Available online at www.sciencedirect.com ScienceDirect Physics Procedia 84 (2016 ) 54 61 International Conference "Synchrotron and Free electron laser Radiation: generation and application", SFR-2016, 4-8 July 2016, Novosibirsk, Russia Superconducting 72-pole indirect cooling 3Tesla wiggler for CLIC damping ring and ANKA image beamline A.Bragin a, Ye.Gusev a, S. Khrushchev a, N. Mezentsev a, V.Shkaruba a *, V.Syrovatin a, O.Tarasenko a, V.Tsukanov a, A.Volkov a, K.Zolotarev a and A.Zorin a Budker Institute of Nuclear Physycs, Lavrentiev ave., 11, Novosibirsk 630090, Russia Abstract One of the directions of BINP activity is the creation of multipole superconducting wigglers with the magnetic field levels from 2 T to 7.5 T which are installed on many SR sources. Special efforts were made by BINP to develop the cryogenic system with zero liquid helium consumption. The next significant step became the design and creation of superconducting full size prototype of damping wiggler for CLIC project where supposed the installation of 104 wigglers. For operation of CLIC damping wiggler it is required the magnetic field of 3 T and the period about 50 mm with a beam vertical aperture of 13 mm. Design features of the wiggler which was proposed and created by BINP is the application of the indirect cooling method. In this case Nb-Ti magnet with the length of 1.9 m and the weight of 700 kg is located in a vacuum and is cooled by four Gifford-McMahon cryocoolers. To maintain the temperature about 4.2 K on the magnet it is used the tubes with circulating liquid helium as the heat conducting elements. To increase the efficiency of pre-cooling down of the magnet it is used nitrogen-based heat pipes of siphon type. The features of the magnetic and cryogenic systems of CLIC damping wiggler full size prototype and test results are presented in this article. 2016 The The Authors. Authors. Published Published by Elsevier by Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SFR-2016. Peer-review under responsibility of the organizing committee of SFR-2016. Keywords: Insertion devices; superconducting wiggler; indirect cooling; * Corresponding author. Tel.: +7-383-329-49-76; fax: +7-383-330-71-63. E-mail address: Shkaruba@inp.nsk.su 1875-3892 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SFR-2016. doi:10.1016/j.phpro.2016.11.010
A. Bragin et al. / Physics Procedia 84 ( 2016 ) 54 61 55 1. Introduction Budker Institute of Nuclear Physics (BINP) creates different superconducting insertion devices during several decades. The first superconducting 20-pole wiggler with the field of 3.5 T was created also in BINP in 1979. Then it was produced some high field (from 7 T to 10 T) superconducting shifters especially designed for shifting photon spectrum to high energy region. However by early 2000 s the interest moved from usual shifters to the side of multipole wigglers which allows increase the photon flux in proportion to number of poles. From that time BINP created by more than ten multipole superconducting wigglers for several storage rings around the world with various magnetic field configuration especially designed to improve spectrum properties of sources by increasing of intensity and rigidity of radiation, see Khrushchev et al. (2014). All these wigglers may be divided into three groups according to their applicability and the user requirements for the various purposes. The first group is the wigglers with very high radiation power with a high field of 7-7.5Т and the period of 150-200 mm which essential increases the rigidity of radiation and allows using independently several beamlines for experiments due to a wide horizontal fan angle of the radiation. The second group of the wigglers which has medium field of 2.5-4.5 Т and can contain an array of 50 and more poles due to enough short period of 48-60 mm is the most demanded because of high photon flux at wide energy range. The spectrum of the third group of the wigglers with low field of 2-2.2 Т and short period of 30-35 mm is already very close to the undulator characteristics in the low photon energy range (K-value is equal to 6). One of the main characteristic of any cryostat with superconducting magnet is liquid helium consumption. It is very important parameter especially for insertion devices which are operated on the storage ring with of restricted accessibility. The helium consumption of BINP wigglers was improved step by step from typical of 1-2 liters per hour for beginning of 2000 s down to zero and even to negative pressure (near half of atmosphere) inside of helium volume. The next logical steps were refusing of immersion the magnet into liquid helium and try to use indirect cooling method for the magnet which is located in vacuum. And it was good opportunity for BINP to design and create the prototype of damping wiggler with indirect cooling method for CLIC project by the order and in collaboration with CERN and KIT. To achieve high luminosity at the collision point of CLIC the emittances of the electron and positron beams must be reduced before the beams enter the linear accelerators. An effective way to obtain ultralow emittances is using of damping rings equipped with superconducting wiggler magnets. The emittance value is decreased due to the decay of the oscillation amplitude due to emission of synchrotron radiation. It is supposed the using of 104 damping wigglers on two CLIC damping rings. 