Three-dimensional beam size compression for external injection of plasma wakefield acceleration

Accelerators based on laser plasma wakefield acceleration are of great interest for a new generation of compact machines. External injection of an electron beam from an RF injector into a plasma accelerating stage has the advantage that a well-controlled and fully characterized beam can be used. The matching of the electron bunches into an accelerating plasma wakefield places high demands on the electron beam quality. The electron beam size must be extremely small to match the field structure inside the plasma wake. The short period of the accelerating field in the plasma requires a bunch length in the (sub-)fs range. These electron beam properties result in a high electron density and strongly space charge dominated bunches. The beamline upstream of the plasma must be able to control the significant effect of space charge on the bunch and to transversely match the beam to the focusing fields of the plasma. Further constraints to the beamline design are given by the in-coupling of the high-power drive laser and the implementation of diagnostic tools. Choosing suitable settings for the beamline elements in order to match the beam thus poses a great challenge. Using multi-objective optimization, suitable settings for the beamline elements can be extracted from Pareto optimum solutions. The development of a universal multi-objective optimization algorithm for beamline matching as well as first optimization results are presented.

Simulations and experiments on external electron injection for laser wakefield acceleration

Via three-dimensional particle-in-cell simulations, the self-mode-transition of a laser-driven electron acceleration from laser wakefield to plasma-wakefield acceleration is studied. In laser wakefield accelerator (LWFA) mode, an intense laser pulse creates a large amplitude wakefield resulting in high-energy electrons. Along with the laser pulse depletion, the electron bunch accelerated in the LWFA mode drives a plasma wakefield. Then, after the plasma wakefield accelerator mode is established, electrons are trapped and accelerated in the plasma wakefield. The mode transition process and the characteristics of the accelerated electron beam are presented.

When an intense Gaussian laser pulse ( I ≫1021w/cm2 ) with Circular Polarization (CP) is incident on a planar thin foil, the foil is curved and is transformed into a “cocoon”. Rayleigh-Taylor (R-T) like instabilities[1] develop much more slowly as the radiation pressure is increased, so that these instabilities can disperse nearly the whole cocoon into low density plasma cloud and only a clump (with a size about 1∼2λ) is left in the center of the target[2,3]. As long as the laser gradually penetrates into the plasma cloud, the photon cavity can be formed and the laser pulse begins to embrace the clump. As the central clump is focused and pushed by the photon cavity (the three-dimensional beam compression), a mono-energetic and highly collimated proton beam is realized. The energy spread is close to the maximum value estimated by the 1D model of PSA given in Ref.[4]: Δw/w r ≅ 2Δp/p r ≅ 2ξ 0 Ωp r , where w r , p r and ξ 0 are the kinetic energy, normalized momentum of the reference particle and the oscillating amplitude, respectively.

Ionization injection in a plasma wakefield accelerator was investigated experimentally using two lithium plasma sources of different lengths. The ionization of the helium gas, used to confine the lithium, injects electrons in the wake. After acceleration, these injected electrons are observed as a distinct group from the drive beam on the energy spectrometer. They typically have a charge of tens of pC, an energy spread of a few GeV, and a maximum energy of up to 30 GeV. The emittance of this group of electrons can be many times smaller than the initial emittance of the drive beam. The energy scaling for the trapped charge from one plasma length to the other is consistent with the blowout theory of the plasma wakefield.

