The AWAKE (Advanced WAKEfield Experiment) facility at CERN is a proof-of-principle experiment, which investigates wakefield plasma acceleration using a proton bunch as a driver, a world-wide first. It aims to accelerate a low-energy witness bunch of electrons from 15 to 20 MeV to several GeV over a short distance (10 m) by creating a high acceleration gradient of several GV/m. Particle accelerators currently in use, like CERN's LHC, use standard or superconductive RF-cavities for acceleration, but they are limited to an acceleration gradient in the order of 100 MV/m.
Circular accelerator machines are not efficient for transporting electrons at high energy due to the large energy loss in synchrotron radiation. Linear accelerators do not have this issue and are therefore better suited for accelerating and transporting electrons at high energies. [1] [2]
AWAKE's high acceleration gradient will allow the construction of a new generation of shorter and less expensive high energy accelerators, representing a big step in the particle accelerators technology, especially for linear electron accelerators.
A plasma consists of positively charged ions and negatively charged free electrons, while remaining macroscopically neutral. If a strong electric field is applied, ions and electrons can be spatially separated. A local electric field is thereby created, thus a charged particle entering a such plasma can be accelerated. [3]
When the driver, the positively charged proton bunch, penetrates the plasma, it attracts the negatively charged plasma electrons, they overshoot and start to oscillate, creating a wakefield. The interaction between the wakefield and a charged particle injected behind the proton can be interpreted as the same as the one between a surfer and a wave. The latter will transfer its energy to the surfer who will thus be accelerated. The wakefield consists of decelerating and accelerating phase, as well as focusing and defocusing phase. The injection position of the electron bunch in the wakefield is thus crucial, since only a fraction (1/4th) of the wakefield is both focused and accelerated, which is needed for the trapping and the acceleration of the electrons. AWAKE is the first plasma wakefield experiment using a bunch of protons as a driver. Protons, as for example the protons that form the CERN Super Proton Synchrotron (SPS), can carry a large amount of energy (~ 400 GeV). Therefore, they can produce wakefields in a plasma for much longer distances than a laser pulse or electron bunch as a driver due to energy depletion. [4]
A plasma can be seen as an ensemble of oscillators with a frequency of the plasma frequency ωp2=4nee2/εme, with ne the plasma electron density, me the electron mass and e the elementary charge. [5] To excite those oscillators resonantly, the driver must contain a Fourier component close to the plasma frequency ωp. [5] Moreover, the length of the drive bunch should be close to the plasma wavelength λp (=2πc/ωp with c is the speed of light). For AWAKE like density (ne ≈ 1•1015 cm−3) this corresponds to approximately λp ≈ 1 mm. The length of currently available proton bunches though exceeds this value significantly. AWAKE profits form the seeded self-modulation (SSM) of the proton bunch travelling through the plasma, which divides the long proton bunch into shorts micro-bunches with the length of the plasma wavelength that can drive the wakefield resonantly. [4] [5]
The AWAKE experiment is installed at CERN, in the former CERN Neutrinos to Gran Sasso (CNGS) facility. This site was selected for its underground location, and it was specifically designed for the use of high-energy proton beams without any significant radiation issue. [1]
The proton bunches for AWAKE are extracted from the CERN SPS and are transported through an ~800-meter beam-line to the 10-meter long vapor source of AWAKE. The electron witness bunches are injected behind the proton bunch. [4] To detect acceleration of the injected electrons, a dipole magnet is installed after the vapor, bending their path. The larger the electron's energy, the smaller curvature of its path. A scintillation screen then detects accelerated electrons. [2]
The vapor source contains Rubidium (Rb) vapor which is ionized by a Ti:Sapphire laser. The vapor source is surrounded by an oil bath. By setting the temperature of the oil, the Rb vapor density can be set and kept uniform along the vapor source.