2. Magnetic system Damping wigglers are intended to use for obtaining of ultra-low emmitance electron and positron beams for CLIC project. The required parameters of the wigglers magnetic system were defined by CLIC team as a compromise between the desired minimal values of emittance and practically available for today methods for generation of magnetic field with maximal achievable magnitude, see Schoerling et al. (2012). Currently parallel are developed two variants of damping wiggler prototype (from Nb-Ti and Nb 3 Sn superconducting wire) and each of them having both advantages and limitations. BINP developed and created the Nb-Ti version of the prototype with the characteristics which is presented in Table 1. Required parameters of magnetic system of Nb-Ti damping wiggler prototype were specified as magnetic field of 3 T, period of 50 mm and beam gap of 13 mm. The main physical advantage of indirect cooled magnet which is located in vacuum is the possibility to increase field level by reducing of magnetic gap due to the removal of vacuum chamber of helium vessel from the gap. In this case it needed to place in the magnetic gap only beam vacuum chamber and protect of superconducting coils from heating generated by beam. Based on these requirements and taking into account both the critical parameters currently available superconducting wires and the technological opportunities for arrangement of vacuum chamber inside of wiggler magnetic gap a reasonable minimum value of the magnetic gap has been identified as 18 mm.
56 A. Bragin et al. / Physics Procedia 84 ( 2016 ) 54 61 Table 1. Main parameters of CLIC damping wiggler prototype. Magnetic Field, T 3 Period, mm 51 Beam gap, mm 13 Magnetic gap, mm 18 Number of poles 68+4 Side poles +¼,-¾,,+¾,-¼ K- value < 16 Magnet length, mm 1930 Length flange to flange, mm 2720 Stored energy, kj 60 Cold mass, kg 700 Maximum ramping time, min < 5 Beam heat load (acceptable), W 50 Period for LHe refill with beam > 1 year LHe boil off for 1 quench, l < 15 Field stability for two weeks ±10-4 The main choice in the design of the magnetic system configuration was between vertical and horizontal racetrack. Both of these variants have as advantages as disadvantages. For example it is obvious that the advantage of horizontal racetrack is minimal values of wire length, inductance and the stored energy. Moreover horizontal racetrack allows reeling up of multi-sectional coils. For example the using of two sections allows increasing the maximum magnetic field by 15% due to the possibility to increase the current in the outer section which is located in lower level magnetic field. An expected problem of horizontal racetrack in contrast to the vertical variant can be multiple connections between the coils which are fabricated separately. However exactly cold welding method for joining of superconducting wires has been well developed in the multi-pole wigglers produced by BINP. The value of residual resistance in this case is less than 10-12 Ohm which corresponds to Joule heating of less than 1 mw at a current of 1 ka flowing through the consecutively connection of several hundreds of splices. In addition in this case will be greatly simplified the problem of replacement of the defective coil in the case of damage that may be especially demanded from the economic point of view for mass production of damping wigglers. Consequently exactly the horizontal racetrack has been chosen for making of the wiggler prototype. It was used Nb-Ti superconducting wire with a diameter of 0.92 mm with a critical current of 520A in a field of 7 T at a temperature of 4.2 T. As a result of current-field relationship optimization of the coil geometry for maximum field of 3 T on the wiggler axis it has been designed two sections with 62 turns in each with a current of 482 A in the inner section and 960 A in the outer section. Critical parameters of used superconducting wire at different temperatures and load lines for each section with the working points for magnetic gap of 18 mm are presented in Fig. 1a. The diagram shows that the calculated margin to the short sample current at 4.2 T is ~14% for the outer section and ~21% in the inner section for the field of 3 T on the magnet axis. In this case the field level in the critical areas of the first and second section (on the first layers of windings in the radius) is 5.3 T and 3.7 T respectively. The maximum field level if the critical current in the wire would have been achieved should be 3.3 T. Transverse field homogeneity on the axis of the magnet is 0.03% in the area of ± 20 mm. Each coil was wound individually using of wet method with impregnation by compound and molded during hot curing. Overall view of ready coils with the attached heat sinks is presented in Fig. 1b.
