The LEP Collider

  • from Design to Approval and Commissioning


    Excerpts from The John Adams Memorial Lecture
    delivered at CERN on 26 November 1990

    ByStephen Myers


    Contents

  • 3.1 Civil engineering
  • 3.2 Magnet and power-converter systems
  • 3.3 Acceleration system
  • 3.4 Beam instrumentation
  • 3.5 Electrostatic separators
  • 3.6 Vacuum system
  • 3.7 Controls system
  • 3.8 Injectors and Pre-injectors
  • 4. Commissioning and first year of operation
  • Conclusions

    THE CONTRIBUTION OF JOHN ADAMS TO THE DEVELOPMENT OF LEP

    The early development of the Large Electron-Positron storage ring (LEP) project is best described by John Adams in his Annual Reports to the CERN Council in 1979 and 1980. I have extracted parts of these reports to highlight his enormous contribution to the early stages of the project.

    'After many years of studies of different types of machines, ... the choice of the European community settled a year or so ago on a very large electron-positron collider called LEP.

    'Studies of the design of the LEP machines started at CERN in 1976 and the first practical design was published in 1978. This machine had a cost-optimized energy of 70 GeV per beam and measured 22 km in circumference. After extensive discussions during the autumn of 1978 it was decided to embark on the design of a somewhat larger machine, 30 km in circumference, with a cost-optimized energy of about 90 GeV per beam. The energy of both these machines could be extended, by using superconducting RF cavities, when these become available, to 100 and 130 GeV respectively.

    'Studies of the 30 km machine were completed during 1979 and a design report was issued in August. These studies ... cover not only machine design but also the design and development of the components of LEP machines. Progress on this part of the studies is most encouraging and already a much cheaper design for the main magnet system has been developed, and also a more economical system for the RF accelerating system using a storage cavity scheme. It was decided to increase the effort on the development of superconducting RF cavities for LEP by setting up a small team at CERN and establishing a collaboration with other European laboratories.... The basic feature of the present LEP design is a large machine circumference in which the machine will be installed in stages corresponding to the new physics events that are predicted by the unified theory of weak and electromagnetic interactions. The first predicted event is the ... Z-zero at an energy of 90 GeV. Since these bosons can be produced singly, the LEP machine energy is about 50 GeV per beam, giving 100 GeV in the centre of mass. The next predicted event is the production of pairs of the charged intermediate boson (W+W-) pairs at an energy of about 180 GeV which requires LEP energies of about 90 GeV per beam.

    '... To explore the Z-zero region, only a part of the RF accelerating cavities and their power amplifiers will be installed in the machine. To reach the next energy stage one has the choice either to install the full set of copper RF accelerating cavities or to install superconducting cavities, if they are ready at that time.... All the other components of the LEP machine would be designed for the top energy envisaged, but items such as power supplies would be installed for the energy stage being used at the time and extended only when the energy was increased....'

    '... The latest development of the LEP Project is to use the PS and SPS machines as the injectors for LEP....

    3.1 Civil engineering

    Chronologically the LEP construction began with the civil-engineering work and the infrastructure. This work accounted for more than half of the total LEP construction budget, and between 1983 and 1988 LEP was the largest civil-engineering undertaking in Europe. The 26.67 km Main Ring tunnel formed the most impressive part of this work, even though it represented less than half of the 1.4 million m3 of material which had to be excavated for the project. The remainder of the underground work consisted of the four experimental caverns, 18 pits, 3 km of secondary tunnel, and some 60 chambers and alcoves. After an extensive campaign of test borings in and around the area proposed for the LEP tunnel it was decided to incline the plane of the tunnel by 1.4%. This decision was made so as to ensure that all underground caverns and the main part of the tunnel would be located in solid rock while, at the same time, limiting the maximum depth of the shafts to less than 150 m.

