Abstract-Magnet programs at BNL, LBNL and FNAL have observed instabilities in high J c Nb 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 10T) cannot sustain these same currents at low fields (1-3T) 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.

Abstract-The Nb 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 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 crosssection 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 costeffective block-coil design for future high-field, acceleratorquality dipoles.Index Terms-High-field accelerator magnets, Nb 3 Sn.

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instruction, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to FUNDING NUMBERS PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)Naval Postgraduate School Monterey, CA 93943-5000 PERFORMING ORGANIZATION REPORT NUMBER SPONSORING /MONITORING AGENCY NAME(S) AND ADDRESS(ES)N 12b. DISTRIBUTION CODE ABSTRACT (maximum 200 words)Strengthening of composite joints is a topic of recent research. The benefits of using locally applied carbon nanotubes to reinforce a carbon fiber composite joint were studied. The effect of carbon nanotubes on enhancing the fracture toughness and joint interface strength was investigated by performing Mode I, Mode II, and Mixed Mode I/Mode II fracture with and without carbon nanotubes applied locally at the joint interface. Furthermore, the effects of seawater absorption on Mode II fracture were investigated. Finally, an optimization of carbon nanotube concentration was performed. During the study, the image correlation technique was used to examine the fracture mechanisms altered by the introduction of carbon nanotubes. The experimental study showed that carbon nanotubes can increase the fracture toughness of the composite interface significantly, especially for Mode II, including a physical change in the fracture mechanism. NUMBER OF PAGES 70 SUBJECT TERMS ABSTRACTStrengthening of composite joints is a topic of recent research. The benefits of using locally applied carbon nanotubes to reinforce a carbon fiber composite joint were studied. The effect of carbon nanotubes on enhancing the fracture toughness and joint interface strength was investigated by performing Mode I, Mode II, and Mixed Mode

Abstract-The U.S. LHC Accelerator Research Program (LARP), a collaboration between BNL, FNAL, LBNL, and SLAC, has among its major objectives the development of advanced magnet technology for an LHC luminosity upgrade. The LBNL Superconducting Magnet Group supports this program with a broad effort involving design studies, Nb 3 Sn conductor development, mechanical models, and basic prototypes. This paper describes the development of a large aperture Nb 3 Sn racetrack quadrupole magnet using four racetrack coils from the LBNL Subscale Magnet (SM) Program. The magnet provides a gradient of 95 T/m in a 110 mm bore, with a peak field in the conductor of 11.2 T. The coils are pre-stressed by a mechanical structure based on a pre-tensioned aluminum shell, and axially supported with aluminum rods. The mechanical behavior has been monitored with strain gauges and the magnetic field has been measured. Results of the test are reported and analyzed.

The Lawrence Berkeley National Laboratory (LBNL) Superconducting Magnet Group has completed the design, fabrication and test of HD1, a 16 T block-coil dipole magnet. State of the art Nb 3 Sn conductor was wound in double-layer racetrack coils and supported by an iron yoke and a tensioned aluminum shell. In order to prevent conductor movement under magnetic forces up to the design field, a coil pre-stress of 150 MPa was required. To achieve this level without damaging the brittle conductor, the target stress was generated during cool-down to 4.2 K by exploiting the thermal contraction differentials between yoke and shell. Accurate control of the shell tension during assembly was obtained using pressurized bladders and interference load keys. An integrated 3D CAD model was used to optimize magnetic and mechanical design and analysis.Index Terms-Nb 3 Sn, superconducting accelerator magnet.