A. Bragin et al. / Physics Procedia 84 ( 2016 ) 54 61 57 a b Fig. 1. (a) Critical parameters of used superconducting wire at different temperatures and load lines for each section with the working points for magnetic gap of 18 mm and period of 51 mm; (b) Overall view of ready coil for CLIC damping wiggler prototype with attached heat links. The main problem of any indirect cooling magnet which is located in a vacuum in contrast to magnets which is immersed into liquid helium is the requirement to provide reliable cooling of superconducting coils by using only heat conductivity of applied materials. For damping wiggler is added the necessity to provide a reliable heat insulation of the magnet from heating by the beam which is generated by image current, electron clouds and synchrotron radiation. In the damping rings the main contribution to heat load on beam pipe is from synchrotron radiation of upstream wigglers which is expected up to ~50 W. Structurally the magnet consists of two halves with coils which are located above and below the wiggler axis (see Fig. 2). Each coil is cooled via flexible copper heat links (see Fig. 3a) which connects iron core with copper heat distributor located along the magnet that simultaneously extracts the heat from whole magnet body. Cooling of the entire magnet body carried out only by thermal contacts which are located on the upper halves of the magnet. The bottom half is cooled only through the copper plates connected with the upper half. Large number of windows on the length of the magnet body makes the possibility of easy access for arrangement of heat sinks which provide uniform cooling along the beam pipe inserted between of two halves of the magnet. And there is an opportunity to place a sufficient number of supports for reliable positioning of beam pipe. Open-able design of the magnet allows if required to exchange of coils and beam vacuum pipe (see Fig. 3b and Fig. 3c). Fig. 2. Overall view of assembled magnetic system with the cooling elements and dump resistors with cold diodes. Beam vacuum pipe is already inserted and positioned inside of the magnet. The presented wiggler has even number of poles. In case of odd number and using for feeding of two independent power supplies it possible to make the first field integral equal to zero by redistributing the currents in the windings. The value of the second field integral must be equal to zero automatically and deviations from zero will be determined only by actual manufacturing inaccuracies. But in the case of even number of poles the first field integral will be zeroing automatically and it does not depend on the value of the second field integral. Really it has some deviation from zero caused by fabrication inaccuracies. To make the second integral equal to zero two power supplies are enough. But for simultaneously first integral zeroing and symmetric deviation beam trajectory from the axis on the entire length of the magnet it needed to use one more power supply, see Khrushchev et al. (2015)
58 A. Bragin et al. / Physics Procedia 84 ( 2016 ) 54 61 a b c Fig. 3. (a) Arrangement of flexible copper cooling heat links of superconducting coils on the magnet; (b) View of beam vacuum pipe assembled with flexible heat sinks and positioning supports; (c) Open-able design of the magnet which allows to exchange of coils and beam vacuum pipe. 3. Cryogenic system One of the special requirements for the cryogenic system of damping wiggler prototype (in additional to indirect cooling) is high requirement for a safety margin during long-term operation in damping rings. It was especially important to provide reliable thermal insulation of superconducting magnet from the heat load on beam pipe. So heat budget of the cryostat has overcooling margin for cooling of especially critical parts of the wiggler such as beam pipe, current lead blocks and coils of the magnet. It is allowed to obtain the lowest possible temperature in the working condition. The main concept of the cryogenic system is the interception any heat inleak inside of the cryostat by heat sinks connected to the cryocoolers stages. The superconducting magnet is located in a safety vacuum of the cryostat having two radiation screen with temperatures of 60 K and 20 K which are cooled by appropriate stages of SRDK-408 and SRDK-415 cryocoolers. Cupper vacuum chamber is cooled along of the whole length throughout heat links with 20 К stages of SRDK-408 cryocoolers. Current lead block is combination of HTSC and brass parts which are cooled through electrically isolated sapphire heat contact by the relevant stages of SRDK-415 cryocoolers. Current leads allow inputting the current of ~1000 A