IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 20, NO. 3, JUNE 2010 543

Stability Test of a Superconducting W7-X Coil WithRespect to Mechanical Disturbances

Dag Hathiramani, Thomas Bergmann, Victor Bykov, Peng Chen, Wolfgang Dänner, Andrzej Dudek,Joris Fellinger, Stefan Freundt, Laurent Genini, Klaus Höchel, Johannes Peter Kallmeyer, Johann Lingertat,

Holger Viebke, Stephan Weber, and Felix Schauer

Abstract—The superconducting magnet system of the Wendel-stein 7-X (W7-X) stellarator experiment consists of 50 non-planarand 20 planar coils which are supported by the central supportstructure and inter-coil structure elements. This highly loaded sup-port system is prone to mechanical disturbances like stick-slip ef-fects. On the other hand, the coils are built up from cable-in-con-duit-conductors (CICC) whose strands are highly compressed byLorentz forces during operation. Residual elastic energy releasewithin a cable can be triggered by shock waves and correspondingfrictional heat generation may occur. The released energy mightcome into the order of the conductor stability limit and possiblycause a quench. An experiment was performed to simulate the im-pact of such mechanical disturbances on W7-X coils with stabilitymargins corresponding to different operation conditions. A non-planar coil installed within the magnet test facility was energizedand then hit by a pendulum via a stainless steel transfer rod. Thetest has shown that mechanical disturbances expected in W7-X arenot able to induce a quench in any of the foreseen W7-X operationscenarios.

Index Terms—Fusion reactors, stellarators, superconductingcoils, superconducting device reliability.

I. INTRODUCTION

T HE goal of the Wendelstein 7-X (W7-X) stellarator ex-periment is to demonstrate that this kind of fusion device

is a viable option for a fusion power plant. At present the W7-Xis in the assembly phase at the Max- Planck-Institut für PlasmaPhysik in Greifswald, Germany [1]. The superconductingmagnet system of the W7-X consists of 50 non-planar and 20planar coils which are supported by the central support struc-ture and inter-coil structure elements [2]. This highly loadedsupport system is designed for certain flexibility in order toreduce mechanical stresses and thus contains as special featuressliding elements with predefined initial gaps and also flangeconnections which gradually close or open during operation.Therefore, the structure exhibits a highly non-linear behaviorand is prone to mechanical disturbances like stick-slip effects.

On the other hand, the coils are built up from cable-in-con-duit-conductors (CICC) [3] whose strands are highly com-pressed by Lorentz forces during operation. Residual elasticenergy release within a cable can be triggered by shock waves

Manuscript received October 15, 2009. First published March 18, 2010; cur-rent version published May 28, 2010.

D. Hathiramani, T. Bergmann, V. Bykov, P. Chen, W. Dänner, A. Dudek, J.Fellinger, S. Freundt, K. Höchel, J. P. Kallmeyer, J. Lingertat, F. Schauer, H.Viebke, and S. Weber are with the Max-Plack-Institut für Plasmaphysik, Greif-swald D-17489, Germany (e-mail: dag.hathiramani@ipp.mpg.de).

L. Genini is with the Commissariat a l’Energie Atomique, DAPNIA, Centrede Saclay, Gif-sur-Yvette 91191, France.

Digital Object Identifier 10.1109/TASC.2010.2040155

and strand movements with corresponding frictional heat gen-eration may occur. The released energy might come into theorder of the conductor stability limit and may cause a quench[3], [4].

The CICCs of W7-X coils are based on NbTi superconduc-tors. As compared to other large NbTi based superconductingmagnet systems, W7-X magnets will be operated closer to thecritical current density [5]. Theory cannot reliably answer thequestion if the W7-X magnet system will be stable with respectto mechanical disturbances once operated in its most criticalmagnetic field configuration.

