Dust particle formation due to interaction between graphite and helicon deuterium plasmas
Introduction
The study of dust particles formed in fusion devices due to the interaction between carbon walls and plasmas has been receiving increasing attention, because appreciable amounts of dust particles have been found in many fusion devices [1], [2], [3], [4], [5], [6], [7], [8], [9]. These carbon dust particles pose potential problems in future long-term operations of fusion devices. For instance, the dust particles can contain a large amount of tritium, and their existence in the device could also lead to deterioration of the plasma confinement [2], [10]. Moreover, the dust-air cloud formed due to air ingress can even be an explosion hazard during normal plasma operation in fusion devices [11]. Therefore, it is important to understand their formation mechanism, their transport as well as their accumulation areas, and to suppress their formation in fusion devices having carbon divertors. Many papers concerning dust particles formed in fusion devices have mainly focused on μm-sized dust particles. We have studied the formation of nm-sized dust particles due to the interaction between a graphite and a hydrogen plasma in the large helical device (LHD) and in a helicon plasma reactor which we have developed to simulate divertor plasmas in the LHD [12], [13], [14]. A large number of spherical particles of nm in size and a relatively small number of their agglomerates whose primary particles are about 10 nm in size have been collected in both the helicon plasma reactor and the LHD. The composition of these dust particles is carbon which is the component of the graphite target of the helicon plasma reactor as well as the divertor material of the LHD. The agreement of the features of such dust particles formed due to the interaction between a hydrogen plasma and a carbon wall is an evidence to support the fact that the helicon plasma reactor simulates the LHD well. Deuterium plasma operation is being planned in the LHD in the immediate future, and hence preliminary experiments concerning dust formation using the helicon deuterium plasma reactor provide important information on the dust formation due to the deuterium plasma – carbon wall interaction in the LHD. In this paper, we report the results of the collection of dust particles having a size of μm–nm, which are formed due to the interaction between a graphite and a helicon plasma of deuterium in the helicon plasma reactor. Finally the formation mechanisms based on these results are discussed.
Section snippets
Experimental
Experiments were carried out with the helicon plasma reactor, shown in Fig. 1. The reactor is composed of a stainless vessel with 267 mm maximum inner diameter and 294 mm length, a quartz tube of 50 mm diameter and 200 mm length, as well as an antenna for the m = 1 helicon mode excitation of 170 mm length placed around the tube. A uniform magnetic field of 150 G was applied along the tube axis with four magnetic coils placed as shown in Fig. 1. Pure deuterium (D2) gas was supplied to the reactor at a
Results and discussion
Fig. 2 shows the discharge power dependence of the ion density and electron temperature. The capacitive to inductive (E–H) mode transition is realized around 0.6–0.8 kW; the ion density ni increases from 1011 cm−3 for 0.3 kW to 4 × 1012 cm−3 for 1.0 kW and above, while the electron temperature Te is in the range of 4.6–11.9 eV for the power of 0.3–1.5 kW. In case of the hydrogen plasma operation, the E–H mode transition is seen at around 0.7–0.9 kW; ni increases from 1011 cm−3 for 0.5 kW to 2.5 × 1012 cm−3
Conclusion
A sampling of dust particles formed due to the interaction between a graphite and a deuterium helicon plasma was carried out in the helicon plasma reactor as a preliminary experiment of deuterium plasma operation in the LHD. The following results were obtained: (1) the dust particles can be classified into small spherical particles below 400 nm in size, agglomerates whose primary particles have a size of about 10 nm, and large flakes of above 1 μm in size. The major composition of these dust
Acknowledgements
This research was supported in part by the NIFS General Collaboration Project (LHD Experiment) of the National Institute for Fusion Science, by the Japan Society for the Promotion of Science, and by the Ministry of Education, Culture, Sports, Science and Technology of Japan. We would like to thank Dr. N. Ashikawa (NIFS) and Prof. S. Masuzaki (NIFS) for useful discussions.
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