J.M. van Oort

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.

Development of a high gradient quadrupole for the LHC interaction regions

A collaboration of Fermilab, Lawrence Berkeley National Laboratory and Brookhaven National Laboratory is engaged in the design of a high gradient quadrupole suitable for use in the LHC interaction regions. The cold iron design incorporates a two-layer, cos(2/spl theta/) coil geometry with a 70 mm aperture operating in superfluid helium. This paper summarizes the progress on a magnetic, mechanical and thermal design that meets the requirements of maximum gradient above 250 T/m, high field quality and provision for adequate cooling in a high radiation environment.

Critical current degradation in Nb/sub 3/Sn cables under transverse pressure

The critical current degradation of a few sample Rutherford-type Nb/sub 3/Sn cables is investigated as a function of transverse pressure. A comparison is made between Nb/sub 3/Sn strands produced by the powder-in-tube, bronze, and modified jelly roll processes. The (keystoned) Rutherford cables are charged at 11 T under transverse pressures up to 250 MPa. Large differences in critical current reduction are observed, ranging from 6 to about 60% at 200 MPa, depending on the type of Nb/sub 3/Sn. It appears that about 40% of the total reduction is irreversible. Moreover, the irreversible part shows relaxation, and a partial recovery is possible by thermal cycling.<<ETX>>

The reduction of the critical current in Nb/sub 3/Sn cables under transverse loads

The degradation of the critical current of impregnated Rutherford type Nb/sub 3/Sn cables was investigated as a function of the applied transverse load and magnetic field. The cable is made of modified jelly-roll-type strand material and has a keystone angle of 1.0 degrees . The voltage-current characteristics were determined for the magnetic field ranging from 2 to 11 T and transverse pressure up to 250 MPa on the cable surface. It was found that the 48-strand cable, made of strands with six elements in the matrix, showed a larger critical current degradation than the 26-strand cable with 36 elements per strand. The global degradation of the 48-strand cable was 63% at 150 MPa, and 40% at 150 MPa for the 26-strand cable. Microanalysis of the cross-section before and after compression is presented, showing significant permanent damage to the superconducting strands.<<ETX>>

A fiber optics sensor for strain and stress measurements in superconducting accelerator magnets

A novel cryogenic interferometric fiber optics sensor for the measurement of strain and stress in the coil windings of superconducting accelerator magnets is described. The sensor can operate with two different readout sources, monochromatic laser light and white light respectively. The sensor head is built up as an extrinsic Fabry-Perot interferometer formed with two cleaved fiber surfaces, and can be mounted in several configurations. When read with laser light, the sensor is an extremely sensitive relative strain or temperature detector. When read with white light the absolute strain and pressure can be measured. Results are presented of tests in several configurations at 77 K and 4.2 K, both for the relative and absolute readout method, Finally, the possible use for quench localization using the temperature sensitivity is described. >

Test results for a Nb/sub 3/Sn dipole magnet

A cosine theta type dipole magnet using Nb/sub 3/Sn conductor have been designed, built and tested. D19H is a two-layer dipole magnet with a Nb/sub 3/Sn inner layer and a recycled NbTi outer layer. Coil-pairs are connected with two of the four Nb/sub 3/Sn splices in a high field region, and compressed by a ring and collet system. The ramp-rate sensitivity and the splice resistances were pleasingly low; and the 4.4 K training was rapid. At 1.8 K, however, the unusually high frequency of outer-coil fast-motion events increased with current, effectively creating a training-ceiling at 90% of the expected outer-layer limit (10.2 T). A low end-load applied to a relatively fluffy outer layer is believed to have caused this training limit. The end-load was increased; but a retest was aborted after the magnet failed a precautionary hipot test.

Fabrication and component testing results for a Nb/sub 3/Sn dipole magnet

At present, the maximum field achieved in accelerator R&D dipoles is slightly over 10 T, with NbTi conductor at 1.8 K. Although Nb/sub 3/Sn has the potential to achieve much higher fields, none of the previous dipoles constructed from Nb/sub 3/Sn have broken the 10 T barrier. We report here on the construction of a dipole with high current density Nb/sub 3/Sn with a predicted short sample limit of 13 T. A wind and react technique, followed by epoxy impregnation of the fiberglass insulated coils, was used. The problems identified with the use of Nb/sub 3/Sn in earlier dipole magnets were investigated in a series of supplemental tests. This includes measurement of the degradation of Jc with transverse strain, cabling degradation, joint resistance measurements, and epoxy strength tests. In addition, coil assembly techniques were developed to ensure that adequate prestress could be applied without damaging the reacted Nb/sub 3/Sn cable. We report here the results of these tests and the construction status of this 50 mm bore dipole.<<ETX>>

Design of a 16 T Nb/sub 3/Sn twin bore accelerator dipole with a window-frame conductor layout

A simplified design study of a 16 T Nb/sub 3/Sn twin bore accelerator dipole magnet is presented. The philosophy behind the study is to design a high field magnet with a coil structure optimized for a reasonable Lorentz-load and ease of construction. The coils are of the rectangular window-frame type with modular flat pancake windings, thus eliminating the need for complex coil return ends. The magnetic and structural design is presented and a comparison is made with existing cell layouts for high field magnets.<<ETX>>

Two alternate high gradient quadrupoles; an upgraded Tevatron IR and a "pipe" design

The present Fermilab IR quadrupole lenses achieve a 50% increase in gradient over their predecessors. This was accomplished by the following developments: (a) a more dense winding (Kapton insulation only, eliminating the fiberglass-epoxy), (b) an improvement in critical current density to J/sub c/(5T, 4.2K)=3 kA/mm/sup 2/ and (c) a higher aspect ratio cable >11/1 and (d) finer strand (0.53 mm diameter). The natural evolution of this design to the CERN 70 mm aperture quadrupole results in the following: (a) a reduction in operational temperature (4.8K-1.9K), (b) improvement of J/sub c/(10 T, 2.0 K)>2 kA/mm/sup 2/ and (c) a higher aspect ratio cable >13/1 and (d) finer strand (0.48 mm diameter). These two-layer designs should achieve the required operational gradient of 250 T/m with at least a 5% margin on the short-sample load-line intersection. These high current densities also require turn-off times <0.2 s after transition to the normal state. Such a short time requires an advanced protection heater design. The status of the design and the experimental development will be reported. New design concept quadrupole calculations are presented as well. This concept, using an active flux return is being proposed as a possible candidate design for a future higher gradient requirement.

The pipe-quadrupole, an alternative for high gradient interaction region quadrupole designs

In the design of interaction region (IR) quadrupoles for high luminosity colliders such as the LHC or a possible upgrade of the Tevatron, the radiation heating of the coil windings is an important issue. Two obvious solutions to this problem can be chosen. The first is to reduce the heat load by added shielding, increased cooling with fins or using Nb/sub 3/Sn to increase the temperature margin. The second solution eliminates the conductor from the areas with the highest radiation intensity, which are located on the symmetry-axes of the midplanes of the coils. A novel quadrupole design is presented, in which the conductor is wound on four half-moon shaped supports, forming elongated toroid sections. The assembly of the four shapes yields a quadrupole field with an active flux return path, and a void in the high radiation area. This void can be occupied by a liquid helium cooling pipe to lower the temperature of the windings from the inside. The coil layout, harmonic optimization and mechanical design are shown, together with the calculated temperature rise for the radiation load of the LHC interaction region quadrupoles.