D. ter Avest

Observation of a periodic pattern in the persistent-current fields of the superconducting HERA magnets

During an experimental study of the time dependence of a HERA dipole, it was found that the sextupole field exhibits a sinusoidal structure along the axis of the magnet. A similar periodic structure was found for the main dipole field with the help of a nuclear magnetic resonance probe. The wavelength of the periodic pattern is compatible with the transposition pitch of the Rutherford-type cable in the magnet coils. The effect is related to superconducting currents in the cable because it vanishes when the magnet is warmed up above its critical temperature. The structure was found to exist in all HERA dipoles measured afterwards ( approximately 10) and also in a superconducting coil without iron yoke. With a specially developed 2-cm-long pickup coil, it was found that all accessible multipole components in dipole and quadrupole magnets are modulated along their axis.<<ETX>>

Experimental results of thermally controlled superconducting switches for high frequency operation

As part of a study to develop thermally controlled switches for use in superconducting rectifiers operating at a few hertz and 1 kA, a theoretical model is presented of the thermal behavior of such a switch. The calculations are compared with experimental results of several switches having recovery times between 40 and 200 ms. A discussion is given of the maximum temperature T/sub N/ that occurs in the normal regions when the switch is in the resistive state. Once T/sub N/ is known, it is possible to predict the recovery time, activation energy, stationary dissipation and minimum propagation current. The calculated and measured results, in good agreement, show that T/sub N/ is approximately 12 K and largely independent of the thickness or material of the insulation layer. Mention is made of some problems, related to the room-temperature equipment which drives the rectifier, that so far have prevented the rectifiers from being used at their design specifications. >

Development of an experimental 10 T Nb/sub 3/Sn dipole magnet for the CERN LHC

An experimental 1-m long twill aperture dipole magnet developed using a high-current Nb/sub 3/Sn conductor in order to attain a magnetic field well beyond 10 T at 4.2 K is described. The emphasis in this Nb/sub 3/Sn project is on the highest possible field within the known Large Hadron Collider (LHC) twin-aperture configuration. A design target of 11.5 T was chosen.

Analysis of the mechanical behaviour of an 11.5 T Nb/sub 3/Sn LHC dipole magnet according to the ring collar concept

According to the CERN-LHC (Large Hadron Collider) reference design, 10-tesla twin-aperture NbTi dipoles will be built with split collars that enclose both apertures. As part of the development program towards an experimental 11.5-tesla Nb/sub 3/Sn LHC dipole magnet, the mechanical implications of an alternative collar concept have been studied with a finite element analysis. In this concept ring shaped collars are shrunk on each finished single aperture coil, thus providing the necessary room-temperature prestress. This system results in a major improvement of the stress distribution in the collars. It is noted that introduction of friction at the sliding planes can cause reopening of the gap between the yoke halves during excitation. This depends strongly on the value of the friction coefficients. >

Three-dimensional computation of magnetic fields and Lorentz forces of an LHC dipole magnet using the method of image currents

Magnetic fields and Lorentz forces of an LHC dipole magnet are calculated using the method of image currents to represent the effect of the iron shield. The calculation is performed for coils of finite length using a parametrization for coil heads of constant perimeter. A comparison with calculations based on POISSON and TOSCA is made.

Predesign study of a 4-5 T superconducting wiggler magnet for the ESRF

The ESRF is currently setting up a beam line for very hard photons well above 250 keV. This requires the installation of a high field three pole wavelength shifter. The nominal and target fields of the wiggler magnet are 4 and 5 tesla respectively while the nominal field integral over the central pole is 0.256 T-m at 4 T. The proposed magnet is a system of superconducting main and two side pole coils connected in series. An additional superconducting correction coil system enables to tune the field integral. The coils, made of high performing NbTi, are enclosed and prestressed in an aluminium coil housing. About 15% field enhancement, shielding and force balancing is achieved with two iron plates sandwiching the coil housing. The cooling concept is a cryostat with a closed helium re-condensation system. Cooling is provided by a mechanical hybrid GM and JT refrigerator with a cooling capacity of about 2.5 W at 4.4 K. The paper reports on the design concepts and optimizations and a preferred solution is presented.

Optimizing the conductor dimensions for a 10-13 T superconducting dipole magnet (for accelerators)

A method to obtain systematic solutions for the conductor dimensions and their layout is described. From these solutions a selection can be made based on a number of practical requirements. The inclusion of a measured J/sub max/(B) relation of the superconductor in order to make the design practical has a large effect on the geometry. A dipole magnet with cables of equal width is not necessarily the best solution; it is often better to have a broader inner-layer cable. The volume of superconductor appeared to vary only within a few percent in the systematic solutions and is therefore not a relevant criterion. In the case of a 13-T dipole magnet and using the J/sub max/(B) relation described, more than two layers of conductors must be used in order to keep the design within practical limits. It appears to be difficult to obtain a convenient solution for a 13-T magnet, although the number of geometrical variants is very high.

A New Method to Calculate Conductor Magnetization in Accelerator Dipoles

During the ramping of superconducting dipole magnets as used in particle accelerators as the proposed SSC or CERN-LHC, magnetization in the conductor will manifest itself, perturbing the field homogeneity of the dipole. A numerical method was developed that covers the calculation of magnetic field inside the conductors, keystoned cables, current redistribution due to the variation of magnetic field with time as well as the multipole distribution in the dipole. For the calculation of current redistribution in a cable a keystoned cable geometry has been assumed, although the method applies to any geometry. The model incorporates saturation in the cable. In the case of an unsaturated cable the current redistribution can be described completely analytically. Also the model allows the calculation of the interstrand coupling loss as a function of the time-varying magnetic field. Finally, field remanency due to persistent currents in the filaments is described.

Magnetic fields and Lorentz forces in an LHC-dipole magnet; 3D analysis using the FEM program TOSCA

This paper describes an investigation of magnetic fields and lorentz forces in the straight section and the coil end of an LHC-dipole magnet using the finite element program TOSCA. Starting from an existing design of the straight section the constant perimeter description was used to generate the coil end geometry. Results will be shown of the local magnetic field and lorentz force in the coil end.

Numerical Analysis of the Voltage Current Transition in Superconducting Cables

A new method is presented to calculate the voltage current transition in a superconducting cable using any E(J) relation for the superconductor in the cable. The method gives the possibility of comparing the voltage current transition in a superconducting wire with the transition in a cable composed of a number of these wires. Results are presented using a distribution function g(J) to describe the relation E(J). Differences in the transition of wires and cables will be shown as a function of a number of parameters.