- Email: [email protected]
Upgrade of the Alpha Magnetic Spectrometer (AMS-02) for long term operation on the International Space Station (ISS)
Nuclear Instruments and Methods in Physics Research A 654 (2011) 639–648
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Technical Note
Upgrade of the Alpha Magnetic Spectrometer (AMS-02) for long term operation on the International Space Station (ISS) a ¨ K. Lubelsmeyer , A. Schultz von Dratzig a,, M. Wlochal a, G. Ambrosi d, P. Azzarello d, R. Battiston e,d, g R. Becker , U. Becker g, B. Bertucci e,d, K. Bollweg h, J.D. Burger g, F. Cadoux b, X.D. Cai g, M. Capell g, V. Choutko g, M. Duranti e,d, C. Gargiulo f, C. Guandalini c, S. Haino d, M. Ionica e,d, A. Koulemzine g, A. Kounine g, V. Koutsenko g, G. Laurenti c, A. Lebedev g, T. Martin h, A. Oliva d, M. Paniccia b, E. Perrin b, D. Rapin b, A. Rozhkov g, St. Schael a, H. Tholen a, S.C.C. Ting g, P. Zuccon d a st
I Physics Institute RWTH Aachen, D-52074 Aachen, Germany DPNC, Universite´ de Gene ve, CH-1211 Geneva 4, Switzerland c INFN-Sezione di Bologna, I-40126 Bologna, Italy d INFN-Sezione di Perugia, I-06100 Perugia, Italy e Universita Degli Studi di Perugia, I-06100 Perugia, Italy f INFN-Sezione di Roma, I-00185 Roma, Italy g Massachusetts Institute of Technology, Cambridge, MA 02139, USA h NASA, JSC Houston, 2101 NASA Road One, Houston, TX 77058, USA b
a r t i c l e i n f o
abstract
Article history: Received 14 April 2011 Received in revised form 6 June 2011 Accepted 7 June 2011 Available online 16 June 2011
Following the decision to maintain the International Space Station (ISS) on orbit until at least 2020 (possibly until 2028) the AMS collaboration decided to correspondingly extend the lifetime of the experiment. Since the limited amount of helium used to cool the superconducting magnet allowed for only a limited run time of the experiment, a change from the superconducting magnet to the permanent magnet used in AMS-01 became necessary. Due to the lower magnetic field, to maintain the resolution the silicon tracker also had to be reconfigured with the installation of a silicon plane on the top of the experiment and a new plane above the electromagnetic calorimeter. & 2011 Elsevier B.V. All rights reserved.
Keywords: Astro_particle physics AMS-02
1. Introduction 1.1. Motivation The Alpha Magnetic Spectrometer AMS-01 [1,2] was flown as an engineering, precursor flight on the space shuttle Discovery (mission STS-91, June 1998). This prototype of the experiment used a permanent magnet with a maximum magnetic field of B¼0.14 T and a bending power of BL2 ¼0.15 T m2. The AMS-02 detector [3,4], to be installed on the International Space Station (ISS), was built with a superconducting magnet cooled by liquid helium. Due to the restricted amount of liquid helium the experiment lifetime would have ended after a limited running time. In early 2010 an announcement by NASA was released according to which the space station was scheduled to be maintained on orbit until at least 2020, and likely until 2028.
Corresponding author. Tel.: þ 49 241 8027162.
E-mail address: [email protected] (A. Schultz von Dratzig). 0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.06.051
To benefit from this extended operational period of the ISS and because it was not possible to refill the helium reservoir, the superconducting magnet had to be exchanged with the AMS-01 permanent magnet. To ensure that the performance of the permanent magnet had not changed since the AMS-01 mission, the B-field was measured again. This showed that there had been no degradation, within 1% (the accuracy of the original measurement), over the last 12 years (see Figs. 1 and 2). Since the support structure of the inner silicon tracker of the precursor experiment was reused in AMS-02, the superconducting magnet had the same inner dimensions (radius, height) as the permanent magnet. Thus the superconducting magnet could be replaced by the permanent magnet by replacing the support structure. The lower magnetic field of the permanent magnet required a reconfiguration of the tracker to maintain the spectrometer resolution (see Fig. 3). The former silicon tracker, designed for use with the superconducting magnet, consisted of five planes equipped with one resp. two layers of double sided silicon strip detectors. Plane 1 underneath the upper time of flight detector (TOF) and plane 5 above the lower TOF each support a single layer of silicon. The planes 2–4, symmetrically arranged in the bore of
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K. L¨ ubelsmeyer et al. / Nuclear Instruments and Methods in Physics Research A 654 (2011) 639–648
the magnet, carry double layers of silicon each, totalling eight layers of silicon detectors. For the permanent magnet the support plane 1N for the first silicon layer was relocated from the existing tracker (former support plane 5) on top of the transition radiation detector (TRD). This required a new plane, 1NS, for support, light tightness and electromagnetic and low energy particle shielding. The inner tracker volume was closed for light tightness with a new support plane, 5N, without silicon, a copy of former support plane 5. A new support plane, 6, for the new silicon layer 9 located above the electromagnetic calorimeter (ECAL), was built (cf. Section 4.2). Fig. 3 shows the reconfiguration.
