Fusion Engineering and Design 58–59 (2001) 337–341

Test of an extruder feed system on the Tore Supracentrifugal pellet injector

A. Geraud a,*, J.F. Artaud a, G. Gros a, C. Pocheau a, S.K. Combs b,C.R. Foust b

a Departement de Recherches sur la Fusion Controlee, Association Euratom-CEA, CEA/Cadarache,F-13108 Saint Paul Lez Durance, France

b Oak Ridge National Laboratory, TN 37831-8071, USA

Abstract

A deuterium ice extruder feed system has been developed by ORNL for adaptation to the existing ORNLCentrifugal Pellet Injector installed on Tore Supra. A detailed assessment of this feed system coupled to thecentrifugal launcher has been carried out in a dedicated test bed at Cadarache, to complete and understand the firstresults obtained at ORNL and to improve them. The results presented in this paper are related to the study of thepellet trajectory from the deuterium ice cutting to the collection of the accelerated pellets in a guide tube. The overallreliability of the injector has been shown to be very sensitive to deviations in the free flight part of the trajectory,before capture by the rotating launcher. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Tore Supra; Centrifugal pellet injector; Deuterium

www.elsevier.com/locate/fusengdes

1. Introduction

In order to control the density of the plasmasassociated with the new CIEL configuration inTore Supra, a pellet injector able to fuel theplasma for long pulse discharges (100–1000 s)with a very high reliability (�95%) is required. Insuch conditions, the total number of pellets whichcan be delivered per plasma pulse, the reliability(i.e. the ratio of the number of pellets entering theplasma to the number of fired pellets), and theinjection frequency must exhibit simultaneously a

high level of performance. In the required range150–600 m/s, the pellet velocity is not reallychallenging, in particular in the High Field Sideinjection configuration for which the velocity islimited by the guide tube necessary to convey thepellets in the inner part of the tokamak chamber.The initial feed system of the injector installed onTore Supra exhibited performances for these threeparameters (respectively 100, 70% and 5 Hz) thatwere not compatible with the new requirementsfor CIEL. The extruder technology allowed thenumber of pellets (� 1.6 mm) to be increased upto 1000 and the associated new punch mechanismwas able to work at frequencies up to 10 Hz. Thereliability is certainly the parameter that requires

* Corresponding author. Tel.: +33-442256188; fax: +33-442252661.

E-mail address: geraud@drfc.cad.cea.fr (A. Geraud).

0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

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the most important effort. For the new design, aswell as for the old, the reliability is mainly influ-enced by the issue of the pellet capture by therotating centrifuge after cutting. Therefore, thetests have been concentrated on the study of thepellet trajectory from cutting of the extruded iceribbon to the end of the acceleration process andfunneling to a guide tube.

2. Experimental equipment

The new feed system is similar to the basicORNL extruder design [1] but working with anangle of 15° relatively to the horizontal plane. Asthe extruder axis was installed normal to thecentrifuge plane, it has been necessary to extrudethe ice around a 90° bend for proper alignmentwith the punch mechanism and centrifuge. Thiswas accomplished by the addition, at the nozzleexit of the basic extruder, of a cooled copperblock in which a 90° curved channel guides theextruded ice. The tests have shown that this addedblock, located outside the extruder cryostat, wasthe cause of a limitation in the duration of theextrusion process [2] that is not discussed in thispaper. The feed system has been adapted to thespin tank in place of the old feed system, based on‘in situ’ deuterium condensation on a disk [3,4].

The centrifugal launcher [1,3,4] was the cate-nary centrifuge used on Tore Supra since 1990. Ithas been installed with its spin tank, in which itrotates (60 Hz during the tests, corresponding to apellet velocity of 500 m/s) on a dedicated test bedin a cryogenics laboratory.

