The AWAKE experiment is designed to study electron acceleration in plasma wakefields driven by self-modulated proton bunches. This project focuses on analyzing the transverse, time-integrated profile of the proton bunches after self modulation in rubidium plasma. The size and shape of these profiles can be used to verify the presence of self-modulation for each event and to study the influence of experimental parameters on the modulation process.
We summarize and explain the realization of witness particle injection into wakefields for the AWAKE experiment. In AWAKE, the plasma wakefields are driven by a self-modulating relativistic proton bunch. To demonstrate that these wakefields can accelerate charged particles, we inject a \unit[10-20]{MeV} electron bunch produced by a photo-injector. We summarize the experimental challenges of this injection process and present our plans for the near future.
The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) is a project at CERN aiming to accelerate an electron bunch in a plasma wakefield driven by a proton bunch. The plasma is induced in a 10 m long rubidium vapor cell using a pulsed Ti:Sapphire laser, with the wakefield formed by a proton bunch from the CERN Super Proton Synchrotron (SPS). A 16 MeV electron bunch is simultaneously injected into the plasma cell to be accelerated by the wakefield to energies in the GeV range over this short distance. After successful runs with the proton and laser beams, the electron beam line was installed and commissioned at the end of 2017 to produce and inject a suitable electron bunch into the plasma cell. To achieve the goals of the experiment, it is important to have reliable beam instrumentation measuring the various parameters of the proton, electron and laser beams. This contribution presents the status of the beam instrumentation in AWAKE and reports on the performance achieved during the AWAKE runs in 2017.
We use particle-in-cell (PIC) simulations to study the effects of variations of the incoming 400 GeV proton bunch parameters on the amplitude and phase of the wakefields resulting from a seeded self-modulation (SSM) process. We find that these effects are largest during the growth of the SSM, i.e. over the first five to six meters of plasma with an electron density of $7 \times 10^{14}$ cm$^{-3}$. However, for variations of any single parameter by $\pm$5%, effects after the SSM saturation point are small. In particular, the phase variations correspond to much less than a quarter wakefield period, making deterministic injection of electrons (or positrons) into the accelerating and focusing phase of the wakefields in principle possible. We use the wakefields from the simulations and a simple test electron model to determine the injection position along the bunch and along the plasma leading to the largest energy gain. This analysis includes the dephasing of the electrons with respect to the wakefields that is expected during the growth of the SSM. We find that the optimum position along the proton bunch is at $\xi \approx -1.5 \; \sigma_{zb}$, and that the optimal range for injection along the plasma (for a highest final energy of $\sim$1.6 GeV after 10 m) is 5-6 m. The latter result is obtained from a PIC simulation that tests different injection points and is also used to validate the model mentioned above.
The stripline BPMs of AWAKE (The Advanced Proton Driven Plasma Wakefield Acceleration Experiment at CERN) are required to measure the position of the single electron bunch with a resolution of 10 um rms, for the bunch charge of 100 pC to 1 nC. This paper presents a AFE-Digital processing system developed at TRIUMF (Canada) which achieved such performance. The design of the electronics readout system is reviewed. The beam test results at CALIFES of CERN are also described.
The AWAKE experiment at CERN aims to use a proton driven plasma wakefield to accelerate electrons from 10–20 MeV up to GeV energies in a 10 m plasma cell. We present the design of the magnetic spectrometer which will measure the electron energy distribution. Results from the calibration of the spectrometer's scintillator and optical system are presented, along with a study of the backgrounds generated by the 400 GeV SPS proton beam.
The AWAKE experiment reached all planned milestones during Run 1 (2016-18), notably the demonstration of strong plasma wakes generated by proton beams and the acceleration of externally injected electrons to multi-GeV energy levels in the proton driven plasma wakefields. During Run~2 (2021 - 2024) AWAKE aims to demonstrate the scalability and the acceleration of electrons to high energies while maintaining the beam quality. Assuming continued success of the AWAKE program, AWAKE will be in the position to use the AWAKE scheme for particle physics applications such as fixed target experiments for dark photon searches and also for future electron-proton or electron-ion colliders. With strong support from the accelerator and high energy physics community, these experiments could be installed during CERN LS3; integration and beam line design studies show the feasibility of a fixed target experiment in the AWAKE facility, downstream of the AWAKE experiment in the former CNGS area. The expected electrons on target for fixed target experiments exceeds the electrons on target by three to four orders of magnitude with respect to the current NA64 experiment, making it a very promising experiment in the search for new physics. Studies show that electrons can be accelerated to 70 GeV in a 130 m long plasma cell installed in an extended TI 2 extraction tunnel from SPS to the LHC and transported to collision with protons/ions from the LHC. The experiment would focus on studies of the structure of matter and QCD in a new kinematic domain. The AWAKE scheme offers great potential for future high energy physics applications and it is the right moment now to support further development of this technology leading to unique facilities.