AMS01_Magnet 0.14
before AMS01-Flight Aachen, 03/2010
0.1 0.08 0.06 0.04
1.2. Performance of the two configurations in test beams and with cosmic rays
0.02 0 -1500
-1000
-500
0
500
1000
1500
On ground the alignment of all nine silicon layers can be achieved with an accuracy of 2 mm (1 mm for the seven inner silicon layers) by means of cosmic ray muons in a time interval of 103 min. Nearly the same accuracy is reached with cosmic protons on the ISS within one orbit, equivalent to 92 min (see Fig. 4). This ensures the design spatial resolution of the tracker in the bending plane of 10 mm. Fig. 5 shows the number of cosmic ray muons per TV plotted vs. the inverse rigidity 1/R. After alignment of all nine silicon layers the measured data lead to a maximum detectable rigidity (MDR) of 2.2TV for the permanent magnet, equal to that obtained with the superconducting magnet. The modifications described above do not affect the AMS-02 acceptance for most of the physics topics, including dark matter searches, for particle rigidities up to approximately 400 GV. Above that and up to the MDR of 2.2 TV the acceptance is reduced by a factor between 1.5 and 2. The exact acceptance vs. the particle rigidity will be published in a separate paper. This reduction in acceptance is, however, minor compared to the extension of the experiment lifetime to that of the ISS. In order to ensure that the lifetime of the AMS detector is comparable to the lifetime of the ISS, evaluations were made on all components including the lifetime of consumables, the variation of the magnetic field, and the materials (to mitigate long term exposure to atomic oxygen and solar ultraviolet rays). The most critical consumable lifetime concerns the transition radiation detector. The TRD straw tubes are filled with a mixture
z-Axis / mm Fig. 1. B-Field of AMS-01 permanent magnet along the z-axis.
Fig. 2. Variation of the B-field of the AMS-01 permanent magnet in the midplane over 12 years. The accuracy of the original measurement was 1%.
1NS
1
1
2.3
2
4.5
3
6.7
4
8
5
Suppsort Planes
Silicon Layerso
with superconducting magnet 8 layers of silicon
with permanent magnet 9 layers of silicon
1N
1
1
2
2
3.4
3
5.6
4
7.8
5N 6
Fig. 3. Reconfiguration of the tracker.
9
Silicon Layerso
Flux density / Telsla
0.12
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641
The support structure of the permanent magnet had to be
10
Alignment accuracy [µm]
integrated into the volume within the vacuum case.
The load paths of the original straps with which the super-
Sim. Layer 1
8
Sim. Layer 9 Data Layer 1
Data Layer 9
6
4
2
0 10
102 103 Measurement time [min]
104
Fig. 4. Alignment accuracy measured with cosmic ray muons at the Kennedy Space Center on the two external layers.
Entries
102
conducting magnet was suspended from the vacuum case rings had to be kept. The tracker support plane 5 had to be used as the new tracker plane 1N on top of the TRD. The new plane 1NS to which the plane 1N is fastened should also serve as a low energy particle shield. The temperature of the tracker layers 1 and 9 had to be kept within their operational range ( 20 1C, þ 40 1C). Electromagnetic interference effects on the new silicon layers should be kept tolerably low.
3. The integration of the permanent magnet into AMS-02 3.1. Hardware 3.1.1. Preparation of the permanent magnet All remnants of glue, foil attachments and cabling on the permanent magnet from the precursor experiment were carefully removed and all surfaces cleaned. The alodine surface treatment was checked and damaged spots repaired. All bolts (the magnet flanges are bolted to the cylindrical part of the magnet) were replaced by new ones. Near the inner periphery of the flanges new holes were drilled for the attachment of the conical flanges of the vacuum case.
10 AMS-Perm. Magnet AMS-S.C.
1
Magnet
MDR= 2.2 TV
-10
-5
0
5
10
1/Rigidity [TV-1] Fig. 5. Cosmic ray measurement of the 1/R distribution after tracker alignment.
of Xe/CO2. The CO2 is lost steadily by diffusion through the straw tube walls. The measured long term leak rate of CO2 amounts to 5 mg=s for the entire TRD. The onboard storage of 5 kg of CO2 at launch corresponds to a lifetime of 30 years. The measurements of the magnetic field of the permanent magnet show that the B-field had not degraded within a margin of o1% in the last 12 years (compare Figs. 1 and 2). We conclude that the field on the ISS will remain the same for the next 20 years. Additional single and double layer beta cloth has been added to cable harnesses to reduce the effects of atomic oxygen and UV degradation.
Fig. 6. AMS-01 permanent magnet.
2. Prerequisites Several requirements had to be fulfilled:
The permanent magnet had to be installed with only very
minor modifications; namely some new holes in the upper and lower flanges. In order to not redesign and rebuild the tubing and cabling of the tracker and other detectors the conical flanges together with the outer cylinder and the outer rings of the vacuum case of the superconducting magnet should be retained.
Fig. 7. New support structure (‘‘double-X-structure’’).
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3.1.2. Preparation of the static test article (STA) vacuum case The spare vacuum case, which was used as a part of the AMS structure for the static and modal tests,1 was prepared for the mounting of the permanent magnet. To this end the inner cylinder was cut right underneath and above the welding where it was connected to the upper and lower conical flanges. The latter were unbolted from the outer rings of the vacuum case. The cutting edge and part of the flanges had to be milled down to allow the permanent magnet to fit in between. Because the
conical flanges had to be bolted to the magnet flanges a series of holes were drilled near the inner periphery that are aligned with those drilled into the magnet flange. The strap ports of the superconducting magnet suspension were prepared for the reception of the new struts that suspend the permanent magnet from the vacuum case rings. The rings and the outer cylinder were mounted into the assembly stand used for the assembly of the superconducting magnet.
