Rudiger Schmidt (CERN)
One of CERN’s most ambitious and successful projects was the search for the intermediate bosons, W and Z [1]. The accelerator part of the project relied on a number of innovations in accelerator physics and technology. The inven-tion of the method of stochastic cooling and the extension by many orders of magnitude beyond the initial proof of principle demonstration allowed the construction of the Antiproton Accumulator. Major modifications to the 26 GeV PS complex and the conversion of the 300 GeV SPS, which had just started up as an accelerator, to ap¯pcollider were required. The SPS collider had to master the beam–beam effect far beyond limits reached before and had to function in a tight symbiosis with the UA1 and UA2 experiments.
1 Introduction
Today, the CERN Super Proton Synchrotron (SPS) is essential in the accel-erator chain for delivering protons and ions to LHC and it provides beams for fixed targets experiments.
The SPS has a long and very successful history. It was designed in the beginning of the 70s [2] as a synchrotron to accelerate protons and to extract them to fixed target experiments. The accelerator has a bending radius of 1100 m and a length of 6911 m. Initially three stages of construction were planned, first for an acceleration to an energy of 200 GeV/c, later to 300 GeV/c and finally either to 400 GeV/c with normal conducting mag-nets, or to 700 GeV/c with superconducting magnets. Later it was decided to build the accelerator in one stage with normal conducting magnets. The SPS has been delivering protons for physics experiments since January 1977.
It started with a circulating beam intensity of 2×1013protons per pulse and a repetition time of 9.6 seconds.
Already during the construction period the idea came up to use the SPS for pp¯collisions. There was some previous experience at CERN with
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a hadron collider, the CERN-ISR constructed in the seventies. In the ISR two proton beams with an energy of up to 31 GeV were circulated in two separate vacuum chambers and collided with an angle, delivering a maximum luminosity of up to 1.3×1032cm−2s−1.
With the experience from the ISR and the development of stochastic cooling, it was possible to accumulate dense ¯p bunches in an accumulator ring and to transform the Super Proton Synchrotron (SPS) into ap¯pcollider.
The first collisions between protons and antiprotons in the SPS were observed in 1981 during the first phase of operation that lasted until 1986. After a substantial upgrade the machine restarted in 1987 and the last year of operation asp¯pcollider was 1990. Table 1 shows the basic parameters of the Sp¯pS collider for the operation from 1988 to 1990.
The SppS¯ was designed as a hadron collider operating with bunched beams. Before its commissioning it was debated if such a machine could ever work, or if beam–beam effects without the presence of damping due
Table 1. Main parameters of theSp¯pS collider.
Typical parameters SPSp¯pcollider
Injection momentum [GeV/c] 26
Top momentum [GeV/c] 315
Integrated luminosity in 1990 [nb−1] 6790
Maximum initial luminosity [cm−2s−1] 5.5×1030
Initial luminosity lifetime [h] 9–12
Proton bunch intensity 12×1010
Antiproton bunch intensity 5×1010
Number of bunches per beam 6
Number of collision points 3
Horizontal emittance≡σ2×4/β(proton) 11
Vertical emittance≡σ2×4/β (proton) 11
Horizontal emittance≡σ2×4/β(antiproton) 12 Vertical emittance≡σ2×4/β (antiproton) 10
βhandβv at IP in 1990 [m] 0.65/0.30
Linear tune shift per interaction point on protons (H, V) 0.0037/0.0026 Linear tune shift per interaction point on antiprotons (H, V) 0.0066/0.0063
Total tune shift on protons (H, V) 0.011/0.008
Total tune shift on antiprotons (H, V) 0.020/0.014
Bunch length in store 4σs[ns] 2.4
Operational tunes (H, V) 26.685/27.680
Bunch intensity lifetime [h] ≥60
Efficiency (time with colliding beam/scheduled time)
(average 1988, 89, 90) 51%
Number of stores/year (average 1988, 89, 90) 110 Number of stores/year lost due to failure (average 1988, 89, 90) 20%
to the emission of synchrotron radiation as in electron–positron colliders would prohibit an operation with high luminosity. Its successful operation demonstrated that beam–beam effects at high energy hadron collider can be mastered and that such machines are excellent tools for experiments in particle physics.
2 The SPS as a synchrotron
The SPS was finished in 1976 after five years of construction on the CERN Prevessin site. The first beam was circulating in spring 1976, and operation as a synchrotron for fixed target experiments started in January 1977.
