Proton Synchrotron
The Proton Synchrotron (PS) is a particle accelerator at CERN, located near Geneva, Switzerland, that serves as a key component in the laboratory's injector chain, accelerating protons, heavy ions, electrons, positrons, and antiprotons to energies of up to 26 GeV within a circular ring of 628 meters in circumference.[1][2] Constructed between 1956 and 1959 under the leadership of John Adams, it became operational on 24 November 1959, when it first accelerated protons to 24 GeV, briefly holding the record as the world's highest-energy particle accelerator at that time.[3][2] Initially designed for fixed-target experiments to probe high-energy particle interactions, the PS pioneered techniques such as strong focusing with 100 combined-function electromagnets (reaching 1.4 tesla at full energy) and beam extraction methods introduced in 1963, enabling it to supply particles to downstream facilities.[2] By the 1970s, its role evolved to primarily feed beams into more powerful accelerators like the Super Proton Synchrotron (SPS) and, later, the Large Hadron Collider (LHC), while also supporting experiments at facilities such as the Antiproton Decelerator (AD), ISOLDE, n_TOF, and fixed-target setups.[1][2] Over its more than six decades of operation, the PS has seen beam intensity increase by a factor of a thousand since 1959, accommodating diverse particle types including alpha particles, oxygen, sulfur, argon, xenon, and lead nuclei, and contributing to CERN's foundational discoveries in particle physics.[1][3]History
Conception and preliminary studies
In the aftermath of World War II, European particle physics faced significant challenges, including the destruction of laboratories and the emigration of scientists, prompting a concerted effort to rebuild through international collaboration and advanced infrastructure. Existing cyclotrons, such as those in national facilities, were limited to energies around 200 MeV, insufficient for replicating cosmic ray phenomena observed at GeV scales in controlled experiments. This need for higher-energy accelerators was underscored by developments in the United States, particularly the Brookhaven Cosmotron, a 3 GeV weak-focusing proton synchrotron that began operations in 1952 and demonstrated the feasibility of synchrotron technology for laboratory-based high-energy physics.[4][5] CERN's Provisional Council, established in 1952 following UNESCO's endorsement of a European laboratory in 1950, prioritized a major accelerator as its flagship project. In May 1952, the Council formed the Proton Synchrotron (PS) Study Group, led initially by Norwegian physicist Odd Dahl, to explore designs beyond scaled-up cyclotrons. The group initially considered a 10 GeV weak-focusing synchrotron similar to the Cosmotron but, after a pivotal visit to Brookhaven in August 1952, adopted the newly proposed alternating-gradient (strong-focusing) principle, invented by Ernest Courant and colleagues earlier that year. This innovative approach promised a more compact machine capable of reaching 25-30 GeV with reduced magnet mass, enabling Europe to compete globally. British physicist John Adams, who joined CERN in 1953, played a crucial role in refining these proposals, becoming Director of the PS Division in 1954 and advocating for the strong-focusing design during feasibility deliberations.[4][5][6] Feasibility studies, greenlit by the Provisional Council in October 1952, evaluated the strong-focusing synchrotron against alternatives, confirming its viability for a 25 GeV proton energy with an initial budget estimate of approximately 25 million Swiss francs. Site selection favored Geneva in the same month, due to its central location facilitating contributions from twelve founding European states and proximity to international borders, a decision ratified by local referendums in 1953. These studies highlighted key engineering challenges, including the development of ultra-high vacuum systems to maintain pressures around 10^{-5} Torr to prevent beam scattering and the precise design of 100 combined-function magnets totaling 3,800 tonnes, requiring field homogeneity within 10^{-3} at 1.2 T peak induction. International input from European laboratories, such as those in France and the UK, alongside exchanges with Brookhaven experts, shaped these assessments, ensuring the PS would serve as CERN's foundational accelerator.[4][7][5]Construction
