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A Super Conducting 60 cm Warm Bore Cosine Theta Dipole Magnet for the Jefferson Lab Super High Momentum Spectrometer (SHMS).
Paul D. Brindza, Steve Lassiter and Michael Fowler
Abstract—The Jefferson Lab 12 GeV upgrade involves building a new 12 GeV/c spectrometer for JLAB Hall C called the Super High Momentum Spectrometer (SHMS). This device achieves 4.5 mStr acceptance at bend angles for 5.5 degrees to 40 degrees by using five magnetic elements. The other magnetic elements of the SHMS including the small SC dipole used to achieve the small 5.5 degree scattering angle are described elsewhere in this conference. The 4.5 TeslaSC dipole provides momentum analysis for particles from 1 to 12 GeV/c and has a bend strength of 13.5 Tesla meters. The magnetic design including multipole strengths will be presented. The dipole cold mass uses a stainless steel shrink fit force collar, Titanium keys and a copper stabilized super conductor consisting of a 36 strand surplus SSC outer cable wave soldered to a copper extruded substrate. This combination provides for a very conservative magnet that can be assembled with little or no tooling and a high degree of stability. The force collar mechanical analysis will be presented as well as details of the magnet cryostat.
Index Terms—Superconducting magnets, Detector magnets, Dipole magnets,
INTRODUCTION
T
homas Jefferson National Accelerator Facility is currently upgrading the CEBAF accelerator from its original design energy of 6 GeV to 12 GeV. The Upgrade Project is in the Project Engineering and Design (PED) phase. Construction is expected to begin in FY10 and the 12 GeV Nuclear Physics program is expected to begin in FY14. The Upgrade involves adding 10 additional cryo modules to the CEBAF accelerator to double the energy, new experimental equipment in JLAB Experimental Hall’s A, B and C and the entirely new Experimental Hall D [1].Super Conducting (SC) magnets feature prominently in the Experimental Equipment of the 12 GeV upgrade as they did in the original CEBAF construction at JLAB. The plan for JLAB hall C requires a new SC spectrometer, the Super High Momentum Spectrometer(SHMS) as a 12 GeV/C companion to the existing High Momentum Spectrometer(HMS) operating at 7.3 GeV/c. The SHMS requires five new SC magnets a small initial Horizontal Bend Dipole (HB), three quadrupoles
Fig. 1. SHMS Dipole magnet shown with warm yoke and mounted at required installation angle of 9.2 degrees. This installation features requires the use of a cryogenic vent from the rear, high point of the magnet.
(Q1Q2Q3 ) and the subject of this paper a 4.5 Tesla 60 cm. warm bore cosine type SC dipole. The dipole is shown in Fig. 1. The HB magnet is required to enable the SHMS to reach the smallest required scattering angle of 5.5 degrees and this magnet is the subject of a paper (4M02) at MT-20. The three quads are also described here at MT 20 ( 4M06, 4M07) and they are the focusing elements of the dQQQD spectrometer. The SHMS achieves a momentum resolution of 1 x10^-3 and has a solid angle acceptance of 4.5 mStr. The SHMS spectrometer has excellent optical properties and is largely immune to the most common field aberrations of the large bore short length magnets. This results in simple SC magnets with relatively large construction tolerances (1 mm). The final construction tolerances will be set entirely by the requirements for magnet assembly and construction rather than field errors. The SHMS Dipole and the Q23 Quadrupoles share many similar design and construction details including the same composite stabilized super conductor, the same cryostat cross section, nearly the same operating current and the same force collar design and method of coil preload. Both the Dipole and Q23 quads use a stainless steel force collar that is a shrink fit over the coil which uses Titanium “keys” to insure coil preload after cool down to 4.4 Kelvin. This type of cold mass assembly does not rely on large, complex and expensive tooling used only once. The main properties of the SHMS Dipole are listed in Table I. The new super conducting magnets that constitute the 12 GeV Upgrade of Jefferson Labs Experimental Equipment in Hall C were reviewed at JLAB in September 2006 by an external Technical Review team. This review was successful in that there were no reservations and only a few recommendations. The subject matter presented at this review and the final report from the review team can be found in reference [2].