The AWAKE experiment had a very successful Run 1 (2016-8), demonstrating proton-driven plasma wakefield acceleration for the first time, through the observation of the modulation of a long proton bunch into micro-bunches and the acceleration of electrons up to 2 GeV in 10 m of plasma. The aims of AWAKE Run 2 (2021-4) are to have high-charge bunches of electrons accelerated to high energy, about 10 GeV, maintaining beam quality through the plasma and showing that the process is scalable. The AWAKE scheme is therefore a promising method to accelerate electrons to high energy over short distances and so develop a useable technology for particle physics experiments. Using proton bunches from the SPS, the acceleration of electron bunches up to about 50 GeV should be possible. Using the LHC proton bunches to drive wakefields could lead to multi-TeV electron bunches, e.g. with 3 TeV acceleration achieved in 4 km of plasma. This document outlines some of the applications of the AWAKE scheme to particle physics and shows that the AWAKE technology could lead to unique facilities and experiments that would otherwise not be possible. In particular, experiments are proposed to search for dark photons, measure strong field QED and investigate new physics in electron--proton collisions. The community is also invited to consider applications for electron beams up to the TeV scale.
Proton-driven plasma wakefield acceleration allows the transfer of energy from a proton bunch to a trailing bunch of particles, the `witness' particles, via plasma electrons. The AWAKE experiment at CERN is pursuing a demonstration of this scheme using bunches of protons from the CERN SPS. Assuming continued success of the AWAKE program, high energy electron or muon beams will become available, opening up an extensive array of future particle physics projects from beam dump searches for new weakly interacting particles such as Dark Photons, to fixed target physics programs, to energy frontier electron-proton, electron-ion, electron-positron and muon colliders. The time is right for the particle physics community to offer strong support to the pursuit of this new technology as it will open up new avenues for high energy particle physics.
In this article, we briefly summarize the experiments performed during the first Run of the Advanced Wakefield Experiment, AWAKE, at CERN (European Organization for Nuclear Research). The final goal of AWAKE Run 1 (2013 - 2018) was to demonstrate that \unit[10-20]{MeV} electrons can be accelerated to GeV-energies in a plasma wakefield driven by a highly-relativistic self-modulated proton bunch. We describe the experiment, outline the measurement concept and present first results. Last, we outline our plans for the future.
In this thesis I study experimentally electron beam loss signals in AWAKE, the Advanced WAKEfield experiment at CERN. In AWAKE, electrons are accelerated by obliquely injecting them into the plasma wakefields driven by a self-modulating relativistic proton bunch. Due to the complexity of the 10 meter long vapor source that provides the plasma, we have to transport and inject the electrons through a 10 mm diameter entrance aperture. I designed, simulated and implemented a diagnostic system to study physics properties of the external injection of the 18MeV/c electron bunch into the plasma. We have installed seven scintillating detectors along the plasma as electron beam loss monitors. Each detector measures the secondary particles produced when the electron bunch interacts with material. To prove the feasibility of the system and to support understanding of the results, I run FLUKA simulations of the setup. According to simulations, secondary particles can exit the vapor source and their energy deposition in the detectors is above the detection threshold of 100 keV. The spatial resolution, determined by the distance between individual detectors, allows to estimate where the beam is lost and whether it interacts with the material surrounding the entrance aperture. We measured the electron transverse beam size at the aperture location, for different beam focus positions and beam charges, scanning the electron beam across the vapor source entrance aperture, while recording the beam loss monitor signals. For the 200 pC electron bunch, the r.m.s. transverse beam size (σx, σy) at the entrance increases from (0.45 ± 0.02, 0.33 ± 0.04) mm to (2.6 ± 0.4, 0.9 ± 0.1) mm as the beam is focused further inside the plasma. Furthermore, I observed the beam size to increase with the charge as σ600 ∼√ (2)σ200 (where σ600 and σ200 are the r.m.s. beam sizes for the 600 and 200 pC beams respectively), as expected from theoretical predictions. Spatial electron, proton and laser beam alignment is one of the crucial issues of the AWAKE experiment; therefore, we were interested in quantifying the deviation caused by the earth magnetic field on the electron beam trajectory in order to precisely overlap the beams. Aligning the electron beam onto a proton reference trajectory and scanning both beams across the aperture, I estimated the deflection from the straight trajectory to be: (1.2±0.1) mm in the horizontal plane (bending to the right) and (0.4 ± 0.1) mm in the vertical plane (bending downward). Beam loss detection gives also information on the beam propagation along the vapor source. I estimated electron beam losses at the entrance for different beam focusing optics and studied the propagation of electrons in vacuum and within the plasma channel. During the acceleration experiment, at the presence of proton driven wakefields, I observed an increase of electron losses downstream the injection point. This may be explained considering defocusing wakefields acting on part of the injected electron bunch. Additionally, studying the background generated by the proton beam on the beam loss monitors, I observed satellite pre-bunches ahead of the main proton bunch delivered by the CERN Super Proton Synchrotron.
