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

Evaluation of Electrode Potential and Heat Load of a Cascade Arc Discharge
for Realization of High Density Hydrogen Plasma Discharge
R. Miyauchi, N. Ezumi, T. Seto, T. Okamoto, K. Takanashi, S. Takahashi,K. Kouno, H. Kawahara, S. Togo, Y. Nakashima, M. Sakamoto
Plasma Research Center, University of Tsukuba, Tsukuba, Japanmiyauchi_reina@prc.tsukuba.ac.jp
A DEMO reactor design is underway in Japan as thenext step after ITER. The design of the crucial divertor for
realizing the DEMO is intended to be carried out using
simulation codes. However, it is anticipated that theDEMO divertor will experience a higher heat load andmore complex physical phenomena compared to ITER,
necessitating an improvement in the accuracy of thesimulation code. Simulation experiments using linear
devices have contributed to enhancing the accuracy of thesimulation code. Nevertheless, the current linear device
cannot conduct DEMO-class divertor simulation
experiments. We are currently constructing a new lineartype device called the "Pilot GAMMA PDX-SC (PilotDevice)", aiming to generate steady-state, high-density
plasma equivalent to a DEMO reactor divertor. The Pilot
device has two superconducting coils, generating a steadymagnetic field of up to 1.5T. The Pilot device uses aplasma source that employs a hot cathode arc discharge togenerate steady-state high-density hydrogen plasmas.
The plasma source consists of a LaB6 cathode with150 mm in diameter, a copper anode, and fiveintermediate floating electrodes. These electrodes
designed along the magnetic field lines generated bysuperconducting coils contribute to the large diameter
plasma. Using a large-diameter cathode matched to themagnetic field lines enhances the thermal electron
emission performance of the cathode. It contributes to thegeneration of high-density plasma. So far, the first plasma
was generated in October 2022 (Figure 1). In this case,argon and hydrogen plasmas were successfully generated
with densities of 1.5 × 1019 m−3 and 0.8 × 1019 m−3 atthe magnetic throat, respectively. However, the hydrogenplasma generated by hot cathode arc hydrogen discharge
was unstable and difficult to achieve higher density.
Understanding the discharge characteristics is important
for achieving steady-state high-density hydrogen plasmas.
To evaluate the potential structure and heat loss inside theplasma source related to the optimization of hydrogen
discharge, the potential of each discharge electrode andthe heat load on each electrode and component weremeasured. As shown in Figure 2, the electrode potentials
were measured as the floating potentials of theintermediate electrodes and the cathode vessel. Eachelectrode and component heat load were evaluated by thecalorimetry method based on the flow rate of the coolingwater and its temperature change. This presentation will
explain the evaluation results of electrode potential andheat load to the electrode and component during discharge,comparing the relationship with discharge characteristics.

Figure 1 The first plasma in Pilot GAMMA PDX-SC(Hydrogen plasma)
Cathode Intermediate
Cathode vessel
electrodes Anodeflange
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CathodeCooling waterto flow meter
Potential measurementwith oscilloscope
DC power supply

Figure 2 Cross-sectional view of the hot cathode DCarc plasma sourceThis work was partly supported by NIFS Collaboration
Research program (NIFS20KUGM148, andNIFS23KUGM174)