    However, owing to the necessarily large diameter of LEP and the constraining distance between Geneva airport and the foothills of the nearby Jura mountains, it was inevitable that some part of the tunnel had to be bored under unfavourable conditions created by the limestone. This part, which consisted of around 3.5 km, was excavated by blasting, and the poor quality of the rock necessitated an increase in the thickness of the roof lining to 40 cm. As anticipated, many geological faults had to be traversed here but most were detected in advance by systematically boring pilot holes ahead of the excavation. Nevertheless, in spite of all the care taken, several major inundations of water (150 l/s at 13 x 10E5 Pa pressure was recorded) and clay occurred and delayed the work by several months. Excavation of the part of the tunnel in the rock ( about 23 km) progressed at a much faster rate, owing to the use of three full-face tunneling machines, each of which progressed at an average rate of about 25 m per day.

    The guidance of the tunnelling machines on their desired trajectory to a precision of about 1 cm and the alignment of the collider components within the LEP tunnel to a required short-range relative precision of less than about 0.1 mm on the scale of many kilometres is worthy of note. The basic reference for this work was provided by a geodesic network between the hills surrounding the site. The base lines of up to 13 km length were measured with precisions of l0E-7 by the use of a two-wavelength laser interferometer (Terrameter). Recently these measurements have been verified with excellent agreement by satellite observations using the NAVSAT satellite system. The first precise measurements with beams indicated that the LEP circumference was in fact more than twice as precise as predicted: better than 1 cm in 26.67 km.

    In addition to the underground civil-engineering work, the construction of LEP necessitated the construction of 71 surface buildings of a total area of some 51'000 m2, situated over eight sites. This was done in a way which preserved the local environment.

    The LEP infrastructure, although often industrial in nature, presented a great challenge owing to its enormous size and the very tight time and budget constraints. This infrastructure consisted of the many types of water cooling, air conditioning and treatment, and the electrical distribution scheme as well as the transport and installation schemes.

    3.2 Magnet and power-converter systems

    The electromagnetic guide field system of LEP consists of dipoles, quadrupoles, sextupoles, horizontal and vertical dipole correctors, rotated quadrupoles, and finally electrostatic dipole deflectors. About three quarters of the LEP circumference is occupied by 'standard cells'. Each of the eight arcs contains 31 of these standard cells, which are comprised of magnets in the following order: a defocusing quadrupole, a vertical orbit corrector, a group of six bending dipoles, a focusing sextupole, a focusing quadrupole, a horizontal orbit corrector, a second group of six bending dipoles, and finally a defocusing sextupole. The length of a standard cell is 79.11 m.

    The electrons and positrons are bent in a piecewise circular trajectory by the strings of dipole magnets. As previously indicated, the bending field of these dipoles has been made unusually low ( about 0.1 T) so as to increase the bending radius and thereby reduce the amount of synchrotron radiation. The low bending field allowed a novel design of the dipole magnet cores, with the 4 mm gaps between the steel laminations ( 1.5 mm thick) filled by mortar. Compared with classical steel cores, this produced a cost saving of around 40%. The quadrupole magnets, which produce fields linear with the transverse position, act as magnetic lenses and focus the beam to be comfortably contained within the vacuum chamber. The alternating polarity of the quadrupoles in the standard cells produces alternating-gradient focusing or 'strong' focusing. The cell sextupoles produce a field which is quadratic in transverse displacement, and they are used to compensate the dependence of the focusing strength on the beam energy ('chromaticity'). The small horizontal and vertical correctors are individually powered so as to allow 'steering' of the beam through the centre of the LEP aperture.

    Each experimental collision point in LEP is surrounded by a large solenoidal magnet used for particle identification. The bunches of each beam must be tightly focused ('squeezed') to very small dimensions in the centre of these detectors in order to increase the luminosity or particle production rate. This is accomplished by a set of superconducting quadrupoles with very strong field gradients that focus the transverse beam dimensions to about 10 micro-m and 250 micro-m in the vertical and horizontal planes respectively. The solenoidal detector magnets produce another effect, however: they cause the horizontal oscillations to be 'coupled' into the vertical plane; if this were uncompensated it would greatly increase the vertical beam size and cause a reduction in the luminosity. For this reason, rotated quadrupoles are installed around each solenoid to compensate this magnetic coupling. These quadrupoles are similar to conventional quadrupoles but rotated about their axis by 45 degrees.

