A.F. Lietzke

The use of pressurized bladders for stress control of superconducting magnets

LBNL is using pressurized bladders in its high field superconducting magnet program Magnet RD3; a 14 T race track dipole, has been assembled and pre-stressed using such a system. The bladder, placed between the coil pack and the iron yoke, can provide 70 MPa of pressure while compressing the coil pack and tensioning a 40 mm thick structural aluminum shell. Interference keys replace the bladders functionality as they are deflated and removed leaving the shell in 140 MPa of tension. During cool down, stress in the shell increases to 250 MPa as a result of the difference in thermal expansion between the aluminum shell and the inner iron yoke. A number of strain gauges mounted onto the shell were used to monitor its strain during assembly, cool-down and testing. This technique ensures that the final and maximum stress in the shell is reached before the magnet is ever energized. The use of a structural shell and pressurized bladders has simplified magnet assembly considerably. In this paper we describe the bladder system and its use in the assembly of a 14 T Nb/sub 3/Sn magnet.

Magnet RaD for the US LHC Accelerator Research Program (LARP)

TUA2OR6 Magnet RD fax: 510-486-5310; e-mail: sagourlay@lbl.gov). G. Ambrosio, N. Andreev, E. Barzi, R. Bossert, V. S. Kashikhin, V. V. Kashikhin, F. Nobrega, I. Novitsky, D. Turrioni, R. Yamada, and A.V. Zlobin are with Fermilab National Accelerator Laboratory, Batavia, IL 3 M. Anerella, A. Ghosh , , R. Gupta, M. Harrison, J. Schmazle, and P. Wanderer are with Brookhaven National Laboratory, Upton, NY.

An approach for faster high field magnet technology development

J 2LC02 SC-MAG#773 LBNL-49918 An Approach for Faster High Field Magnet Technology Development R. R. Hafalia, S. Caspi, L. C hiesa, M . Coccoli , D.R. Die tde rich, S. A. Gourlay, A.F. Lietzke, l .W. ONeill, G. Sabbi, and R.M. Scanlan Abstract- The Superconducting Magnet Program at LBNL has developed a magnet design supporting our new Subscale Magnet Program, that facilitates rapid testing of small superconducting racetrack coils in the field range of 10·12 Tesla. Several coils have been made from a variety of NbJSnlCu cables, insulated, wound, reacted, potted and assembled into a small reusable yoke and shell loading structure. Bladder and key technology have provided a rapid, efficient means for adjusting coil pre-stress during both initial assembly, and between thermal cycles. This affords the opportunity to test moderately long cable samples under magnet conditions on a time scale considerably closer to that for traditional short-sample cable tests. We have built and tested four coils with the initial aim of determining the feasibility of reducing overall conductor costs with mixed-strand cables. Details of cost reduction improvements, coil construction, magnet structure, and assembly procedures are reported, along with the relative performance of the mixed-strand coil. II. THE CONDUcrOR The Subscale Magnet Program utilizes superconducting Nb3Sn strands produced by Oxford and lac. T he cables were wou nd with 20 strands ofO.7mm diameter. The nominal cable cross-section used by the Subscale Magnet Program is 7.9mm x 1.3mm. The cable is sheathed in a continuous, woven, fiberglass sleeve. Coil modules are wound in 2 layers in flat racetrack coil configuration around an iron pole-island - each layer having 22 turns each. (Fig. 1). Each double-layer coi l module requires 22m of cable (5 kg). The two baseline coil modules (SCOI & SC02) were used in the inaugural test of the new Subsea Ie Magnet Test Faci lity. They were both wound with Nb3Sn cable - with the same strand as was used in LBLs 14.7 Tesla RD-3B large-scale magnet. The winding and assembly, prior to reacti on, took about 2-3 days. Index Terms- Common Coil Magnet, mixed-strand cable, Nb)Sn, racetrack coil. I. I NTRODUcrlON awrence Berkeley National Laboratory has implemented a Subscale Magnet Program. The new Subscale Magnet Test Facility was fabricated to supporJ the program. The facility includes a 38 lmm-lD x - 1524mm-deep vertical cryostat, wi th supporJ structure and magnet supporJ header. T he program tests the performance of various types of superconducting cables in a medium-to-high field environment as well as investigates magnet structural design modifications, L in small scale, before implementing them into our large-scale program. The magnet structural components were mass- Fig. I . 2 layers wound around iron pole. prod uced and standardized for rapid coil mod ule assembly. Three of the four coil modules initially built (two baseline coils, one mi x.ed strand coil and a comparison coil), have been tested. Presently, two more coil modul es with mi xed Nb3SnlCu strand coil variations have been reacted and are presently being assembled. Also, a coil mod ule with a new woven ceramic insulator is being reacted. Manuscript received August 6, 2002. Thi s was supported by the Director, Office of Energy Research, O ffi ce of High Energy and N uclear Physics, Hi gh Energy Physics D ivision , U. S. Department of Energy. under Contract No. The next coil module (SC03) was wound with a mixed- strand cable consisting of 2 1 strands - 14 Nb3Sn strands cabled with 7 strands of pure copper. Due to the difference in elastic modulus between the SC and the Cu strands, decabling was evident throughout the whole length of the cable. Coil winding tension was cut in half, from 178N to 89N, to minimize decabling. Even with the lower tension, popped strands were observed. DE-AC03--76SFO0098. R.R. Hafalia is with the Lawrence Berkeley National Lab. Berkeley, CA 94720 USA (telephone: 5 10-486-5712, (e-rnail: rrhafalia @Ibl.gov). Coi l module SC04 was wound with 20 Nb3Sn strands and was intended as a comparison to the SC03 mixed strand modul e.

