Signal Processing for Beam Position Monitors

At the first sight the problem to determine the beam position from the ratio of the induced charges of the opposite electrodes of a beam monitor seems trivial, but up to now no unique solution has been found that fits the various demands of all particle accelerators. The purpose of this paper is to help "instrumentalists" to choose the best processing system for their particular application, depending on the machine size, the input dynamic range, the required resolution and the acquisition speed. After a general introduction and an analysis of the electrical signals to be treated (frequency and time domain), the definition of the electronic specifications will be reviewed. The tutorial will present the different families in which the processing systems can be grouped. A general description of the operating principles with relative advantages and disadvantages for the most employed processing systems is presented. Special emphasis will be put on recent technological developments based on telecommunication circuitry. In conclusion, an application example will show how to choose the correct solution for a particular cas

The comparison of signal processing systems for beam position monitors

At first sight the problem of determining the beam position from the ratio of the induced charges of the opposite electrodes of a beam monitor seems trivial, but up to now no unique solution has been found that fits the various demands of all particle accelerators. The purpose of this paper is to help instrumentalist in choosing the best processing system for their particular application. The paper will present the different families in which the processing systems can be grouped. A general description of the operating principles with relative advantages and disadvantages for the most employed processing systems is also presented

From narrow to wide band normalization for orbit and trajectory measurements

The beam orbit measurement (BOM) of the LEP collider makes use of a narrow band normalizer (NBN) based on a phase processing system. This design has been working fully satisfactory in LEP for almost 10 years. Development work for the LHC, requiring beam acquisitions every 25 ns, has led to a new idea of a so-called "Wide Band Normaliser" (WBN) which exploits most of the P.U.'s differentiated pulse spectrum. In the WBN the beam position information is converted into a time difference between the zero crossing of two recombined and shaped electrode signals. A prototype based on the existing NBN unit has been developed and tested to prove the feasibility of this new idea. For this the B.P. filters and the 90° hybrids are replaced by L.P. filters and delay lines

A logarithmic processor for Beam Position Measurements applied to a Transfer Line at CERN

The transfer line from the CERN proton synchrotron (PS) to the super proton synchrotron (SPS) requires a new beam position measurement system in view of the LHC. In this line, the single passage of various beam types (up to 7), induces signals with a global signal dynamics of more than 100 dB and with a wide frequency spectral distribution. Logarithmic amplifiers, have been chosen as technical solution for the challenges described above. The paper describes the details of the adopted solutions to make beam position measurements, with a resolution down to few 10-4 of the full pickup aperture over more than 50 dB of the total signal dynamics. The reported performances has been measured on the series production cards, already installed into the machine and on one pickup in the transfer line

A new wide band time normaliser circuit for bunch position measurements with high bandwidth and wide dynamic range

Trajectory and closed orbit measurements are vital for commissioning and operation of accelerators. With the push for high luminosities at modern colliders, the azimuthal bunch distribution becomes very complex, so that various phenomena (beam-beam forces, wake fields) strongly affect the orbits of individual bunches. Hence a system with high bandwidth capable of measuring the transverse position of any bunch is desirable. With the current techniques a bandwidth above 100 MHz can only be achieved by individual integration and digitisation of the pick-up signals. The drawback of such an approach is the limited dynamic range of typically 30 dB. In the context of the development of an orbit system for the LHC at CERN a high bandwidth could be achieved by extending the principle of phase normalisation to a wide band time normalisation of the position monitor signals. The circuit described in this paper (Wide band time normalizer) combines the signals of two pick-up electrodes with different delays and converts the beam position information into a pulse width modulation of the digital output signal. This way a bandwidth of more than 40 MHz and a dynamic range of about 50 dB could be achieved. The paper introduces the requirements for the LHC orbit system, compares various technical solutions and finally explains the working principle of the wide band time normalizer including some laboratory tests results

Protection and Diagnostic Systems for High Intensity Beams

This paper presents a summary of the facilities for beam interlocks and diagnostics to protect the CERN SPS machine. An overview of the existing systems is given, which are based on beam loss and beam current monitors and large beam position excursion in the horizontal plane. The later system mainly protects the system against a failure of the transverse damping system. The design for a new large excursion interlock for both transverse planes is also presented in some detail. For this system a digital approach is being taken to allow post-mortem analysis of the behaviour of the beam prior to the activation of the interlock

The second generation of optimized beam orbit measurement (BOM) system of LEP: hardware and performance description

The BOM System with its 504 Beam Position Monitors and 40 Processing Electronics Stations, distributed along the 27 km of the LEP tunnel, has been optimized for all beam conditions and modes of operation. The description of the Beam Position Monitors (or PU) behavior in the tunnel is given. The guiding approaches for obtaining both main aspects of the critical BOM performances were: a) high reliability, since most of the electronics is not accessible during operation, and b) resolution, precision and stability of the signal processing equipment for the management of the LEP optics, polarization and energy calibration. The finalized analog signal processing chains, both Wide-Band and Narrow-Band, are described. Since local memories allow for the recording of data at each bunch passage during more than 1000 revolutions, it can be followed by a powerful digital signal processing allowing for many modes of beam observation. Examples are presented of beam and machine behavior studies. The BOM System has been a key instrument for the success of LEP operation

Incremental Convex Planarity Testing

AbstractAn important class of planar straight-line drawings of graphs are convex drawings, in which all the faces are drawn as convex polygons. A planar graph is said to be convex planar if it admits a convex drawing. We give a new combinatorial characterization of convex planar graphs based on the decomposition of a biconnected graph into its triconnected components. We then consider the problem of testing convex planarity in an incremental environment, where a biconnected planar graph is subject to on-line insertions of vertices and edges. We present a data structure for the on-line incremental convex planarity testing problem with the following performance, where n denotes the current number of vertices of the graph: (strictly) convex planarity testing takes O(1) worst-case time, insertion of vertices takes O(log n) worst-case time, insertion of edges takes O(log n) amortized time, and the space requirement of the data structure is O(n)

Performance of BPM Electronics for the LEP Spectrometer

At the LEP e+/e- collider at CERN, Geneva, a Spectrometer is used to determine the beam energy with a relative accuracy of 10-4. The Spectrometer measures the change in bending angle in a well-characterised dipole magnet as LEP is ramped. The beam trajectory is obtained using three beam position monitors (BPMs) on each side of the magnet. The error on each BPM measurement should not exceed 1 micron if the desired accuracy on the bending angle is to be reached. The BPMs used consist of an aluminium block with an elliptical aperture and four capacitive button pickup electrodes. The button signals are fed to customised electronics supplied by Bergoz. The electronics use time multiplexing of individual button signals through a single processing chain to optimise for long-term stability. We report on our experience of the performance of these electronics, describing measurements made with test signals and with beam. We have implemented a beam-based calibration procedure and have monitored the reproducibility of the measurements obtained over time. Measurements show that a relative accuracy better than 300 nm is achievable over a period of 1 hr