CERN Accelerating science

The electron spectrometer for the Advanced Wakefield (AWAKE) experiment at CERN has been tested using an electron beam derived from partially-stripped ions accelerated in the Super Proton Synchrotron (SPS). The remaining electrons are stripped by passage of the beam through a thin screen upstream from the spectrometer, and using knowledge of the ion beam charge and energy, models of the spectrometer response could be verified.

The commissioning of the AWAKE electron beam line was successfully completed in 2018. Despite a modest length of about 15 m, this low-energy line is quite complex and several iterations were needed before finding satisfactory agreement between the model and the measurements. The work allowed to precisely predict the size and positioning of the electron beam at the merging point with the protons inside the plasma cell, where no direct measurement is possible. All the key aspects and corrections which had to be included in the model, precautions and systematic checks to apply for the correct setup of the line are presented. The sensitivity of the ∼18 MeV electron beam to various perturbations, like different initial optics parameters and beam conditions, energy jitters and drifts, earth’s magnetic field etc., is described.

Demonstration of the electron acceleration through the proton beam-driven plasma wakefield has been successfully done by the AWAKE RUN 1 experiment. Moreover, next AWAKE experiment is scheduled. Main goal of the AWAKE RUN 2 experiment is to achieve the capturing efficiency and the energy gain over 90%, and 10 GeV. In order to accomplish the goal, beam size, and its length have to be less than 50 μm, and 100 fs rms, respectively. Since the conventional beamline cannot meet the AWAKE RUN 2 requirements, we are focusing on designing new type of the electron beamline. In this paper, we present simulation results of the beam size, and the bunch length along the new beamline.

We briefly describe the basic physics principles considered for planning of the AWAKE Run 2 experiment. These principles are based on experimental results obtained during Run 1 and knowledge obtained from numerical simulation results and other experiments. The goal of Run 2 is to accelerate an electron bunch with a narrow relative energy spread and an emittance sufficiently low for applications. The experiment will use two plasmas, electron bunch seeding for the SM process, on-axis external injection of an electron bunch and electron bunch parameters to reach plasma blow-out, beam loading and beam matching.

The self-modulation instability is fundamental for the plasma wakefield acceleration experiment of the AWAKE (Advanced Wakefield Experiment) collaboration at CERN where this effect is used to generate proton bunches for the resonant excitation of high acceleration fields. Utilizing the availability of flexible electron beam shaping together with excellent diagnostics including an RF deflector, a supporting experiment was set up at the electron accelerator PITZ (Photo Injector Test facility at DESY, Zeuthen site), given that the underlying physics is the same. After demonstrating the effect [1] the next goal is to investigate in detail the self-modulation of long (with respect to the plasma wavelength) electron beams. In this contribution we describe parameter studies on self-modulation of a long electron bunch in an argon plasma. The plasma was generated with a discharge cell with densities in the 1013 cm−3 to 1015 cm−3 range. The plasma density was deduced from the plasma wavelength as indicated by the self-modulation period. Parameter scans were conducted with variable plasma density and electron bunch focusing.

We describe an external electron injection scheme for the AWAKE experiment. We use scattering in two foils, that are necessary as vacuum window and laser beam dump, to decrease the betatron function of the incoming electron beam for injection and matching into plasma wakefields driven by a self-modulated proton bunch. We show that, for a total aluminum foil thickness of $\sim 280\, \mu$m, multiple Coulomb scattering increases the beam emittance by a factor of $\sim 10$ and decreases the betatron function by a factor of $\sim 3$. The plasma in the accelerator is created by a ionizing laser pulse, counter-propagating with respect to the electron beam. This allows for the electron bunch to enter the plasma through an "infinitely" sharp vapor-plasma boundary, away from the foils.

The advanced wakefield experiment (AWAKE) at CERN is the first proton beam-driven plasma wakefield acceleration experiment. The main goal of AWAKE RUN 1 was to demonstrate seeded self-modulation (SSM) of the proton beam and electron witness beam acceleration in the plasma wakefield. For the AWAKE experiment, a 10-meter-long Rubidium-vapor cell together with a high-power laser for ionization was used to generate the plasma. The plasma wakefield is driven by a 400 GeV/c proton beam extracted from the super proton synchrotron (SPS), which undergoes a seeded self-modulation process in the plasma. The electron witness beam used to probe the wakefields is generated from an S-band RF photo-cathode gun and then accelerated by a booster structure up to energies between 16 and 20 MeV. The first run of the AWAKE experiment revealed that the maximum energy gain after the plasma cell is 2 GeV, and the SSM mechanism of the proton beam was verified. In this paper, we will present the details of the AWAKE electron injector. A comparison of the measured electron beam parameters, such as beam size, energy, and normalized emittance, with the simulation results was performed.

The Advanced Wakefield Experiment (AWAKE) develops the first plasma wakefield accelerator with a high-energy proton bunch as driver. The 400GeV bunch from CERN Super Proton Synchrotron (SPS) propagates through a 10m long rubidium plasma, ionized by a 4TW laser pulse co-propagating with the proton bunch. The relativistic ionization front seeds a self-modulation process. The seeded self-modulation transforms the bunch into a train of micro-bunches resonantly driving wakefields. We measure the density modulation of the bunch, in time, with a streak camera with picosecond resolution. The observed effect corresponds to alternating focusing and defocusing fields. We present a procedure recovering the charge of the bunch from the experimental streak camera images containing the charge density. These studies are important to determine the charge per micro-bunch along the modulated proton bunch and to understand the wakefields driven by the modulated bunch.

We present a method to measure transverse size and position of an electron or proton beam, close to the injection point in plasma wakefields, where other diagnostics are not available. We show that transverse size measurements are in agreement with values expected from the beam optics with a $< 10\%$ uncertainty. We confirm the deflection of the low-energy 18 MeV electron beam trajectory by the Earth's magnetic field. This measurement can be used to correct for this effect and set proper electron bunch injection parameters. The AWAKE experiment relies on these measurements for optimizing electron injection.

We describe the main ideas, promises and challenges related to proton-driven plasma wakefield acceleration. Existing high-energy proton beams have the potential to accelerate electron beams to the TeV scale in a single plasma stage. In order to drive a wake effectively the available beams must be either highly compressed or microbunched. The self-modulation instability has been suggested as a way to microbunch the proton beams. The AWAKE project at CERN is currently the only planned proton-driven plasma acceleration experiment. A self-modulated CERN SPS beam will be used to drive a plasma wake. We describe the design choices and experimental setup for AWAKE, and discuss briefly the short-term objectives as well as longer-term ideas for the project.

We show that for beam-driven wakefield acceleration, the linearly ramped, equally spaced train of bunches typically considered to optimise the transformer ratio only works for flat-top bunches. Through theory and simulation, we explain that this behaviour is due to the unique properties of the plasma response to a flat-top density profile. Calculations of the optimal scaling for a train of Gaussian bunches show diminishing returns with increasing bunch number, tending towards saturation. For a periodic bunch train, a transformer ratio of 23 was achieved for 50 bunches, rising to 40 for a fully optimised beam.