa, and thepressure separation capability was sustained for more than
1 hour, as shown in Figure 1 (c). Hβ Stark broadening
measurement and the Thomson scattering measurement
showed that the electron density and temperature inside
the channel reached 1019-1020 m-3 and 1.6 eV, respectively.
The power balance analysis on the electron thermal energy revealed that the neutral density and temperature inside the channel, which are the critical parameter for assessing the pressure separation capability, were as high as
1023 m-3 and 4000 K, respectively.
We will introduce the detail of the newly developed PW
apparatus and the quantitative analysis on the pressure
separation capability of the PW.

This work supported by JSPS KAKENHI under Grant
No. JP20H00141, O. S. G. Fund, Furukawa research fund,and NIFS Collaboration Research Program under Grant
No. NIFS NIFS22KIIH003, NIFS22KIIH011, andNIFS23KIIH016.

Figure 1 Schematic diagram of the indirectly heatedhollow cathode (a), the plasma window apparatus (b).
(c) Time evolution of the pressures at the gas tube andthe vacuum vessel.
References[1] A. Hershcovitch, PoP 5(5), 2130-2136 (1998).
[2] B. T. Pinkoski et al., RSI 72(3), 1677-1679 (2001).
[3] F. Marti, Proceedings of LINAC2012, 1050-1054(2012)