The lattice is of separated function type and F0D0 configuration with 108 F0D0 cells. This is in contrast to the first generation of alternating syn-chrotrons which were of combined function type. In the separated function type the bending is provided by dipole magnets, focusing is by relatively few quadrupole magnets. With the separated function lattice the field in the dipole magnets is considerably higher than with a combined function lattice and allows to achieve higher energy with a given circumference.
3 From SPS to Sp¯pS
Two concepts led to CERN’spp¯collider: the concept of particle–antiparticle colliders as it had been demonstrated with electrons and positrons, and beam cooling. The first realistic scheme for colliding beams was discussed in 1956 by D. W. Kerst [3]. Seven years after the first experimental confirmation of antiprotons in 1955, K. Johnson at CERN worked on the possibility of colliding protons and antiprotons in the ISR, but these studies did not result into a concrete proposal [4].
The first realistic proposal for a p¯p collider seems to have been made by Budker and Skrinsky at Orsay in 1966 [5]. The proposal was based on Budker’s idea of electron cooling specifically for the production of antiproton beams dense enough to makepp¯colliders viable. In 1968 Simon van der Meer had the idea of stochastic betatron cooling but only published it in 1972 [6].
Both electron and stochastic cooling were experimentally proven in 1974 at the NAP-M storage ring in Novosibirsk. In October 1974, stochastic cooling was first observed at the ISR [7].
The discovery of neutral currents provoked Carlo Rubbia and collabo-rators to propose a colliding beam experiment at both CERN and Fermi-lab in 1976 [5] with the specific aim of producing W and Z bosons. Such machines require only a single ring, a concept already practiced at that time with electron–positron colliders. Rubbia, realizing the potential offered by
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the projected SPS as a 300 GeV machine proposed in 1976 to use it as a p¯p collider [8].
4 Modification of the SPS for collider operation
As soon as the project was decided in June 1978, the modifications of the SPS started [1]. TheSppS¯ had a six-fold symmetry with two physics experiments UA1 and UA2 at the interaction points 4 and 5.
• A new beam line was constructed, to transfer the antiprotons from the CERN Proton Synchrotron (PS) to the SPS, and a new injection system for counter-clockwise injection was added in the SPS.
• The SPS had been built for an injection energy of 14 GeV/c. The proton transfer line, TT10, and the injection system had to be upgraded to 26 GeV/c.
• A drastic improvement of the SPS beam vacuum system was needed. The design pressure of 2×10−7 torr was only adequate for a synchrotron and needed to be improved by two orders of magnitude for the long beam storage time required in a 2 beam collider.
• The machine lattice had to be modified to include low-beta insertions, squeezing proton and antiproton beams to small sizes at the interaction points, for achieving design luminosity.
• The accelerating RF system with its traveling-wave structures had to undergo modifications for simultaneous acceleration of protons and antiprotons. Precise synchronization between proton and antiproton bunches had to be implemented, for collisions to occur at the centre of the detectors. The RF system was also upgraded to achieve lower noise levels which are required for long storage times.
• Beam diagnostics had to be adapted to very low beam intensities, and new devices added, such as directional couplers for independent obser-vation of protons and antiprotons.
Further modifications to the Sp¯pS collider were made to operate with the ten-fold increase in the antiproton production rate in 1987.
5 SPS as a p¯p collider
During 1981 to 1990, the CERN SPS was operated as a pp¯ collider, pro-viding high energy collisions for two major experiments located in adjacent sextants of the accelerator. This operation was almost always with three dense bunches of protons in collision with three rather weak bunches of antiprotons, with no separation of the beams at the unused crossing points.
The first proton–antiproton collisions were recorded in the summer of 1981. The first physics run took place at the end of that year when 0.2 nb−1 of integrated luminosity was produced. Between 1982 and 1986 the peak luminosity was pushed up to 3.9×1029 cm−2s−1 and more than 1.2 pb−1 of integrated luminosity was accumulated at each of the two main experimental interactions.
Until 1983 the centre-of-mass energy was limited to 546 GeV due to resis-tive heating of the magnet coils. The addition of further water cooling allowed the machine energy to be pushed up to 630 GeV in 1984. The 1986 run was prematurely interrupted because of a major fault of the UA1 detector.