3.1.3. Permanent magnet support The support interfaces of the permanent magnet (see Fig. 6) are oriented in the x- and y-axes of the AMS coordinate system. The locations where the struts are connected with the vacuum case rings (the former strap ports), however, lie along the bisection lines of the quadrants. Therefore, a new support structure had to be designed which allows to suspend the magnet at locations which guarantee the preservation of the load paths. This so-called ‘‘double-X-structure’’ is shown in Fig. 7 indicating also the ‘‘struts’’ with rod end bearings and clevises. Due to the relatively large distance between the fixations of the struts and the magnet interface this kind of support introduces bending moments which are the main reason for the rather large crosssection of the ‘‘double-X-structure’’.2 Fig. 8. Assembly of the ‘‘double-X-structure’’. Table 2 Margins of safety of the permanent magnet support. Table 1 Static load factors used in the FEA. Nx g
4.1
Ny g
Nz g
Rx rad=s2
Ry rad=s2
Rz rad=s2
3.6
8.6
33.5
54.3
14.9
Component
Max. tensile/compressive stress (MPa)
Margin of safety (FSu ¼3, 3.45)
X-structure Clevis (to X-structure) Clevis (to outer ring) Strut buckling
47.6 97.6 55.5
1.7 0.3 1.3 0.85
Fig. 9. Total displacement of permanent magnet support.
1
IABG, Einsteinstrasse 20, 85521 Ottobrunn, Germany.
2
BF Maschinen GmbH, Wallensteinstraße 7, 82538 Geretsried, Germany.
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643
Fig. 10. Most highly stressed location of the permanent magnet support.
Fig. 11. First eigenfrequency of the permanent magnet support.
3.1.4. Assembly The four principal members of the ‘‘double-X-structure’’ were connected to form a square around the permanent magnet, the distance between the magnet interfaces and the ‘‘double-X-structure’’ was measured and the gap filled with shims, adjustable shear pins were installed, and finally the ‘‘double-X-structure’’ was bolted to the magnet (Fig. 8). As a next step the upper conical flange was bolted to the upper magnet flange. With the help of a lifting tool both the magnet, structure and the attached conical flange were lifted with a crane and lowered from above into the prepared vacuum case. The upper conical flange was then bolted to the upper ring of the vacuum case. The struts were assembled and mounted between the principal members and the two vacuum case rings and torqued with a low preload. Next the lifting tool was removed and the lower conical flange was bolted to the magnet and the lower ring.
3.2. Structural verification3 3.2.1. Static finite element analysis Static and modal finite element analyses (FEA) of the magnet and structure were carried out for the space shuttle launch loads. Only one set of load factors has been chosen because of the symmetry of the arrangement (see Table 1). Fig. 9 shows the total displacement under the above loads. A summary of the margins of safety is shown in Table 2. An ultimate (i.e., against yielding) factor of safety (FSu) of 3 was used, except for strut buckling where it was 3.45. The critical location was found at one of the clevis tangs. A detailed investigation is shown in Fig. 10.
3
ISATEC Engineering GmbH, Rathausstrasse 10, 52072 Aachen, Germany.
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Fig. 12. Overview of uppermost tracker arrangement.
Fig. 13. Mounting principle of plane 1NS.
4. New supports for the additional tracker planes
Table 3 Overview of margins of safety of plane 1NS. Component
Margin of safety (FSu ¼3, 3.45)
Bracket Face sheet HC core C-channel GPS antenna bracket
0.44 4.74 (kick load) 0.32 (kick load) 17.7 2.2
As discussed earlier, three new tracker planes were introduced; plane 1NS above the TRD, plane 5N to close the inner tracker volume and plane 6 on top of the ECAL. As plane 5N was a copy of the previous plane 5, it is just needed to be fabricated.4 Therefore, new supports had to be designed and constructed for planes 1NS and 6. 4.1. Support plane 1NS4
3.2.2. Modal analysis The result of the modal analysis gives a lowest frequency of 73.4 Hz in drum mode, well above the desired 50 Hz (see Fig. 11) for space shuttle payloads.
4.1.1. Mechanics of support plane 1NS for the uppermost tracker plane The zenith radiator for the thermal management of the cryogenic components of the superconducting magnet is no 4
ADCO GmbH, Werkstraße 6, 52076 Aachen, Germany.
K. L¨ ubelsmeyer et al. / Nuclear Instruments and Methods in Physics Research A 654 (2011) 639–648
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Fig. 14. Total displacement of plane 1NS.
Fig. 15. Most stressed component of plane 1NS.
longer needed and therefore removed. The former tracker plane 5 with a reduced number of silicon ladders is moved on top of the AMS-02 experiment and called plane 1N holding silicon layer 1. It is supported by a new sandwich plane 1NS of the same octagonal shape as the top cover of the TRD. The thickness is designed to satisfy stability requirements and also to serve as a low energy particle shield. The face sheets (carbon fiber/epoxy resin T700) are therefore 2.5 mm thick each and the core is a 70 mm high aluminum honeycomb (Al 3/16-0.001 in.). Plane 1N is bolted to it from below with the silicon layer facing down towards the TRD. Along the circumference between the upper cover of the TRD and the plane 1NS a C-channel made from carbon fiber guarantees
light tightness. Four tracker cabling feedthroughs and four venting chicanes are placed at eight locations in the C-channel (see Fig. 12). In cold environments the silicon has to be heated to be kept within its operational range ( 20 1C, þ40 1C). Therefore, heater pads are glued to the lower side of the support plane 1NS more or less equally distributed above the plane 1N.5 The heat locally produced by the front-end electronics is removed via copper braids that are connected to the lower side of plane 1NS.
5
Carlo Gavazzi Space SpA, Via Gallarate 150, 20151 Milano, Italy.
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K. L¨ ubelsmeyer et al. / Nuclear Instruments and Methods in Physics Research A 654 (2011) 639–648
Fig. 16. First eigenfrequency of plane 1NS.
Fig. 17. Tracker plane 6 supporting silicon layer 9.
