Digital Signal Processing: State-of-the-Art at CERN and Recommendations

Dramatic hardware performance improvements over the last decades have paved the way to the ascent of digital techniques for processing signals, with a concurrent and parallel interest in Digital Signal Processing (DSPing) and in the use of Digital Signal Processors (DSPs). Recent discussions within PS showed that there are needs for DSP-qualified manpower in new projects that cannot be fully satisfied internally. In order to determine how PS can best profit from the growing importance and efficiency of DSP technologies, with an effort compatible with the available divisional resources, a DSP working group was created. Its mandate is to advise PS management on the best way to proceed in the DSPs and DSPing domains. In particular, the issues targeted are wide-ranging, from evaluating the state-of-the-art at CERN to hardware standardisation and required training. This report gives the findings of the working group and presents its closing recommendations

Bunch Trains for LEP

Since 1995 LEP has been operated with a bunch train scheme which allows head-on collisions of four trains of up to four bunches within a train. The proposal and its implementation are presented, and the consequences for the beam dynamics are discussed in detail. In particular the side effects due to the separation scheme itself and the parasitic beam-beam encounters are computed. The necessity of a self consistent treatment is shown and emphasis is placed on a comparison between the expectations and the observations

Experience with Bunch Train in LEP

Since 1995 LEP is operated with the new bunch train scheme. This scheme allows head-on collisions of four trains of up to four bunches within a train. The first experience with this new scheme and the problems encountered during the commissioning and the operation are reviewed and discussed. The performance of LEP and the results from dedicated experiments are shown and compared with expectations. The modifications and improvements to allow a successful operation at LEP2 energies are discussed and the performance at energies above 80 GeV is presented

Calibration of centre-of-mass energies at LEP 2 for a precise measurement of the W boson mass

The determination of the centre-of-mass energies for all LEP 2 running is presented. Accurate knowledge of these energies is of primary importance to set the absolute energy scale for the measurement of the W boson mass. The beam energy between 80 and 104 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is defined in the NMR model, which is calibrated against precise measurements of the average beam energy between 41 and 61 GeV made using the resonant depolarisation technique. The validity of the NMR model is verified by three independent methods: the flux-loop, which is sensitive to the bending field of all the dipoles of LEP; the spectrometer, which determines the energy through measurements of the deflection of the beam in a magnet of known integrated field; and an analysis of the variation of the synchrotron tune with the total RF voltage. To obtain the centre-of-mass energies, corrections are then applied to account for sources of bending field external to the dipoles, and variations in the local beam energy at each interaction point. The relative error on the centre-of-mass energy determination for the majority of LEP 2 running is 1.2 x 10^{-4}, which is sufficiently precise so as not to introduce a dominant uncertainty on the W mass measurement.Comment: 79 pages, 45 figures, submitted to EPJ

Operating Experience with the LEP200 Superconducting RF System

By the beginning of 1999, after several stages of installation, the RF system in LEP had gained a final total of 288 four-cell SC cavities. For 2000, the last year of LEP running, eight original LEP1 copper cavities were re-installed to bring their total to 56. During 1999 and 2000, the RF system was pushed to its absolute maximum limits for physics. By mid-2000 maximum total RF voltages of well over 3600 MV could be sustained, allowing beam energies of up to and even over 104 GeV for new particle searches. This corresponded to average gradients approaching 7.2 MV/m in the SC cavities, well above the design value of 6 MV/m. This level of performance was achieved due to the very successful high-field conditioning of the niobium-copper sputtered SC cavities, the many RF system improvements made in previous years and by a cryogenics system cooling power upgrade. Operation at very high energies however brought new difficulties, many related to the high fields and increased RF power levels. Running with the RF system at its limit required new operational procedures and facilities as well as constant follow up of cavity and RF system performance. LEP high energy running proved very successful, the beam energies and integrated luminosities obtained largely exceeded the most optimistic expectations. Finally, a vast amount of experience has been gained during the construction and operation of the LEP SC RF system. Some critical design issues in SC RF systems can be reviewed in the light of this experience

Performance of the LEP200 superconducting RF system

The LEP Superconducting RF system has reached its maximum configuration of 288 four-cell cavities powered by 36 klystrons. This has allowed the beam energy to be raised from 45.6 GeV where physics of the Z-particle was studied to well above 80.5 GeV the threshold of W pair production. The search for Higgs bosons and other new particles requires even higher beam energies. Currently the maximum operational energy achieved is 101 GeV with the RF system supplying a circumferential voltage of 3500 MV. This requires not only operating the cavities well beyond their design gradient but also demands a very high operational reliability from the entire system. The major developments necessary to achieve this performance are described

Ultimate Performance of the LEP RF System

The LEP Superconducting RF system reached its maximum configuration of 288 four-cell cavities powered by 36 klystrons in 1999. In 2000, this system, together with 56 cavities of the original copper RF system, routinely provided more than 3630 MV, allowing the beam energy to be raised up to 104.5 GeV. This not only required operating the cavities more than 15% above their design gradient, but has also demanded a very high operational reliability from the entire system. This paper will describe the operation of the LEP RF system during 2000, including new features, operational procedures and limitations

Evaluation of the LEP Centre-of-Mass Energy Above the W-Pair Production Threshold

Knowledge of the centre-of-mass energy at LEP2 is of primary importance to set the absolute energy scale for the measurement of the W-boson mass. The beam energy above 80 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is calibrated against precise measurements of the average beam energy between 41 and 55 GeV made using the resonant depolarisation technique. The linearity of the relationship is tested by comparing the fields measured by the probes with the total bending field measured by a flux loop. This test results in the largest contribution to the systematic uncertainty. Several further corrections are applied to derive the the centre-of-mass energies at each interaction point. In addition the centre-of-mass energy spread is evaluated. The beam energy has been determined with a precision of 25 MeV for the data taken in 1997, corresponding to a relative precision of 2.7x10^{-4}. This is small in comparison to the present uncertainty on the W mass measurement at LEP. However, the ultimate statistical precision on the W mass with the full LEP2 data sample should be around 25 MeV, and a smaller uncertainty on the beam energy is desirable. Prospects for improvements are outlined.Comment: 24 pages, 10 figures, Latex, epsfig; replaced by version accepted by European Physical Journal

Report of the 1995 bunch train study group

In order to raise the luminosity LEP was operated in 1995 with four equidistant trains of bunches in each beam, instead of the usual four or eight single bunches. Each train consisted of up to four bunches. The bunch spacing was about 74 m. A comparison is made between the plans and expectations in the 1994 Bunch Train Report, and the actual implementation and observations in 1995. The observations made during machine development sessions and during routine operation for physics are discussed. The effects of the scheme on the background in the LEP experiments are anlysed. The performance of LEP equipment, in particular of beam instrumentation electrostatic separators, and the RF systems is presented. The plans for running LEP with bunch trains in 1996 are briefly outlined