Magnetic Field Measurements in Beam Guiding Magnets

Electromagnets used as beam guiding elements in particle accelerators and colliders require very tight tole-rances on their magnetic fields and on their alignment along the particle path. This article describes the methods and equipment used for magnetic measurements in beam transport magnets. Descriptions are given of magnetic resonance techniques, various induction coil methods, Hall generator measurements, the fluxgate magnetometer as well as the recently developed method of beam based alignment. References of historical nature as well as citations of recent work are given. The present commercial availability of the different sensors and asso-ciated equipment is indicated. Finally we shall try to analyze possible future needs for developments in those fields

Field Measurements

The measurement of the magnetic field is often the final verification of the complex design and fabrication process of a magnetic system. In several cases, when seeking high accuracy, the measurement technique and its realization can result in a considerable effort. This note describes most used measurement techniques, such as nuclear magnetic resonance, fluxmeters and Hall generators, and their typical range of application. In addition some of less commonly used techniques, such as magneto-optical, SQUIDs, or particle beams methods, are listed

Beam position monitor offset determination at LEP

For the performance of an electron positron storage ring it is important that the beam orbit passes well centred through the quadrupole magnets. Beam position monitors (BPM) aligned relative to the magnets can still have a residual mechanical or electronical offset with respect to the magnetic axis. A beam-based method is used at LEP to measure these offsets. During the machine operation for physics the gradient of selected quadrupoles is modulated with low frequencies (few Hz) and very small amplitudes (of the order of 10-4). The effect on the beam is observed with a high sensitivity pick-up. The observed effect passes through a minimum when the beam is centred in the quadrupole. Offsets for about 70 different BPM's were determined. Systematically different offsets were found for different type of BPM electronics and different types of quadrupoles. Simulations based on the past results show that the level of spin polarisation can be increased by further offset measurements

Signal Conditioning for Cryogenic Thermometry in the LHC

Temperature measurement is a key issue in the LHC, as it will be used to regulate the cooling of the superconducting magnets. The compromise between available cooling power and the coil superconducting characteristics leads to a restricted temperature control band, around 1.9 K. An absolute accuracy DeltaT < 10 mK below 2.2 K, and DeltaT < 5 K above 25 K, is necessary. For resistive thermometers covering the full temperature range, and having a negative dR/dT sensitivity, this is typically equivalent to a relative accuracy DeltaR/R of 3 10**-3 over 3 resistance decades. Also, to limit the thermometer's self-heating, the sensing current must be limited to few muA. Furthermore, the radiation levels next to the accelerator are expected to degrade significantly the performance of conventional analogue electronics. As these stringent requirements are not met by commercial conditioners, three different architectures have been developed at CERN. The first compresses the input dynamic range using a logarithmic transfer function; the second partitions the input range into three linear regions; the third converts resistance linearly into the frequency of a square wave. They fulfil the above specifications and provide industrial robustness in terms of thermal drift, galvanic protection, and compact packaging, while optimising cost-to-performance ratio. This paper describes the principles of their design, compares their characteristics and shows results of field tests. Future developmens include ASIC versions, Fieldbus interfacing, and radiation tolerant re-design