with minimal heat inleak to cryostat. Conceptual diagram and 3-D model of indirect cooling cryogenic system are presented in Fig. 4a and Fig. 4b respectively. a b Fig. 4. (a) Conceptual diagram of indirect cooling cryogenic system; (b) 3-D model of indirect cooling damping wiggler cryostat. The basic cooling of superconducting magnet realized by heat contacts with two syphon tubes with circulated helium. The tubes are connected with 60 liter helium vessel allocated above the magnet body. Liquid helium flows down through inlet end of the tubes attached to lower part of the vessel and after heating by magnet and evaporation gas helium returns through outlet end to upper part of the vessel. In upper part of the helium vessel a gas is recondensed by two gilded copper heat exchangers which are connected by heat sinks with 4 K stages of SRDK 415 cryocoolers.
A. Bragin et al. / Physics Procedia 84 ( 2016 ) 54 61 59 One of the expected problems of indirect cooling cryostat operating is how to speed up the primary cooling of the cold mass weighing about 700 kg only by cryocoolers power without using of cryogenic liquids. To speed up the cooling would be convenient to use not only 4 K stages of SRDK-415 cryocoolers with a power of only 1.5 W but also more powerful 60 K stages of all four cryocooler with a total power of ~200 W. However this heat contact makes sense until the magnet temperature is still higher than cryocooler stage temperature. And after this the heat contact must be disconnected because cryocooler becomes a heater for magnet. For the simultaneous execution of both of these requirements it was proposed to use the system of thermo-siphon heat pipes with two-phase nitrogen as a working substance which is operated as a thermal switch. Upper end of each heat pipe (recondensing part) has a mechanical thermo-contact with 60 K stage of SRDK 415 cryocooler. The lower end (evaporating part) it connected with copper heat distribution plate on the magnet body. There is a possibility to extract heat flows from the magnet right down to freezing of nitrogen inside of the heat pipe. So the heat pipe is operated as a thermal switch and automatically cut off the thermal connection with the magnet body after cooling down to the temperature of ~64 K. To prolong the working condition of the cooling process and prevent the premature freezing of nitrogen (due to excess cooling capacity) it is used the heating elements which installed on the upper recondensing part of heat pipes. The optimal powering of the heaters by current is realized with using of feedback loop. Maximal extracted power for each pipe reaches a value of ~100 W. More detailed description of heat pipes operation for cooling of superconducting magnet is presented by Khrushchev et al. (in this issue). In the beginning of cooling down the helium vessel was connected with a source of gas helium (for example gas exhaust line for evaporated helium). During the process of cooling down by 4 K stages of SRDK 415 cryocooler there was occurred simultaneous dropping of the temperature and the pressure inside of helium vessel. It was accompanied with permanent supplying of gas helium from the exhaust gas line and increasing of density of helium right until the moment of beginning of recondensation and accumulation of liquid helium. So it was not required liquid and enough only gas helium for initial cooling of the magnet. Moreover during normal operation it takes only 1-2 days for recondensation of gas helium into liquid inside of helium vessel up to maximum level. It takes 5 days to cool the magnet in this manner down to working temperatures and after that wiggler can operate without any service during some years. 4. Test results For preliminary testing of indirect cooling conception a short model with 10 pairs of poles was produced. The short model was directly attached to 1 W cooling stage of Sumitomo SRDK-408 D2 cryocooler by copper thermal links (see Fig. 5a and Fig. 5b) and cooled down to ~3 K during ~2 days. During testing it was set different field level and then the temperature of the magnet was increased by heaters up to quench. As it is shown in Fig. 5c the field level was depended from temperature in good agreement with critical current of superconducting wire. In particular it was obtained the maximum field of 3.3 T and 2.5 T for the temperature of 3.7 K and 5.75 K respectively. a b c Fig. 5. (a) View of one half of 10-pole short prototype; (b) Short prototype prepared for testing with indirect cooling method; (c) Dependence of maximal currents achieved on short prototype from magnetic field in inner and outer sections as a result of quenching at different temperatures.
