B. Ploeckl

Spring-driven high speed valve for massive gas injection in tokamaks

A new high speed gas valve was developed for disruption mitigation studies in the tokamak ASDEX Upgrade. The valve was designed to operate inside the vacuum vessel to reduce the time of flight of the injected gas and to prevent dispersion of the gas cloud before the gas reaches the plasma. A spring-driven mechanism was chosen for the valve as it is robust against the high magnetic fields and electromagnetic disturbances inside the vessel. The internal gas reservoir (128 cm3) of the valve, which holds the mitigation gas, is opened within 1.5 ms, and the maximal stroke between the valve plate and nozzle (diameter 13 mm) is 4.5 mm. This allows a peak flow rate of 72 kPam3/s after 1 ms which was determined both analytically and numerically. The highest gas velocity (approximately 560 m/s) is reached 0.6 ms after the valve is opened. The gas cloud expands in a pear shape with an opening angle of 49°.

Core Fueling of DEMO by Direct Line Injection of High-Speed Pellets From the HFS

Pellet injection represents to date the most realistic candidate technology for core fueling of a demonstration fusion power reactor tokamak fusion reactor. Modeling of both pellet penetration and fuel deposition profiles, for different injection locations, indicates that effective core fuelling can be achieved launching pellets from the inboard high field side at speeds not less than ~1 km/s. Inboard pellet fueling is commonly achieved in present tokamaks, using curved guide tubes; however, this technology might be hampered at velocities ≥1 km/s. An innovative approach, aimed at identifying suitable inboard “direct line” paths, to inject high-speed pellets (in the 3 to 4 km/s range), has recently been proposed as a potential complementary solution. The fuel deposition profiles achievable by this approach have been explored using the HPI2 simulation code. The results presented here show that there are possible geometrical schemes providing good fueling performance. The problem of neutron flux in a direct line-of-sight injection path is being investigated, though preliminary analyses indicate that, perhaps, this is not a serious problem. The identification and integration of straight injection paths suitably tilted may be a rather difficult task due to the many constraints and to interference with existing structures. The suitability of straight guide tubes to reduce the scatter cone of high-speed pellets is, therefore, of main interest. A preliminary investigation, aimed at addressing these technological issues, has recently been started. A possible implementation plan, using an existing Italian National Agency for New Technologies, Energy and Sustainable Economic Development-Oak Ridge National Laboratory facility is shortly outlined.

A Flexible Pellet Injection System for the Tokamak JT-60SA: The Final Conceptual Design

Abstract The research plan of the JT-60SA, a superconducting tokamak device currently under construction, requests a powerful pellet injection system for its particle fueling and edge-localized-mode (ELM) pacing experiments. These investigations, foreseen to answer basic questions with respect to the operation of ITER and a future fusion power plant like DEMO, need pellets with flexible parameters delivered precisely and reliably for control purposes. Here, we present a conceptual design of this system based on classical pellet technology. Analysis showed pellets will show the best performance for fueling and most likely also for ELM pacing when injected from the torus inboard side, despite the limited maximum pellet speed caused by this approach. This is due to constructional constraints rising from the fact the JT-60SA vacuum vessel is already under construction, enforcing inboard injection via a multibend guiding-tube system and limiting the maximum pellet speed to about 470 m/s. To match this boundary condition and fulfill the need for precise control, a centrifuge accelerator has been chosen. Based on the stop cylinder principle and equipped with a double accelerator arm, it can host up to six steady-state ice extruders working simultaneously for pellet production. This way, all system requirements expressed in the research plan can be well covered, providing even some headroom for better flexibility during the planned investigations. Details of our design and the reasoning for the layout chosen are provided in this paper.