The heat is finally radiated off to space from a circular region ð 0:5 m2 Þ in the center of the upper side of plane 1NS. Because of the modified configuration at the top of the experiment a new multi-layer insulation (MLI) hood had to be designed and produced. It is not totally closed but has an opening over the radiator area. A 50 mm aluminum foil covers the top side of the TRD top cover and the top side of plane 1NS which are electrically connected to the MLI hood to prevent any electromagnetic interference from and to the TRD and silicon layer 1. This also serves as the radiator material. Fig. 13 shows the mounting principle. Plane 1NS is supported by eight brackets which are fastened to the top cover of the TRD, using the existing threaded holes in the C-profiles which form the outer edge of the TRD top cover. These were originally designed for the lifting brackets of the TRD.
4.1.2. Static and modal analysis of plane 1NS All 64 load case combinations have been investigated in the static analysis including acoustic loads. The maximum values of the translational and rotational loads have been combined as a worst case in the analysis. The total displacement in this worst case is shown in Fig. 14. The maximum displacement occurs in the center of plane 1NS in the AMS z-direction and is equal to 1.57 mm, the circular region where the plane 1N is attached to plane 1NS shows a displacement of around 1 mm. The lowest margin of safety, 0.44, is found at the brackets and is seen in the detail in Fig. 15. As could be expected, the highly stressed region is very narrow and can be considered as a local effect only of no concern. Table 3 gives an overview of the margins of safety for plane 1NS. They represent the lowest margins of the respective components and belong to different load cases. An ultimate factor of safety of 3 was used, except for the GPS antenna bracket where 3.45 was used. The first eigenfrequency of plane 1NS is 68.4 Hz and occurs in drum mode (see Fig. 16). It lies well above 50 Hz and therefore no further testing is required. Details of the cabling and cooling of the new tracker planes as well as the results of the measured performance with test beams of high energy particles will be published in a separate paper. 4.2. Support plane 6 4.2.1. Mechanics of support plane 6 for the lowest tracker silicon layer 9 To complete this report on the upgrade of the AMS-02 detector a short description of the support for the new tracker layer 9, plane 6, shall be given here. The mechanical design was done at the INFN Bologna in Italy, and the finite element analysis at the engineering company ISATEC, Aachen. Plane 6 consists of a sandwich support plate6 made from carbon fiber skins (M55J/CE)4 with an aluminum 6
EUROMEC S.R.L., Via dalla costa Raimondo 165, 41100 Modena, Italy.
K. L¨ ubelsmeyer et al. / Nuclear Instruments and Methods in Physics Research A 654 (2011) 639–648
647
LC 3, Design Limit Load Factors Nx = 4.3 g Ny = 2.3 g Nz = 7.3 g Rx = 72.4 rad/s Ry = -53.1 rad/s Rz = 13.1 rad/s
Fig. 18. Total displacement of tracker plane 6.
LC 3, Design Limit Load Factors Nx = 4.3 g Ny = 2.3 g Nz = 7.3 g Rx = 72.4 rad/s² Ry = -53.1 rad/s² Rz = 13.1 rad/s²
Seqv = 40.63 MPa
Margins of Safety MSu = Ftu / ( FSu x Seqv ) - 1 = 393 / ( 3 x 40.63) - 1 = 2.2 Support Arms, Al 7050-T7351-4‘‘ Strength Values according to Mil. Spec.
MSy = Fcy / ( FSy x Seqv ) -1 = 317 / ( 2 x 40.63) -1 = 2.9
Fig. 19. Location of the highest stress of tracker plane 6.
honeycomb core. This plane carries the ladder structures of the silicon detector. Light tightness is provided by a thin carbon fiber sandwich cover on top and carbon fiber profiles around the edges in between the two sandwich plates, which allow for venting and cable feedthroughs. The support plane is attached to the ECAL fixation blocks by eight aluminum arms, two at each corner (see Fig. 17). They are connected to the honeycomb support plane by means of sliding blocks. This allows movements caused by differential thermal expansion without inducing stress on the plane 6 plus silicon layer 9 assembly. A photograph of the completed assembly is shown in Fig. 17.
4.2.2. Static and modal analysis of plane 6 As mentioned in Section 4.1.2, the worst combination of loads out of the 64 possibilities was chosen for the static FEA of the plane 6. The total displacement is shown is Fig. 18 and turns out to be small. The most stressed location appears at the arms but is far from being critical (see Fig. 19). The result from the modal analysis is seen in Fig. 20. The lowest eigenfrequency of 67.5 Hz is a horizontal movement in the AMS x-direction close to the second eigenfrequency which is in the AMS y-direction. The frequencies are far above 50 Hz and hence no testing was needed.
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K. L¨ ubelsmeyer et al. / Nuclear Instruments and Methods in Physics Research A 654 (2011) 639–648
Horizontal (x-) Displacement [mm]
Fig. 20. First eigenfrequency 67.5 Hz of tracker plane 6.
Acknowledgments The realization of the upgrade of AMS-02 with the permanent magnet and the rearrangement of the tracker has benefitted from the contributions of many individuals. The authors wish to thank Phil Mott, AMS-02 Chief Engineer7 and the numerous engineers of the structural support group of Jacobs Technology. Special thanks go to the mechanics team of the Ist Physics Institute, RWTH Aachen and to the teams at INFN Sezione of Bologna and Perugia. Last but not least, we wish to express our gratitude to the four companies ISATEC,3 ADCO,4 BF Maschinen,2 and Euromec6 for their fast and excellent work.
7
Jacobs Technology, AMS ESCG, 2222 Bay Area Boulevard, Houston TX, USA.