AWAKE uses a laser pulse to ionize the Rb vapor. By propagating the laser pulse co-linearly within the proton bunch, the hard edge of the beam/plasma interaction seeds the self-modulation of the proton bunch, enforcing the grow over the 10m long plasma It also allows to create a phase reference for the start of the wakefield, which is needed to inject the witness bunch at the right phase for trapping and acceleration. The electrons are produced by sending the laser onto an RF-gun photo-cathode. [6]
The first run lasted from 2016 to 2018. The ten metre-long vapor source was installed 11 February 2016 and the first proton beam was sent through the beam-line and the empty vapor source on 16 June 2016. The first data with a proton bunch inside the plasma was acquired in December 2016. [4] [2] On 26 May 2018, AWAKE accelerated an electron beam for the first time. The beam was accelerated from 19 MeV to 2 GeV over a distance of 10 m. [7]
A second run is planned for 2021 to 2024. The acceleration gradient will be increased and the emittance is expected to shrink. It is planned to increase the electron energy to 10 GeV. After this phase the goal is to increase the energy to at least 50 GeV and provide beams for first applications. [8]
A cyclotron is a type of particle accelerator invented by Ernest Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. A cyclotron accelerates charged particles outwards from the center of a flat cylindrical vacuum chamber along a spiral path. The particles are held to a spiral trajectory by a static magnetic field and accelerated by a rapidly varying electric field. Lawrence was awarded the 1939 Nobel Prize in Physics for this invention.
The Tevatron was a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory, east of Batavia, Illinois, and was the highest energy particle collider until the Large Hadron Collider (LHC) of the European Organization for Nuclear Research (CERN) was built near Geneva, Switzerland. The Tevatron was a synchrotron that accelerated protons and antiprotons in a 6.28 km (3.90 mi) circumference ring to energies of up to 1 TeV, hence its name. The Tevatron was completed in 1983 at a cost of $120 million and significant upgrade investments were made during its active years of 1983–2011.
A linear particle accelerator is a type of particle accelerator that accelerates charged subatomic particles or ions to a high speed by subjecting them to a series of oscillating electric potentials along a linear beamline. The principles for such machines were proposed by Gustav Ising in 1924, while the first machine that worked was constructed by Rolf Widerøe in 1928 at the RWTH Aachen University. Linacs have many applications: they generate X-rays and high energy electrons for medicinal purposes in radiation therapy, serve as particle injectors for higher-energy accelerators, and are used directly to achieve the highest kinetic energy for light particles for particle physics.
The Large Electron–Positron Collider (LEP) was one of the largest particle accelerators ever constructed. It was built at CERN, a multi-national centre for research in nuclear and particle physics near Geneva, Switzerland.
A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The magnetic field which bends the particle beam into its closed path increases with time during the accelerating process, being synchronized to the increasing kinetic energy of the particles.
A charged particle beam is a spatially localized group of electrically charged particles that have approximately the same position, kinetic energy, and direction. The kinetic energies of the particles are much larger than the energies of particles at ambient temperature. The high energy and directionality of charged particle beams make them useful for many applications in particle physics.
The Compact Linear Collider (CLIC) is a concept for a future linear particle accelerator that aims to explore the next energy frontier. CLIC would collide electrons with positrons and is currently the only mature option for a multi-TeV linear collider. The accelerator would be between 11 and 50 km long, more than ten times longer than the existing Stanford Linear Accelerator (SLAC) in California, US. CLIC is proposed to be built at CERN, across the border between France and Switzerland near Geneva, with first beams starting by the time the Large Hadron Collider (LHC) has finished operations around 2035.
High-energy nuclear physics studies the behavior of nuclear matter in energy regimes typical of high-energy physics. The primary focus of this field is the study of heavy-ion collisions, as compared to lighter atoms in other particle accelerators. At sufficient collision energies, these types of collisions are theorized to produce the quark–gluon plasma. In peripheral nuclear collisions at high energies one expects to obtain information on the electromagnetic production of leptons and mesons that are not accessible in electron–positron colliders due to their much smaller luminosities.
The Super Proton Synchrotron (SPS) is a particle accelerator of the synchrotron type at CERN. It is housed in a circular tunnel, 6.9 kilometres (4.3 mi) in circumference, straddling the border of France and Switzerland near Geneva, Switzerland.