    The strengths of all magnets in the LEP ring are very accurately adjusted by controlling the current flowing in their coils. This is accomplished by the use of more than 750 precisely stabilized d.c. power supplies ranging from less than 1 kW to a maximum of 7 MW. The specifications for these power supplies are extremely tight, both in their individual operation and, during energy ramping, in their precise synchronization. For the main dipole and quadrupole supplies, absolute accuracies down to 2 parts in 10E5 have been achieved with a resolution typically three times better.

    Each magnet has, of course, its own cooling circuit. For the majority, the cooling is provided by demineralized water circuits, which are connected to a total of 10 cooling towers with a capacity of 10 MW each. Some of the small corrector magnets are air-cooled, whilst the superconducting quadrupoles and the superconducting experimental solenoids are cooled by liquid helium at 4.2 K from the cryogenics installation.

    3.3 Acceleration system

    The RF acceleration system installed at present consists of 128 five-cell copper cavities powered by sixteen 1 MW klystrons via a complex of waveguides and circulators. Each accelerating cavity is coupled to a spherical low-loss storage cavity in such a way that the electromagnetic power continuously oscillates between the two sets of cavities. The coupling is arranged so that the power is at its peak in the acceleration cavities at the instant of the passage of the beam bunches. In this way, the bunches receive the maximum possible accelerating gradient, but the power loss due to heating of the copper cavity walls is greatly reduced since the electromagnetic power spends half of its time in the very-low-loss storage cavities. The operating frequency is 352.21 MHz, which corresponds to 31,320 times the revolution frequency of a beam circulating in LEP. The present system allows for a peak RF voltage of 400 MV per revolution.

    The sinusoidal electric field that is generated in each accelerating cavity cell produces a 'potential well', inside which each particle of each bunch can perform stable oscillations with respect to the particle at the centre of the bunch, i.e. the synchronous particle. These oscillations are around the energy of the synchronous particle and, in azimuthal distance, ahead of or behind this particle. When represented in a phase-plane plot of normalized energy as a function of normalized distance, the trajectories form circles for the case of small oscillation amplitudes while the maximum stable oscillation possible inside the potential well forms a closed contour which, in accelerator jargon, is called an RF bucket. It may now be appreciated that there can possibly be as many synchronous particles and hence stable regions (buckets) as there are RF oscillations in the revolution time of LEP. As previously stated, this means a total of 31,320 buckets for LEP, with the possibility of stable oscillations in each. However, in order that the e+e- bunches collide at the centres of the experimental detectors, they must be injected and accumulated in precisely the correct buckets. This is achieved by a very precise synchronization system between the RF systems of LEP and its injector, the SPS.

    3.4 Beam instrumentation

    The LEP beam-instrumentation system is used to observe the position, shape, or other relevant properties (such as polarization or electrical current) of the beam. There are two ways of 'observing' the beam. The first and most simple is by placing a monitor directly in the path of the primary beam: an example of this is the luminescent screen that is used to observe the position and shape of the injected beam. The beam particles interact with the chromium in the screen and produce a luminescent image of the cross-section of the beam which is transmitted as a television signal to the control room. These monitors are very useful in the early days of commissioning when the other, more sophisticated monitors are perhaps not yet fully tested, and in the case of LEP they were used to steer the beam through the first turn. The beam electrical current is measured in LEP as in other accelerators by current transformers placed around the vacuum chamber. These transformers are capable of measuring the current of a single injection or of a steady circulating beam. In the latter case the beam lifetime can be evaluated by accurate measurement of the current as a function of time.

    In order to position the beam accurately in the middle of the vacuum aperture, it is essential to measure the transverse beam positions at many azimuthal locations on the circumference. In the case of LEP this 'closed orbit' is measured by 504 monitors fairly evenly distributed around the circumference. Each monitor forms part of the vacuum chamber and consists of four electrostatic pick-up 'buttons'; these are positioned in housings which, for alignment and accuracy reasons, are connected directly to the end faces of the quadrupole magnets. The electromagnetic field of the bunches induces voltages on each of the four buttons; when properly analysed, these can give an accurate measurement of the horizontal and vertical beam positions relative to the centre of the monitor. This system has been designed to be capable of measuring the positions of all eight individual bunches during more than 1,000 revolutions to an accuracy of better than a millimetre.