Plasma wakefield accelerators promise to deliver orders of magnitude higher accelerating gradients than conventional accelerator technology. Whether the technology is used for even higher energy accelerators than exist today, or more compact accelerators, the promise of high gradients has sparked a number of plasma wakefield experiments over the last few decades. The Advanced Wakefield Experiment (AWAKE) is the first to exploit the self-modulation instability in long particle bunches in plasma in combination with a proton bunch from an existing high energy synchrotron. The experiment is located at CERN and connected to the Super Proton Synchrotron (SPS). The first run of AWAKE saw electrons accelerated from 19 mega-electronvolts (MeV) to 2 giga-electronvolts (GeV) in just 10 metres of ionised Rubidium vapour, achieving a gradient of nearly 200 MV/m. A challenge facing plasma wakefield accelerator designs is the final quality of the accelerated bunch in terms of its spread in energy and its emittance. In order to minimise both these parameters while retaining a high accelerating gradient – goals that are to an extent in conflict – the electron bunch needs to load the generated fields in such a manner that it is as uniform as possible over the length of the bunch. Computer simulations are needed to pinpoint the parameters that balance these opposing goals. Part of the work included integration of the experiment into the control system at CERN. However, most of the work presented in this thesis seeks, through computer simulations, to inform design choices for the next run of AWAKE, scheduled to start in 2021. The simulations show that it is, under otherwise ideal conditions, possible to accelerate 30 to 70 pico-Coulomb (pC) of electrons in an accelerator like AWAKE up to 1.8 to 2 GeV in a 4 metre plasma stage, with an energy spread of less than 2 percent and no significant emittance growth. Low energy spread is achieved by finely tuning the witness bunch size and density to fit the plasma parameters as well as the wakefields generated by the drive bunch. Low emittance growth is achieved by exploiting the wake generated by the head of the witness bunch to create a stable condition for the tail of the bunch.
A magnetic spectrometer has been developed for the AWAKE experiment at CERN in order to measure the energy distribution of bunches of electrons accelerated in wakefields generated by proton bunches in plasma. AWAKE is a proof-of-principle experiment for proton-driven plasma wakefield acceleration, using proton bunches from the SPS. Electron bunches are accelerated to $\mathcal{O}$(1 GeV) in a rubidium plasma cell and then separated from the proton bunches via a dipole magnet. The dipole magnet also induces an energy-dependent spatial horizontal spread on the electron bunch which then impacts on a scintillator screen. The scintillation photons emitted are transported via three highly-reflective mirrors to an intensified CCD camera, housed in a dark room, which passes the images to the CERN controls system for storage and further analysis. Given the known magnetic field and determination of the efficiencies of the system, the spatial spread of the scintillation photons can be converted to an electron energy distribution. A lamp attached on a rail in front of the scintillator is used to calibrate the optical system, with calibration of the scintillator screen's response to electrons carried out at the CLEAR facility at CERN. In this article, the design of the AWAKE spectrometer is presented, along with the calibrations carried out and expected performance such that the energy distribution of accelerated electrons can be measured.