HERA was a particle accelerator at DESY in Hamburg. It was operated from 1992 to 30 June 2007. At HERA, electrons or positrons were brought to collision with protons at a center-of-mass energy of 320 GeV. HERA was used mainly to study the structure of protons and the properties of quarks, laying the foundation for much of the science done at the Large Hadron Collider (LHC) at the CERN particle physics laboratory today. HERA is the only lepton–proton collider in the world to date and was on the energy frontier in certain regions of the kinematic range.
A particle beam is a stream of charged or neutral particles. In particle accelerators, these particles can move with a velocity close to the speed of light. There is a difference between the creation and control of charged particle beams and neutral particle beams, as only the first type can be manipulated to a sufficient extent by devices based on electromagnetism. The manipulation and diagnostics of charged particle beams at high kinetic energies using particle accelerators are main topics of accelerator physics.
Plasma acceleration is a technique for accelerating charged particles, such as electrons or ions, using the electric field associated with electron plasma wave or other high-gradient plasma structures. These plasma acceleration structures are created using either ultra-short laser pulses or energetic particle beams that are matched to the plasma parameters. The technique offers a way to build affordable and compact particle accelerators.
The Proton Synchrotron is a particle accelerator at CERN. It is CERN's first synchrotron, beginning its operation in 1959. For a brief period the PS was the world's highest energy particle accelerator. It has since served as a pre-accelerator for the Intersecting Storage Rings (ISR) and the Super Proton Synchrotron (SPS), and is currently part of the Large Hadron Collider (LHC) accelerator complex. In addition to protons, PS has accelerated alpha particles, oxygen and sulfur nuclei, electrons, positrons, and antiprotons.
The Alternating Gradient Synchrotron (AGS) is a particle accelerator located at the Brookhaven National Laboratory in Long Island, New York, United States.
A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies to contain them in well-defined beams. Small accelerators are used for fundamental research in particle physics. Accelerators are also used as synchrotron light sources for the study of condensed matter physics. Smaller particle accelerators are used in a wide variety of applications, including particle therapy for oncological purposes, radioisotope production for medical diagnostics, ion implanters for the manufacture of semiconductors, and accelerator mass spectrometers for measurements of rare isotopes such as radiocarbon.
A Fixed-Field alternating gradient Accelerator is a circular particle accelerator concept that can be characterized by its time-independent magnetic fields and the use of alternating gradient strong focusing.
The INFN National Laboratory of Frascati (LNF) was founded in 1954 with the objective of furthering particle physics research, and more specifically to host the 1.1 GeV electrosynchrotron, the first accelerator ever built in Italy. The Laboratory later developed the first ever electron-positron collider: from the first prototype AdA, which demonstrated the feasibility, to the ring ADONE and later on to DAΦNE, still operative today (2024). LNF was also the proposed site of the cancelled particle accelerator SuperB.
Chandrashekhar Janardan Joshi is an Indian–American experimental plasma physicist. He is known for his pioneering work in plasma-based particle acceleration techniques for which he won the 2006 James Clerk Maxwell Prize for Plasma Physics and the 2023 Hannes Alfvén Prize.
CTF3 was an electron accelerator facility built at CERN with the aim of demonstrating the key concepts of the Compact Linear Collider accelerator. The facility consisted in two electron beamlines to mimic the functionalities of the CLIC Drive Beam and Main Beam. The facility stopped its operation in December 2016, and one of its beamlines has been converted into the new CERN Linear Electron Accelerator for Research (CLEAR) facility.
The Super Proton–Antiproton Synchrotron was a particle accelerator that operated at CERN from 1981 to 1991. To operate as a proton-antiproton collider the Super Proton Synchrotron (SPS) underwent substantial modifications, altering it from a one beam synchrotron to a two-beam collider. The main experiments at the accelerator were UA1 and UA2, where the W and Z bosons were discovered in 1983. Carlo Rubbia and Simon van der Meer received the 1984 Nobel Prize in Physics for their contributions to the SppS-project, which led to the discovery of the W and Z bosons. Other experiments conducted at the SppS were UA4, UA5 and UA8.