J. Billan

Design and test of the benches for the magnetic measurement of the LHC dipoles

The magnetic measurement of more than 1300 LHC dipoles comprises the content of higher harmonic field components, field direction and field integrals. The measurements will be carried out along a warm bore installed inside the magnet cold bore, thus allowing the use of rotating coils at room temperature. This coil, together with Hall and NMR detectors is mounted at one end of a 12.5 m long shaft which is specially designed for very high rotational stiffness and which is controlled from its far end by a motor, an angular encoder and a level meter, all standard components placed outside the magnetic field without space restrictions. Particular emphasis has been put on the user-friendliness of the bench and its automated, computer-controlled operation requiring a minimum of staff, an important issue during production measurements of large series of magnets. The bench and its performance and precision achieved during its commissioning are described. >

The eight superconducting quadrupoles for the ISR high-luminosity insertion

Eight superconducting quadrupoles for a high-luminosity insertion in the ISR have been produced by industrial firms according to CERN design and manufacturing specifications, and assembled and tested at CERN. The horizontal cylindrical cryostats, which contain windings and steel yoke in a boiling helium bath, have a 173 mm warm bore. For 31 GeV beam energy, the maximum operating gradient on the quadrupole axis is 43 T m−1 and the maximum field in the windings is 5.5 T. Sextupole windings provide a linear variation of the gradient of up to 4% over the bore width and dodecapole windings trim the field pattern as a function of excitation. This paper reports about production history, acceptance tests, and performance. The results of magnetic measurements are also summarized. The insertion will be installed into the ISR as from August 1980.

Test of 1 m long model magnets for LHC

Power tests and magnetic measurements have been performed on various 1-m-long LHC (Large Hadron Collider) dipole model magnets. Single- and twin-aperture magnets were energized up to peak fields above 9 T. Quench behavior, improvements in the test facility, and losses in the superconductors were examined. The field quality was measured with rotating coils immersed directly in the 1.8 K bath of the vertical cryostat. The design of these coils and the achieved measurement precision are presented. Preliminary measurements of the persistent currents show an unexpected time-dependent decay of the field harmonics. >

A Superconducting High-Luminosity Insertion in the Intersecting Storage Rings (ISR)

A focusing insertion will be constructed at an intersection of the ISR, to achieve a reduction of the effective beam height by about a factor 6 in the interaction diamond and a corresponding increase of the luminosity. It will contain eight superconducting quadrupole magnets, capable of producing gradients of 43 Tm-1 in a warm bore of 173 mm diameter: four of them have a magnetic length of 1.15 m and four of 0.65 m. The magnets are also equipped with sextupole windings, to match the gradients to particle momenta over the beam width, and with dodecapole correction windings. After construction and successful operation of a prototype magnet in the laboratory, competitive tenders were obtained from industry on the basis of technical specifications and manufacturing drawings. The magnets proper and the cryostats are being manufactured under two separate contracts. The insertion will be installed within 1980 at intersection I8, for use in physics experiments with a large magnetic spectrometer.

Ambient temperature field measuring system for LHC superconducting dipoles

Performs acceptance tests including field measurements of the collared coils assembly of the LHC superconducting dipoles to estimate, at an early production stage, the possible significant deviations from the expected multipole component value of these magnets. A sensitive measuring probe and efficient data acquisition are the consequence of a low magnetizing current necessary to limit the coil heating. This demands a high signal sensitivity and an enhanced signal-to-noise ratio to retrieve the higher multipole component. Moreover, the correlation with the multipole content of the magnets at cryogenic temperature and nominal excitation current need to be identified before the manufacturing process may continue. The field probe of the mole-type is equipped with three radial rotating search coils, an angular encoder and gravity sensor. It has been designed to slide inside the bore of the dipole coils and to measure the local field at fixed positions. The field analysis resulting in terms of multipole components, field direction and field integrals, measured on four 10 m long, twin-aperture LHC dipole prototypes, will be described together with the performance of the measuring method.

