CERN Accelerating science

In the AWAKE experiment a high-power laser pulse ionizes rubidium atoms inside a 10 m long vapor cell thus creating a plasma for proton-driven wakefield acceleration of electrons. Propagating co-axial with the SPS proton beam the laser pulse seeds the self-modulation instability within the proton bunch on the front of plasma creation. The same laser will also generate UV-pulses for production of a witness electron beam using an RF-photoinjector. The experimental area formerly occupied by CNGS facility is being modified to accommodate the AWAKE experiment. A completely new laser laboratory was built, taking into account specific considerations related to underground work. The requirements for AWAKE laser installation have been fulfilled and vacuum beam lines for delivery of laser beams to the plasma cell and RF-photoinjector have been constructed. First results of laser beam hardware commissioning tests following the laser installation will be presented.

The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) aims at studying plasma wakefield 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 wakefield acceleration experiment. The AWAKE experiment is currently being installed in the former CNGS facility and will use the 400 GeV/c proton beam bunches from the SPS to drive the wakefields in the plasma. The first experiments will focus on the self-modulation instability of the long (rms ~12 cm) proton bunch in the plasma. These experiments are planned for the end of 2016. Later, in 2017/2018, low energy (~15 MeV) electrons will be externally injected to sample the wakefields and be accelerated with GeV/m gradients. The main goals of the experiment will be summarized. A summary of the AWAKE design and construction status will be presented.

The longitudinal single bunch instability observed in the SPS leads to uncontrolled emittance blow-up and limits the quality of high intensity beams required for the High Luminosity LHC and AWAKE projects at CERN. The present SPS impedance model developed from a thorough survey of machine elements was used in macro-particle simulations (with the code BLonD) of the bunch behavior through the acceleration cycle. Comparison of simulations with measurements of the synchrotron frequency shift, performed on the SPS flat bottom to probe the impedance, show a reasonable agreement. During extensive experimental studies various beam and machine parameters (bunch intensity, longitudinal emittance, RF voltage, with single and double RF systems) were scanned in order to further benchmark the SPS impedance model with measurements and to better understand the mechanism behind the instability. It was found that the dependence of instability threshold on longitudinal emittance and beam energy has an unexpected non-monotonic behavior, leading to islands of (in)stability. The results of this study are presented and can be used to define possible parameter settings for the future CERN projects.

The AWAKE experiment at CERN will use a proton bunch with an momentum of 400 GeV/c from the SPS to drive large amplitude wakefields in a plasma. This will require a ~830 m long transfer line from the SPS to the experiment. The prepa- rations for the beam commissioning of the AWAKE proton transfer line are presented in this paper. They include the detailed planning of the commissioning steps, controls and beam instrumentation specifications as well as operational tools, which are developed for the steering and monitoring of the beam line. The installation of the transfer line has been finished and first beam is planned in summer 2016.

The AWAKE project at CERN was approved in August 2013 and since then a big effort was made to be able to probe the acceleration of electrons before the "2019-2020 Long Shutdown". The next steps in this challenging schedule will be a dry run of all the beam line systems, at the end of the HW commissioning in June 2016, and the first proton beam sent to the plasma cell one month later. The current status of the project is presented together with an outlook over the foreseen works for operation with electrons in 2018.

For a low energy, high intensity beam, total beam emittance is dominated by defocusing space charge force. This is most commonly observed in photo-injectors. In this low energy regime, emittance measurement techniques such as quadrupole scans fail as they consider the beam size only depends on optical functions. The pepper-pot method is used for 2D emittance measurements in a single shot manner. In order to measure the beam emittance in space charge dominated regime by quadrupole scans, space charge term should be carefully incorporated into the transfer matrices. On the other hand, methods such as divergence interferometry via optical transition radiation (OTRI), phase space tomography using 1D projections of quadrupole scans can be suitably applied for such conditions. In this paper, the design of a versatile pepper-pot system for AWAKE experiment at CERN is presented for a wide range of bunch charges from 0.1 to 1nC where the space charge force increases significantly. In addition, other aforementioned methods and respective algorithms are introduced as alternative methods.