Studies on the quench stability of CICCs with respect to me-chanical parameters are quite limited. Systematical critical cur-rent measurements on single strands and multi-stage CICC asa function of both the mechanical strain and the magnetic fieldwere performed on an especially developed test rig [6]. The onlyimpact experiment known to the authors is on a superconductingmagnet with an outer diameter of 140 mm where mechanicaldisturbances were investigated with respect to the quench sta-bility [7]. However, the mentioned experiments are not relevantfor the W7-X conditions.

Consequently, the presented experiment is quite unique, espe-cially with respect to its scale and the complexity of the geom-etry of the non-planar coils. Therefore, the results are not onlyof high interest for the Wendelstein 7-X project, but might alsobe valuable as reference for designers of magnets with compa-rable stability requirements, design and size.

The complete test program was accompanied by complex fi-nite element (FE) dynamic calculations. Stick-slip events oncoil support elements were simulated to derive the character-istics and order of magnitude of mechanical disturbances whichare considered possible in the W7-X magnet structure. Further-more, the experiment was simulated to verify that the experi-mental concept will be able to provide mechanical disturbancescomparable to those predicted for W7-X. The paper focuses onthe experimental results, the accompanied calculations will bepublished in a separate paper [8].

II. EXPERIMENTAL

A non-planar coil type 2 was chosen for the stability test be-cause the maximal field at the conductor is higher than at anyplanar coil, whereas the maximal currents are the same. In dy-namic simulations it could be shown that the response to impactson different types of non-planar coils is quite similar.

A. Coil Parameter and Stability Margins

The stability margin concerning a quench of the W7-X CICCis a function of the temperature T, the magnetic field B, and the

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544 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 20, NO. 3, JUNE 2010

Fig. 1. Principle of the impact unit to study the quench stability of a W7-X coilwith respect to mechanical disturbances. A pendulum mass hits a rod whichtransfers the impact to the superconducting energized coil.

coil current I with respect to their distances to the critical sur-face of the superconductor. Two stability margins of the CICC,determined with a zero-dimensional approach [3], were investi-gated during the impact test campaign: first a stability margin of50 (related to strand volume) and in a second phase astability margin of 20 . Only the conditions in the testbed had to be compared to those in W7-X; therefore, this methodto calculate the margins is sufficient.

Since at the coil test facility [9] no external magnetic fieldcan be provided and thus the induction at the conductor is lowerthan in W7-X, an equivalent stability margin has to be set usingthe two remaining parameters: current and temperature. In addi-tion, it has also to be considered that the maximal possible coilcurrent is limited to , and the lowest achievabletemperature is about . The maximal current 17.6 kAcorresponds to a maximal field at the conductor of 5.0 T.

1) Stability Margin 50 : This nominal stabilitymargin value corresponds approximately to W7-X operationwith 3 T at the plasma axis in standard field configuration. Thefield and current in W7-X are 6.2 T and 16.2 kA, respectively,at 4.2 K. The corresponding safety margin is in the order of 50

. For the impact tests the equivalent safety margin wasestablished at the coil test facility using: and

.2) Stability Margin 20 : This nominal stability

margin corresponds to the W7-X plasma axis field of 3 T at lowshear operation. Under these conditions, the maximal field atthe conductor is , , and the nominaltemperature is . For the impact test the equivalentsafety margin was established using: and

.

B. Impact Unit

The principle of the impact mechanism to provide mechanicaldisturbances to the superconducting coil is shown in Fig. 1. Im-pacts were applied using a pendulum system with a pendulumlength of 1.54 m. A cylindrical stainless steel pendulum massof 2, 10 or 30 kg with a length of 30 cm each was lifted to adefined dropping height and hits after release the warm side ofa transfer rod.