The support of INFN, Italy, the University of Geneva, Switzerland, the RWTH Aachen, Germany, the US DOE and MIT and NASA is gratefully acknowledged. Support from the space agencies of Germany (DLR) and Italy (ASI) played important roles in the success of the reconfiguration of AMS-02. References [1] S. Ahlen, et al., Nucl. Instr. and Meth. A 350 (1994) 351. [2] S.C.C. Ting, in: K. Huitu, M. Chaian, R. Orava (Eds.), The First Arctic Workshop on Future Physics and Accelerators, World Scientific, Singapore, 1995, pp. 1–26. [3] M. Aguilar, et al., Phys. Rep. 366 (2002) 331. [4] B. Alpat, et al., Nucl. Instr. and Meth. A 613 (2010) 207.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Technical Note
Upgrade of the Alpha Magnetic Spectrometer (AMS-02) for long term operation on the International Space Station (ISS) a ¨ K. Lubelsmeyer , A. Schultz von Dratzig a,, M. Wlochal a, G. Ambrosi d, P. Azzarello d, R. Battiston e,d, g R. Becker , U. Becker g, B. Bertucci e,d, K. Bollweg h, J.D. Burger g, F. Cadoux b, X.D. Cai g, M. Capell g, V. Choutko g, M. Duranti e,d, C. Gargiulo f, C. Guandalini c, S. Haino d, M. Ionica e,d, A. Koulemzine g, A. Kounine g, V. Koutsenko g, G. Laurenti c, A. Lebedev g, T. Martin h, A. Oliva d, M. Paniccia b, E. Perrin b, D. Rapin b, A. Rozhkov g, St. Schael a, H. Tholen a, S.C.C. Ting g, P. Zuccon d a st
I Physics Institute RWTH Aachen, D-52074 Aachen, Germany DPNC, Universite´ de Gene ve, CH-1211 Geneva 4, Switzerland c INFN-Sezione di Bologna, I-40126 Bologna, Italy d INFN-Sezione di Perugia, I-06100 Perugia, Italy e Universita Degli Studi di Perugia, I-06100 Perugia, Italy f INFN-Sezione di Roma, I-00185 Roma, Italy g Massachusetts Institute of Technology, Cambridge, MA 02139, USA h NASA, JSC Houston, 2101 NASA Road One, Houston, TX 77058, USA b
a r t i c l e i n f o
abstract
Article history: Received 14 April 2011 Received in revised form 6 June 2011 Accepted 7 June 2011 Available online 16 June 2011
Following the decision to maintain the International Space Station (ISS) on orbit until at least 2020 (possibly until 2028) the AMS collaboration decided to correspondingly extend the lifetime of the experiment. Since the limited amount of helium used to cool the superconducting magnet allowed for only a limited run time of the experiment, a change from the superconducting magnet to the permanent magnet used in AMS-01 became necessary. Due to the lower magnetic field, to maintain the resolution the silicon tracker also had to be reconfigured with the installation of a silicon plane on the top of the experiment and a new plane above the electromagnetic calorimeter. & 2011 Elsevier B.V. All rights reserved.
Keywords: Astro_particle physics AMS-02
1. Introduction 1.1. Motivation The Alpha Magnetic Spectrometer AMS-01 [1,2] was flown as an engineering, precursor flight on the space shuttle Discovery (mission STS-91, June 1998). This prototype of the experiment used a permanent magnet with a maximum magnetic field of B¼0.14 T and a bending power of BL2 ¼0.15 T m2. The AMS-02 detector [3,4], to be installed on the International Space Station (ISS), was built with a superconducting magnet cooled by liquid helium. Due to the restricted amount of liquid helium the experiment lifetime would have ended after a limited running time. In early 2010 an announcement by NASA was released according to which the space station was scheduled to be maintained on orbit until at least 2020, and likely until 2028.
Corresponding author. Tel.: þ 49 241 8027162.
E-mail address: [email protected] (A. Schultz von Dratzig). 0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.06.051
To benefit from this extended operational period of the ISS and because it was not possible to refill the helium reservoir, the superconducting magnet had to be exchanged with the AMS-01 permanent magnet. To ensure that the performance of the permanent magnet had not changed since the AMS-01 mission, the B-field was measured again. This showed that there had been no degradation, within 1% (the accuracy of the original measurement), over the last 12 years (see Figs. 1 and 2). Since the support structure of the inner silicon tracker of the precursor experiment was reused in AMS-02, the superconducting magnet had the same inner dimensions (radius, height) as the permanent magnet. Thus the superconducting magnet could be replaced by the permanent magnet by replacing the support structure. The lower magnetic field of the permanent magnet required a reconfiguration of the tracker to maintain the spectrometer resolution (see Fig. 3). The former silicon tracker, designed for use with the superconducting magnet, consisted of five planes equipped with one resp. two layers of double sided silicon strip detectors. Plane 1 underneath the upper time of flight detector (TOF) and plane 5 above the lower TOF each support a single layer of silicon. The planes 2–4, symmetrically arranged in the bore of
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K. L¨ ubelsmeyer et al. / Nuclear Instruments and Methods in Physics Research A 654 (2011) 639–648
the magnet, carry double layers of silicon each, totalling eight layers of silicon detectors. For the permanent magnet the support plane 1N for the first silicon layer was relocated from the existing tracker (former support plane 5) on top of the transition radiation detector (TRD). This required a new plane, 1NS, for support, light tightness and electromagnetic and low energy particle shielding. The inner tracker volume was closed for light tightness with a new support plane, 5N, without silicon, a copy of former support plane 5. A new support plane, 6, for the new silicon layer 9 located above the electromagnetic calorimeter (ECAL), was built (cf. Section 4.2). Fig. 3 shows the reconfiguration.
