J. Plasma Fusion Res. SERIES, Vol.3 (2000) 467469

A New centrifuge Pellet lnjector with a Screw Extruder
for Steady State Fuelling
VINIAR Igor*
PELIN laboratory, Ltd., Moscow, 125212, Russia(Received: 18 January 2000 / Accepted: 13 May 2000)

Abstract
A new conceptual design of a centrifuge injector for steady state plasma refuelling by solidhydrogen isotope pellets is presented and discussed. The apparatus has three new components: a screw
extruder for continuous pellet production, a new rotating curved barrel for pellet acceleration, whose
entrance section is placed on the axis of the centrifuge rotor, and a new pellet chopping unit. In
preliminary tests, the screw extruder with the pellet chopping unit delivered a series of about 10,000
deuterium pellets of 2 mm in size (at the rate up to 15 Hz and velocities 100-150 m/s) into the curved
barrel with a 99Vo reliabllitv.

Keywords:
fuelling, pellet, injector, extruder, centrifuge

1. Introduction

reliability of screw extruders, the simplicity of

One of the main tasks still to be solved before thesuccessful operation of a fusion reactor such as ITER isto refuel the plasma for long pulse lengths over 1000 s,
practically for steady state operation. There are threetechniques for plasma refuelling: by gas puffing, bycompact toroids and by injection of pellets produced
from solidified hydrogen isotopes. The key problem ofpneumatic injectors and the absence of propellant gas incentrifuges, we have suggested a new conceptual designof a centrifuge pellet injector to be discussed below.

2. Injector Design and Operating lssues

A schematic diagram of the centrifuge pellet
injector and its operation principle is illustrated in Fig.
l. The main parts of the injector are an extrusion

the injection is to develop a reliable and much simpler
pellet injector capable of injecting, in the steady statemode, an unlimited number of pellets into a plasma

cryostat with a screw extruder and a vacuum chamberwith a pellet chopping unit and a rotating curved barrel
attached to the centrifuge rotor. A solid deuterium rod ispushed out from the screw extruder continuously. The
chopping unit cuts off a piece of the rod and directs thepellet into the rotating barrel. The pellet is accelerated tothe barrel periphery and ejected into the exaust tube.

core. A screw extruder [1] is still most suitable forreliable pellet production in the steady state mode. Twokinds of injection techniques are now in use, those withgas guns and centrifuges. Generally speaking, pneumatic
injectors are more reliable and simple devices thancentrifuges, providing the pellet speed up to 4.5 km/s [2]exceeding the maximum speed for centrifuges almost by

The prime feature of the centrifuge is the entrance
section of the rotating curved barrel, which is alignedwith the rotational axis of the rotor. This means that theentrance section ofthe barrel, into which a pellet is fed,a factor of four. However, centrifuges have a majoradvantage over guns because there is no propellant gas

to handle, process and recover. To combine the*@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchC o rre sp ondin g autho r' s e - rnail : n p e I in @ de lfa. ne t467

Viniar I., A New Centrifuge Pellet Injector with a Screw Extruder for Steady State Fuelling

500

Et

4oo
3oo0LcmFig.3 A deuterium pellet (3 mm) velocity Valong thebarrel length L without and with gas (pressure 0.1
MPa, temperatures 25 and 300 K) acceleration.

Fig. 1 A centrifuge pellet injectorq

500 m/slro
20\r1000 m/s

10\0.02

dV/V
Fig. 4 The acceptable area of variation in the initial pellet

velocities and the time of the pellet throw-in into
the barrel entrance.

oscillations in the pellet velocities from the gas inside
Lrcm

the barrel, we intend to use a special chopping unit. Thisunit will cut out a pellet and create a gas pressure in thebarrel inlet. The gas will accelerate the pellet effectively
at the beginning of the barrel, where the centrifugal
acceleration a" is small, as is illustrated in Fig. 2. The
pneumatic acceleration cp on the periphery of theFig. 2 A deuterium pellet (3 mm) acceleration a along
the barrel length L as a sum of the pneumatic ao

and centrifugal a" acceleration components.

