A fast sextupole probe for snapback measurement in the LHC dipoles

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Abstract-In superconducting particle accelerators significant changes occur in tune and chromaticity during the injection of particles and their subsequent acceleration. This behavior is caused by the decay of magnetic field components in the superconducting accelerator magnets during injection and their so-called "snapback" to the original hysteresis curve during the first few seconds after the start of the energy ramp. The two effects are closely related to a spatially periodic modulation of all field components along the magnet axis. In order to avoid a loss of particles the sextupole component in the dipoles has to be compensated with a very high accuracy. Standard magnetic measurements using rotating coils do not have the time resolution required to completely resolve the snapback. For this reason we have developed a fast system for sextupole measurements consisting of three Hall plates mounted on a ring. A new calibration procedure for the first time allows quantitative results. The detector is used to measure decay and snapback as a function of several parameters in the operation cycle. The results obtained help to better understand the interaction between current distribution and magnetization in the coil.

Section: The Detectormentioning

confidence: 99%

Section: The Detectormentioning

confidence: 99%

Field decay and snapback measurements using a fast Hall plate detector

Haverkamp

Bottura

Benedico

et al. 2002

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“…Multipoles are expressed in units of with respect to the main dipolar component . As explained in detail in [5], [6], it is possible to measure the -order harmonic of the dipole field using an appropriate arrangement of Hall plates. The Hall probes are placed tangentially to a ring at a radial distance and at the azimuthal With this ideal geometry the total signal, , from Hall plates as described above is given by: (2) where is an arbitrary integer.…”

Section: The Sensormentioning

confidence: 99%

Field Decay and Snapback Measurements Using a Fast Hall Probe Sensor

Pieloni

Sanfilippo

Bottura

et al. 2004

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Abstract-During beam injection the components of the magnetic field inside the magnets decay in time. At the re-start of the ramping, this decay is cancelled resulting in a fast change of the harmonics called "snapback". This sudden variation affects mainly the sextupole and decapole components and can induce significant changes in the machine chromaticity, thus causing particle beam loss. Standard magnetic measurements with rotating coils do not have a sufficient time resolution to properly characterize the snapback phenomenon. A system based on Hall probes has been developed from an existing prototype to measure with a moderate acquisition frequency (3-10 Hz) the decay and snapback of the sextupole and, for the first time, of the decapole fields. The present system also provides local measurements of these field harmonics along a wavelength of the superconducting cable twist pitch. In this paper, we describe the assembly features of this detector and of the measurement chain. The performances are demonstrated based on preliminary measurements performed on the LHC pre-series dipoles in operating conditions.

“…They were mounted on two rings at a radius of 17 mm on the set-up dedicated for the snap-back study in LHC dipoles [8]. Hall probes were connected in series and the input current value was fixed to 50 mA.…”

Section: ) Hall Probes Set-up To Study the Magnetic Field Of Shortmentioning

confidence: 99%

Investigation of the periodic magnetic field modulation in LHC superconducting dipoles

Pugnat

Schreiner

Siemko

2002

Abstract-The windings of high-field accelerator magnets are usually made of Rutherford-type superconducting cables. The magnetic field distribution along the axis of such magnets exhibits a periodic modulation with a wavelength equal to the twist pitch length of the cable used in the winding. This effect, resulting from quasipersistent currents, was investigated with a Hall probes array inserted inside the aperture of the LHC superconducting dipoles, both in short models and full-scale prototypes. The amplitude and the time dependence of this periodic field oscillation have been studied as a function of the magnet current history. The origin and the impact on the LHC dipoles stability of the nonuniform current redistribution producing such a field modulation are discussed.Index Terms-LHC, periodic field pattern, persistent eddy-current, quench performance, Rutherford type cable, superconducting magnets.