Cryostat for production and fast ejection of deuterium filaments

A cryostat for production and fast ejection o f filaments o f solidified gas is described. It is to be used particularly for feeding deuterium filaments into centrifuges with which D 2 pellets are fired at high speed and repetition rate (> 500 m s- 1 > 1 Hz) into plasma devices, fusion reactors and the like. The cryostat consists o f an extrusion part and a storage part whose temperatures can be/dependently controlled. The extruded deuterium filament can thus be temperature conditioned for optimum mechanical strength for the large forced acceleration in the guiding channel o f the centrifuge. The D 2 pellets thus accelerated have to be o f a defined length, which means that the feeding method has to meet extremely high requirements as regards the starting, stopping and speed of the filaments to ensure the exact entry of the pellets. Feed rates of up to 3.5 m s- 1 have been demonstrated.

Cryostat for production and fast ejection of deuterium filaments W. Amenda and R.S. Lang Key words: cryostat,deuterium, fusion reactor Plasma and fusion machines are to be replenished by injection of deuterium pellets using ultracentrifuges and other acceleration devices. 1-a Large machines will probably require quasi-continuous replenishment, lasting several seconds. It is advantageous to feed the centrifuge by direct insertion a,4 of a deuterium filament into the intake channel of the centrifuge, so the pellet is cut by the upper edge of the acceleration tube wall (Fig. 1). Pellets injected into the plasma require speeds exceeding a few hundred metres per second. Such pellet speeds call for centrifuge rotors up to 1 m in diameter in the frequency range 100 to 500 Hz. During a rotation the deuterium filament has to be inserted to the length required for the pellet. A pellet length of up to 4 mm maximum will thus require a feed rate of up to 2 m s -1 . If a pellet is not to be accelerated at every rotation, the filament should not be advanced continuously, but intermittently. If accelerated pellets have to be of a defined length, the scenario in handling the deuterium filament has to be exactly defined as regards starting, stopping and feed rate. Previously it was attempted to solve this problem by fast extrusion of the solidified deuterium, this method has proved inadequate, however, in providing a sufficient extrusion rate and reproducible timing of it. a Moreover, deuterium filaments produced by fast extrusion are necessarily of lower inherent stability due to the higher temperature involved. The device presented here is intended to afford both a much higher feed rate of the deuterium filaments, and a free choice of temperature for these filaments irrespective of the extrusion process, so that the necessary inherent stability of the D2 pellets can be achieved. This is important in view of the high forced acceleration exerted by the centrifuge on the pellet. Furthermore, this device is intended to allow intermittent direct feeding into the intake of the centrifuge. The authors are from the Max-Planck-lnstitut f~Jr Ptasmaphysik, Library, 8046 Garching, be/ M~nchen, FRG. Paper received 20 December 1981.

Deuterium filament source

Deuterium filament source

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Acceleration tube

1 / " Accelereted pallet

Fig. 1 a - Schematic diagram o f the apparatus for pallet acceleration. X l - r o t o r position during the cutting of the pellet; X2-rotor position after pellet acceleration, b -- Cross-sectional view o f the pellet cutting details in rotor position X l

Cryostat The device basically comprises two cryostats (T1 and T2) axially aligned with one another (Fig. 2). The two cooling blocks are largely thermally separate from one another to allow them to be set at different temperatures (insulation ring). With the device it is possible to solidify deuterium, extrude it, store the extruded material and eject it quickly. This calls for an operating temperature range of approximately 3 to 20 K. The extrusion cryostat T1 forms a unit with the fasteners, the supply lines and the vacuum vessel cover. The storage cryostat T2 is located in the extension of the extrusion aperture (nozzle) of the extrusion cryostat. The extrusion ram is a coaxial piston

0011-2275/82/007364-03 $03.00 © 1982 Butterworth & Co (Publishers) Ltd. 364

CRYOGENICS . JULY 1982

Ejection piston-rod He exhaL He exhaust ('

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through the nozzle aperture into the storage cryostat, where it can perform the necessary stroke in the storage space.

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in which the ejection ram is accomodated (Fig. 3). From the vessel cover the rams can be actuated independently of one another, it being possible to introduce the ejection ram

The cryostats work on the principle of the evaporator cryostat. 7 The coolant enters the cooling block T1, where it evaporates, causes cooling and is transported from the device via the external radiation shield by using the enthalpy of the helium gas. To cool the cooling block T2, part of the coolant fed to the device is tapped off and passed to the outside through a separate exhaust gas line. The coolant throughput in the cooling blocks TI and T2 can be set by means of dosing valves at the coolant outlet. Furthermore, the cooling blocks are provided with electrical heaters which allow the temperature to be regulated. With the coaxial piston raised and with cooling block T1 sufficiently cooled, the deuterium gas is admitted to the block T1 and is condensed and solidified in the condensation space. Once the condensation space is filled with solid deuterium the coaxial piston, which has meanwhile descended (the ejection ram being retracted), slowly extrudes a deuterium filament from the nozzle if the pressure and heating of the cooling block T1 are sufficient. When the storage cryostat is at a suitable temperature, the deuterium filament can enter the storage space and fill its whole length. After further heating of the cooling block T1 (expulsion or softening of the remaining deuterium in the condensation space) it is then possible to advance the ejection ram slowly through the nozzle into the storage space. When a temperature at which deuterium has favourable properties has been set, the ejection ram (continuously or intermittently) expels the deuterium from the storage with the necessary speed.