    The betatron-tune value is defined as the number of transverse oscillations, around the closed orbit, made by the beam per revolution. The measurement and correction of this parameter is of paramount importance for the stability of a beam in a storage ring. Owing to non-linear resonances driven by magnetic imperfections in the guide field, there are many undesirable values for the horizontal and vertical betatron tunes as well as undesirable combinations of the two. In LEP the betatron-tune values (or the Q values) are measured by two Q meters, one for each plane. The Q-meter system consists of a magnetic beam 'shaker', which excites the beam, and of dedicated electrostatic pickups, which measure the phase and amplitude of the induced transverse oscillations as a function of the excitation frequency. Since the beam acts as a high-quality resonator, the amplitude of the response is at a maximum when the excitation frequency equals the natural resonant frequency of the beam; this gives the betatron-tune value. In addition, the excitation can be 'phase-lock looped' to the beam response (acting in the same way as a modern FM receiver), thereby giving a continuous readout of the betatron-tune values. This mode of operation is particularly beneficial during energy ramping, where the tune values may wander. A system is now operational which continuously maintains the tune values at their previously prescribed values by automatically adjusting the excitation of the focusing system.

    As previously described, when charged particles are bent in a circular trajectory they radiate photons. Consequently, the beams can be 'seen' by measuring this flux in the ultraviolet (UV) frequency range. Four UV monitors are used in LEP to measure the transverse dimensions of both beams at two different locations. The UV range is preferred because it produces a sharper image on the digital TV camera The images are transmitted to the control room to give a real-time view of the beam, while the digital signals are processed to provide numerical values for the beam sizes. The UV synchrotron radiation monitors cannot give the absolute beam dimensions; they are therefore calibrated using the 'wire scanner monitor': a 37 micro-m diameter carbon fibre traverses the beam at 0.5 m/s and thereby creates photons which are detected outside the vacuum chamber. The beam profile is given by the density of the detected photons, plotted as a function of the position of the flying wire, which can be measured to an accuracy of 10 micro-m. The synchrotron light signal can also be used to measure the length of the bunch with picosecond precision.

    The synchrotron radiation results in another problem: background originating from the high-energy spectrum of the photon emissions. In order to reduce this background, collimators are installed around each experimental point. Each of these collimators consists of remotely movable jaws of tungsten and copper, which can intercept and absorb the high-energy photons. Since these collimators can be placed very close to the beam, they were designed to accommodate, inside each horizontal jaw, a mini-calorimeter consisting of tungsten absorbers and silicon detectors. These mini-calorimeters are used to measure the relative luminosity in each experimental point by counting the number of Bhabha events at very small angles to the beam trajectory. In addition, other collimators are located far from all the experiments: these define the LEP aperture and remove any beam halo that might otherwise end up in one of the detectors. The system of collimators has proved invaluable in LEP and has resulted in low background conditions in the detectors practically from the first physics run.

    3.5 Electrostatic separators

    Under certain circumstances it is essential that the beams of electrons and positrons do not collide. This is particularly true at injection energy, where the electromagnetic fields associated with each bunch would destroy the opposing bunch long before a sufficient number of particles could be accumulated. If electrons and positrons, travelling in opposite directions are subjected to the same transverse electrostatic field, they will be displaced in opposite directions, thereby avoiding collisions and greatly reducing the beam-beam effect. In LEP this has been achieved by equipping each of the eight possible collision points (four bunches could make eight collision points) with four electrostatic separators, each of which is 4 m long and produces a vertical electric field of 2.5 MV/m between the plates, which are separated by 11 cm. This produces a separation between the bunches of electrons and positrons of more than 40 standard deviations of the vertical beam size. The separators are powered in all eight possible collision points during injection, accumulation, and energy ramping. Some time before physics data taking starts the separators in the experimental points are switched off to allow collisions. At higher energies in LEP however, the bunches may not necessarily collide perfectly 'head-on', even if the separators are off. For this reason the separators have been equipped with a vernier adjustment, which allows vertical steering of the beam positions in the collision points. The LEP separators have been designed to have a very low 'spark-over' rate: less than one spark per 1000 hours of operation even in the presence of synchrotron radiation. In addition, owing to their proximity to the experimental detectors, they must be compatible with the ultrahigh-vacuum requirements imposed by the low-background conditions needed in the collision areas.

