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Abstract-A new dc current reference standard has been developed for high-current power converter calibration in the large hadron collider (LHC) project at the European Organization for Nuclear Research (CERN). This standard provides a near ideal 10-mA dc current with long-term drift of one part in 10 6 per year. The paper describes the requirements and the detailed design and evaluation of the unit. Since similar 10-V standards are commercially available, the paper concentrates on the unique current output capability of this device.

Abstract-The Large Hadron Collider (LHC) particle accelerator project will require an accuracy of a few parts in 10 6 in the control of the current to the superconducting magnets. A new infrastructure for calibration is being built at CERN based on dc current reference standards rather than voltage. The paper describes the rationale, the infrastructure, the transfer methods, and the performance.Index Terms-Accelerator control systems, calibration, current comparator, dc, transducer. I. BACKGROUNDT HE Large Hadron Collider (LHC) project [1] aims to build a new proton collider at CERN, working at 7 TeV/beam. This machine is scheduled to come into operation in the year 2007. To achieve this aim, superconducting magnets will be used, working with 13 kA current at 1.9 K. The dipole magnets produce a bending field of 8.4 T over the 27-km circumference tunnel, 100-m underground.The current in the several thousand magnets must be controlled very accurately to minimize particle losses. Even very small losses would deposit enough energy in the superconducting coils to cause a quench. It is also very important to maintain tracking between the many circuits during the accelerator cycle. Special high-precision dc current transducers (DCCT) are used to control the switch-mode power converters producing the magnet currents.The main dipole and quadrupole magnet circuits in a conventional accelerator are always powered in series to ensure synchronism and homogeneity in the magnetic field around the circumference. Due to constraints in the protection of the superconducting magnets and the very high stored magnetic energy (12 GJ), the main circuits in the LHC are divided into eight sectors. It, therefore, becomes fundamental to successful operation that the individual sector currents can be controlled with absolute accuracy in amplitude and time. The periodic calibration of the DCCTs and the following analog-to-digital (A/D) converters will be essential to the successful operation of the LHC.The control systems for the highest precision converters have been planned in clusters to facilitate in-situ calibration and the ensuing environmental control. A complete calibration system has been conceived [2], which is based on the availability of an accurate 10mA dc current standard [3] and a complete infrastructure to maintain traceability to national standards. II. DCCT CONSIDERATIONSThe new accelerator technology needs higher current and unprecedented accuracy in the current control. The state-of-the-art in DCCT design and calibration needed improvement to achieve this. Experience from DCCT technology shows that current ratios are limited in range, but have long-term stability and are much more precise than the current-to-voltage conversion (in burden resistors) at their output. It is also evident that current-tovoltage conversion is more accurate the lower the power is.Two avenues were available and the latter was chosen for reasons of reliability and economy.• Develop a two-stage DCCT, such that current-to-voltage conversion could b...

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