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

Current Status of Development of High-Density Radio Frequency Plasma
Source with Flat Loop Antennas for Pilot GAMMA PDX-SC
Takumi SETO, Naomichi EZUMI, Reina MIYAUCHI, Takuma OKAMOTO,Satoshi TAKAHASHI, Kosuke TAKANASHI, Keishi KOUNO, Hiroto KAWAHARA,
Mafumi HIRATA, Junko KOHAGURA, Satoshi TOGO, Mizuki SAKAMOTO,Takeru FURUKAWA1 and Shunjiro SHINOHARA2.
Plasma Research Center, University of Tsukuba,
1Kobe University, 2Tokyo University of Agriculture and Technology.
e-mail (speaker): seto_takumi@prc.tsukuba.ac.jp
For realizing the demonstration power plant (DEMO)and future fusion reactors, it is critical to reduce the heatload to the divertor. Currently, divertor simulation
experiments are being conducted using many linear
devices to investigate the physics of detachment plasma,
which is one of the methods used to reduce the heat load[1]. In order to investigate detachment plasma in a more
relevant plasma condition, which is expected in theentire scrape-off layer (SOL) and divertor region inDEMO, higher electron densities of the order of 1020 –1021 m-3 and electron and ion temperatures above 100 eVin the steady state are required [1]. In addition, a large
plasma diameter under a strong magnetic field isexpected in a real-scale DEMO-grade divertor region [2].
As one of the radio frequency (RF) plasma sources,
helicon plasma (HP) sources are expected to generate thesteady-state, high-density and large diameter plasma by
high-power discharge with a flat type antenna. However,hydrogen plasma generation, which is important fordivertor fundamental research, has thermal problems dueto the effects of high temperature atoms caused bydissociation, or other factors. In addition, the physicalproperties of HP sources in the high-density, such as
density saturation due to neutral particle depletion, arecomplex and remain unclear. Therefore, our group has
been developing a novel HP source using flat antennas toinvestigate these fundamental challenges [3].
The novel plasma source has two-turn flat loop
antenna, which is one of the flat-type antennas (Fig. 1).
The antenna is made of cooper and has a water channelfor cooling to realize high-power and high-density
plasma generation. An RF power supply, 30 kW, 13.56
MHz continuous wave, and a matching box with twovariable condensers were connected to the antenna. The
plasma source development was carried out using theCTP (Compact Test Plasma) device, and argon andhydrogen discharges were performed by varying amagnetic field configuration and its field strength using
two magnetic coils up to about 0.1T. Figure 1 shows thepicture of argon discharge experiment on the CTP. Itshows the strong (blue light) intensity, which showsargon visible lines, and a characteristic of helicon plasma.
The plasma source can be installed to the Pilot
GAMMA PDX-SC (Pilot device), which is underconstruction for fundamental research of the DEMO
relevant divertor plasma. The Pilot device has two
superconducting magnets, generating strong magnetic
field of up to 1.5 T. To the best of our knowledge, it is
the first example of HP plasma generation by flat loop
antenna under such a strong magnetic field.
In this presentation, we will report the experimental
results during the discharge under different magnetic
fields by the CTP and the Pilot device.
Acknowledgement
This work was partly supported by NIFS Collaboration
Researchprogram(NIFS20KUGM148,NIFS20KUGM150, and NIFS23KUGM174).
References[1] N Ohno, Plasma Phys. Control. Fusion 59 034007(2017).
[2] N. Asakura et al., Nucl. Fusion 61 126057 (2021).
[3] T. Seto, et al., Plasma Fusion Res., 18 2401054(2023).

O.D. = 5 and 10 cm
Figure 1. The novel plasma source with two-turn flat loop antenna, and the CTP photograph during argon discharge.
The antenna outer diameter (O.D.) was 5 and 10 cm. The antenna was installed on left hand side of picture.