The construction of the Proton Synchrotron (PS) commenced with groundbreaking on 17 May 1954 at CERN's Meyrin site near Geneva, Switzerland.[4] Excavation of the underground tunnel followed, with the 628-meter circumference ring—a reinforced concrete structure embedded in molasse rock—completed by mid-1957.[1][4] The project, spanning 1954 to 1959, represented a major international engineering endeavor under the leadership of J.B. Adams as project engineer, drawing on the synchrotron principle for high-energy proton acceleration.[8] Key engineering challenges were overcome in fabricating and installing the machine's core components. The ring featured 100 dipole electromagnets, each a combined-function unit weighing approximately 30 tons and providing a bending radius of about 100 meters for the proton beam path.[1][9] These magnets, totaling around 3,800 tons, were supplied by international firms including Brown Boveri (Switzerland), Ansaldo (Italy), and ACEC (Belgium).[4][8] The vacuum system, essential for minimizing beam-gas interactions, achieved an operating pressure of 10^{-5} Torr in the thin stainless-steel chamber encircling the ring.[10] The radiofrequency (RF) acceleration system comprised 16 ferrite-loaded cavities operating in the 2–16 MHz range to boost protons from injection energy to 28 GeV.[4] The effort engaged a dedicated team that grew to 143 members by late 1956, supplemented by contributions from European laboratories such as Harwell in the UK and facilities in France, alongside contracted industrial expertise.[4][8] Construction faced delays from precise magnet alignment requirements and power supply integration, contributing to budget overruns beyond the initial estimate of 130 million Swiss francs.[4] Safety measures incorporated radiation shielding via the tunnel's concrete ring beams and pillars, with later enhancements addressing beam-induced irradiation concerns.[4]Commissioning and early operations
The commissioning of the CERN Proton Synchrotron (PS) began in earnest in the summer of 1959, following the completion of its construction. Protons were first injected from the Linac 1 linear accelerator at an energy of 50 MeV, achieving the initial beam circulation on 16 September 1959.[4] By the end of November, significant progress had been made, with the first full acceleration of the proton beam to a kinetic energy of 24 GeV occurring on 24 November 1959, marking the machine's breakthrough through the transition energy barrier.[11] A few weeks later, in December 1959, adjustments to the magnetic field cycle enabled the PS to reach its design energy of 28 GeV kinetic energy, establishing it briefly as the world's highest-energy particle accelerator.[11] Early operations were marked by several technical challenges that required innovative solutions to achieve stable performance. Beam instability, particularly due to interactions at integer resonances around 12 kG magnetic fields, led to significant losses during acceleration, which were mitigated through the addition of pole face windings to maintain focusing up to 14 kG.[11] Vacuum leaks and inadequate pressure levels also posed issues, with initial vacuum conditions reaching only about 10^{-5} Torr, necessitating ongoing improvements to the chamber design and pumping systems for reliable beam storage.[4] Injection from Linac 1 proved tricky, with capture efficiencies starting at 20-25% for injected currents up to 1 mA, but these hurdles were gradually overcome through refinements in the single-turn injection process.[11] Key milestones in the initial years underscored the PS's rapid maturation. The first fixed-target experiments commenced in June 1960, utilizing beams with intensities around 6 \times 10^{10} protons per pulse to probe particle interactions in the South Hall.[4] By 1963, the machine had stabilized at its peak energy of 28 GeV, and the implementation of the first fast extraction system allowed for more efficient delivery of high-energy protons to experimental areas, enabling mid-year data collection for early neutrino studies.[4] The operational setup during commissioning featured a 1-second acceleration cycle, with an initial beam intensity of 10^{10} protons per pulse and a repetition rate of approximately 20 pulses per minute, corresponding to a roughly 3-second overall cycle including preparation and extraction phases.[11] Intensities quickly improved, reaching 3 \times 10^{11} protons per pulse by the end of 1960, supporting stable physics runs.[4] By 1964, the PS had transitioned to routine operations, with beam intensities exceeding 10^{12} protons per pulse and the introduction of slow extraction techniques using integer resonances, facilitating the first international user runs and a growing experimental program.[4] This period solidified the PS's role as a cornerstone of high-energy physics at CERN, overcoming initial teething issues to deliver reliable performance for global researchers.[4]Design and Technical Specifications
