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

Development of High Density and Large Diameter Plasma Sourcein Superconducting mirror device Pilot GAMMA PDX-SC
N. Ezumi1, R. Miyauchi1, T. Seto1, J. Kohagura1, M. Yoshikawa1, M. Hirata1, S. Togo1,Y. Nakashima1, and M. Sakamoto1
1
Plasma Research Center, University of Tsukuba
e-mail (speaker): ezumi@prc.tsukuba.ac.jp
As a platform for DEMO divertor development, a linear
plasma experimental device that can simulate the highdensity divertor plasma expected in the DEMO reactor isrequired [1]. To contribute to developing such an
experimental device, the Plasma Research Center at theUniversity of Tsukuba is constructing a simple mirrortype device, Pilot GAMMA PDX-SC (Pilot device), using
superconducting (SC) coils shown in Figure 1. In thispresentation, we will report on the outline and initial
results of a DC arc discharge plasma source using a hot
cathode, which has been developed as a steady-state highdensity plasma source for this system.
The aim of this research is to generate steady, highdensity hydrogen plasma with an electron density of >1019 m-3 and a diameter of ~10 cm (FWHM ~5 cm) in adivertor simulated region while maintaining the gaspressure in the main vacuum vessel, which is the plasma
confinement region, at about 10-4 Pa. To achieve this goal,
we have developed a thermal cathode arc discharge
plasma source optimized for the magnetic field
configuration of the Pilot GAMMA PDX-SC, based on
the knowledge of the TPD-type plasma source as shownin Figure 2. The cathode is a 15 cm diameter LaB6 disk,heated to the temperature of enough thermal electron
emission by a carbon heater installed behind the disk, anda steady-state plasma is generated by a DC arc discharge
between the cathode and the anode, each of which isplaced between insulated intermediate electrodes. The
plasma passing through the anode flows into the main
vacuum vessel along the magnetic field created by the SC
coil and through the differential pumping chamber. Eachelectrode is forced water-cooled for heat removal, and theinner diameter was designed and fabricated to follow themagnetic field lines with a radius of 1 cm at the throat atthe center of the SC coil.
A discharge test using hydrogen was conducted during
the rated operation of the superconducting coil (1.5 T
magnetic field at the throat), and hydrogen plasma was
successfully generated with a discharge current of 20 A
(Figure 3). Despite preliminary low current discharge, theplasma density achieved 1.2 x 1019 m-3 for argon and 0.8
x 1019 m-3 for hydrogen at the magnetic throat of 1.5T.
Additionally, we are in the process of developing ahigh-power helicon discharge plasma source with acapacity of 30kW as an alternative option. During thispresentation, we will be providing a detailed overview ofthe plasma source and its properties, as well as discussing
the challenges associated with achieving a higher-density
and higher-temperature plasma.
This work was partly supported by the NIFS
Collaboration Research program (NIFS20KUGM148 andNIFS23KUGM174).
Reference[1] K. Okano et al., Fusion Eng. Design 136 (2018).
500.3

400.2

rthroat [cm] 30

B [T]

200.1
100-400-390-380-370-360-350-340-330-320-3100-300z [cm]LaB6
CathodeFloating Electrodes AnodePlasma Source Region

Differential Pumping Region

Figure 2. DC arc plasma source for Pilot GAMMAPDX-SC and magnetic field structure (orange lines
are magnetic field lines with a throat radius of 1 cm).

Figure 1. Superconducting mirror device Pilot
GAMMA PDX-SC.

Figure 3. First hydrogen plasma viewed from theobservation port of the differential pumping chamber(discharge current 20A).