It is the intention of JLAB to acquire all five of these magnets from commercial or institutional sources thru the competitive award of fixed price best value type contracts. All five SHMS magnets will feature significant materials (GFM) and equipment(GFE) to be furnished by JLAB. It is the intention of JLAB to supply the tested processes superconductor, the cryogenic control reservoir, the DC power system including energy dump, the instrumentation and control system including software, the warm iron yoke and support services for the on site acceptance test at JLAB. This is a significant departure from past practices at JLAB where in general turn-key magnet systems were procured by fixed price performance contracts. The main reasons for this approach is to enforce standardization of external components such as DC power , cryogenic control reservoir and Instrument and Control systems and to help control the overall cost by sharing in a significant way the overall technical responsibility and project risk. The overall magnet project delivery schedule is also expected to be shorter and more predictable is many major systems can be provided earlier especially the composite superconductors.
I.Magnetic Design of the SHMS Dipole
The magnetic design of the SHMS Dipole has been designed using the TOSCA family of computer codes. The magnet is designed as a two current sector cosine (theta) coil with constant perimeter (CP) ends. TOSCA generated coils have been used extensively in the design and optimization. The coil configuration that is being modeled consists of 6 winding layers arranged as double cylindrical pancake windings. Since the TOSCA generated coils do not correspond exactly to the classic two sector optimization, the coils are subdivided into 6 blocks in each double layer or 18 subcoils in all. This allows a simple magnetic model of the SHMS Dipole using Tosca generated CP coils to approach closely the actual shape. Since even this coil shape is not an exact cosine dipole coil a small amount of magneto-static trimming is required to minimize the first few multipoles on the mid plane. Table II shows the result of this optimization and the corresponding sensitivity of the first few allowed multipoles is listed in Fig. 3. As you can clearly see the magnetic design is very forgiving of coil displacements and in fact the field quality is acceptable for +/- 1 conductor coil changes. Since the conductor is 5 mm wide in the phi direction that corresponds to a tolerance of 0.8 degrees in the angles defining the current blocks! Further studies of magnetic field errors involving a set of symmetric and asymmetric coil displacements have concluded that construction errors could be as large as 2 mm which is clearly much larger than mere assembly considerations would allow. The SHMS Dipole final coil tolerances will be set almost entirely by construction and assembly consideration alone with the nominal dimensions selected to optimize the field.
II.SHMS Dipole Superconductor and stability
The super conductor for the SHMS Dipole has been selected to be a soldered composite of a copper substrate plus cable. This superconductor exhibits classic Stekely stability in the SHMS Dipole and also in the SHMS Q23 quadrupole magnets. The cable used is surplus SSC outer conductor obtained thru the generosity of USDOE/OHEP.A sufficient quantity of the cable is at JLAB to permit its use in all new SC magnets required for the 12 GeV upgrade including all five of the SHMS magnets and the large new SC Torus and SC Solenoid to be built for JLAB Hall B’s Upgrade and even small quantities have been used in the rebuilding of the now ancient MEGA/LASS Solenoid for JLAB Hall D. We estimate that the present value of all the cable to be used in the 12 GeV upgrade has saved JLAB over 4 million dollars not to mention the time to fabricate completely new conductor. The untimely end of the SSC resulted in this large quantity of cable remaining untested as cable. JLAB has engaged the excellent cable test facility at BNL and operated by Arup Ghosh as our testing site and all cable required for SHMS magnets has already been tested. The short sample testing results from the BNL measurements are presented in Fig. 2 superimposed on the conductor short sample nominal curve and the SHMS Dipole load line. The SSC cable exhibits large operating margins which together with the stabilizer and fully clamped coils should result in a conservative and highly stable magnet that has a very high probability of reaching full excitation without quenching or training. The current HMS magnets were designed in a similar conservative fashion and after 13 years of operation they have never quenched. The JLAB Reference Design for the SHMS Dipole has the composite conductor insulated with half lapped Kapton tape and over wrapped with B-stage glass epoxy tape. The conductor layers and ground insulation used are sheets of 1mm G10. The use of B stage tape allows the coils to be handled after winding and curing during assembly and installing of the force collar. The super conductor properties are listed in Table III.
Fig. 2. Short Sample curve and SHMS Dipole load line with the BNL cable test results superimposed.