Principles of operation
The Proton Synchrotron (PS) functions as a circular particle accelerator employing the synchrotron principle, in which charged particles, primarily protons, are confined to a fixed orbital radius by a time-varying magnetic field while their energy is simultaneously increased to maintain the orbit. The centripetal force required for circular motion is provided by the Lorentz force, given by \mathbf{F} = q (\mathbf{v} \times \mathbf{B}), where q is the particle charge, \mathbf{v} is its velocity, and \mathbf{B} is the magnetic field perpendicular to the velocity. For relativistic protons, the orbital radius r is related to the momentum p = \gamma m v (with \gamma the Lorentz factor and m the rest mass) via p = q B r, ensuring that as energy E rises, B is ramped proportionally to keep r constant at approximately 100 m. This strong-focusing design, using alternating-gradient magnets, stabilizes the beam transversely while minimizing aperture requirements.[12] The acceleration cycle begins with injection of protons at an initial kinetic energy of 2 GeV from the Proton Synchrotron Booster (PSB) since the LHC Injectors Upgrade, though previously 1.4 GeV and historically 50 MeV from a linear accelerator. Protons are accelerated over a cycle time of 1.0–1.2 seconds, reaching a maximum kinetic energy of 26 GeV (momentum 26 GeV/c) for injection into higher-energy machines like the Large Hadron Collider (LHC). Energy gain occurs via radio-frequency (RF) cavities that provide an accelerating electric field E_{rf}, yielding a per-turn energy increment \Delta E = q V_{rf} \sin \phi, where V_{rf} is the RF voltage (up to several kV) and \phi is the phase at which the particle crosses the cavity. The PS employs 16 ferrite-loaded cavities tunable from 3 MHz to 10 MHz to match the increasing orbital frequency as particle velocity approaches the speed of light, with the RF frequency modulated synchronously with the magnetic field ramp. The maximum energy is constrained by E_{\max} \approx q B_{\max} \rho c, where \rho \approx 70 m is the dipole bending radius, nominal B_{\max} = 1.2 T (12 kG) for 25 GeV, though up to 1.4 T for 28 GeV, yielding approximately 25 GeV nominally.[12][13] Beam dynamics in the PS are governed by collective effects and single-particle motion, with synchrotron radiation losses negligible for protons due to their high mass compared to electrons, unlike in electron synchrotrons where such radiation dominates energy loss and damping. Instead, space charge effects—arising from electrostatic repulsion among protons in the bunch—are significant at low energies, limiting beam intensity and requiring mitigation through higher injection energies (e.g., via the ongoing LHC Injector Upgrade) and careful aperture design. Longitudinal and transverse instabilities, such as those from impedance mismatches, are managed through Landau damping via controlled beam distribution and octupole magnets for nonlinear focusing. Injection utilizes H⁻ charge exchange, where negatively charged hydrogen ions from the PSB are stripped of electrons upon passing through a thin foil, converting them to protons that merge with the circulating beam over multiple turns to build intensity without significant emittance growth. Extraction employs fast kicker magnets to deflect the beam into transfer lines toward targets or subsequent accelerators, or slow extraction via nonlinear resonances for fixed-target experiments, ensuring efficient delivery of up to $10^{13} protons per pulse.[12]Key components and layout
The Proton Synchrotron (PS) features an underground circular tunnel with a circumference of 628 meters, housing the accelerator ring on a floating concrete floor to minimize vibrations. The layout consists of 100 main combined-function magnets (among 277 total electromagnets) arranged in a regular pattern, spaced approximately every 6.28 meters, interleaved with 100 straight sections that accommodate auxiliary equipment. Among these, four longer straight sections are dedicated to critical functions such as radiofrequency (RF) acceleration and beam injection/extraction systems, enabling efficient beam manipulation within the compact design.[14][2][15][1] The magnet system comprises 100 C-shaped combined-function units that integrate dipole bending and quadrupole focusing capabilities, constructed with an iron yoke and copper coils for excitation. These room-temperature electromagnets achieve a maximum field strength of up to 1.4 tesla at full energy of 28 GeV, with pole-face windings allowing precise adjustments to the magnetic field profile. The total electrical power consumption for the magnet system peaks at around 40 MW during acceleration cycles, supported by a solid-state power converter with capacitive energy storage for rapid cycling. Water cooling systems maintain the magnets at operational temperatures, dissipating heat from the coils and yoke.