LEP Couplers...a Troubled Story of Success

LEP couplers were a success. 288 couplers have been used for years in LEP without even one serious failure. I will briefly resume the history of the development, the problems that appeared during tests and how all these problems were solved , turning the project into a brilliant success

LHC Collimators Low Level Control System

The low level control system (LLCS) of the LHC collimators is responsible for accurate synchronization of 500 axes of motion at microsecond level. Stepping motors are used in open loop ensuring a high level of repeatability of the position. In addition, a position survey system based on Resolver and LVDT sensors and operating at approximately 100 Hz, verifies in real-time the position of each axis with some tens of micrometers accuracy with respect to the expected position. The LLCS is characterized by several challenging requirements such as high reliability, redundancy, strict timing constraints and compactness of the low level hardware because of the limited space available in the racks underground. The National Instruments PXI platform has been proposed and evaluated as real-time low level hardware. In this paper the architecture of the LHC collimators LLCS is presented. The solution adopted for implementing motion control and positioning sensors reading on the PXI platform are detailed

Coupled Analysis of Electromagnetic, Thermo-mechanical Effects on RF Accelerating Structures

One of the main problems to the designer of RF structures used in particle accelerators is to estimate their sensitivity to thermo-mechanical effects. These parameters are the basis for the choice of the tuning strategy and for the determination of the feedback architecture. We explored the possibility of using ANSYS [1] to perform such simulations in a single environment. Some examples of the most common problems of interaction between electromagnetic fields and thermo-mechanical effects are presented and, when possible, compared to measurements. Problems encountered are outlined

Physical parameters activating electrical signal distortions in polluted soils

Laboratory investigations and field measurements show that the electrical behaviour of polluted soils is strongly non-linear at low frequencies. This phenomenon can be related to the class and the amount of pollutants. To measure this non-linearity, we used only monochromatic voltage waveform as input signal and analysed the current signals at first by means of the classical spectral analysis. In particular, the Total Harmonic Distortion % (THD%) and the Harmonic Distortion %(?) measure the non-linearity level and identify the frequency interval where the non-linear electrical behaviour is activated. This frequency interval can be related to the pollutant molecular size. Open interpretative problems were the following: 1) phase localization of the signal deformation; 2) «local» amplitude of the applied signal activating the distortion, and 3) numerical fit of the distortion. We employed the wavelet analysis to study the phenomenon. The wavelet technique breaks up a signal into shifted and scaled versions of the original wavelet, which is a waveform of limited duration. These features of the wavelets allow us to obtain current components that can be interpreted on the bases of a real physical meaning. By using the wavelet analysis, we obtained the phase localization of the ‘oscillations’ of the details and consequently the phase and amplitude of the applied signal. The sum of nine details provides a good numerical fit of the distorted signal. Starting from the wavelet analysis, we determined the physical conditions activating each distortion, testing some parameters on experimental data. The parameters that resulted most significant are the phase ? of the distortion activation and the product Vin?t (Vs) (where ?t is the time interval corresponding to the said ? and Vin is the integral tension applied to the sample on ?t). The latter parameter is in a very good agreement with field data of Advanced Monochromatic Spectral Induced Polarization (AMSIP) and restricts the physical interpretative hypotheses of distorted signals that are measured in the field. Typical experimental results will be shown as examples

The UA9 Experiment at the CERN-SPS

The UA9 experiment intends to assess the possibility of using bent silicon crystals as primary collimators to direct coherently the beam halo onto the secondary absorber, thus reducing out-scattering, beam losses in critical regions and radiation load. The experiment will be performed in the CERN-SPS in storage mode with a 120 or 270 GeV/c proton beam. The otherwise stable beam will be perturbed to create a diffusive halo. The setup consists of four stations: the crystal station with two goniometers for crystals, two tracking stations at about 90 degrees phase advance with detectors for single particle tracking and the collimation (TAL) station with a 600 mm long tungsten absorber. The observables are the localization of the losses in the collimation area, the collimation efficiency and the shape of the deflected beam phase space. We discuss the experimental layout and the way we expect to collect data in 2009

Final Implementation and Performance of the LHC Collimator Control System

The 2008 collimation system of the CERN Large Hadron Collider (LHC) included 80 movable collimators for a total of 316 degrees of freedom. Before beam operation, the final controls implementation was deployed and commissioned. The control system enabled remote control and appropriate diagnostics of the relevant parameters. The collimator motion is driven with time-functions, synchronized with other accelerator systems, which allows controlling the collimator jaw positions with a micrometer accuracy during all machine phases. The machine protection functionality of the system, which also relies on function-based tolerance windows, was also fully validated. The collimator control challenges are reviewed and the final system architecture is presented. The results of the remote system commissioning and the overall performance are discussed

Cesium-Telluride Photocathode No. 166

In the CERN photoemission laboratory, a Cs2 Te photocathode has been produced in December 2006. The co-evaporation of Cs and Te onto a copper substrate is observed with two quartz oscillator thickness monitors. The calibration of these monitors and the resulting Cs and Te layer thicknesses are described, and the calculated stoichiometric ratio of the sample is given. The quantum efficiency of cathode No. 166, measured using the cathode in a DC gun, has been found to be 6.2%

DSP-Based Stepping Motor Drivers for the LHC Collimators

The control electronics of the LHC collimators stepping motors will be located in radiation safe zones up to 800 meters far from the motors. With such cable lengths the standard chopping drivers do not work properly because of the voltage losses on the cable and even more because of the high cable capacitance. The capacitance in fact produces a ringing phenomenon on the driver feedback current that limits the control chopping frequency to the point of being incompatible with the tight EM emissions requirements of the LHC tunnel. In some cases the feedback loop may even become unstable and the driver would fail. The problem was solved by accurately modeling the overall motor-cable system taking into account non-linearities due to hysteresis and eddy currents and by designing an adaptive digital controller, self-tuning to the real cable length. The controller will aim at increasing the chopping frequency to reduce the spectral density of the emissions and at damping the oscillations of the feedback current to avoid instability. In this paper the model of the motor-cable system as well as the digital controller are described. Particular attention is devoted to the hardware implementation based on a TI Cx2000 DSP