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Centrifuge Fuel Pellet Injector

#2319


Development of Centrifuge Pellet Injector for Fuelling Tokamaks and Stallarators

Tech Area / Field

  • FUS-MCS/Magnetic Confinement Systems/Fusion

Status
3 Approved without Funding

Registration date
26.09.2001

Leading Institute
NIIEFA Efremov, Russia, St Petersburg

Supporting institutes

  • TsKBM, Russia, St Petersburg

Collaborators

  • National Institute for Fusion Science, Japan, Gifu\nMax-Plank-Institut für Plasmaphysik / IPP Greifswald, Germany, Greifswald

Project summary

Thermonuclear plasma fuelling by means of solid hydrogen pellets is investigated intensively in a number of research centers in the USA, Germany, Japan, France, Great Britain and Russia. This activity is concerned with the controlled thermonuclear fusion program and is aimed at the construction of the thermonuclear fusion reactor as a source of the electric energy. The reactor should operate for a long period of time during which the deuterium-tritium fuel burns out and the resultant helium ash then must be replaced with the new deuterium-tritium mixture. The fuel should be delivered into the plasma by means of pellets, which have the characteristic size of several millimeters and the velocity of the order of one kilometer per second.

The fuelling system is an essential part of a thermonuclear reactor and it must work properly during the whole operation period.

The fuelling systems designed and constructed in the leading countries are oriented mainly at research problems. Although these systems have demonstrated possibility of production and acceleration of fuelling pellets with parameters close to the reactor-relevant values, their reliability is lower than that required for the thermonuclear fusion reactor (higher than 95%).

For such thermonuclear devises as JET, JT-60, ASDEX Upgrade, Tore Supra, LHD, W7-X it is efficient to use the centrifuge method of pellet acceleration. Compared to the light gas injectors, centrifuge injectors yield a significantly lower gas load on the pumping system due to absence of the high-pressure propellant gas and enable a high-frequency injection.

Vast experience of the centrifuge injector development has been accumulated in several laboratories such as ORNL (USA), IPP (Germany), Mitsubishi (Japan) and NIIEFA (Russia).

Modern centrifuge injectors have the following injection parameters:

– pellet velocity – 0.5ё 1.3 km/s;


– pellet size – 1ё2 mm;
– injection frequency – 1ё40 Hz;
– injection time – 1ё10 s;
– reliability – < 70ё90 %.

Pellets are accelerated by series (up to 500 pellets in a series). For production of pellets the extruder technology is used.

The existing injectors still don’t satisfy two substantial requirements: reliability and operating period (longer than 15 mins).

To overcome these difficulties, it is necessary to perform an additional investigation aimed at development of an appropriate extruding system and of mechanisms for pellet formation and delivery to the centrifuge acceleration channel, which rotation rate is of the order of 10,000 rpm.

The main purpose of this project is to develop a centrifuge fueling pellet injector for large tokamaks and stellerators with a long enough operation period.

The acceleration arm with a straight acceleration channel, made of a strength alloy with the toughness limit of 150ё200 kg/mm2, will be used in the injector being constructed. Such launcher will provide pellet acceleration up to і 650 m/s velocities. A turbomolecular pump with a vertical axis of rotation will be used as a centrifuge drive. The acceleration arm will be installed on the axis of the turbomolecular pump.

Supports of the rotor unit should provide rotation of the acceleration arm in vacuum at the rate of (10ё15)×103 rpm in continuous regime. For this purpose, the capability of the radial supports as well as contactless radial supports (magnetic) will be evaluated. This will allow us to improve the injector reliability substantially.

The maximal pellet velocity depends on the mechanical strength of solid hydrogen, which is sensitive to the temperature and to mechanical forces affecting the pellet during its acceleration. Optimization of the forces affecting the pellet will allow us to provide the pellet integrity with the reliability і 95% for the velocities over 650 m/s.

The solid hydrogen rod from which pellets are cut off will be prepared from gas hydrogen. The pellet temperature will be about 7 K, the pellet size will be і2 mm.

The pellet formation unit will contain several piston extruders working in sequence for a single outlet channel. The extrusion rate up to 50 mm/s will allow us to prepare pellets at a frequency of up to 20 Hz. Several methods of pellet cutting, including traditional cutting by means of a knife, will be tested and optimized.

To deliver pellets into the acceleration channel of the acceleration arm, a synchronized electromagnetic drive will be installed at the central zone of the centrifuge injector. The expected scattering angle for pellets at the centrifuge injector exit will be of the order of ±4 °.

The centrifuge injector will have the following parameters:

– pellet velocity – і 650 m/s;


– pellet size – і 2 mm;
– injection frequency – 20 Hz;
– scattering angle – ± 4;
– material – H2;
– injection time – і 15 min;
– reliability – і 95%.

The following works should be done to fulfill the project:

– to make a technical design and to perform experimental tests of the extruder unit for continuous injector operation;


– to develop a cutter for pellet formation;
– to develop an interface for pellet loading into the centrifuge injector;
– to issue the technical documentation on the injector;
– to perform, manufacture, assemble the injector and put it into operation;
– to optimize the injector operation regimes;
– to measure the main injector parameters: velocity, frequency, scattering angle and to make pellet photographs at the injector outlet.

As a background of the project, it is efficient to use the NIIEFA and STU experience in pellet injection development, the CDBMB experience in the centrifuge rotors design and fabrication.


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