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4M06 1 The Cosine Two Theta Quadrupole Magnets for the Jefferson Lab Super High Momentum Spectrometer. Paul D. Brindza, Steven R. Lassiter and Michael J. Fowler Abstract— The Jefferson Lab 12 Gev/c upgrade involves building a new 12 Gev/c spectrometer for JLAB Hall C called the SHMS. This device achieves 4.5 mStr acceptance at bend angles for 5.5 degrees to 40 degrees by using five magnetic elements in a dQQQD configuration. The Q1 SC quad is described elsewhere in this Conference and is an evolution of a cold iron magnet used previously for the existing JLAB 7.5 Gev/c HMS spectrometer. The pair of identical Cosine Two Theta quads are an entirely new design with a large 60 cm warm bore and 13 Tesla/meter gradient. These 5 Tesla Quads provides focusing for particles from 1 to 12 Gev/c and have an integral gradient strength of 23.5 (T/M) M. The magnetic design including multipole strengths will be presented. The quadrupole 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 assemble 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, Quadrupole Magnets I. INTRODUCTION T HOMAS Jefferson National Accelerator Facility is currently upgrading the CEBAF accelerator from its original design energy of 6 Gev/c to 12 Gev/c. The Upgrade Project is in the Project Engineering and Design (PED) phase. Construction is expected to begin in FY10 and the 12 Gev/c 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, C, and an entirely new Experimental Hall, Hall D. Super Conducting (SC) magnets feature prominently in the Experimental Equipment of the 12 Gev/c upgrade as they did in the original CEBAF construction at JLAB. The plan for JLAB Hall C Manuscript received August 28, 2007. Notice: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC0506OR23177. 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: [email protected]). . Fig. 1. SHMS Q2 Quadrupole General Arrangement. The Q3 magnet is identical except for the slotted yoke. requires a new SC spectrometer, the Super High Momentum Spectrometer (SHMS) [1] as a 12 Gev/c companion to the existing High Momentum Spectrometer (HMS) [2] operating at 7.5 Gev/c. The SHMS requires five new SC magnets, a small initial Horizontal Bend Dipole (HB), three quadrupoles (Q1,Q2,Q3 ) of which the subject of this paper are the Q2, Q3 magnets (collectively identified as Q23), with 13 Tesla/meter gradient and 60 cm warm bore cosine (2theta) type SC Quads. Fig. 1 shows Q2 with the notch in the warm return yoke to allow for the primary beam line. The SHMS achieves a momentum resolution of 1 x10^-3 and has a solid angle acceptance of 4.5 mStr. This spectrometer has excellent optical properties and is largely immune to the most common field aberrations of the SHMS 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 Q23 Quadrupoles and the SHMS Dipole 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. 4M06 2 The Q23 Quadrupoles 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 Q23 Quadrupoles are listed in Table I and Table II. TABLE I Q23 QUADRUPOLE PARAMETERS Parameter Quantity Warm Bore 0.600 m Cryostat Length 2.33 m Yoke Length 1.90 m Current Density 5050 A.T/cm2 Mega Amp Turns 6.50 M. A.T Turns 1288 Operating Current 5050 A Stored Energy 9.78 MJ Inductance 0.77 H Magnet Weight 60 tons TABLE II Q23 MAGNETIC RESULTS Parameter Quantity Gradient 13.1 T/m Effective Field Length 1.79 m Peak Yoke Field 2.72 T Peak Coil Field Integral Gradient Momentum Range Integral Harmonic N=3 % of N=1 5.87 T 23.5 (T /m)m 1 to 12 Gev/c -0.15 % Integral Harmonic N=5 % of N=1 -0.60 % Integral Harmonic N=9 % of N=1 -0.10 % TABLE III Q23 CONDUCTOR PARAMETERS Parameter Quantity Conductor Size 2.00 cm x 0.50 cm Copper size 1.89 cm x 0.44 cm SC Cable 36 strand SSC outer Stability Alpha 0.93 Energy Margin 0.480 Joules Field Margin It is the intention of JLAB to acquire all five of the SHMS 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 processed 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 [3]. 