In this article, we present the underlying physics and the present status of high gradient and high-energy plasma accelerators. With the development of compact short pulse high-brightness lasers and electron and positron beams, new areas of studies for laser/particle beam-matter interactions is opening up. A number of methods are being pursued vigorously to achieve ultra-high-acceleration gradients. These include the plasma beat wave accelerator (PBWA) mechanism which uses conventional long pulse ( approximately 100 ps) modest intensity lasers (I approximately 10(14)-10(16) W cm(-2)), the laser wakefield accelerator (LWFA) which uses the new breed of compact high-brightness lasers (<1 ps) and intensities >10(18) W cm(-2), self-modulated laser wakefield accelerator (SMLWFA) concept which combines elements of stimulated Raman forward scattering (SRFS) and electron acceleration by nonlinear plasma waves excited by relativistic electron and positron bunches the plasma wakefield accelerator. In the ultra-high intensity regime, laser/particle beam-plasma interactions are highly nonlinear and relativistic, leading to new phenomenon such as the plasma wakefield excitation for particle acceleration, relativistic self-focusing and guiding of laser beams, high-harmonic generation, acceleration of electrons, positrons, protons and photons. Fields greater than 1 GV cm(-1) have been generated with monoenergetic particle beams accelerated to about 100 MeV in millimetre distances recorded. Plasma wakefields driven by both electron and positron beams at the Stanford linear accelerator centre (SLAC) facility have accelerated the tail of the beams.

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 meters in length. To probe 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. Results of beam tests of the scintillator screen output are presented, along with tests of the resolution of the proposed optical system. The results are used together with a BDSIM simulation of the spectrometer system to predict the spectrometer performance for a range of possible accelerated electron distributions.

Plasma wakefield acceleration has the potential to dramatically shrink the size and cost of particle accelerators. Research at the SLAC National Accelerator Laboratory has demonstrated that plasmas can provide 1,000 times the acceleration in a given distance compared with current technologies. Developing revolutionary and more efficient acceleration techniques that allow for an affordable high-energy collider is the focus of FACET, a National User Facility at SLAC. The existing FACET National User Facility uses part of SLAC’s two-mile-long linear accelerator to generate high-density beams of electrons and positrons. FACET-II is a new test facility to develop advanced acceleration and coherent radiation techniques with high-energy electron and positron beams. It is the only facility in the world with high energy positron beams. FACET-II provides a major upgrade over current FACET capabilities and the breadth of the potential research program makes it truly unique. It will synergistically pursue accelerator science that is vital to the future of both advanced acceleration techniques for High Energy Physics, ultra-high brightness beams for Basic Energy Science, and novel radiation sources for a wide variety of applications. The design parameters for FACET-II are set by the requirements of the plasma wakefield experimental program. To drive the plasma wakefield requires a high peak current, in excess of 10kA. To reach this peak current, the electron and positron design bunch size is 10μ by 10μ transversely with a bunch length of 10μ. This is more than 200 times better than what has been achieved at the existing FACET. The beam energy is 10 GeV, set by the Linac length available and the repetition rate is up to 30 Hz. The FACET-II project is scheduled to be constructed in three major stages. Components of the project discussed in detail include the following: electron injector, bunch compressors and linac, the positron system, the Sector 20 sailboat and W chicanes, and experimental area and infrastructure.

We propose a Plasma Accelerator Research Station (PARS) based at proposed FEL test facility CLARA (Compact Linear Accelerator for Research and Applications) at Daresbury Laboratory. The idea is to use the relativistic electron beam from CLARA, to investigate some key issues in electron beam transport and in electron beam driven plasma wakefield acceleration, e.g. high gradient plasma wakefield excitation driven by a relativistic electron bunch, two bunch experiment for CLARA beam energy doubling, high transformer ratio, long bunch self-modulation and some other advanced beam dynamics issues. This paper presents the feasibility studies of electron beam transport to meet the requirements for beam driven wakefield acceleration and presents the plasma wakefield simulation results based on CLARA beam parameters. Other possible experiments which can be conducted at the PARS beam line are also discussed.

The influence of initial position on the acceleration of externally injected electrons in a plasma wakefield is investigated. Test-particle simulations show previously unobserved complex structure in the parameter space, with quasi-stable injection channels forming for particles injected in narrow regions away from the wake centre. Particles injected into these channels remain in the wake for a considerable time after dephasing and as a result achieve significantly higher energy than their neighbours. The result is relevant to both the planning and optimisation of experiments making use of external injection.