The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) aims at studying plasma wakefield 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 wakefield acceleration experiment. The AWAKE experiment will be installed in the former CNGS facility and uses the 400 GeV/c proton beam bunches from the SPS. The first experiments will focus on the self-modulation instability of the long (r.m.s ~12 cm) proton bunch in the plasma. These experiments are planned for the end of 2016. Later, in 2017/2018, low energy (~15 MeV) electrons will be externally injected to sample the wakefields and be accelerated beyond 1 GeV.

The self-modulation instability (SMI) can be used to transform a long, charged particle bunch into a train of periodically spaced shorter bunches. The SMI occurs in a plasma when the plasma wake period is much shorter than the bunch length. The train of short bunches can then resonantly drive wakefields to much larger amplitude that the long bunch can. The SMI will be used in the AWAKE experiment at CERN, where the wakefields will be driven by a high-energy (400GeV) proton bunch. However, most of the SMI physics can be tested with the electron and positron bunches available at SLAC-FACET. In this case, the bunch is ~10 plasma wavelengths long, but can drive wakefields in the GV/m range. FACET has a meter-long plasma and is well equipped in terms of diagnostic for SMI detection: optical transition radiation for transverse bunch profile measurements, coherent transition radiation interferometry for radial modulation period measurements and energy spectrometer for energy loss and gain measurement of the drive bunch particles. The latest experimental results will be presented.

The AWAKE experiment at CERN aims at studying plasma wakefield generation and acceleration driven by proton bunches. The first experiments will focus on the self-modulation instability of the long (~12cm, rms) proton bunch in the plasma. This instability is used to transform the incoming bunch into a train of short bunches with a period approximately equal to the plasma wavelength, ~1.2mm at a nominal plasma electron density of 7·10¹⁴/cc. These experiments are planned for the end of 2016. Later, low energy (~15MeV) electrons will be externally injected to sample the wakefields and be accelerated beyond 1GeV. The main goals of the experiment will be summarized and the progress with the plasma source, beam diagnostics and injection method will be presented.

The AWAKE experiment is to be constructed at the CERN Neutrinos to Gran Sasso facility (CNGS). This will be the first experiment to demonstrate proton-driven plasma wakefield acceleration. The 400 GeV proton beam from the CERN SPS will excite a wakefield in a plasma cell several metres in length. To observe the plasma wakefield, electrons of 10–20 MeV will be injected into the wakefield following the head of the proton beam. Simulations indicate that electrons will be accelerated to GeV energies by the plasma wakefield. The AWAKE spectrometer is intended to measure both the peak energy and energy spread of these accelerated electrons. Improvements to the baseline design are presented, with an alternative dipole magnet and quadrupole focussing, with the resulting energy resolution calculated for various scenarios. The signal to background ratio due to the interaction of the SPS protons with upstream beam line components is calculated, and CCD camera location, shielding and light transport are considered.

The AWAKE project at CERN is planned to study proton driven plasma wakefield acceleration. The proton beam from the SPS will be used in order to drive wakefields in a 10 m long Rb plasma cell. In the first phase of this experiment, scheduled in 2016, the self-modulation of the proton beam in the plasma will be studied in detail, while in the second phase an external electron beam will be injected into the plasma wakefield to probe the acceleration process. The installation of AWAKE in the former CNGS experimental area and the required optics flexibility define the tight boundary conditions to be fulfilled by the electron beam line design. The transport of low energy (10-20 MeV) bunches of 1.25·10⁹ electrons and the synchronous copropagation with much higher intensity proton bunches (3E11) determines several technological and operational challenges for the magnets and the beam diagnostics. The current status of the electron line layout and the associated equipments are presented in this paper.

The on-axis injection of electron bunches in the proton-driven plasma wake at the AWAKE experiment at CERN implies co-propagation of a low-energy electron beam with the long high-energy proton beam in a common beam pipe over several meters upstream of the plasma chamber. The possible effects of the proton-induced wakefields on the electron bunch phase space in the common beam pipe region may have crucial implications for subsequent electron trapping and acceleration in plasma. We present the CST Studio simulations of the tentative common beam pipe setup and the two beam co-propagating in it. Simulated effects of the proton wakefields on electrons are analysed and compared to analytical predictions.