The main limitations to the machine performance during the initial oper-ation phase were the scarcity of antiprotons, space charge and beam–beam effects, longitudinal stability and intra-beam scattering; all imposing con-straints at different operational stages during injection, acceleration and storage.
During 1987 and early 1988, the CERN ¯p complex upgrade was com-pleted and provided significantly more antiprotons. During 1987 another ring, the antiproton collector (ACOL), was added to the existing antiproton accumulator in order to increase the acceptance for ¯pof the complex.
In 1988 the available stack of antiprotons in the accumulation machine was normally between 4 and 7×1011particles, reaching a maximum intensity of 8.5×1011 compared with the previous best stack before the upgrade of 4.5×1011. Furthermore, the average accumulation rate was about 3.3×1010 particles per hour, with a maximum rate of over 3.8×1010, compared with 1.2×1010 achieved previously.
To make the best use of this increased supply of antiprotons, the number of bunches of both protons and antiprotons injected into the collider was increased in 1988 from three to six.
• To reduce the limiting beam–beam effect, electrostatic deflectors (“sep-arators”) separated the beams at 9 of 12 collision points.
• A new RF system at 100 MHz, half the accelerating frequency of 200 MHz, increased the longitudinal acceptance at injection. The more intense bunches (up to 1011 p/bunch) had a larger longitudinal emit-¯ tance. By making them longer, the momentum spread was kept the same, to stay within the SPS momentum acceptance.
• The scheme for chromaticity correction was upgraded.
• In 1990, when UA2 took its last data, a super-squeezed low-beta scheme boosted the luminosity by a further factor of 2.
The yearly integrated luminosity from 1982 to 1990 is shown in Fig. 1.
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Fig. 1. Integrated annual luminosity in units of nb−1 from 1982 to 1990 [10].
6 Operational cycle
After stacking antiprotons for many hours, the PS and SPS prepared for a fill. The first step was the optimisation of the injection cycle with proton and low intensity antiproton pilot bunches. The filling sequence was as follows:
firstly, three (and later six) proton bunches were fabricated in the CERN PS complex and sent to the SPS at 2.4 s intervals and stored at 26 GeV.
These bunches were placed symmetrically around the SPS circumference.
Then a single antiproton bunch was unstacked from the ¯p accumulator and injected into the PS at 3.5 GeV. The PS accelerated the bunch to 26 GeV, and the bunch was transferred to the SPS at an azimuthal position to within a fraction of a nanosecond for collisions in the physics experiments. This was followed by three (or six) bunches of antiprotons extracted from the antiproton stack at 2.4 s and transferred to SPS. The SPS accelerated both beams to an energy of 315 GeV.
During the first two seconds at 315 GeV, the beta functions in the two low-β insertions for the physics detectors UA1 and UA2 were reduced down to their final values for physics operation of typically 1 m horizontally and 0.5 m vertically [11] to increase the luminosity. The collider then passed into storage, and two electrostatic separators were activated, to achieve separa-tion at the unused crossing points away from the experiments. In the experi-ments, the beams were colliding head-on. Horizontal and vertical emittances were similar for both protons and antiprotons. The beams collided for many hours (see Fig. 2).
Fig. 2. The injection sequence, the energy ramp and the initial operation at 315 GeV/cof the SPS collider when operating with 6 bunches per beam. The horizontal axis shows time, the step shaped curve shows the total stored beam intensity, and the solid curve shows the evolution of the main magnet current. The beam energy starts at injection values on the left and reaches collision values on the right.
The entire process of injection, acceleration and beta function squeez-ing was achieved in a 43.2-second cycle, and the separation required for operating with colliding beams took a further few seconds to complete. The experiments could then raise the magnetic fields in the detectors, and data taking by the experiments started within a few minutes.
7 Beam–beam effects
The SPS is an accelerator with normal conducting magnets with little non-linear magnetic field perturbations. Still, resonances of second, third and fourth order turned out to be dangerous and had to be strictly avoided. The beam–beam interaction introduced higher order resonances. During the first years of operation it was understood that it is important to limit the tune spread of the particles to values that could fit between resonances of higher order [12] (see Fig. 3).
The betatron tunes of an ensemble of particles differ from the tune of a single particle because:
• The tune spread due to the beam–beam interaction is in the order of the linear tune shift. In a hadron collider the tune spread is independent of the beam energy. The tune in the SPS for a particle with small betatron amplitude is shifted upwards.