The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE), based at CERN, explores the use of a proton driven plasma wake-field to accelerate electrons at high energies over short distances. This paper introduces the Beam Position Measurement (BPM) system of the proton beamline and its performance. This BPM system is composed of 21 dual plane button pickups distributed along the 700m long transfer line from the CERN Super Proton Synchrotron (SPS) extraction point to beyond the plasma cell. The electrical pulses from the pickups are converted into analogue signals proportional to the intensity and displacement of the beam using logarithmic amplifiers, giving the system a high dynamic range (>50dB). These signals are digitized and processed by an FPGA-based front-end card featuring an ADC sampling at 40Msps. Each time a bunch is detected, the intensity and position data is sent over 1km of copper cable to surface electronics through a serial link at 10 Mbps. There, the data is further processed and stored. The dynamic range, resolution, noise and linearity of the system as evaluated from the laboratory and 2016 beam commissioning data will be discussed in detail.
The AWAKE collaboration prepares a proton driven plasma wakefield acceleration experiment using the SPS beam at CERN. A long proton bunch extracted from the SPS interacts with a high power laser and a 10 m long rubidium vapor plasma cell to create strong wakefields allowing sustained electron acceleration. The electron beam to probe these wakefields is created by an electron accelerator consisting of an rf-gun and a booster structure. This electron source should provide beams with intensities between 0.1 and 1 nC, bunch lengths between 0.3 and 3 ps and an emittance of the order of 2 mm mrad. The booster structure should accelerate the electrons to 16 MeV. The electron line includes a series of diagnostics (pepper-pot, BPMs, spectrometer, Faraday cup and screens) and an optical transfer line merges the electron beam with the proton beam on the same axis. The installation of the electron line started in early 2017 and the commissioning will take place at the end of 2017. The first phase of operation is called RUN1. After the long shutdown of LHC a second phase for AWAKE is planned starting 2021 called RUN2. In this phase the aim is to demonstrate the acceleration of high quality electron beams therefore a bunch length of the order of 100 fs rms is required corresponding to a fraction of the plasma wavelength. The AWAKE collaboration is studying the design of such an injector either based on classical rf-gun injectors or on laser wake-field acceleration. The focus for the RF accelerator is on a hybrid design using an S-band rf-gun and x-band bunching and acceleration cavities. The layout of the current and the future electron accelerator and transfer line, including the diagnostics will be presented.
Proton Beam-Driven Plasma Wakefield Accelerator (PBD-PWFA) has been actively investigated at CERN within the AWAKE experiments to study the electron beam acceleration using plasma wake fields of the order of GV/m. In the AWAKE RUN 1 experiment an electron beam with an energy of 19 MeV and a bunch length of 2.2 ps rms has been used for the first demonstration of electron beam acceleration in the plasma wake fields. It has been observed that the energy gain of the electron beam is up to 2 GeV, and electron capture efficiency is few percent. Higher capturing efficiency and emittance preservation could be achieved by making the electron beam short enough to be injected only into the acceleration and focusing phase of the plasma wake fields. The electron accelerator needs to be upgraded for AWAKE RUN 2 experiments to obtain a bunch length less than 100 fs which corresponds to a quarter of the plasma wavelength. Planned electron beam parameters for the AWAKE RUN 2 are a beam charge of 100 pC, and a beam energy larger than 50 MeV. In this paper, we show the electron beam parameters for RUN 2, and the parameters of the transfer line such as Twiss parameters, beam envelope, and emittance.
The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) aims at studying plasma wake field generation and electron acceleration driven by proton bunches. It is a proof-of-principle R&D; experiment at CERN and the world's first proton driven plasma wake field acceleration experiment. The experiment uses the 400 GeV proton beam from the SPS which travels through a 10 m long Rb-vapour plasma cell where it gets self-modulated and generates the plasma wake fields. Eventually an electron beam is injected externally to probe the wake-fields. AWAKE has completed several experimental campaigns starting in 2016. Results from the initial characterization of the plasma cell and measurements of the seeded self-modulation of the proton beam will be presented. First results on electron witness beam acceleration using the proton driven plasma wake field have been obtained recently and are presented in this paper.
The AWAKE experiment uses an ultra-high energy proton beam to create large amplitude wakefields for accelerating electrons in plasma. The proton beam is much longer than the plasma wavelength, and must be formed into small, sub- wavelength sized beamlets before it can effectively drive the wake. These beamlets are referred to as micro-bunches and are formed by the plasma self-modulation instability. An im- portant aspect of AWAKE is to measure the depth, frequency, and stability of the modulation, as this provides critical in- formation for establishing the presence of a high-amplitude wakefield driven by a self-modulation proton bunch. This paper discusses Fourier Analysis techniques for measuring the modulation frequency and compares error estimation techniques that work for both small and large datasets.