is fixed relative to the other injector units and also
relative to the extrusionnozzle. Despite the rotation, thebarrel entrance section does not move relative to thesolid fuel rod extruded from the nozzle. This allows theadjustment of the pellet chopping unit by changing theposition of the barrel entrance section, rather than of its
exit section, relative to an exhaust unmovable guiderotating barrel (the barrel length Z is 60 cm) isnegligible and the pellet will be accelerated by thecentrifugal forces. The gas influence on the final pellet

velocity V is small, as seen from Fig. 3. Here thedeuterium gas in amount of 20Vo of the pellet mass (attemperature of 25 and 300 K and pressure of 0.1 MPa)
was chosen for the acceleration of the deuterium pellet

tube. This considerably simplifies the injector designand performance. The curved barrel for pelletof 3 mm in diameter and length. However, the highinitial pellet velocity at the entrance of the rotating
barrel is useful because it reduces the pellet spread

acceleration by centrifugal forces was chosen instead ofan arbor or a disc [3], because during the extrusion,pellet chopping and acceleration processes, there appear
gas sources due to solid deuterium ablation, which canangles at the barrel exit. The acceptable area ofvariationin the initial pellet velocities and the time of the pellet
throw-in into the barrel entrance are shown in Fie. 4.

make up 30Vo of the pellet mass. To eliminate468

Viniar I., A New Centrifuge Pellet Injector with a Screw Extruder for Steady State Fuelling

The oscillations of the time and initial velocities shouldbe less than 0.0lms and 18 m/s to achieve the final
velocity of I km/s with the spread angle less than 2". It
allows injection of intact pellets because the normal
velocity during the pellet blow into the exaust guidetube walls will be less than 20 mls. By controlling theinitial speed of the pellet fed into the barrel within thearea shown in Fig. 4, the residence time of the pellets inthis tube can be adjusted more accurately and thereliability of the intact pellet transport along the injector

the gas pressure. After the acceleration in the curved
barrel, all the pellets were found to be intact. The tests
will be continued after the centrifuee chamber and rotor
are manufactured.

4. Summary
An original centrifuge injector with a screw
extruder for steady state plasma refuelling has been
designed and is being patented now. The injector with
one screw extruder contains l0 cm3 of solid deuterium

and is capable to produce up to 140 mm3/s ofexhaust tube axis can be enhanced.

transparent deuterium ice for pellet production. The
chopping unit injects over 10,000 deuterium pellets of 2mm diameter (at a frequency of l-15 Hz and speed
100-200 m/s) into the fixed barrel entrance section with
a99Vo reliabllity. Tests for the pellet acceleration in the3. Test Results
The centrifuge injector is being manufactured now,so a limited number of preliminary tests have been
carried out with the screw extruder, the chopping unit
and the model curved barrel. Detailed results of thescrew extruder tests were presented in [4]. A continuous

rotating barrel are being prepaired.

1000 s extrusion of transparent deuterium ice at a speed

References
I. Viniar and A. Lukin. Techn. Phys., 45 (l), 106

up to 45 mm/s was demonstrated. The chopping unit cut[]out pellets at frequencies from I to 15 Hz with a step ofI Hz. No broken pellets were found in the diagnosticchamber after the chopping. Less than 17o of 10,000tzl A. Geraud, J.P. Perin and G. Gros. Fusion Technology 1998: Proceedings of the 20'h Symposium,(2000).

pellets were lost during the acceleration in the model
barrel by helium gas imitating the evaporated deuteriumin the amount of 30Vo of the pellet mass. The pellet
velocities were as high as 100-150 m/s depending on

Marseille, 2, 941 (1998).

[3] S. Combs. Rev. Sci. Instrum., 64 (7),1679 (1993).
[4] I. Viniar, S. Skoblikov and A. Lukin. J. Plasma Fusion Res. SERIES, Vol. 3 (2000).

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