    3.6 Vacuum system

    The duration of a typical operation to fill LEP with particles for a physics run is 12 h. During this time each of the 10El2 particles in the beams will have traversed the complete 26.67 km of the LEP vacuum chamber about 500 million times. In order to minimize particle losses due to collisions with residual gas molecules, the whole vacuum chamber must be pumped down to very low pressures. The achieved static pressure for LEP is 8 x 10E-l2 Torr whereas in the presence of beam the pressure rises to about 10E-9 Torr. This pressure rise is due to gas desorption from the inner vacuum-chamber wall, provoked by the synchrotron radiation of the circulating beam, and has had a profound influence on the design of the LEP vacuum system.

    The two main components of the vacuum system are the vacuum chamber itself and the pumping system. Of the 27 km of LEP vacuum chamber, a length of about 22 km passes through the dipole and quadrupole magnets, and is subjected to the heating due to synchrotron radiation. Although this heating represents a mere 100 W/m for phase 1, it rises to more than 2000 W/m for phase 2. Therefore the chambers need water-cooling channels and are constructed from aluminium because of its good thermal conductivity. However, only about half the radiated power would be absorbed by the aluminium; the remainder would normally escape into the tunnel and produce such a high radiation dose that organic materials such as gaskets, cables, electronic components, etc., would be rapidly destroyed. In addition, severe damage could result from the formation of ozone and nitric oxides, which produce highly corrosive nitric acid in the presence of humid air. For these reasons, the aluminium chamber is covered with a lead cladding of a thickness varying between 3 and 8 mm, which greatly reduces the radiation that escapes into the tunnel during operation. These chambers are interconnected by bolted flanges with aluminium gaskets and stainless-steel bellows, which allow for minor misalignments during installation and thermal expansion during machine operation and high-temperature 'baking' of the chambers. Other types of chambers are used in special regions such as the injection, RF, electrostatic separators, and the detector regions. For the main part these are made of stainless steel except for the detector regions where, for reasons of transparency to particles, they are fabricated from beryllium, thin-walled aluminium, or carbon-fibre composites.

    For reasons of reliability the 26.7 km of the LEP vacuum system is subdivided into smaller 'vacuum sectors' with a maximum length of 474 m. During shutdown periods, when there is no circulating beam and work is often going on in the tunnel, these vacuum sectors are isolated from each other by full-aperture gate vacuum 'sector valves'. Consequently, if an accident occurs, only 474 m of vacuum will be affected and not the full 26.7 km.

    There are two independent pumping systems for each of these sectors: a rough system, which provides pressures down to the 10-4-10-5 Torr range; and the second system needed to provide and maintain ultrahigh vacuum. In previous electron storage rings, the ultrahigh vacuum was normally produced by linear sputter-ion pumps operating in the field of the bending magnets. However, this proved impossible in LEP since the bendingfield strength is below the threshold for efficient operation of such pumps. Consequently, a novel type of ultrahigh-vacuum pumping system was required for the LEP storage ring. The solution adopted, for the first time in an accelerator, was the use of non-evaporable getter (NEG) strips, installed in pumping channels running parallel to the beam channel with pumping holes between the two. The NEG strip is 3 cm wide, and extends over 22 km. It is fabricated by coating constantan with a zirconium-aluminium alloy. The NEG material forms stable chemical compounds with the majority of the active gases; consequently the residual gas molecules inside the pumping channel simply 'stick' to the NEG ribbon. During long periods of pumping, the getter surface becomes progressively saturated and loses some of its pumping capacity. An essential operation is therefore reconditioning; this consists of heating the getter up to 400 degrees C for about 15 min and results in the diffusion of O2 and N2 from the saturated surface layer into the bulk of the material.