The stainless steel transfer rod with a length of 2.6 m and adiameter of 30 mm, firmly fixed on its cold end to the coil sur-face, allowed the impact to propagate. The hitting position onthe coil is of minor relevance for exciting disturbances withinthe winding pack [8]. The transfer rod, sealed by double O-rings,was fed through a vacuum port via two flexible bellows into the

Fig. 2. Non-planar coil equipped with additional sensors for the impact teststo measure the impact profile (SG), the acceleration in impact direction �� �

close to the impact point, the accelerations (��

� and��

� ), and lengths of boththe short and long coil axes. Impact point and direction are indicated by thetransfer rod.

cryo-vacuum. The two bellows system allowed the transfer rodto follow the motion of the coil during cool down, electromag-netic loading and impact, and provided an intermediate vacuumstage as a safety system to protect the cryostat from an unwantedventing in the case of an accident.

C. Measurement Systems for the Impact Tests

The applied impacts were passed via a transfer rod onto thecoil surface. Therefore, at the hitting point the coil had to face anadditional thermal load of about 0.3 W which has no influenceon the test.

Impact as well as rebound velocities of the pendulum masswere measured using light barriers. The velocity was deter-mined at the position were the pendulum mass had its lowestpotential energy. At the bottom of the pendulum masses a smallstick was installed. Knowing the distance between the two lightbarriers and the time difference the stick needed to pass thetwo sensors, one could derive the impact as well as reboundvelocities of the pendulum mass.

The impact profile was derived from the measured strainat the end of the transfer rod close to the hitting point.Two pairs of temperature compensated strain gauges (TMLCFCA-1-350-11) were used, positioned opposite to each otheron the circumference of the transfer rod. The pair was con-nected as a full bridge operated at a constant bridge voltage of8.13 V.

Vibration excitations of the coil, induced by impacts, wererecorded by measurement of the lengths of the two main coilaxes (see Fig. 2). For each axis a 0.4 mm Cu wire was spannedand fixed on one side, whereas on the opposite side connectedto a strain gauge based extensometer (Cantilever CE-10, TokyoSokki Kenkyujo).

Fig. 2 shows three different locations on the inner side ofthe coil where accelerations were measured. Piezo-electrical

HATHIRAMANI et al.: STABILITY TEST OF SUPERCONDUCTING W7-X COIL 545

Fig. 3. Sequence of applied impacts. The differently shaded areas indicate thedifferent coil configurations with corresponding nominal stability margins ofthe superconducting cable. Impacts applied with a pendulum mass of: 2 kg(squares); 10 kg (circles); and 30 kg (triangles). The last two impacts were per-formed on a non-energized coil.

accelerometers from Endevco company for cryogenic environ-ment were used. At location the acceleration was mea-sured (with type 7722) approximately along the direction of theimpact, whereas on the other two locations the accelerationswere measured in all three directions using three identical or-

thogonal aligned accelerometers (type 2272 for location

and 2271A for location ).All data acquisition systems were triggered by a short circuit

signal generated once the pendulum mass was in physical con-tact with the transfer rod . All signals were recordedfor 5 s at a sampling rate of 100 kHz.

III. RESULTS

Regular coil acceptance tests [10], [11] with currents up to17.6 kA at 5.7 K were carried out on the specimen before im-pact tests. One of the initial tests included a temperature inducedquench at the maximal coil test current. The quench was ob-served, as expected, 20 minutes after the helium inlet tempera-ture of the winding pack was increased in the last temperaturestep from 6.1 to 6.2 K. After the whole impact test campaignthe regular coil acceptance tests were repeated to show that thecoil did not experience any damage by the performed impacts.All regular acceptance tests before and after the impacts weresuccessfully passed, indicating no unusual behavior comparedto other non-planar W7-X coils.

The impact test sequence was executed in three phases: atfirst the coil parameters were set to achieve a stability marginof 50 (see Section II-A), in the second phase the sta-bility margin was reduced to 20 , and finally two im-pacts were performed without electromagnetic load on the coil.The impact sequence is chronologically summarized in Fig. 3.The impact energy was stepwise increased starting with ener-gies of few Joules using a pendulum mass of 2 kg. The maximalhit using a pendulum mass of 10 kg was always repeated withidentical impact energy but using a pendulum mass of 30 kg.