The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) is a proof-of-principle R&D; experiment at CERN. It is the world’s first proton driven plasma wakefield acceleration experiment, using a high-energy proton bunch to drive a plasma wakefield for electron beam acceleration. The AWAKE experiment will be installed in the former CNGS facility and uses the 400 GeV proton beam bunches from the SPS, which will be sent to a plasma source. An electron beam will be injected into the plasma cell to probe the accelerating wakefield. Challenging modifications in the area and new installations are required for AWAKE. First proton beam to the experiment is expected late 2016. The accelerating electron physics will start late 2017. This paper gives an overview of the project from a physics and engineering point of view, it describes the main activities, the milestones, the organizational set-up for the project management and coordination.

The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) aims at studying plasma wakefield 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 wakefield acceleration experiment. The AWAKE experiment will be installed in the former CNGS facility and uses the 400 GeV/c proton beam bunches from the SPS. The first experiments will focus on the self-modulation instability of the long (rms ~12 cm) proton bunch in the plasma. These experiments are planned for the end of 2016. Later, in 2017/2018, low energy (~15 MeV) electrons will be externally injected to sample the wakefields and be accelerated beyond 1 GeV. The main goals of the experiment will be summarized. A summary of the AWAKE design and construction status will be presented.

We present the results of numerical studies of laser pulse propagating in a 3.5 cm Rb vapor cell in the linear dispersion regime by using a 1D model and a 2D code that has been modified for our special case. The 2D simulation finally aimed at finding laser beam parameters suitable to make the Rb vapor fully ionized to obtain a uniform, 10 m-long, at least 1 mm in radius plasma in the next step for the AWAKE experiment.

This report describes the conceptual steps in reaching the design of the AWAKE experiment currently under construction at CERN. We start with an introduction to plasma wakefield acceleration and the motivation for using proton drivers. We then describe the self-modulation instability --- a key to an early realization of the concept. This is then followed by the historical development of the experimental design, where the critical issues that arose and their solutions are described. We conclude with the design of the experiment as it is being realized at CERN and some words on the future outlook. A summary of the AWAKE design and construction status as presented in this conference is given in [1].

A method to accurately measure the density of Rb vapor is described. We plan on using this method for the Advanced Wakefield (AWAKE)~\cite{bib:awake} project at CERN , which will be the world's first proton driven plasma wakefield experiment. The method is similar to the hook~\cite{bib:Hook} method and has been described in great detail in the work by W. Tendell Hill et. al.~\cite{bib:densitymeter}. In this method a cosine fit is applied to the interferogram to obtain a relative accuracy on the order of $1\%$ for the vapor density-length product. A single-mode, fiber-based, Mach-Zenhder interferometer will be built and used near the ends of the 10 meter-long AWAKE plasma source to be able to make accurate relative density measurement between these two locations. This can then be used to infer the vapor density gradient along the AWAKE plasma source and also change it to the value desired for the plasma wakefield experiment. Here we describe the plan in detail and show preliminary results obtained using a prototype 8 cm long novel Rb vapor cell.

AWAKE, the Advanced Proton-Driven Plasma Wakefield Acceleration Experiment, is a proof-of-principle R&D; experiment at CERN using a 400 GeV/c proton beam from the CERN SPS (longitudinal beam size sigma_z = 12 mm) which will be sent into a 10 m long plasma section with a nominal density of approx. 7x10^14 atoms/cm3 (plasma wavelength lambda_p = 1.2mm). In this paper we show that by measuring the time integrated transverse profile of the proton bunch at two locations downstream of the AWAKE plasma, information about the occurrence of the self-modulation instability (SMI) can be inferred. In particular we show that measuring defocused protons with an angle of 1 mrad corresponds to having electric fields in the order of GV/m and fully developed self-modulation of the proton bunch. Additionally, by measuring the defocused beam edge of the self-modulated bunch, information about the growth rate of the instability can be extracted. If hosing instability occurs, it could be detected by measuring a non-uniform defocused beam shape with changing radius. Using a 1 mm thick Chromox scintillation screen for imaging of the self-modulated proton bunch, an edge resolution of 0.6 mm and hence a SMI saturation point resolution of 1.2 m can be achieved.