AMS01_Magnet 0.14
before AMS01-Flight Aachen, 03/2010
0.1 0.08 0.06 0.04
1.2. Performance of the two configurations in test beams and with cosmic rays
0.02 0 -1500
-1000
-500
0
500
1000
1500
On ground the alignment of all nine silicon layers can be achieved with an accuracy of 2 mm (1 mm for the seven inner silicon layers) by means of cosmic ray muons in a time interval of 103 min. Nearly the same accuracy is reached with cosmic protons on the ISS within one orbit, equivalent to 92 min (see Fig. 4). This ensures the design spatial resolution of the tracker in the bending plane of 10 mm. Fig. 5 shows the number of cosmic ray muons per TV plotted vs. the inverse rigidity 1/R. After alignment of all nine silicon layers the measured data lead to a maximum detectable rigidity (MDR) of 2.2TV for the permanent magnet, equal to that obtained with the superconducting magnet. The modifications described above do not affect the AMS-02 acceptance for most of the physics topics, including dark matter searches, for particle rigidities up to approximately 400 GV. Above that and up to the MDR of 2.2 TV the acceptance is reduced by a factor between 1.5 and 2. The exact acceptance vs. the particle rigidity will be published in a separate paper. This reduction in acceptance is, however, minor compared to the extension of the experiment lifetime to that of the ISS. In order to ensure that the lifetime of the AMS detector is comparable to the lifetime of the ISS, evaluations were made on all components including the lifetime of consumables, the variation of the magnetic field, and the materials (to mitigate long term exposure to atomic oxygen and solar ultraviolet rays). The most critical consumable lifetime concerns the transition radiation detector. The TRD straw tubes are filled with a mixture
z-Axis / mm Fig. 1. B-Field of AMS-01 permanent magnet along the z-axis.
Fig. 2. Variation of the B-field of the AMS-01 permanent magnet in the midplane over 12 years. The accuracy of the original measurement was 1%.
1NS
1
1
2.3
2
4.5
3
6.7
4
8
5
Suppsort Planes
Silicon Layerso
with superconducting magnet 8 layers of silicon
with permanent magnet 9 layers of silicon
1N
1
1
2
2
3.4
3
5.6
4
7.8
5N 6
Fig. 3. Reconfiguration of the tracker.
9
Silicon Layerso
Flux density / Telsla
0.12
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The support structure of the permanent magnet had to be
10
Alignment accuracy [µm]
integrated into the volume within the vacuum case.
The load paths of the original straps with which the super-
Sim. Layer 1
8
Sim. Layer 9 Data Layer 1
Data Layer 9
6
4
2
0 10
102 103 Measurement time [min]
104
Fig. 4. Alignment accuracy measured with cosmic ray muons at the Kennedy Space Center on the two external layers.
Entries
102
conducting magnet was suspended from the vacuum case rings had to be kept. The tracker support plane 5 had to be used as the new tracker plane 1N on top of the TRD. The new plane 1NS to which the plane 1N is fastened should also serve as a low energy particle shield. The temperature of the tracker layers 1 and 9 had to be kept within their operational range ( 20 1C, þ 40 1C). Electromagnetic interference effects on the new silicon layers should be kept tolerably low.
3. The integration of the permanent magnet into AMS-02 3.1. Hardware 3.1.1. Preparation of the permanent magnet All remnants of glue, foil attachments and cabling on the permanent magnet from the precursor experiment were carefully removed and all surfaces cleaned. The alodine surface treatment was checked and damaged spots repaired. All bolts (the magnet flanges are bolted to the cylindrical part of the magnet) were replaced by new ones. Near the inner periphery of the flanges new holes were drilled for the attachment of the conical flanges of the vacuum case.
10 AMS-Perm. Magnet AMS-S.C.
1
Magnet
MDR= 2.2 TV
-10
-5
0
5
10
1/Rigidity [TV-1] Fig. 5. Cosmic ray measurement of the 1/R distribution after tracker alignment.
of Xe/CO2. The CO2 is lost steadily by diffusion through the straw tube walls. The measured long term leak rate of CO2 amounts to 5 mg=s for the entire TRD. The onboard storage of 5 kg of CO2 at launch corresponds to a lifetime of 30 years. The measurements of the magnetic field of the permanent magnet show that the B-field had not degraded within a margin of o1% in the last 12 years (compare Figs. 1 and 2). We conclude that the field on the ISS will remain the same for the next 20 years. Additional single and double layer beta cloth has been added to cable harnesses to reduce the effects of atomic oxygen and UV degradation.
Fig. 6. AMS-01 permanent magnet.
2. Prerequisites Several requirements had to be fulfilled:
The permanent magnet had to be installed with only very
minor modifications; namely some new holes in the upper and lower flanges. In order to not redesign and rebuild the tubing and cabling of the tracker and other detectors the conical flanges together with the outer cylinder and the outer rings of the vacuum case of the superconducting magnet should be retained.
Fig. 7. New support structure (‘‘double-X-structure’’).
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3.1.2. Preparation of the static test article (STA) vacuum case The spare vacuum case, which was used as a part of the AMS structure for the static and modal tests,1 was prepared for the mounting of the permanent magnet. To this end the inner cylinder was cut right underneath and above the welding where it was connected to the upper and lower conical flanges. The latter were unbolted from the outer rings of the vacuum case. The cutting edge and part of the flanges had to be milled down to allow the permanent magnet to fit in between. Because the
conical flanges had to be bolted to the magnet flanges a series of holes were drilled near the inner periphery that are aligned with those drilled into the magnet flange. The strap ports of the superconducting magnet suspension were prepared for the reception of the new struts that suspend the permanent magnet from the vacuum case rings. The rings and the outer cylinder were mounted into the assembly stand used for the assembly of the superconducting magnet.