    The very low static pressure of less than 10E-11 Torr requires initially very clean internal surfaces. This was achieved by careful chemical cleaning of all chambers, followed by storage under chemically inert conditions. After installation in the tunnel all chambers were 'baked out' at 150 degrees C for 24 h. The bakeouts were performed by pumping superheated water at 150 degrees C and at a pressure of 5 x 10E5 Pa into the cooling channels of the aluminium-lead chambers. Sputter-ion pumps, valves, gauges, all stainless-steel chambers, and special equipment such as the electrostatic separators and the feedback system, were baked out by electrical heating elements and jackets.

    3.7 Controls system

    Almost every single LEP component and piece of equipment must be remotely controllable from the main control room by means of the LEP computer control system, which consists of more than 160 computers and microprocesssors distributed over 24 underground areas and 24 surface buildings. Communication between the computers and microprocessors is provided by a data network and a synchronization timing system. Many of the design choices for this system have been dictated by the size and topology of the project. For example, the prohibitive cost of laying many dedicated cables around the 27 km circumference led to the decision to replace cables by a Time-Division Multiplex (TDM) system, which allows many communication channels on a single cable. The network is composed of two logical levels: the upper level consists of the central consoles and servers, which are situated in the control room, and the lower level consists of local consoles and process computers. This network is based on the Token Passing Ring principle (specification IEEE 802.5) using TCP/IP as the high-level protocol.

    The synchronization of control and data taking is a very important aspect of accelerator control. Two important examples of this are: firstly, the energy-ramping process, where all 750 power converters and the RF system must be controlled in perfect synchronization; and secondly, the acquisition, on the same machine revolution, of the closed-orbit data coming from the electrostatic pick-ups. The former example necessitates the use of the General Machine Timing (GMT) system, which consists of pulses at 1 ms intervals with coded events interlaced. The latter example requires the Beam Synchronous Timing (BST) system, which permits tagging of each of the eight bunches with a relative revolution number. This system allows the measurement and storage of the beam position for each bunch, at each pick-up, on a total of more than 1000 LEP revolutions. Measurements may be made over a much longer period by programming the BST to make measurements with a prescribed number of revolutions between each.

    3.8 Injectors and pre-injectors

    The LEP storage ring is the last accelerator in a chain of five, each of which handles the same electrons and positrons generated on every pulse by the electron gun and the positron converter. The LEP injectors consist of two linacs of 200 MeV and 600 MeV followed by a 600 MeV Electron-Positron Accumulator (EPA), which injects into the CERN Proton Synchrotron (PS) operating as a 3.5 GeV e+e- synchrotron. The PS then injects into the CERN Super Proton Synchrotron (SPS), which operates as a 20 GeV electron-positron injector for LEP. The decision to use the two already existing CERN proton synchrotrons (the SPS and the PS) and all the infrastructure associated with them, resulted in significant economies both in cost and in time. The PS, originally designed for 28 GeV protons and commissioned in 1959, was modified to allow acceleration of electrons and positrons from 600 MeV to 3.5 GeV. The SPS, designed to accelerate protons to 450 GeV and first brought into operation in 1976, was modified to accept electrons and positrons from the PS at 3.5 GeV, accelerate them to 20.0 GeV, and finally transfer them to the LEP collider. In order to serve LEP with electrons and positrons, both the PS and the SPS operate in multicycle mode. In this mode, a supercycle is used which incorporates four cycles of electrons/positrons followed by one cycle of protons. Consequently, owing to the fact that the electrons/positrons are accelerated in the dead-time between the proton cycles, the filling of LEP has had little or no effect on the 450 GeV SPS stationary-target physics, which runs in parallel.

    4. COMMISSIONING AND FIRST YEAR OF OPERATION

    July and August 1989

    The first injection into the LEP collider took place on 14 July 1989, one day earlier than scheduled. First collisions of electrons and positrons were provided almost exactly one month later, on 13 August 1989. In the following four months of interleaved operation for physics and machine studies the collider performance allowed more than 30,000 Z-zero particles to be detected in each of the four experiments. During 1990, the LEP performance allowed the detection of around 200,000 Z-zero's in each detector.