Welcome to this meeting in Taipei. I am very happy to see many of world's active leading researchers in this field are gathering here. This is indeed a time of great excitement in the plasma acceleration. In April, three articles appeared in Nature demonstrating the acceleration of nearly mono-energetic electrons by short pulsed lasers when they reached the intensity threshold for relativistic self-focusing. In May, the acceleration of SLAC 28.5 GeV electron beam by the beam driven wakefield in plasma was published in PRL. It is fitting in this centennial year of publication of Einstein's four papers that relativistic laser plasma interaction becomes a hot topic and relativistic plasma physics made a big stride. In this talk, I would like to give a personal perspective of particle acceleration in plasmas and survey some of the outstanding recent results. Then I would like to pose some questions and show some of the preliminary work by my colleagues here notably that of Prof. S. H. Chen in simulation. Our universe now is believed to be dominated by Dark Matter and Dark Energy. But the visible universe is dominantly in the plasma state and plasma is a very effective accelerating medium. Recent advances in high energy astrophysics have demonstrated amply the explosive and efficient particle acceleration by plasma processes — Radio jets, X ray sources, pulsars, and Gamma ray bursts. The particle acceleration is most clearly seen in the acceleration of Cosmic Rays. Eugene Parker once remarked the fact the Nature seems to be so unfair that it gives most of the energies to just a few electrons — so plasma as an accelerating medium can be very capitalistic. The Cosmic Ray spectrum is very rich in different characteristic slopes

Beam-driven, underdense thin plasma lenses are a compact and tunable means of providing strong focusing to relativistic electron beams. It is necessary to focus electron beams to small sizes in a beam-driven plasma wakefield accelerator to preserve the beam emittance, or volume in phase space. The thin plasma lens can be used to achieve these small beam sizes that are otherwise out of reach of conventional electromagnetic beam delivery systems. Ultrashort (~ 30 fs), low energy (~ 10 mJ) laser pulses can be used to ionize a small region of gas to generate the thin plasma lens. The interplay between the ionized gas and the focusing of the laser pulse are modeled in detail to calculate experimentally achievable 3-D plasma lens profiles, which are then used in simulations of beam focusing. Plasma lens focal lengths and minimum beam sizes achievable are estimated, considering the effects of radiation reaction in both the classical and quantum regimes.

Electron dephasing is a major gain-inhibiting effect in plasma-based accelerators. A novel method is proposed to overcome dephasing, in which the modulation of a modest ( O(10 kG)), axial, uniform magnetic field in the acceleration channel leads to densification of the plasma through magnetic compression, enabling direct, time-resolved control of the plasma wave properties. The methodology is broadly applicable and can be optimized to improve the leading acceleration approaches, including plasma beat-wave, plasma wakefield, and laser wakefield acceleration. The advantages of magnetic compression compared to other proposed schemes to overcome dephasing are identified.

If the index of a refraction of a dispersive medium, such as a plasma, changes in time, it can be used to change the frequency of light propagating through the medium. This effect is called photon acceleration. It has been predicted in both theory and simulations, and also been demonstrated experimentally for the case of moving ionization fronts in gases (the so‐called ionization blueshift) as well as for laser‐driven wakefields.Here, we present studies of photon acceleration in laser‐driven plasma wakefields. The unique spectral characteristics of this process will be discussed, to distinguish it from e.g. photon acceleration by ionization fronts, frequency domain interferometry or self‐phase modulation. The dynamics of the photons in laser‐wakefield interaction are studied through both regular particle‐in‐cell and wave‐kinetic simulations. The latter approach provides a powerful, versatile, and easy‐to‐use method to track the propagation of individual spectral components, providing new insight into the ...

The E162 experiment at the Stanford Linear Accelerator Center was the first experiment in which a positron beam gained energy in a plasma wakefield accelerator [B. Blue et al., Accepted for Publication in Physical Review Letters.]. A single positron bunch both excited (gave energy to) and witnessed (extracted energy from) the plasma wakefield. The energy dynamics within the single positron bunch were measured in a dispersive section of the beamline with both time-resolved (1 ps streak camera) and time-integrated (CCD camera) diagnostics. This paper will correlate the energy gain and loss measurements from both diagnostics.