3.1.3. Permanent magnet support The support interfaces of the permanent magnet (see Fig. 6) are oriented in the x- and y-axes of the AMS coordinate system. The locations where the struts are connected with the vacuum case rings (the former strap ports), however, lie along the bisection lines of the quadrants. Therefore, a new support structure had to be designed which allows to suspend the magnet at locations which guarantee the preservation of the load paths. This so-called ‘‘double-X-structure’’ is shown in Fig. 7 indicating also the ‘‘struts’’ with rod end bearings and clevises. Due to the relatively large distance between the fixations of the struts and the magnet interface this kind of support introduces bending moments which are the main reason for the rather large crosssection of the ‘‘double-X-structure’’.2 Fig. 8. Assembly of the ‘‘double-X-structure’’. Table 2 Margins of safety of the permanent magnet support. Table 1 Static load factors used in the FEA. Nx g
4.1
Ny g
Nz g
Rx rad=s2
Ry rad=s2
Rz rad=s2
3.6
8.6
33.5
54.3
14.9
Component
Max. tensile/compressive stress (MPa)
Margin of safety (FSu ¼3, 3.45)
X-structure Clevis (to X-structure) Clevis (to outer ring) Strut buckling
47.6 97.6 55.5
1.7 0.3 1.3 0.85
Fig. 9. Total displacement of permanent magnet support.
1
IABG, Einsteinstrasse 20, 85521 Ottobrunn, Germany.
2
BF Maschinen GmbH, Wallensteinstraße 7, 82538 Geretsried, Germany.
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Fig. 10. Most highly stressed location of the permanent magnet support.
Fig. 11. First eigenfrequency of the permanent magnet support.
3.1.4. Assembly The four principal members of the ‘‘double-X-structure’’ were connected to form a square around the permanent magnet, the distance between the magnet interfaces and the ‘‘double-X-structure’’ was measured and the gap filled with shims, adjustable shear pins were installed, and finally the ‘‘double-X-structure’’ was bolted to the magnet (Fig. 8). As a next step the upper conical flange was bolted to the upper magnet flange. With the help of a lifting tool both the magnet, structure and the attached conical flange were lifted with a crane and lowered from above into the prepared vacuum case. The upper conical flange was then bolted to the upper ring of the vacuum case. The struts were assembled and mounted between the principal members and the two vacuum case rings and torqued with a low preload. Next the lifting tool was removed and the lower conical flange was bolted to the magnet and the lower ring.
3.2. Structural verification3 3.2.1. Static finite element analysis Static and modal finite element analyses (FEA) of the magnet and structure were carried out for the space shuttle launch loads. Only one set of load factors has been chosen because of the symmetry of the arrangement (see Table 1). Fig. 9 shows the total displacement under the above loads. A summary of the margins of safety is shown in Table 2. An ultimate (i.e., against yielding) factor of safety (FSu) of 3 was used, except for strut buckling where it was 3.45. The critical location was found at one of the clevis tangs. A detailed investigation is shown in Fig. 10.
3
ISATEC Engineering GmbH, Rathausstrasse 10, 52072 Aachen, Germany.
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Fig. 12. Overview of uppermost tracker arrangement.
Fig. 13. Mounting principle of plane 1NS.
4. New supports for the additional tracker planes
Table 3 Overview of margins of safety of plane 1NS. Component
Margin of safety (FSu ¼3, 3.45)
Bracket Face sheet HC core C-channel GPS antenna bracket
0.44 4.74 (kick load) 0.32 (kick load) 17.7 2.2
As discussed earlier, three new tracker planes were introduced; plane 1NS above the TRD, plane 5N to close the inner tracker volume and plane 6 on top of the ECAL. As plane 5N was a copy of the previous plane 5, it is just needed to be fabricated.4 Therefore, new supports had to be designed and constructed for planes 1NS and 6. 4.1. Support plane 1NS4
3.2.2. Modal analysis The result of the modal analysis gives a lowest frequency of 73.4 Hz in drum mode, well above the desired 50 Hz (see Fig. 11) for space shuttle payloads.
4.1.1. Mechanics of support plane 1NS for the uppermost tracker plane The zenith radiator for the thermal management of the cryogenic components of the superconducting magnet is no 4
ADCO GmbH, Werkstraße 6, 52076 Aachen, Germany.
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Fig. 14. Total displacement of plane 1NS.
Fig. 15. Most stressed component of plane 1NS.
longer needed and therefore removed. The former tracker plane 5 with a reduced number of silicon ladders is moved on top of the AMS-02 experiment and called plane 1N holding silicon layer 1. It is supported by a new sandwich plane 1NS of the same octagonal shape as the top cover of the TRD. The thickness is designed to satisfy stability requirements and also to serve as a low energy particle shield. The face sheets (carbon fiber/epoxy resin T700) are therefore 2.5 mm thick each and the core is a 70 mm high aluminum honeycomb (Al 3/16-0.001 in.). Plane 1N is bolted to it from below with the silicon layer facing down towards the TRD. Along the circumference between the upper cover of the TRD and the plane 1NS a C-channel made from carbon fiber guarantees
light tightness. Four tracker cabling feedthroughs and four venting chicanes are placed at eight locations in the C-channel (see Fig. 12). In cold environments the silicon has to be heated to be kept within its operational range ( 20 1C, þ40 1C). Therefore, heater pads are glued to the lower side of the support plane 1NS more or less equally distributed above the plane 1N.5 The heat locally produced by the front-end electronics is removed via copper braids that are connected to the lower side of plane 1NS.
5
Carlo Gavazzi Space SpA, Via Gallarate 150, 20151 Milano, Italy.
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Fig. 16. First eigenfrequency of plane 1NS.
Fig. 17. Tracker plane 6 supporting silicon layer 9.
The heat is finally radiated off to space from a circular region ð 0:5 m2 Þ in the center of the upper side of plane 1NS. Because of the modified configuration at the top of the experiment a new multi-layer insulation (MLI) hood had to be designed and produced. It is not totally closed but has an opening over the radiator area. A 50 mm aluminum foil covers the top side of the TRD top cover and the top side of plane 1NS which are electrically connected to the MLI hood to prevent any electromagnetic interference from and to the TRD and silicon layer 1. This also serves as the radiator material. Fig. 13 shows the mounting principle. Plane 1NS is supported by eight brackets which are fastened to the top cover of the TRD, using the existing threaded holes in the C-profiles which form the outer edge of the TRD top cover. These were originally designed for the lifting brackets of the TRD.