A schematic drawing showing the track en-trance in the centrifuge and the puncher cuttingthe extruded ice is presented on Fig. 1. The exitport of the spin tank has been modified in orderto be able to follow the trajectory of the pelletsafter acceleration in the rotating arbor. The initialdesign was adapted to the collection of the pelletsin a guide tube just at the centrifuge exit. The porthas been extended in the centrifuge plane and anew diagnostic chamber has been installed asshown on Fig. 2. The trajectories are determinedfrom two spatially resolved light barriers. The firstone is constituted by a laser diode (5 mW) lighting

a linear array of 18 optical fibers connected to afast electronics. The second one, located 40 cmaway, is installed on a specific chamber equippedwith two large windows (� 120 mm) and a laserdiode (15 mW) lighting a linear array of 72 opti-cal fibers. Trajectories having an exit angle be-tween −4.5 and 4.5° can be resolved. To limit thenumber of required channels on the fast acquisi-tion device (90 simultaneous channels would benecessary), a specific electronics has been devel-oped to encode and mix the signals. The pelletposition at the centrifuge entrance was monitoredwith a CCD camera.

3. Experimental results

Due to small ice inhomogeneity and small irreg-ularity in the cutter motion, the pellets after cut-ting have an angular and temporal spreadrelatively to the rotating arbor, as shown on Fig.1. This spread is deduced from the photographsobtained with the CCD camera viewing the trackentrance. The position of the pellets for a typicalrun is presented on Fig. 3. Clearly the dispersionis important and, due to the relatively large dis-tance between the nozzle axis and the bottom ofthe track (22.5 mm), covers a zone larger than theacceptance zone in the track (4 mm radially and

Fig. 1. Schematic drawing of the track entrance on the arborrotating in a plane quasi perpendicular to the figure plane.

A. Geraud et al. / Fusion Engineering and Design 58–59 (2001) 337–341 339

Fig. 2. Modified exit port with the two light barriers.

11.7 mm or 4° in the direction of rotation). Thiszone can be reached only by pellets whose trajec-tories are inside a cone of half angle of 3°. Thestatistics from video recording shows an averagepercentage of pellets in a good position to becaptured by the centrifuge of 60–70%. This analy-sis highlights a first important cause of reductionof the overall injector reliability since the designof the system does not allow to reduce the freeflight distance.

The second part of the study concerns the effectof the large spread observed at the arbor entranceon the dispersion angle after acceleration. Thelight barrier diagnostic, described on Fig. 2, al-lowed us to rebuild the trajectories. Raw andcalculated data issued from this diagnostic arepresented on Fig. 4 for a typical sequence of 102pellets. On the first curve (a) the position of thepellets at the second barrier is plotted versus theposition at the first one, which allows to controlwhether a pellet has a free trajectory (circles) ornot (crosses). The angle distribution can be de-duced and is plotted on the curve (b). A gaussianfit, taking into account the true limit of the diag-nostic (−4.7 and 2.5°, indicated on the figure bythe vertical lines) is also plotted. Knowing thedistance between the light barriers, the velocitydistribution can be calculated and is also plottedon Fig. 4c. The mean value of 506 m/s is consis-tent with the exact solution of the motion equa-tion for the measured rotation frequency of 59.8Hz. The last curve (d) shows the reconstructedtrajectories with the six pathological pellets hittingthe spin tank and identified in the first curve. Thefact that the angle distribution is not centeredindicates probably that the timing adjustment for

the reference time corresponding to the arbor inposition to receive pellets, and therefore the posi-tion of the feed system relatively to the launcher,was not accurate enough.

The gaussian fit gives a standard deviation of 2°with a significant number of pellets exiting withan angle up to �4°. We will see in the nextsection that such angles are a second cause oflimitation of the overall reliability.

4. Comparison with modelling

The accepted angular dispersion on Fig. 1 cor-responds to a circle of radius 2 mm for capturedpellets. Since the pellets slide on the track, assum-ing no friction, the position variation is equivalentto a phase variation of �2° of the centrifuge,

Fig. 3. (a) Pellet distribution at the centrifuge entrance in theplane of the CCD camera view. A drawing of the arborentrance is superimposed. The ‘o’ and ‘x’ symbols refer, re-spectively to pellets seen as unbroken and broken on the CCDpicture and a ‘+ ’ indicates that the pellet has been seen on thepressure signal in a target tank after acceleration. When apellet was detected by the light barriers, a thick circle isplotted; (b) Statistics on the angle from the cutting location.

A. Geraud et al. / Fusion Engineering and Design 58–59 (2001) 337–341340

Fig. 4. Raw and calculated data from the light barriers diag-nostic for a typical run of 102 pellets.

the considerations on the geometry. The accelera-tion in the centrifuge by itself is not expected tocause pellet breaking since the margin deducedfrom deuterium elastic and plastic properties isquite large.