Details of storage cryostat The cross-section of the storage space is square, so that the cylindrical deuterium filaments in the ideal case only touch the walls of the storage space along four lines in order to avoid icing up if possible and to minimize friction between the filaments and the wails of the storage space (Fig. 4). If,

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CRYOGENICS. JULY 1982

Deuterium filament (~lmm) Fig. 4 Cross-sectional view of the inner parts of the storage cryostat

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nevertheless, adhesion to the wails does occur, ejection may involve the following difficulties: one, deformation or destruction of the deuterium filament; two, poor timing of the transport; and three, piston seizure. For these reasons a fast electrical heating facility was provided for ejection. This was accomplished by ensuring that the copper block enclosing the storage space has little thermal contact with the other parts of the cryostat T2 and, furthermore, has low heat capacitance. This is intended to ensure that a temperature rise only occurs in the adhesion regions between deuterium filaments and the walls of the storage space. Owing to poor heat conduction of solid deuterium the interior of the deuterium filament thus remains as cold as possible and hence is inherently stable during the entire ejection process.

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With an ultracentrifuge as accelerating device, the inherent stability of the deuterium pellet is of decisive importance owing to the compulsive forces thereby exerted on the pellet.

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The deuterium filament with circular cross-section stored in the storage cryostat T2 is 10 cm in length and 1 mm in diameter. It consists of dear, homogeneous solid D~. This filament could be quickly ejected by the hammer-driven ejection ram. In the interior of the storage cryostat the deuterium filament is subjected to just the right amount of friction so that it is force-locked and keeps in pace with the ejection ram without undergoing any permanent deformation on ejection. In addition, the pressure acting on the end of the filament to overcome the friction should be much smaller than about 5 bar (5 x 10 -s N m-2). s This is the static tensile strength of deuterium, the dynamic crushing strength being unknown as yet. The friction in the storage cryostat and at its narrow exit can be reduced to a limited extent by pulse heating the surface of the filament. A method which ensures the necessary force locking despite the diminished friction consists of freezing the deuterium filament to the end of the ram. This possibility was demonstrated. For storing long deuterium filaments with larger inertia, the friction must necessarily be increased to ensure force locking. Here the critical load could easily be reached. Fig. 5 shows the experimental setup for measuring the ejection speed. The lens L2 produces a shadow image of the deuterium filament on the slit, which is ejected from the storage cryostat. The image converter camera with streak picture facility is triggered by the ejection ram and in conjunction with a polaroid camera yields the pictures shown in Fig. 6. From these the ejection speed can be determined by means of the angle of the shadow wedges. Ejection speeds of up to 3.5 m s-1 were measured, depending on the impact of the hammer.

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Fig. 6 Streak pictures taken with the experimental arrangement shown in Fig. 5. Average ejection speed: a -- 0.7 m s-1, b - 1.9 m s-1, C --2.2 m s-1

intermittent feeding of such a filament into an ultracentrifuge, the ejection ram can be transported with a fast step motor or some other suitable propulsion mechanism. Fast provision of deuterium filaments with longer length and greater thickness seems possible. References

1

2 3 4 5

Conclusions

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The principle of producing, storing, conditioning and fast ejection of a deuterium filament was successfully demonstrated with the cryostat presented here. For direct,

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Dimock, D., Jensen, K., Jensen, V.O., J~rgensen, L.W.,

Pdcseli, ILL, S~rensen, IL, 0ster, F. Ris~ Nat Lab, Roskilde, Report 332 (1975) Milora,S.L JFusion Energy 1 (1981) 15 Amenda,W., Lang, R.S. Max-Planck-Inst f Plasmaphys, Garching, Report 1/187 (1981) Foster, C.A., Oak Ridge Nat. Lab., Oak Ridge, T, private Communication Friedman,W.D., Haipern, G.M., Brinker, B.A. Rev Sci Instrum 45 (1974) 1245 Amenda,W. Max-Phnck-Inst f Plasmaphys, Garching, Report 4/151 (1977) Klipping,G. Chem Ing Tech 36 (1964) 430 Borshutkin, D.N., Stetsenko, Yu.E., Alekseeva, LA. SovPhysSolState 12 (1970) 119

CRYOGENICS. JULY 1982