60 A. Bragin et al. / Physics Procedia 84 ( 2016 ) 54 61 Then it was produced full size magnet with absolutely the same design but longer (~1.9 m). First testing was conducted in liquid helium and after training it was obtained maximum field of 3.2 T. After this the magnet was tested with indirect cooling method in own wiggler cryostat and the result was rather unexpected. The fact is that during ramping field the maximum achieved field level was more than 3.1 T. But if the ramping was stopped before the quench it was unpredictable quenches after some period from 1 minute to some hours. The period before each the quench was not correlated with the magnet temperature and the quenched coils were different every quench. Maximal stable field level during long operation was only 2.7 T. The history of training of full size damping wiggler prototype at different stages is presented in Figure 6a. a b Fig. 6. (a) History of quenching. The groups of quenches with number 1-16 and 62-67 was obtained in bath cryostat with liquid helium; (b) View of additional heat interceptors from splices between superconducting coils. In order to determine what exactly was the reason of such behavior of the magnet it were step by step amended accordingly changes in the design of magnetic and cryogenic systems of the wiggler. Firstly the most frequently quenched coils were replaced. The main focus of the next actions was directed to improving of cooling conditions, interception of heat in-leak from outside and removing of heat power generated by current. In particular each of ~300 splices between each coil was additionally thermally connected to heat sinks through electrical insulator with a very good thermal conductivity (see Fig. 6b). After that the additional layers of super-insulation were imposed at doubtful places and were added more interceptions for heat inleak through signal wires and some elements of the cryostat. Moreover it was installed a lot of additional sensors for monitoring of temperatures at the questionable places of the magnet. However all of taken actions haven't resulted in elimination of premature quenching and the maximal stable field at a long time operation remained only 2.8 T. After that keeping in mind that it is required to get as close to the level of the field 3 T the decision was made to decrease magnetic gap from existed 18 mm down to 17 mm. It should be noted that although was required a reduction of the mechanical clearance between the coils and the beam pipe from beginning 1mm down to 0.5 mm that significantly tightened requirements for mechanical accuracy however the size of the gap remains still sufficient for reliable heat isolation coils. Besides between each coil were inserted additional copper foils with the thickness of 0.2 mm which were connected with cold surfaces to improve the cooling condition of SC coils. So the period was increased from beginning 51 mm to 51.4 mm. But the beam gap remained the same required 13 mm. As a result the reached stable field quite predictable increased to 2.95 T. But the reason of unstable operation magnet with indirect cooling is still not explained and remained the subject for investigation. As for performance of the cryogenic system the minimal temperature reached ~3 K on the magnet and ~8 K on the beam vacuum pipe. In Fig. 7a is presented the screen of control system showing main parameters of cryostat and superconducting wiggler at steady state of normal mode of operation at field of 2.95 T. In the damping rings the heat load from synchrotron radiation from upstream wigglers on beam pipe expected up to ~50 W. So heat load test with near twice more power (~90 W) was conducted with simulation of heating by resistive heaters attached to the beam pipe. The temperature was stabilized at ~100 K at beam pipe and ~5 K at the magnet. It should be noted that these measurements were carried out directly with the full magnetic field of 2.9 T and there was no quench despite such stringent conditions, see Bernhard et al. (2016).