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 as many major systems can be provided earlier especially the composite superconductor. II. MAGNETIC DESIGN OF THE SHMS Q23 QUADRUPOLES The Q23 magnets are designed as a two current sector cosine two theta coil. The coil configuration that is being modeled consists of standard Tosca generated constant perimeter coils, with 8 winding layers arranged as double cylindrical pancake windings. Return flux is contained by the warm iron yoke. The BH curve used in the models came from 1006 iron. A notch is required in Q2’s yoke to accommodate the primary beamline at small forward angles. The notch is symmetrical placed on either side of the magnet to minimize harmonics and forces. Table II shows the result for the Q3 magnet. A Fourier fit is performed on the field along the warm bore diameter to extract the field harmonics. These harmonics are then integrated along the axis of the magnet and compared to the main quadrupole field. The N used follows the TOSCA convention so the pole count is 2*(N+1). Coil placement studies were performed and the magnetic design was found to be very forgiving of coil misalignments. Acceptable field quality is maintained for +/- 1 conductor turn width per layer changes. This corresponds to a tolerance of 0.8 degrees in the angles defining the current blocks. Studies showed that coil displacements as large as 2mm, both symmetric and asymmetric, produced magnetic field errors that are within acceptable range. Magnetic studies conclude that the fields and field integrals are very linear over the full excitation, with the effective field length (EFL) falling off by only ~1 cm. The multipoles are stable with only a small saturation effect on the Octupole component. Inductance of Q23 is nearly constant up to currents of 3000A and dropping to 0.77H at 5000 amps. 1.43 T Critical Current Margin 1217 A III. Q23 QUADRUPOLE SUPERCONDUCTOR AND STABILITY Temperature Margin 2.18 K Kapton Thickness 0.1 mm B-stage Epoxy Thickness 0.2 mm The SHMS Q23 Quadrupole uses 7.7Km of surplus SSC outer conductor Rutherford cable per magnet. The Rutherford cable will be wave soldered to a copper substrate. The 4M06 3 Conductor critical current and load lines 25000 fit @ 4.4 K SHMS quad fit at 6.623 K Tested Average Current Amps 20000 15000 10000 5000 0 0 2 4 6 8 10 12 Peak field Tesla Fig. 2. SHMS Q23 Quadrupole Short Sample and load line IV. SHMS QUADRUPOLE MECHANICAL DESIGN The SHMS Q23 Quadrupole cold mass design uses an innovative coil preloading system which includes a shrink fit stainless steel force collar and Titanium keys [6]. The FEA analysis of the SHMS Q23 Quadrupole Force Collar is ongoing at Accel [7] under contract from JLAB. They are analyzing the SHMS quadrupole magnets force collars and coil clamping mechanism in a parallel effort with JLAB. Final results are expected soon but already there is evidence that the general technique is sound. A somewhat related technique now being used by a team at CEA/Saclay to design the R3BGlad Dipole [8] was the inspiration for the selection of this cold mass clamping technique. The cryostats for the SHMS Q23 Quadrupoles are straightforward cylindrical assemblies. Many of the design features of the High Momentum Spectrometer (HMS) Q2/Q3 of Hall C are incorporated into this design. The stainless steel Helium containment vessel and insulating vacuum vessel are based upon the Q2/Q3 design of the HMS. The new design uses inflated stainless steel panels as the LN2 shields. The cryogenic control reservoir for all the SHMS magnets will be based on a standard JLAB design. The reservoir has the capability of providing several hours of Helium and Nitrogen cooling to the magnet in the event of cryogen feed disruption. It also contains the “burnout proof” current leads, JLAB standard cryogenic control valves, bayonets, and some of the magnet instrumentation. V. SHMS Q23 “NO BURNOUT” CURRENT LEADS The SHMS magnets will all be equipped with nominal 5000 Amp vapor cooled current leads that are a “no burnout” design. A pair of these current leads was designed, built and tested at American Magnetics Inc. under contract from JLAB [9]. These tests were conducted by Gregory J. Laughon of AMI and Paul D. Brindza. These vapor cooled current leads are similar in construction to standard super conducting magnet optimized vapor cooled leads but they have a longer effective length and a larger conducting cross section. The extra current lead mass permits a much longer thermal runaway time in the absence of cooling. The output of DC power supplies available for testing was limited to 4900 Amps. The vapor cooled current leads were tested at this current with no cooling. The time to reach the maximum safe temperature and corresponding voltage was measured. The current leads were also slow discharged at different rates and again the temperature and voltage profiles were measured. The main conclusions from these tests are that the leads can remain at a