Finally these ice properties have been used in amodel developed to calculate the maximum veloc-ity that can be tolerated by a pellet hitting asurface [6]. A maximum perpendicular speed of 20m/s was found for an ice temperature in the 6–14K range, which corresponds to an angle of 2.3°for the velocity of 500 m/s of the experiments.This limit is very close to the limit angle of 2.2°found experimentally by Combs et al. [1].

As a guide tube at the spin tank exit must beintroduced to convey the pellets towards theplasma, only the fraction of the angle distributionhaving an angle less than 2.3° will be collectedwithout breaking. For the tested configurationthis fraction was 71% as shown on Fig. 5.

Taking into account the fraction of pellets cap-tured at the centrifuge entrance (70% from theCCD image), we find finally a typical overallreliability of the injector of about 50%. This cal-culated value indeed corresponds to the best per-formances achieved during both the present test atCadarache and the initial tests at ORNL.

corresponding to the angle at which the pellet willtouch the bottom of the track. In the same way,the maximum acceptable timing variation corre-sponds to an angle of �2°; the total entrancephase dispersion is thus �4°. This entrance phasedispersion is conserved during the acceleration inthe rotation arbor and thus pellets exit with anangular dispersion of �4°. The exact calculationis obtained by solving the Newton’s law equationdescribing the pellet motion in a rotating referen-tial, knowing the shape of the track:

md2Rbdt2 =2m

dRbdt

��b +m�2Rb +Fb contact+Fb friction,

with Ffriction=� ·Fcontact where � is the frictioncoefficient for which an upper limit has beenestimated from measurements performed on theASDEX-U pellet injector [5]: ��1.3×10−3.Therefore, the friction term can be neglected. Thecontact force is calculated allowing to estimate thepressure on the pellet during the acceleration:considering a surface of contact corresponding tothe surface of the median plane of a cylindricalpellet, a maximum value of 0.15 MPa is found fora rotation frequency of 60 Hz, which is about30% of the pellet breaking limit deduced fromdeuterium ice characteristics.

The experimental angle distribution found inthe previous section is therefore consistent with Fig. 5. Exit angle distribution and calculated angle limit.

A. Geraud et al. / Fusion Engineering and Design 58–59 (2001) 337–341 341

5. Conclusion

It has been shown that, due to the relativelylarge distance between the extrusion nozzle andthe track entrance in the centrifuge, space andtime dispersion of the pellets after cutting leads toan important angular dispersion at the exit afteracceleration. The corresponding angle distributionhas been measured, using a new diagnostic basedon multi-light barriers, and compared to the resultof a model giving the breaking velocity limit forpellets hitting a plate or a guide-tube: a significantpart of the distribution (�30%) is shown toexceed this limit. We can thus conclude that anaverage overall reliability of �50% would be thetypical performance of the injector, which indeedcorresponds to the best performances achievedexperimentally at Cadarache.

Thus the performances required for sustainingfuelling in Tore Supra cannot be reached with theinjector in its present configuration. Neverthelessno experimental evidence of intrinsic pellet lossesin the acceleration process itself has been found,

as the overall losses at the exit appear to beentirely justified by the observed mismatch at thearbor entrance. Negligible acceleration lossesmeans in turn that a highly reliable long pulsecentrifugal injector could be designed, providedthat the initial pellet dispersion is reduced and/orthe acceptance concept is improved. At least forshort sequences in the 100-pellet range, this is thecase for the Asdex-U and JET injectors with thestop-cylinder concept [7].

References

[1] S.K. Combs, C.R. Foust, Rev. Sci. Instrum. 68 (12) (1997)4448 December.

[2] A. Geraud, Association Euratom-CEA report NT �/133,September 1998.

[3] C.A. Foster, J. Vac. Sci. Technol. A1 (1983) 953.[4] C.A. Foster et al., IAEA-TECDOC-534, 1989, p. 275.[5] C. Andelfinger, et al., Rev. Sci. Instrum. 64 (1993) 4 April.[6] J.F. Artaud, A. Geraud, Fus. Technol. 1998, 20th SOFT,

Vol. 2, p. 971.[7] M.J. Watson et al., in: Proc. 1999 IEEE/NPSS 18th SOFE,

IEEE Cat. No. 99CH37050, Piscataway, NJ, p. 326