TOSCAParam. / N=2/N=0 / N=4/N=0 / N=6/N=0 / N=8/N=0
Alpha 1 / -0.7 % / -0.4% / weak / weak
Alpha 2 / weak / +0.2 % / -0.1 % / weak
B 2 / -0.4 % / weak / weak / weak
Fig. 3. Sensitivity of lowest multipoles to variation in TOSCA coil parameters as a per cent of the dipole field.
III.SHMS Dipole Design
The SHMS Dipole cold mass design uses an innovative coil preloading system which includes a shrink fit stainless steel force collar and Titanium keys. This system with the copper stabilized composite superconductor provides for the coil preload against magnetic forces after cool down. A modest 50 degree C shrink fit temperature is used to assemble the coil mass at room temperature. The use of dissimilar materials in the coil and key structure provides for the preload against magnetic forces. The Titanium keys must be segmented and finger jointed along the length of the coil to avoid an overstress situation in the end turns. The analysis of the SHMS Dipole (Q23) is ongoing and is being conducted at four separate sites. Teams at Accel, FermiLab, Novatech and JLAB are analyzing the SHMS magnets force collars and coil clamping mechanism in a parallel FEA effort under contract with JLAB. Final results for all four efforts are expected soon but already there is evidence that the general technique is sound.[7,8,9] An illustration showing the cold mass stresses for the JLAB developed model of the cold powered dipole is shown in Fig.4to illustrate the preliminary results. A somewhat related technique now being used by a team at CEA/Saclay to design the R3b-Glad dipole[10] was the inspiration for the selection of this cold mass clamping technique.
Fig. 4. JLAB 1/8 SHMS Dipole FEA model showing coil forces at 4.5 Kelvin
The cryostat for the SHMS Dipole and the Q23 magnets are very similar and are straightforward cylindrical assemblies as shown in a cutaway view in Fig.5. It is planned to use inflated stainless steel cryo panels as the LN2 shields. The remaining cryostat including the Liquid Helium vessel and vacuum vessel are typical stainless steel construction. The cryo control reservoir for all the SHMS magnets will be based on a standard JLAB design as used in all HMS SC magnets, the G0 SC Torus and the pair of compact SC septum magnets used in JLAB hall A. This device contains the Helium and Nitrogen reservoirs for some stand alone hold time, the burnout proof current leads, JLAB standard cryogenic control valves and bayonets and some of the magnet instrumentation. All five SHMS magnets will use an identical cryogenic control reservoir furnished by JLAB as Government Furnished Equipment (GFE).
IV.Magnet Quench Protection
The SHMS dipole and the similar Q23 Quadrupoles will be protected by an external fast discharge circuit to remove the magnets stored energy in the event of a magnet quench, unsafe voltage anywhere in the system, extreme loss of coolant or vacuum and life safety considerations. Historically in Jefferson Lab Hall C, fast discharge events have all been “nuisance” type events caused by spurious interlocks. The choice to use an external energy dump and thus avoid the repeated magnet heating that results from the various internal energy dump systems available will result in less lost time for physics. Jefferson Lab Hall C has a standard DC power system that has an integral fast discharge switch and energy dump. All future DC system s for the SHMS will also feature an integral quench detection circuit that is hard wired to the power supply. The SHMS Dipole and Q23 Quads operate at similar currents near 5000 amps and a discharge voltage of 200 volts has been selected on the basis of safe adiabatic hot spot temperatures at a reasonably low voltage. This implies a discharge resistance of 0.040 Ohms for both magnets resulting in final hot spot temperatures of 69 Kelvin and 46 Kelvin respectively. Real magnet quenches in the SHMS Dipole and Q23 are expected to be extremely rare and in fact will be difficult to induce even for test purposes. In the unlikely event of an undetected quench the final temperatures in the Dipole and Q23 assuming that a single coil in each absorbs the entire stored energy are 78 Kelvin and 69 Kelvin respectively.
