M. Wake

Superbunch Hadron Colliders

A novel concept of a high luminosity hadron collider is proposed. This would be a typical application of an induction synchrotron being newly developed. Extremely long bunches, referred to as superbunches, are generated by a multibunch stacking method employing barrier buckets at the injection into the collider and are accelerated with a step voltage induced in the induction gaps. Superbunches intersect with each other, yielding a luminosity of more than 10(35) cm(-2) sec(-1). A combination of vertical crossing and horizontal crossing must be employed in order to avoid any significant beam-beam tune shift.

Feasibility Study of

Feasibility study of Cu stabilized Nb3Al strand and Rutherford cable for the application to high field accelerator magnets are being done at Fermilab in collaboration with NIMS. The Nb3Al strand, which was developed and manufactured at NIMS in Japan, has a non-copper Jc of about 844 A/mm2 at 15 Tesla at 4.2 K, a copper content of 50%, and filament size of about 50 microns. Rutherford cables with 27 Nb3Al strands of 1.03 mm diameter were fabricated and tested. Quench tests on a short cable were done to study its stability with only its self field, utilizing a high current transformer. A pair of 2 meter long Nb3Al cables was tested extensively at CERN at 4.3 and 1.9 K up to 11 Tesla including its self field with a high transport current of 20.2 kA. In the low field test we observed instability near splices and in the central region. This is related to the flux-jump like behavior, because of excessive amount of Nb in the Nb3Al strand. There is possibility that the Nb in Nb3Al can cause instability below 2 Tesla field regions. We need further investigation on this problem. Above 8 Tesla, we observed quenches near the critical surface at fast ramp rate from 1000 to 3000 A/sec, with quench velocity over 100 m/sec. A small racetrack magnet was made using a 14 m of Rutherford cable and successfully tested up to 21.8 kA, corresponding to 8.7 T.

{\rm Nb}_{3}{\rm Al}

A 43.5 mm aperture dipole magnet with a nominal field of 11 T is being fabricated at Fermilab. The design is based on a two-layer shell-type coil structure made of Rutherford-type Nb/sub 3/Sn cable with wind and react technology. The mechanical support structure consists of vertically split iron yoke locked by two aluminum clamps and a 8 mm thick stainless steel skin. This paper summarizes the fabrication details of the first dipole model and test results from a 2110 mm long mechanical model.

Rutherford Cable for High Field Accelerator Magnet Application

Long Nb3Al strands with copper stabilizer are promising for future high field accelerator magnets. A 1.2 kilometer Nb3Al strand with Cu stabilizer was fabricated at the National Institute for Materials Science in Japan. Using this strand a 30 meter Cu stabilized Nb3Al Rutherford cable was made for the first time by a collaboration of NIMS and Fermilab. The Nb3Al strands extracted from cable with a relatively low packing factor showed almost no Jc degradation. But the extracted strands from the highly compacted cable showed some degradation in both Jc and n value, which may be caused by local separation of the copper stabilizer. Still, its Jc degradation is lower than that of typical Nb3Sn strands. The current limit due to magnetic instability in low field is about 500 A at 4.2 K. The magnetization of the strands, which was measured with balanced coils at 4.2 K, showed large flux jumps, usually around 1.5 T. This value is much larger than the Bc2 (4.2 K) of the Nb matrix, which is around 0.4 Tesla. The magnetic instability of the Nb3Al strand at low field is not completely understood, but it might be explained by the superconducting coupling current through the Nb matrix.

Fabrication of the shell-type Nb/sub 3/Sn dipole magnet at Fermilab

High field accelerator magnets for a future Very Large Hadron Collider are being developed at Fermilab in collaboration with LBNL and KEK. The goal of this work is to elaborate a cost-effective Nb/sub 3/Sn dipole magnet design and technology which provide the nominal field of 11 T. This paper presents a description of the magnetic design of the first short model including cable and strand parameters, coil and yoke cross-section and magnet end geometry. The results of field distribution and field quality calculations including geometrical and random harmonics, coil magnetization and iron saturation effects are also reported.