A. Bragin et al. / Physics Procedia 84 ( 2016 ) 54 61 61 5. Current status and conclusion The maximum magnetic field level during ramping is 3.2 T. But the stable field during long operation is limited to 2.95 T. And the physical reason of this instability is not explained satisfactorily yet. Despite of this field limitation the wiggler operates very reliably at the field level of 2.9 T. The view of the wiggler during testing on the ANKA site is presented in Fig. 7b. Indirect cooling and using of nitrogen heat pipes allows to operate on the storage ring without cryogenic infrastructure. The CLIC damping wiggler prototype with indirect cooling was successfully installed and commissioned in ANKA storage ring at 2016 February. Now wiggler is under study of effects on the beam dynamics. The wiggler will be used in parallel as synchrotron radiation source with photon energy of 20-50 KeV for MIQA hard X-ray microscope on ANKA image beamline and for testing as damping wiggler prototype for CERN. a b Fig. 7. (a) The screen of control system showing main parameters of cryostat and superconducting wiggler at steady state of normal mode of operation at field of 2.90 T; (b) View of CLIC damping wiggler prototype during testing on ANKA site. Acknowledgements This work was supported by grant 14-50-00080 of the Russian Science Foundation and was done using the infrastructure of the Shared-Use Center "Siberian Synchrotron and Terahertz Radiation Center (SSTRC)" based on VEPP-3/VEPP-4M/NovoFEL of BINP SB RAS. The authors would like to thank CERN and ANKA-KIT teams for provided opportunity to contribute to the creation of an important element of the future collider CLIC. References Khrushchev S., Mezentsev N., Lev V., Shkaruba V., Syrovatin V., Tsukanov V., Superconducting multipole wigglers: state of art. Proceedings of IPAC2014, Dresden, Germany, 4103-4106. Daniel Schoerling, Fanouria Antoniou, Axel Bernhard, Alexey Bragin, Mikko Karppinen, Remo Maccaferri, Nikolay Mezentsev, Yannis Papaphilippou, Peter Peiffer, Robert Rossmanith, Giovanni Rumolo, Stephan Russenschuck, Pavel Vobly and Konstantin Zolotarev. Design and System Integration of the Superconducting Damping Wigglers for the CLIC Damping Rings. Physical Review Special Topics Accelerators and Beams, vol.15 (2012), p.042401. S.V.Khrushchev, V.A.Shkaruba, N.A.Mezentsev, V.M.Tsukanov, V.K.Lev, Zeroing Magnetic Field Integrals for Wigglers and Undulators with Even Numbers of Poles. Bulletin of the Russian Academy of Sciences. Physics, 2015, Vol. 79, No. 1, pp. 44 48. S.Khrushchev, V.Lev, N.Mezentsev, V.Shkaruba, V.Syrovatin, V.Tsukanov, K.Zolotarev, Cooling of the superconducting magnet with nitrogenfilled heat pipes. (in this issue) A.Bernhard, J.Gethmann, S.Casalbuoni, S.Gerstl, A.W.Grau, E.Huttel, A.-S.Mueller, D.Saez de Jauregui, N.J.Smale, A.V.Bragin, S.V. Khrushchev, N.A.Mezentsev, V.A.Shkaruba, V.M.Tsukanov, K.V.Zolotarev, P.Ferracin, L.Garcia Fajardo, Y.Papaphilippou, H.Schmickler, D.Schoerling, A CLIC damping wiggler prototype at ANKA: commissioning and preparation for a beam dynamics experimental program. Proceedings of IPAC-2016, Busan, Korea. p.2412-2415.