    The speed and efficiency with which the LEP collider was commissioned was the result of careful planning and co-ordination of the testing of components as they were installed in the tunnel and, later, of the extensive programme of global testing without beam, just before the official turn-on date. In the nine months before July, all machine components had been installed and tested in situ in more than 24 km of the tunnel. This work involved in particular the installation of all magnets, vacuum chambers, RF cavities, beam instrumentation, the control system, injection equipment, electrostatic separators, electrical cabling, water cooling, and ventilation. The installation was followed by individual testing of more than 800 power converters and their connection to their corresponding magnets. Great care was taken to check and double check that all magnets had the correct polarity. In parallel, the vacuum chambers were baked at high temperature (either by superheated water or by electrical jackets) and then leak tested. The RF accelerating units situated around interaction regions 2 and 6 were commissioned and the cavities conditioned by raising them to their maximum power of 16 MW. Careful co-ordination of all work was essential in order to avoid conflicts between testing of the different systems and the transport needed for installation of the final octant 3-4.

    In parallel with hardware installation and testing, a great effort, with limited manpower, went into the preparation of the software necessary for the operation of LEP. The software was prepared in close collaboration with the accelerator physicists and the collider operators. This allowed a clear definition of priorities so as to ensure that software became available as it was needed.

    On 7 July, just one week before the scheduled switch on, the whole of the LEP collider was put through a complete 'cold check-out', which involved operation of all the accelerator components under the control of the available software. In particular, the energy ramping proved invaluable for the debugging of the complete system of hardware and software. The second cold check-out, scheduled for 14 July, turned out to be a 'hot check-out', since beams of positrons were already available from the SPS injector.

    The period between 14 July and 13 August was, at the same time, crucial and exciting for the LEP collider. The accelerator work done during this period brought about the transition from the first successful completion of a single turn of the particles to complete physics data taking. For this reason it is worth while itemizing the major accelerator milestones in their order of chronology.

    Following the first stable beams run of 13 August, a pilot physics run was scheduled to cover a five-day period but, owing to various technical problems, only 15 hours of physics were possible during this period. Nevertheless, this pilot run allowed the 'debugging' of the experimental detectors with a maximum luminosity of 5 x lOE28 cmE-2 sE-l. About 20 Z-zero's per experiment were successfully detected during this period.

    A period of three weeks of machine studies was scheduled after the first pilot physics run. The accelerator performance was greatly improved during this period. In particular, the low beta was reduced to the 'back-up' design value of 20 cm, a new optics with less transverse coupling was commissioned, and injection studies gave higher filling rates and maximum intensities. The very last shift of this period was foreseen as a physics preparation run and gave a maximum total beam current of 1.6 mA at 45.5 GeV with the low beta squeezed to 20 cm.

    The first LEP physics run started on 20 September, slightly more than two months after the final testiIlg of the installed accelerator components. The period between this first run and the Christmas shutdown was interleaved with physics data taking and machine studies aimed at increasing the luminosity. The physics running period was subdivided into three types of running. The first subperiod, lasting for five days, was scheduled for operating at the Z-zero peak (45.5 GeV per beam). During the second subperiod a mini-scan of the Z-zero was performed involving five different beam energies 0, +/-1, and +/-2 GeV (centre of mass) around the peak. The final and longest period was devoted to scanning the peak by spending 50% of the time on the peak and 50% off peak. The maximum luminosity achieved during this period was 5x10E30 cmE-2 sE-1, about one third of the design luminosity.

    7. CONCLUSIONS

    From a hardware point of view, LEP phase 1 has been successfully completed and every effort is now being made to increase the luminosity up to and hopefully beyond the design value. The future development of LEP to phase 2 has already been approved by the CERN Council and will, as previously stated, take the beams up to energies that will allow the study of W pairs.

    The present LEP collider, with its energy upgrade and the future programmes of higher luminosity and polarization, is providing and will continue to provide for the physicists of Europe and the rest of the world, a unique and powerful physics tool for fundamental research in the 1990's.

    Acknowledgement

    This lecture is an overview of the work done by a very large number of scientists and technicians who dedicated a large fraction of their professional life to the successful design, construction, and commissioning of the LEP collider. More detailed information on the individual contributions can be found in specialized reports at many conferences on accelerator physics, for example The European Particle Accelerator Conference held in Nice in June 1990.


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