Fig. 5. SHMS Dipole CAD model cutaway view showing cryostat details
V.Conclusions
The super conducting SHMS Dipole and the similar design Q23 pair of quadrupoles form the heart of the SHMS spectrometer. Their large 60 cm warm bore aperture is essential to achieving the relatively high 4.5 Mstr solid angle acceptance of the SHMSD. The nominal 5 Tesla bend field and 14 Tesla/meter gradients are essential to achieve focusing type resolution at momentum up to 12 GeV/c. The challenge for JLAB is to optimize the design around the performance and at the same time to hone the design to ensure a reasonable cost out come for the very limited “one off” contracts that are a necessary fact of life for typical large detector magnet projects. Personal courage is not usually part of the SC magnet engineer’s job description except maybe during acceptance testing but it certainly helps when it comes to letting fixed price contracts! JLAB has been fortunate to have found the services of highly qualified individuals and companies in the past and I have no doubt that we will do so again so that on some future date when I am just a bit grayer I can report here at an MT conference of the future on yet another success story of the worlds superconducting magnet industry in providing exciting and unique magnets for Jefferson Labs Nuclear Physics program.
TABLE I DIPOLEMAGNET PARAMETERS
Parameter / QuantityWarm Bore / 0.600 m
Cryostat Length / 4.11 m
Yoke Length / 3.16 m
Current Density / 4500 A.T/cm2
Mega Amp Turns / 4.33 M. A.T
Turns / 944
Operating Current / 4500 A
Stored Energy / 18.3 MJ
Inductance / 1.81 H
Magnet Weight / 140 tons
TABLE II MAGNETIC RESULTS
Parameter / QuantityDipole Field / 4.7 T
Effective Field Length / 2.96 m
Peak Yoke Field / 2.52 T
Peak Coil Field / 5.89 T
Integral Dipole Field / 13.8 T m
MomentumRange / 1 to 12 Gev/c
Integral Harmonic N=2
% of N=0 / -2.5 %
Integral Harmonic N=4 % of N=0 / -0.63 %
Integral Harmonic N=6 % of N=0 / -0.29 %
TABLE IIICONDUCTOR PARAMETERS
Parameter / QuantityConductor Size / 2.00 cm x 0.50 cm
Copper size / 1.89 cm x 0.44cm
SC Cable / 36 strand SSC outer
Stability Alpha / 0.69
Energy Margin / 0.528 Joules
Field Margin / 1.68 T
Critical Current Margin / 1552 A
Temperature Margin / 2.34 K
Kapton Thickness / 0.1 mm
B-stage Epoxy Thickness / 0.2 mm
Acknowledgment
The authors would like to acknowledge the generous support of the United States Department of Energy Office of High Energy Physics and the technical support of Dr. Bruce Straiuss (USDOE/OHEP) and Dr. Ron Scanlon and Dr. Dan Dietderick (LBNL) for providing Jefferson Lab with the Super Conducting Cable for this project.
References
[1]The Science and Experimental Equipment for the 12 GeV Upgrade of CEBAF, Jan. 2005,
[2]P. D. Brindza et al, “Commissioning the Superconducting Magnets for the High Momentum Spectrometer (HMS) at TJNAF”, IEEE Transaction on Applied Superconductivity, vol. 7, June 1997, p. 755.
[3]12 GeV Upgrade Project Conceptual Design and Safety Review of Superconducting Magnets, Sep. 2006, Mag Rev -final/
[4] John J. LeRose private communication, JLAB January 2007 Optics Study of SHMS using Raytrace and TOSCA fields.
[5] Arup Ghosh, Private communication, BNL, Report. August 2007 “Short Sample tests of SSC Cables for JLAB”.
[6]Gregory J. Laughon, private communication,American Magnetics Inc.Test Report AMI 5000 Amp Vapor Cooled Current Leads Operated at Full Power Without Cooling Flow.American Magnetics, Inc. March, 2005
[7]Robert Wands Private Communication, FermiLab, August 2007
[8]Greg Markham Private Communication, Novatech Inc, August 2007
[9]Steve Lassiter Private communication, Jefferson Lab August 2007
[10]C.Mayri et al, R3B-GLAD Technical Report CEA/Saclay reference I-008406 ver.1,June 29,2006.
Manuscript received August 28, 2007. Notice: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. The U.S. Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce this manuscript for U.S. Government purposes.
P.D. Brindza, S. Lassiter and M. Fowler are with the Thomas Jefferson national Accelerator Facility, Newport New, Virginia (phone: 757-269-7588; fax: 757-269-6273; e-mail: ).