Influence of mortar-induced stresses on the magnetic characteristics of the LEP dipole cores

The steel-concrete cores of the Large Electron Positron colliding-beam accelerator dipole magnets are built of regularly spaced soft-magnetic-steel laminations, the spaces being filled with cement mortar. The effect of compressive stresses on the permeability and the coercivity of soft magnetic steel has been measured and the consequences for magnetic characteristics of the cores have been evaluated. By controlled straining of the aged cores in a hydraulic device, the tension in the mortar is brought locally to exceed the rupture limit. Thus, the compressive stresses in the steel laminations are partially relieved and the loss of bending strength in the dipoles is reduced. The excitation characteristics of the cores are measured by means of a fixed set of excitation windings and measuring coils into which the cores are moved after stress relieving. These measurements include determination of the equivalent surface areas of flux loops, which are embedded in the lower poles and will permit measurement of the field integral on the center line during LEP operation. >

Operational Experience with the Superconducting High-Luminosity Insertion in the CERN Intersecting Storage Rings (ISR)

The eight superconducting quadrupoles and their cryogenic equipment for this insertion were installed in the ISR at the end of 1980. The insertion has been used to assess the problems of running a superconducting insertion in a storage ring as well as to provide high luminosity for physics. The luminosity is increased at intersection 8 by a factor of 7. By means of dedicated collimators and orbit corrections, safe working conditions could be established for the superconducting magnets during injection, accumulation, stable beam periods and when dumping the beams. Quenches were mainly caused by large accidental beam losses. Operating parameters for all standard beam energies, including acceleration to 31.4 GeV/c, have been established. At 26.6 GeV/c, with currents of 30.6 A in ring 1 and 30.3 A in ring 2, a luminosity of 1.4 1032 cm-2s-1 was obtained in the insertion. This is the highest luminosity reached so far in storage rings and it was obtained during a physics run. Satisfactory beam conditions could also be provided for antiproton physics at 26.6 GeV/c in ISR with both low-ß insertions on, in I1 and I8, respectively.

On the Way to the Series Production of Steel-Concrete Cores for the LEP Dipole Magnets

CERN has now launched the series production of the steel-concrete cores for the LEP dipole magnets after a three-year development programme during which 14 prototypes were built in collaboration with industry. This paper describes the mechanical, geometrical and magnetic characteristics of these cores and the fabrication techniques which have been developed. Particular emphasis is put on the following points: - the controlled mixing of steel in order to reduce the field variations between cores. - the influence of the thickness and of the pitch of the laminations on the field quality, - the composition and working conditions of the mortar in relation to the tolerances on core geometry.

Magnetic performance of the LEP bending magnets

The 3304 steel-concrete cores of the LEP (Large Electron Positron colliding beam accelerator) bending magnets have been individually measured to determine the excitation characteristics (field integral on central orbit versus current), and one out of ten has been submitted to a complete measurement of the field pattern in the aperture. The end and junction effects, as well as the field distortion due to the vacuum chamber both in static conditions (due to residual magnetism of the chamber materials) and during field ramping (influence of eddy currents), have been determined on some average cores. The authors briefly describe the measuring benches and give the average values and RMS (root-mean-square) dispersions of the dipole, quadrupole, sextupole, octupole, and decapole components seen by the beam at different field levels and during ramping. The effects on the machine parameters are analyzed for each component and compared with results obtained in the first injection tests.<<ETX>>

Determination of the particle momentum in LEP from precise magnet measurements

The absolute beam energy of LEP (Large Electron-Positron Collider) is obtained by measuring the magnetic field in a reference dipole connected in series with the bending magnets of the machine. Periodic calibrations are carried out by measuring flux variations in a one-turn induction coil embedded in the lower pole of all bending magnets. The measurement equipment is described and the results of periodic calibrations are reported. This work illustrates the value of systematic magnetic measurements of accelerator magnets. The measurement results not only provided a precise knowledge of the beam energy, but also permitted the sorting and subsequent positioning of magnets in the machine in a cost-efficient way.<<ETX>>