constant current of 4900 Amps for 11.3 minutes before reaching 200 milli Volts and 325 Kelvin maximum temperature. The current leads exhibit a stable temperature profile that is everywhere safe at currents up to 3400 Amps, the maximum no cooling DC current. The leads are capable of very slow ramp downs of 2.5 Amps/sec or 2000 seconds discharge time. When the current reaches the no cooling maximum DC current limit, 3400 A, the temperature and voltage begins to decrease during the slow discharge. The time and temperature performance of these “no burnout” vapor cooled current leads are shown in Fig. 3 and Fig. 4. The no burnout vapor cooled current leads are cryogenically reoptimized and they require cooling that is only 9 % higher than conventional vapor cooled leads. It is this author’s opinion that this is a very small cost to bear in exchange for the safety and reliability. It is the plan of Jefferson Lab to equip all five of the SHMS magnets with a pair of these “no burnout” current leads as GFE. 100 90 Voltage Drop (mVolts) properties of the Q23 Conductor are listed in Table III. This superconductor exhibits classic Stekly stability in the SHMS Q23 Quadrupole magnets, with an alpha of 0.93. Short sample testing of the Rutherford cable to be used in Q23 was performed at Brookhaven National Laboratory under the supervision of Arup Ghosh [5]. The average results of these tests for the spools to be used for Q23 are shown in Fig. 2 along with a nominal short sample curve and the load line for Q23. Each turn of the copper stabilized conductor is insulated with half lapped Kapton tape and over wrapped with B-stage glass epoxy tape. A 1mm thick sheet of G10 will be inserted between the layers and used as ground insulation. The use of B-stage tape requires that the coil assembly be cured on the winding table. This rigid coil package allows for handling of the coil assembly and installation of the force collar. Lead 1 80 Lead 2 70 60 50 40 30 20 10 0 0 500 1000 1500 2000 2500 Time (seconds) Fig. 3. Voltage profile during two discharges without cooling at 5 Amp/sec and 2.5 Amp/sec. 4M06 4 (USDOE/OHEP), Dr. Ron Scanlon (LBNL) and Dr. Dan Dietderich (LBNL) in acquiring the surplus SSC cable for Jefferson Lab. We would also like to acknowledge the excellent work of Dr. Arup Ghosh whose superb test facility provided the cable test data that will allow Jefferson Lab to use the cable in these magnets with confidence. 300 200 150 100 REFERENCES Sensor 1 Sensor 2 Sensor 3 Sensor 4 Sensor 5 (K) Temperature (K) 250 [1] [2] 50 0 0 500 1000 1500 2000 2500 3000 Time (seconds) Fig. 4. Temperature profile during two discharges without cooling at 5 Amp/sec and 2.5 Amp/sec [3] [4] [5] VI. CONCLUSIONS The super conducting SHMS Q23 Quadrupoles and the similar SHMS Dipole design form the heart of the SHMS spectrometer. The large 60 cm warm bore aperture of the Q23 magnets is essential to achieving the relatively high 4.5 mStr. solid angle acceptance of the SHMS. The nominal 13 Tesla/meter gradients are essential to achieve focusing and good resolution at momentum up to 12 GeV/c. Jefferson Lab expects to award the fixed price best value contract for the construction of the Q23 Quadrupoles in the second quarter of FY 10 with nominal SHMS commissioning planned for FY14 and the start of the era of 12GeV Nuclear Physics at JLAB. ACKNOWLEDGMENT The authors would like to acknowledge the support of the United States Department of Energy Office of High Energy Physics and the technical support of Dr. Bruce Strauss [6] [7] [8] [9] The Science and Experimental Equipment for the 12 Gev/c Upgrade of CEBAF, Jan. 2005, http://www.jlab.org/12GeV/development.html. 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. 12 Gev/c Upgrade Project Conceptual Design and Safety Review of Superconducting Magnets, Sep. 2006, http://www.jlab.org/~brindza/SC Mag Rev -final/ John J. LeRose private communication, JLAB, January 2007, “Optics Study of SHMS using Raytrace and TOSCA fields” Arup Ghosh, Private communication, BNL, Report. August 2007 “Short Sample tests of SSC Cables for JLAB”. P.B. Brindza et al, “A Super Conducting 60 cm Warm Bore Cosine Theta Dipole Magnet for the Jefferson Lab Super High Momentum Spectrometer (SHMS)”, IEEE Magnetic Technology, submitted for publication. Bernhard Fischer and A. Hobl, private communication, Accel Inc. August 2007 C.Mayri et al, R3B-GLAD Technical Report CEA/Saclay reference I008406 ver.1, June 29,2006. 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