Presently available high-energy proton beams in circular accelerators carry enough momentum to accelerate high-intensity electron and positron beams to the TeV energy scale over several hundred meters of the plasma with a density of about 1e15 1/cm^3. However, the plasma wavelength at this density is 100-1000 times shorter than the typical longitudinal size of the high-energy proton beam. Therefore the self-modulation instability (SMI) of a long (~10 cm) proton beam in the plasma should be used to create the train of micro-bunches which would then drive the plasma wake resonantly. Changing the plasma density profile offers a simple way to control the development of the SMI and the acceleration of particles during this process. We present simulations of the possible use of a plasma density gradient as a way to control the acceleration of the electron beam during the development of the SMI of a 400 GeV proton beam in a 10 m long plasma. This work is done in the context of the AWAKE project --- the proof-of-principle experiment on proton driven plasma wakefield acceleration at CERN.

Status Report of the AWAKE Experiment

Besides the Long Shut-Down 1 (LS1), some projects are still progressing at CERN. Among them, it has to be mentioned LINAC4, a future essential part of the LHC injector chain, AWAKE, a project to verify the approach of using protons to drive a strong wakefield in a plasma, ELENA, a small compact ring for cooling and decelerating antiproton and HIE-Isolde, not to forget the long term studies for CLIC and FCC. This paper describes the status of these projects from the survey and alignment point of view.

Particle-in-cell simulations were performed by using PARMELA to characterise an electron injector with a booster linac for the AWAKE project in order to provide the baseline specifications required by the plasma wakefield experiments. Tolerances and errors were investigated. A 3 GHz travelling wave structure designed by using CST code. Particles were tracked by using the field maps acquired from these electromagnetic simulations. These results are pre- sented in comparison with the generic accelerating structure model within PARMELA.

The goal of the AWKAKE collaboration is the study of plasma wakefields driven by proton (p⁺) bunches through experiments, simulations and theory. Proton bunches are interesting wakefield drivers because they can be ultra-relativistic (TeVs/p⁺) and carry large amounts of energy (>kJ). It was demonstrated in simulations* that acceleration of an electron (e-) bunch from 10GeV to >500GeV can be achieved in ~500m of plasma driven by a 1TeV, 100micron-long, bunch with 10¹¹ p+. Such short p⁺ bunches do not exist today. It was suggested** that a p⁺ bunch long compared to the plasma period can transversely self-modulate and resonantly drive wakefields to large amplitudes (~GV/m). Initial experiments based on self-modulation instability (SMI) will use single 12cm-long CERN SPS bunches with 1-3·10¹¹, 450GeV p⁺ to study physics of SMI. With a plasma density of 7·10¹⁴/cc the plasma wave and modulation period is 1.3mm. The SMI saturates after ~3m with amplitude in the GV/m range. Later a low energy (~10MeV) witness e⁻ bunch will be injected at the SMI saturation point. Energy gain over ~7m of plasma can reach the GeV level. Translation from physics to experimental plan and setup will be presented.

AWAKE is a proton driven plasma-wakefield acceleration at CERN*, that uses long proton bunches ~ 400 ps from the SPS. In a dense plasma, a long proton bunch is subject toμbunching at plasma period due to the self-modulation instability, SMI**. The self-modulated proton bunch generates large amplitude charge separation through resonant wakefield excitation. Numerical simulations show that when seeded the SMI can grow and saturate over ~4 m in a plasma with density in the (1-10) *10¹⁴/cc range. Seeding also allows for deterministic injection of witness bunches in the focusing and accelerating phase of the wakefields. The SPS proton bunch carrying kJ of energy is a unique driver for generation of ~ GeV/m wakefields through 10’s of meters of plasma. The side-injected electrons ~15 MeV can reach GeV energies. The AWAKE experimental layout, the physics of self-modulation, simulation results, plasma source under study, diagnostics plan for bunch modulation measurement using transverse coherent transition radiation***, and phasing of the witness bunch respect to the wave and synchronisation with diagnostics will be presented.****

The AWAKE experiment recently approved at CERN will use the self-modulation instability (SMI) of long (12 cm), relativistic (400 GeV/c) proton bunches in dense plasmas to drive wakefields with accelerating gradients at the GV/m level. These accelerating gradients will be probed by externally injected electrons. In order to preserve the plasma uniformity required for the SMI the first experiments will use on-axis injection of a low energy 10-20 MeV electron beam collinearly with the proton beam. In this article we describe the physics of electron injection into the proton driven SMI wakefields. Requirements on the injected electron beam are determined and the final accelerated beam parameters are obtained via numerical simulations.