Characteristics of Round and Extracted Strands of

Abstract Current leads for superconducting magnets are studied to simplify the design into a single chart. A new parameter, obtainable with a brief calculation, is introduced. Characteristics of a wide variety of current leads, namely geometric dimension, heat flow into liquid helium and pressure drop of the cooling gas flow are represented in the form of a single design chart by using this parameter. Current leads can be optimized without explicit consideration of quantities such as cross-sectional area and length of conductor, heat transfer coefficient and cooling perimeter. As the burn-out of current leads is expected to occur with the accidental decrease or stoppage of cooling gas flow, this serious situation must be taken into account in the design. An adiabatic approximation is used to estimate the temperature rise. Further improvement of the performance can be obtained if the cross-sectional area decreases along the length of the conductor.

{\hbox{Nb}}_{3}\hbox{Al}

Abstract The electric field on the outermost shell of superconducting filaments depends directly on the current distribution inside a composite. Calculations are carried out taking into account the axial diffusion due to the resistivity of the matrix. Moreover, the critical state model is modified to account for the dependence of the local current density on the electric field. It is shown that a simple self-field measurement is a very good way to evaluate the average transverse resistivity in a multi-filament composite. Our samples constructed of very fine filaments show a resistivity in the range of a few 10 −9 ϒ m, that is, much higher resistivity than that of the copper matrix itself. This resistivity decreases as the filament diameter increases. This fact shows that there exists a high resistive barrier at the interface between the superconductor and the copper.

Rutherford Cable

The Fermilab Mu2e experiment seeks to measure the rare process of direct muon to electron conversion in the field of a nucleus. Key to the design of the experiment is a system of three superconducting solenoids; a muon production solenoid (PS) which is a 1.8 m aperture axially graded solenoid with a peak field of 5 T used to focus secondary pions and muons from a production target located in the solenoid aperture; an “S shaped” transport solenoid (TS) which selects and transports the subsequent muons towards a stopping target; a detector solenoid (DS) which is an axially graded solenoid at the upstream end to focus transported muons to a stopping target, and a spectrometer solenoid at the downstream end to accurately measure the momentum of the outgoing conversion electrons. The magnetic field requirements, the significant magnetic coupling between the solenoids, the curved muon transport geometry and the large beam induced energy deposition into the superconducting coils pose significant challenges to the magnetic, mechanical, and thermal design of this system. In this paper a conceptual design for the magnetic system which meets the Mu2e experiment requirements is presented.

Magnetic design of the Fermilab 11 T Nb/sub 3/Sn short dipole model

A quench program using ANSYS is developed for the high field collider magnet for 3-D analysis. Its computational procedure is explained. The quench program is applied to a one meter Nb/sub 3/Sn high field model magnet, which is epoxy impregnated. The quench simulation program is used to estimate the temperature and mechanical stress inside the coil as well as over the whole magnet. It is concluded that for the one meter magnet with the presented cross section and configuration, the thermal effects due to the quench is tolerable. But we need much more quench study and improvements in the design for longer magnets.

Optimization method for superconducting magnet current leads

The most recent results in an ongoing study of the influence of interstrand crossover contact resistance, Rc, strand coating, and cable design, on coupling (eddy current) loss in NbTi-strand Rutherford cables for advanced accelerator applications are presented and discussed. Inductive measurements of AC loss in six-layer stacks of Rutherford cable have been made with the applied field both normal to and parallel to the plane of the cable. Cables studied had bare-Cu, Ni-plated, and stabrite-coated strands; the latter were also furnished with metallic or insulating interlayers (cores) of, respectively, unalloyed Ti, stainless steel, and kapton ribbon. The cable packs were cured under pressure (-90 MPa) at temperatures of 150 to 250°C. After pressure release the AC loss was measured under pressures of 0, 36, and 78 MPa. Lowest coupling loss was obtained with bare Cu cable provided the curing temperature was kept below 200°C. The stabrite cable, which exhibited relatively low loss when cured below 170°C and measured under zero pressure, became extremely lossy under 36 MPa and even more so under 78 MPa. Insertion of any of the ribbon interlayers into the stabrite cable tended to eliminate the Rc-based coupling loss, resulting in a cable whose AC loss was small, controllable, and independent of final pressure.