Correlation between strand stability and magnet performance

Magnet programs at BNL, LBNL and FNAL have observed instabilities in high J/sub c/ Nb/sub 3/Sn strands and magnets made from these strands. This paper correlates the strand stability determined from a short sample-strand test to the observed magnet performance. It has been observed that strands that carry high currents at high fields (greater than 10 T) cannot sustain these same currents at low fields (1-3 T) when the sample current is fixed and the magnetic field is ramped. This suggests that the present generation of strand is susceptible to flux jumps (FJ). To prevent flux jumps from limiting stand performance, one must accommodate the energy released during a flux jump. To better understand FJ this work has focused on wire with a given sub-element diameter and shows that one can significantly improve stability by increasing the copper conductivity (higher residual resistivity ratio, RRR, of the Cu). This increased stability significantly improves the conductor performance and permits it to carry more current.

Test results for a high field (13 T) Nb/sub 3/Sn dipole

A Nb/sub 3/Sn dipole magnet (D20) has been designed, constructed, and tested at LBNL. Previously, we had reported test results from a hybrid design dipole which contained a similar inner Nb/sub 3/Sn and outer NbTi winding. This paper presents the final assembly characteristics and parameters which will be compared with those of the original magnet design. The actual winding size was determined and a secondary calibration of the assembly pre-load was done by pressure sensitive film. The actual azimuthal and radial D20 pre-loading was accomplished by a very controllable novel stretched wire technique. D20 reached 12.8 T (4.4 K) and 13.5 T (1.8 K) the highest dipole magnetic fields obtained to date in the world.

Test Results of LARP Nb3Sn Quadrupole Magnets Using a Shell-based Support Structure (TQS)

Amongst the magnet development program of a large-aperture Nb3Sn superconducting quadrupole for the Large Hadron Collider luminosity upgrade, six quadrupole magnets were built and tested using a shell based key and bladder technology (TQS). The 1 m long 90 mm aperture magnets are part of the US LHC Accelerator Research Program (LARP) aimed at demonstrating Nb3Sn technology by the year 2009, of a 3.6 m long magnet capable of achieving 200 T/m. In support of the LARP program the TQS magnets were tested at three different laboratories, LBNL, FNAL and CERN and while at CERN a technology-transfer and a four days magnet disassembly and reassembly were included. This paper summarizes the fabrication, assembly, cool-down and test results of the six magnets and compares measurements with design expectations.

Design of HD2: a 15 tesla Nb/sub 3/Sn dipole with a 35 mm bore

The Nb/sub 3/Sn dipole HD1, recently fabricated and tested at LBNL, pushes the limits of accelerator magnet technology into the 16 T field range, and opens the way to a new generation of HEP colliders. HD1 is based on a flat racetrack coil configuration and has a 10 mm bore. These features are consistent with the HD1 goals: exploring the Nb/sub 3/Sn conductor performance limits at the maximum fields and under high stress. However, in order to further develop the block-coil geometry for future high-field accelerators, the bore size has to be increased to 30-50 mm. With respect to HD1, the main R&D challenges are: (a) design of the coil ends, to allow a magnetically efficient cross-section without obstructing the beam path; (b) design of the bore, to support the coil against the pre-load force; (c) correction of the geometric field errors. HD2 represents a first step in addressing these issues, with a central dipole field above 15 T, a 35 mm bore, and nominal field harmonics within a fraction of one unit. This paper describes the HD2 magnet design concept and its main features, as well as further steps required to develop a cost-effective block-coil design for future high-field, accelerator-quality dipoles.

Recent Test Results of the High Field

The 1 m long Nb3Sn dipole magnet HD2, fabricated and tested at Lawrence Berkeley National Laboratory, represents a step towards the development of block-type accelerator quality magnets operating in the range of 13-15 T. The magnet design features two coil modules composed of two layers wound around a titanium-alloy pole. The layer 1 pole includes a round cutout to provide room for a bore tube with a clear aperture of 36 mm. After a first series of tests where HD2 reached a maximum bore field of 13.8 T, corresponding to an estimated peak field on the conductor of 14.5 T, the magnet was disassembled and reloaded without the bore tube and with a clear aperture increased to 43 mm. We describe in this paper the magnet training observed in two consecutive tests after the removal of the bore tube, with a comparison of the quench performance with respect to the previous tests. An analysis of the voltage signals recorded before and after training quenches is then presented and discussed, and the results of coil visual inspections reported.

{\rm Nb}_{3}{\rm Sn}

As a first step toward the development of a large-aperture Nb3Sn superconducting quadrupole for the Large Hadron Collider (LHC) luminosity upgrade, two-layer technological quadrupole models (TQS01 at LBNL and TQC01 at Fermilab) are being constructed within the framework of the US LHC Accelerator Research Program (LARP). Both models use the same coil design, but have different coil support structures. This paper describes the TQC01 design, fabrication technology and summarizes its main parameters

Dipole Magnet HD2

The U.S. LHC Accelerator Research Program (LARP) has started the fabrication of 3.7-m long Nb3Sn quadrupole models. The Long Quadrupoles (LQ) are ldquoProof-of-Principlerdquo magnets which are to demonstrate that Nb3Sn technology is mature for use in high energy particle accelerators. Their design is based on the LARP Technological Quadrupole (TQ) models, developed at FNAL and LBNL, which have design gradients higher than 200 T/m and an aperture of 90 mm. The plans for the LQ R&D and a design update are presented and discussed in this paper. The challenges of fabricating long accelerator-quality Nb3Sn coils are presented together with the solutions adopted for the LQ coils (based on the TQ experience). During the fabrication and inspection of practice coils some problems were found and corrected. The fabrication at BNL and FNAL of the set of coils for the first Long Quadrupole is in progress.