AWAKE is a plasma wakefield acceleration experiment using the 12cm-long, 400GeV proton bunch of the CERN SPS. In order to reach an acceleration gradient in the GeV/m range, the plasma electron density is 7$\times$10$^{14}$cm$^{-3}$. The transverse self-modulation, strongly seeded by an laser ionization front (seeded self-modulation or SSM), turns the long bunch into a train of micro-bunches at the plasma wavelength scale ($\sim$1mm) that resonantly drives the wakefields to large amplitude. Low energy electrons ($\sim$15MeV) can then be externally injected and accelerated to GeV energies.The plasma source is a laser-ionized rubidium vapor source. The vapor density is measured with $<0.5\%$ accuracy at both ends of the source. The detection of the SMI is based on diagnostics aimed at measuring the proton bunch modulation: fluorescent screens for measuring the proton bunch transverse density profile at two locations, optical transition radiation (OTR) and streak camera for direct observation of the modulation, and coherent transition radiation (CTR) for modulation frequency measurements.The first experiments focus of the study of the SMI. Experimental results obtained in late 2016 show signs of self-modulation on all diagnostics. Further SMI experiments will be conducted in 2017, together with the installation of the RF-gun and of the electron spectrometer. Injection and acceleration experiments will be conducted in 2018.After a general introduction to AWAKE and to its physics, the experimental apparatus will be briefly described and the most recent experimental results will be presented. Mid- and long-term plans, including future experiments, the development of scalable plasma sources and possible applications to HEP will be discussed.
AWAKE will be the first proton driven plasma wakefield acceleration experiment worldwide. The facility is located in the former CNGS area at CERN and includes a proton, laser and electron beam line merging in a 10 m long plasma cell, which is followed by the experimental diagnostics. In the first phase of the AWAKE physics program, which started at the end of 2016, the effect of the plasma on a high energy proton beam is being studied. A proton bunch is expected to experience the so called self-modulation instability, which leads to the creation of micro-bunches within the long proton bunch. The plasma channel is created in a rubidium vapor via field ionization by a TW laser pulse. This laser beam has to overlap with the proton beam over the full length of the plasma cell, resulting in tight requirements for the stability of the proton beam at the plasma cell in the order of 0.1 mm. In this paper the beam commissioning results of the 810 m long transfer line for proton bunches with $3 \cdot 10^{11}$ protons/bunch and a momentum of 400 GeV/c will be presented with a focus on the challenges of the parallel operation of the laser and proton beam.
Using parameters from the AWAKE project and particle-in-cell simulations we investigate beam loading of a plasma wake driven by a self-modulated proton beam. Addressing the case of injection of an electron witness bunch after the drive beam has already experienced self-modulation in a previous plasma, we optimise witness bunch parameters of size, charge and injection phase to maximise energy gain and minimise relative energy spread and emittance of the accelerated bunch.
For positron studies in plasma wakefield accelerators such as AWAKE, the development of new, cheaper, and compact positron beam sources is necessary. Using the GBAR experiment's positron trap as an example source, this paper explores converting that trapped positron plasma into a usable beam. Bunching is initially accomplished by an electrostatic buncher and the beam is accelerated to 148 keV by pulsed electrostatic accelerators. This is necessary for injection into the $\beta$-matched rf cavities operating at 600 MHz, which bring the positron beam to a transverse emittance of 1.3 $\pi$ mrad mm, a longitudinal emittance of 93.3 $\pi$ keV mm, $\sigma_z$ of 1.85 mm and an energy of 22 MeV. The beamline used here is far simpler and less expensive than those at many facilities, such as SLAC, allowing for a cheap source of positron beams, potentially opening up positron beam studies to many facilities that could not previously afford such a source.
In the AWAKE experiment, the electron beam is used to probe the proton-driven wakefield acceleration in plasma. Electron bunches are produced using an rf-gun equipped with a Cs$_2$Te photocathode illuminated by an ultraviolet (UV) laser pulse. To generate the UV laser beam a fraction of the infrared (IR) laser beam used for production of rubidium plasma is extracted from the laser system, time-compressed to a picosecond scale and frequency tripled using nonlinear crystals. The optical line for transporting the laser beam over the 24 m distance was built using rigid supports for mirrors and air-evacuated tube to minimize beam-pointing instabilities. Construction of the UV beam optical system enables appropriate beam shaping and control of its size and position on the cathode, as well as time delay with respect to the IR pulse seeding the plasma wakefield.