[15][16][17] The RF acceleration system originally utilized four double-gap cavities positioned in the dedicated straight sections, each delivering up to 200 kV for a total accelerating voltage of around 800 kV per turn to boost proton energy from injection to extraction levels. Over time, the setup evolved to include 11 ferrite-loaded cavities operating at around 20 kV peak each, with additional auxiliary cavities at 80 MHz for bunch-length manipulation in modern operations. These components provide the necessary electric fields for synchronous acceleration while compensating for beam loading effects.[15][7] The vacuum system employs stainless steel chambers integrated into the magnet units to maintain ultra-high vacuum conditions, essential for minimizing beam-gas interactions. Ion pumps, distributed along the ring, achieve average pressures around 10^{-8} mbar, supplemented by roughing pumps for initial evacuation; no bake-out is required in the original design. Beam diagnostics include beam position monitors (BPMs) installed at regular intervals in the straight sections to track transverse beam trajectories non-destructively, alongside current transformers for measuring beam intensity and bunch charge with high precision.[18][19][20][21] Auxiliary systems operate without cryogenics, as the entire machine functions at room temperature, relying instead on conventional water cooling circuits for the magnets, RF components, and power converters to manage thermal loads effectively. This design choice simplifies maintenance and aligns with the PS's role as a versatile injector in CERN's accelerator chain.[1]Performance parameters and upgrades
The Proton Synchrotron (PS) was originally designed to achieve a maximum proton energy of 25 GeV, with capabilities extending to 28 GeV at reduced operational rates, delivering approximately $10^{11} to $10^{12} protons per pulse over a basic cycle time of 1.2 seconds and an overall efficiency around 50%. These parameters enabled initial fixed-target experiments and served as a foundational benchmark for CERN's accelerator complex. In the 1970s, upgrades focused on improving injection for the Intersecting Storage Rings (ISR), including the implementation of multi-turn injection from the newly commissioned PS Booster at 800 MeV, which boosted beam intensity to up to $5 \times 10^{12} protons per pulse and enhanced overall brightness. Further enhancements included the introduction of γ-transition jump systems in 1973 for better stability during acceleration and new RF cavities (C10 in 1972 and C200 in 1977–1979) to support higher intensities and controlled beam blow-up. By the early 1990s, extraction systems like the SE61 slow extraction achieved over 95% efficiency at 24 GeV/c with $2 \times 10^{11} protons per pulse, laying groundwork for improved beam quality in subsequent operations. The LHC Injectors Upgrade (LIU) project in the 2010s significantly advanced PS capabilities to meet LHC demands, incorporating charge-exchange injection of H⁻ ions from the upgraded PS Booster for higher beam brightness and support for 25 ns bunch spacing via RF gymnastics and longitudinal feedback systems.[22] Integration with Linac4, operational since 2020, raised injection energy to 160 MeV H⁻, enabling 2 GeV transfer to the PS and reducing emittance growth.[22] Following the second Long Shutdown (LS2) in 2022, enhancements included a new transverse damper with wideband feedback to improve stability against instabilities, alongside beam intensity increases to $2 \times 10^{13} protons per pulse and a stabilized maximum energy of 26 GeV for SPS injection. These upgrades ensure the PS serves as a critical pre-accelerator in the LHC chain, delivering batches with up to $2.9 \times 10^{11} protons per bunch in configurations supporting the High-Luminosity LHC (HL-LHC) goals of 120 fb⁻¹ integrated luminosity per year. Despite these advances, the PS faces limitations such as space charge effects at low energies, which cause emittance growth and require tune adjustments in the PS Booster, and radiation damage to components like kickers and vacuum systems from beam-induced heating.| Parameter | Original (1959–1960s) | Current (Post-2022) |
|---|---|---|
| Max Energy | 25–28 GeV | 26 GeV |
| Protons per Pulse | $10^{11}–$10^{12} | $2 \times 10^{13} |
| Cycle Time | 1.2 s (basic) | ~1.2 s (with supercycles) |
| Injection Energy | ~50 MeV (Linac1) | 2 GeV (from PSB/Linac4) |
| Efficiency | ~50% | >95% (extraction) |