• At injection energy the Laslett space charge detuning must be consid-ered for intense bunches, with the tune of the central particle shifted
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Fig. 3. Tune diagram for three-bunch operation.Q0 is the single particle tune. Due to the coherent tune shift, the measured proton tune isQm. The tune shift due to Laslett detuning for small amplitude protons isQp.Qp¯is the tune for small amplitude antiprotons, shifted due to the beam–beam effect.
downwards. The detuning decreases with 1/γ2 and is therefore negligible at collision energy.
With three proton and antiproton bunches colliding at six points, the tune shift of the antiprotons from the protons was about 0.003 per colli-sion point. The total tune spread was about 0.018. At injection the Laslett incoherent tune spread was about δQh = −0.03 and δQv = −0.05, larger than the beam–beam tune spread but only present at injection when the beam–beam resonances were not yet excited by the beam–beam collisions.
For a good transmission the particles had to be kept clear from resonances of third and fourth order during the time required for injection.
At top energy the destructive effect of 10th order resonances was observed [13]. Figure 4 shows a scan of the tune diagram with three pro-ton bunches and a single weak antipropro-ton bunch performed in the early days of theSppS¯ collider: proton and antiproton bunch intensities recorded with a chart recorder are shown as a function of the tunes. The decay rate of
Fig. 4. Intensity of the proton and antiproton beams during a scan of the betatron tunes [14]. One can see the sharp and fast intensity losses when crossing a resonance.
the antiproton was extremely sensitive to the tunes. When they touched resonances of order 10 or lower, the lifetime dropped from some 100 hours to only 25 hours. At that time no effects from 13th and 16th order resonances were noticed.
During the first years of operation three proton bunches were colliding with three antiproton bunches. The intensity of the antiproton bunches was about 10 times less than the proton bunch intensity due to limitations in the ¯p production.
With a substantially increased ¯p production rate it was later possible to operate with six bunches per beam. To limit beam–beam effects the orbits of the two beams were separated using electrostatic separators at the unwanted collision points (Fig. 5). The intensity of the antiproton bunches was increased to about 60% of the proton bunch intensity and the luminosity increased by nearly one order of magnitude.
The electrostatic separators were installed close to the experiments. The beams were separated by about 6σ at the unwanted crossing points in the horizontal plane during physics operation (the separation is defined as the distance between the beams, σ is the rms horizontal beam size). During injection and ramping one separator created an orbit deformation around the ring with opposite sign for both beams, and the separation between the beams differed for the 12 crossing points between 1.5–6σ [15].
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Fig. 5. Layout of the SPS proton–antiproton collider with the proton and antiproton beams separated by electrostatic deflectors.
During the commissioning of the high luminosity operation theSppS¯ was operated with beams of unequal emittances. The beam–beam effect with beams of different sizes was found to be much more destructive and caused an unacceptable level of background in the physics detectors and a low lifetime for the proton beam, despite the lower intensity of the ¯pbunches. Operation with unbalanced emittances leads to an increase of the strength of high order resonances. After the emittances of both beams were balanced, the adverse effects the of beam–beam interaction were reduced to an acceptable level.
A betatron tune modulation due to unavoidable power supply ripples was always present. With a tracking program it was demonstrated that the stability of the beam decreases due to the beam–beam interaction when such modulation was introduced in the simulation [16]. Without such modulation the particles were much more stable.
8 Pulsed Sp¯pS collider
The SPS was limited to 315 GeV per beam when running as a proton–
antiproton collider due to resistive heating of the coils of the main magnets.
The limit can be overcome by pulsing the magnets between 100 GeV and 450 GeV. During one physics run to study pp¯ collisions at 900 GeV c.m.
energy, beams of protons and antiprotons were collided under these pulsed
Fig. 6. The storage cycle with a length of 21.6 s of the SppS¯ with two plateaus at 100 GeV/cand 450 GeV/c.
conditions. By colliding beams of protons and antiprotons whilst ramping the SPS between 100 GeV and 450 GeV an overview of hadron physics in a very large range of energies could be obtained. Figure 6 shows the machine cycle for stored beams [17].
The first pp¯ collisions at 900 GeV c.m. were observed in March 1985.
Data taking by UA1 on colliding beams whilst ramping between 100 GeV and 450 GeV was done for a total of 95 hours. Initial luminosities were about
Data taking by UA1 on colliding beams whilst ramping between 100 GeV and 450 GeV was done for a total of 95 hours. Initial luminosities were about