Magnetic measurement with coils and wires

Accelerator magnets steer particle beams according to the field integrated along the trajectory over the magnet length. Purpose-wound coils measure these relevant parameters with high precision and complement efficiently point-like measurements performed with Hall plates or NMR probes. The rotating coil method gives a complete two-dimensional description of the magnetic field in a series of normal and skew multipoles. The more recent single stretched wire is a reference instrument to measure field integrals and to find the magnetic axis.Comment: 29 pages, 26 figures, presented at the CERN Accelerator School CAS 2009: Specialised Course on Magnets, Bruges, 16-25 June 2009. For higher-resolution figures see http://cdsweb.cern.ch/record/134099

Field Errors Decay and "Snap-Back" in LHC Model Dipoles

The magnetic field in accelerator magnets decays when the current is kept constant during the particles injection phase, and returns quickly (snaps back) to the original values as soon as ramping is restarted. Here we show results of measurements of the decay of the field errors in 10 m long LHC model dipole magnets. In accordance with previous findings, precycles and stops at intermediate current levels influence the decay. We discuss a possible mechanism causing the decay and snap-back, based on the internal field change in the cable

Experimental Evidence of Boundary Induced Coupling Currents in LHC Prototypes

The field quality of 10 m long LHC dipole models has been measured with short rotating coils to explore its dependence on time and position. Multipoles exhibit a longitudinal periodic variation, with period equal to the twist pitch length. This periodicity is shown here to have at least two components with very different time constants. The amplitude of the component with the shorter time constant, in the range of 100 to 300 s, depends on position and time. Larger amplitudes are measured at early times after a ramp and close to regions with incomplete cable transposition with respect to the non-uniform external field change. As the multipoles periodicity is due to current imbalance in the cables, we attribute the short time scale variations to the presence of space and time decaying boundary induced coupling currents (BICC's) in the cable. An estimate of their value is give

The Multipoles Factory: An Element of the LHC Control

The measurements performed at CERN on prototypes and first pre-series main dipole magnets confirm the need of an active control of the Large Hadron Collider to compensate the dynamic field changes during the proton beam injection and acceleration. This control requires in turn an accurate forecast of the magnetic field in the accelerator. We plan to predict the field on the basis of two elements: theoretical field models tailored through the accumulated knowledge of the main magnets during series tests, and an on-line measurement system running on few reference magnets tracking the LHC current cycle. Data coming from this "Multipoles Factory" will result from the fusion of the two sources. Based on this system we foresee to deliver calibration information for pre-defined accelerator cycles as well as real time information for the active control. In this paper we report the conceptual design of the system, and we discuss the features and performance of the models that we have developed for the field forecast

Magnetic Field Quality of Short Superconducting Dipole Model Magnets for LHC

A series of 1-m long, 56 mm aperture dipole models has been built and tested at CERN within the scope of the R&D program for LHC. Here we report a summary of results of warm and cold steady state field measurements in these models, concentrating on the contribution of the coil geometry. The first allowed harmonics are clearly correlated to the coil azimuthal size, and the slope of the correlation can be predicted accurately

Sensitivity and Accuracy of the Systems for the Magnetic Measurements of the LHC Magnets at CERN

Beam optics of the LHC accelerator require stringent control of the field quality of the main dipole and quadrupole magnets. The field quality measurements need challenging accuracy given the small size of the aperture (50 mm) : relative strength of the magnets within 2×10-4, harmonics in the ppm range, axis determination within 0.1 mm, main field direction within 0.2 mrad. We present a detailed analysis of the accuracy and reproducibility obtained with the equipment presently available for the qualification tests of the first series magnets

A VME-based LabVIEW system for the magnetic measurements of the LHC prototype dipoles

A magnetic measurement system based on a set of rotating harmonic coils has been integrated together with the coil positioning and rotation control, the associated data acquisition and the power supply control using a PC. This PC is a mono-board VME module with its networking connection, local hard disk and serial interfaces. The PC communicates with its peripheral devices (the controller embedded in the power converter, the coil positioning PLC and the coil rotation hardware) via RS-232C lines and acquires data using VME modules: in-house designed voltage integrators for the magnetic measurement and a commercial ADC for real-time measurements. The software is a LabVIEW application: it handles and synchronizes the peripheral devices of the measurement system and the real-time tasks related to the data acquisition; it constitutes a man-machine interface for the operator and also directly stores field maps onto a file server. The system is operational on the test benches and has proved reliable, user-friendly and performed as expected.

Coil Size and Geometric Field Quality in Short Model Dipoles for LHC

We have measured the magnetic field at room temperature and at 1.8 K on more than twenty, 1-m long, single aperture LHC superconducting dipole models. The magnets feature either a 5-block coil geometry or the baseline 6-block geometry foreseen for the LHC. Comparison of warm and cold measurements show that the coil geometry is essentially unchanged during cooldown. We have therefore used mechanical measurements taken on the coil and collars during assembly to estimate the azimuthal coil length. Based on these measurements we show here that the sensitivity of allowed harmonics on coil size is in good agreement with the prediction obtained from the numerical model used for designing the LHC magnets