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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...

The LHC will require over 1700 magnet power converters, some of which will need an unprecedented precision of about 1 ppm (of 13 kA). This paper presents the approach taken, prototype methods, initial results and charts future design directions. These results confirm that such performance can be obtained reliably and at a reduced cost compared to conventional methods. Developments of a real-time controls infrastructure needed to support on-line beam feedback are outlined. BACKGROUNDFor the LHC machine to achieve its full potential, the power converter system needs to attain a peak performance of about 1 part per million (ppm) in terms of resolution, stability and reproducibility. This represents an improvement over current practice of approximately a factor of ten. In addition, the very large electrical time constants presented by super-conducting magnets, coupled with the need to remove dynamic errors required a new approach. In order to meet this challenge a number of studies and practical tests have taken place over the last few years aimed at proving that such increased performance can be obtained reliably. A strategy for obtaining such improvement was presented in an earlier paper [1]. In brief this strategy was :• Employ digital regulation methods rather than analogue methods.• Apply digital corrections of known errors.• Employ real-time feedback mechanisms (both magnetic and beam related).• Incorporate in-situ calibration techniques (this subject is not covered further in this paper).This approach required an extensive revision of present practice for the regulation and remote control. This paper presents the design of the prototype system, the results obtained and outlines future design directions. PROTOTYPE IMPLEMENTATIONThe digital regulation hardware, shown in figure 1, has been built using a commercial digital signal processor (DSP) card as the heart of the system. This processor, a Texas TMS320C32, 32 bit floating-point device computes the current reference value every milli-second, compares this to the actual measured current and then computes, using a regulation algorithm, the required signal to drive the power converter to the correct value. This signal is converted into analogue form via a digital to analogue converter (DAC). The output current is measured by a purpose designed, Sigma-Delta, analogue to digital converter (ADC), which has better than ±1 ppm performance and is directly interfaced to the DSP. The interface between the DSP, the remote control system and the power converter hardware is provided by a second commercial micro-controller card. This card employs a Motorola MC68HC16Z1 which acts as the 'master' for the DSP.The software for both processors is written in 'C', using commercial software development tools. Only minor use of assembly language has proven necessary. Both systems employ 'direct to processor' connections to the PC (BDI and JTAG), allowing code down-load and extensive debug capabilities. The reference and regulation algorithms operate as a single repetitive t...

Tune, chromaticity and orbit of the LHC beams have to be precisely controlled by synchronising the magnetic field of quadrupole, sextupole and corrector magnets.This is a challenging task for an accelerator using superconducting magnets, whose field and field errors will have large dynamic effects.The accelerator physics requirements are tight due to the limited dynamic aperture and the large energy stored in the beams.The power converters need to be programmed in order to generate the magnetic functions with defined tolerances. During the injection process and the energy ramp the magnetic performance cannot be predicted with sufficient accuracy, and therefore real-time feedback systems based on magnetic measurements and beam observations are proposed. Beam measurements are used to determine a correction factor for some of the power converters. From magnetic measurements the excitation of small magnets to compensate the sextupolar (b 3 ) and decapolar (b 5 ) field components in the dipole magnets will be derived. To meet these requirements a deterministic control system is envisaged.

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