Fig. 4. Force profile on the coil surface due to impact with parameters given inthe diagram.

Fig. 5. Excited frequencies measured across the short coil axis. Impact param-eters are given in the diagram. Four pronounced peaks between 70 and 150 Hzare linked to eigenmodes of the whole coil body.

The overall maximal impact energy applied onto the energizedcoil was 167 J, corresponding to a momentum of 100 Ns. Noquench of the superconductor had been triggered even with thismaximal impact load at the lowest stability margin setting of 20

.A typical force profile acting on the coil at the hitting point

is shown in Fig. 4. The shock wave needs about 0.5 ms to prop-agate through the transfer rod. The profile lasts for roughly 2ms on the coil surface and shows two saw tooth peaks. The firstslope of the force profile is quite steep and hence able to excitealso frequencies in the kHz range.

A typical frequency spectrum of the short axis coil length os-cillation, induced by an impact, is shown in Fig. 5. The mainexcited frequencies are in the range of 50 to 200 Hz. The ampli-tudes are in the order of 10 and can be easily linked to thepredicted eigenfrequencies of the whole coil.

Accelerations of several thousand were obtained afterexciting the coil by an impact. A typical behavior of the acceler-ation versus time is given in Fig. 6. Oscillations of less than one

546 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 20, NO. 3, JUNE 2010

Fig. 6. Impact-provoked acceleration �� � close to the hitting point. Impactparameters given in the diagram.

Fig. 7. Comparison of experimentally derived and for W7-X expected dis-placements in the frequency domain. Shown are results from the maximal im-pact energy test, and from simulations of the expected maximal stick-slip eventin the magnet structure of W7-X.

millisecond are visible and the high amplitudes of accelerationsare damped within about 20 ms.

The comparison between measured and expected W7-X me-chanical disturbances on displacement versus frequency graphis presented in Fig. 7. The experimental values are derived frommeasured accelerations induced by the applied maximal impactenergy close to the hitting point. The mechanical disturbance ex-pected for W7-X is conservatively generated by a sudden releaseof the shear force on the highest loaded inter-coil narrow supportelement (sliding bearing) within the W7-X magnet structure.Displacements for both the experimental as well as the simu-lated data were derived by fast Fourier transformations (FFT)of the induced accelerations at the location showing the highestvalues. To obtain the displacements the amplitudes of the FFTsare normalized to the square of the respective angular frequency.Details concerning the calculations as well as further relatedsimulations will be published in a separate paper [8].

Both curves in Fig. 7 develop similarly above frequencies of200 Hz where they show comparable characteristics. From the

figure can be seen that the displacements in W7-X in critical di-rection transverse to CICC filaments do not surpass 2 underworst case assumptions, but the displacements during the testwere an order of magnitude higher. This means that a sufficientsafety margin is given.

IV. CONCLUSION

No quench of the superconductivity was triggered even withthe maximal impact of 167 J at the lowest nominal stabilitymargin of 20 . Shocks applied on the energized coilwere similar to the conservatively determined worst mechan-ical disturbances expected within the magnet structure of W7-X.The studied lowest stability margin corresponds to the W7-Xmaximal magnetic field operation with 3 T on the plasma axis inlow shear configuration. Since under these conditions no quenchwas triggered, one can be quite confident that also no quenchwill be initiated by mechanical disturbances like stick-slip ef-fects during the operation of Wendelstein 7-X.

ACKNOWLEDGMENT

The authors thank J.-H. Feist, D. Naujoks, K. Risse, G. Ehrke,P. v. Eeten, G. Croari, J. Tretter, L. Michaelsen, J. Sachtleben,M. Marquardt, G. Marlow for fruitful discussions and valuablecontributions. Special thanks are given to the coil test teams atCommissariat a l’Energie Atomique, DAPNIA and at IPP. Theauthors also gratefully acknowledge the work of KRP-MechatecEngineering and L.T. Calcoli.

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