4.1.2. Static and modal analysis of plane 1NS All 64 load case combinations have been investigated in the static analysis including acoustic loads. The maximum values of the translational and rotational loads have been combined as a worst case in the analysis. The total displacement in this worst case is shown in Fig. 14. The maximum displacement occurs in the center of plane 1NS in the AMS z-direction and is equal to 1.57 mm, the circular region where the plane 1N is attached to plane 1NS shows a displacement of around 1 mm. The lowest margin of safety, 0.44, is found at the brackets and is seen in the detail in Fig. 15. As could be expected, the highly stressed region is very narrow and can be considered as a local effect only of no concern. Table 3 gives an overview of the margins of safety for plane 1NS. They represent the lowest margins of the respective components and belong to different load cases. An ultimate factor of safety of 3 was used, except for the GPS antenna bracket where 3.45 was used. The first eigenfrequency of plane 1NS is 68.4 Hz and occurs in drum mode (see Fig. 16). It lies well above 50 Hz and therefore no further testing is required. Details of the cabling and cooling of the new tracker planes as well as the results of the measured performance with test beams of high energy particles will be published in a separate paper. 4.2. Support plane 6 4.2.1. Mechanics of support plane 6 for the lowest tracker silicon layer 9 To complete this report on the upgrade of the AMS-02 detector a short description of the support for the new tracker layer 9, plane 6, shall be given here. The mechanical design was done at the INFN Bologna in Italy, and the finite element analysis at the engineering company ISATEC, Aachen. Plane 6 consists of a sandwich support plate6 made from carbon fiber skins (M55J/CE)4 with an aluminum 6
EUROMEC S.R.L., Via dalla costa Raimondo 165, 41100 Modena, Italy.
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LC 3, Design Limit Load Factors Nx = 4.3 g Ny = 2.3 g Nz = 7.3 g Rx = 72.4 rad/s Ry = -53.1 rad/s Rz = 13.1 rad/s
Fig. 18. Total displacement of tracker plane 6.
LC 3, Design Limit Load Factors Nx = 4.3 g Ny = 2.3 g Nz = 7.3 g Rx = 72.4 rad/s² Ry = -53.1 rad/s² Rz = 13.1 rad/s²
Seqv = 40.63 MPa
Margins of Safety MSu = Ftu / ( FSu x Seqv ) - 1 = 393 / ( 3 x 40.63) - 1 = 2.2 Support Arms, Al 7050-T7351-4‘‘ Strength Values according to Mil. Spec.
MSy = Fcy / ( FSy x Seqv ) -1 = 317 / ( 2 x 40.63) -1 = 2.9
Fig. 19. Location of the highest stress of tracker plane 6.
honeycomb core. This plane carries the ladder structures of the silicon detector. Light tightness is provided by a thin carbon fiber sandwich cover on top and carbon fiber profiles around the edges in between the two sandwich plates, which allow for venting and cable feedthroughs. The support plane is attached to the ECAL fixation blocks by eight aluminum arms, two at each corner (see Fig. 17). They are connected to the honeycomb support plane by means of sliding blocks. This allows movements caused by differential thermal expansion without inducing stress on the plane 6 plus silicon layer 9 assembly. A photograph of the completed assembly is shown in Fig. 17.
4.2.2. Static and modal analysis of plane 6 As mentioned in Section 4.1.2, the worst combination of loads out of the 64 possibilities was chosen for the static FEA of the plane 6. The total displacement is shown is Fig. 18 and turns out to be small. The most stressed location appears at the arms but is far from being critical (see Fig. 19). The result from the modal analysis is seen in Fig. 20. The lowest eigenfrequency of 67.5 Hz is a horizontal movement in the AMS x-direction close to the second eigenfrequency which is in the AMS y-direction. The frequencies are far above 50 Hz and hence no testing was needed.
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Horizontal (x-) Displacement [mm]
Fig. 20. First eigenfrequency 67.5 Hz of tracker plane 6.
Acknowledgments The realization of the upgrade of AMS-02 with the permanent magnet and the rearrangement of the tracker has benefitted from the contributions of many individuals. The authors wish to thank Phil Mott, AMS-02 Chief Engineer7 and the numerous engineers of the structural support group of Jacobs Technology. Special thanks go to the mechanics team of the Ist Physics Institute, RWTH Aachen and to the teams at INFN Sezione of Bologna and Perugia. Last but not least, we wish to express our gratitude to the four companies ISATEC,3 ADCO,4 BF Maschinen,2 and Euromec6 for their fast and excellent work.
7
Jacobs Technology, AMS ESCG, 2222 Bay Area Boulevard, Houston TX, USA.
The support of INFN, Italy, the University of Geneva, Switzerland, the RWTH Aachen, Germany, the US DOE and MIT and NASA is gratefully acknowledged. Support from the space agencies of Germany (DLR) and Italy (ASI) played important roles in the success of the reconfiguration of AMS-02. References [1] S. Ahlen, et al., Nucl. Instr. and Meth. A 350 (1994) 351. [2] S.C.C. Ting, in: K. Huitu, M. Chaian, R. Orava (Eds.), The First Arctic Workshop on Future Physics and Accelerators, World Scientific, Singapore, 1995, pp. 1–26. [3] M. Aguilar, et al., Phys. Rep. 366 (2002) 331. [4] B. Alpat, et al., Nucl. Instr. and Meth. A 613 (2010) 207.