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An ep collider based on proton-driven plasma wakefield acceleration

An ep collider based on proton-driven plasma wakefield acceleration

nosity value is significantly below that of conventional LHeC designs and so raises the question as to whether this can be increased, by e.g. increasing the repetition rate or decreasing the size of the electron beam at the interaction point. Alternatively, physics at high energy, but lower luminosity should be considered. As plasma wakefield acceleration has clearly demonstrated high accelerat- ing gradients, there are prospects for a future ep of e + e − collider at the high energy frontier, but possibly with reduced luminosity than can be achieved with RF acceleration. Brief studies have started [10] considering the physics that could be investigated at such colliders, such as classicali- sation in electroweak and gravity, and should be further pursued.
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Proton driven plasma wakefield acceleration in AWAKE

Proton driven plasma wakefield acceleration in AWAKE

The support of the Max Planck Society is gratefully acknowledged. This work was supported in parts by the Siberian Branch of the Russian Academy of Science (project No. 0305-2017- 0021), a Leverhulme Trust Research Project Grant RPG-2017-143 and by STFC (AWAKE- UK, Cockroft Institute core and UCL consolidated grants), United Kingdom; a Deutsche Forschungsgemeinschaft project grant PU 213-6/1 “Three-dimensional quasi-static simulations of beam self-modulation for plasma wakefield acceleration”; the National Research Foundation of Korea (Nos. NRF-2015R1D1A1A01061074 and NRF-2016R1A5A1013277); the Portuguese FCT— Foundation for Science and Technology, through grants CERN/FIS-TEC/0032/2017, PTDC- FIS-PLA-2940-2014, UID/FIS/50010/2013 and SFRH/IF/01635/2015; NSERC and CNRC for TRIUMF’s contribution; and the Research Council of Norway. M. Wing acknowledges the support of the Alexander von Humboldt Stiftung and DESY, Hamburg. The AWAKE collaboration acknowledge the SPS team for their excellent proton delivery.
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Directions in plasma wakefield acceleration

Directions in plasma wakefield acceleration

Plasma-based wakefield acceleration driven by particle beams (PWFA) is particularly attractive, since it allows a constant phase relation between accelerating driver beam and accelerated witness beam, and long acceleration distances in a single plasma wakefield stage. This allows large energy gains to be reached while the relative position and the conditions experienced by the accelerated witness beam inside the bubble remain essentially constant. The witness beam can therefore be matched to the specific acceleration scenario and phase e.g. in terms of its size and charge profile, which allows optimization of the quality of the output beam. PWFA-driven plasma blowouts [8] also can propagate with a velocity even closer to the speed of light than LWFA-driven plasma bubbles due to the slowed down propagation velocity of laser light in plasma. This facilitates dark-current free operation of PWFA. A further fundamental difference between electromagnetic waves and particle beam drivers is that particle beams have unipolar electric fields instead of oscillating ones. Only at very high laser pulse intensities and associated extremely high oscillating electric peak fields, the ponderomotive force pushes plasma electrons effectively outside, rather than merely making plasma electrons oscillate back and forth within the laser pulse envelope. In contrast, the non-
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AWAKE:a proton driven plasma wakefield acceleration experiment at CERN

AWAKE:a proton driven plasma wakefield acceleration experiment at CERN

The AWAKE Collaboration has been formed in order to demonstrate proton-driven plasma wakefield acceleration for the first time. This acceleration technique could lead to future colliders of high energy but of a much reduced length when compared to proposed linear accelerators. The CERN SPS proton beam in the CNGS facility will be injected into a 10 m plasma cell where the long proton bunches will be modulated into significantly shorter micro- bunches. These micro-bunches will then initiate a strong wakefield in the plasma with peak fields above 1 GV / m that will be harnessed to accelerate a bunch of electrons from about 20 MeV to the GeV scale within a few meters. The experimental program is based on detailed numerical simulations of beam and plasma interactions. The main accelerator components, the experimental area and infrastructure required as well as the plasma cell and the diagnostic equipment are discussed in detail. First protons to the experiment are expected at the end of 2016 and this will be followed by an initial three-four years experimental program. The experiment will inform future larger-scale tests of proton-driven plasma wakefield acceleration and applications to high energy colliders.
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AWAKE, The Advanced Proton Driven Plasma Wakefield Acceleration Experiment at CERN

AWAKE, The Advanced Proton Driven Plasma Wakefield Acceleration Experiment at CERN

AWAKE will use a rubidium vapor source [15] ionized by a short laser pulse (see Table 1). Rubidium plasma consists of heavy ions, that mitigate plasma ions motion e ff ects. The plasma cell is 10 m long and has a diameter of 4 cm. The den- sity uniformity is achieved by imposing a uniform temperature (within 0.2 %) along the source. For that purpose a heat ex- changer with su ffi cient heat carrying fluid flow is used. Syn- thetic oil is circulated inside a thermal insulation around the tube containing the rubidium vapor. The oil temperature can be stabilized to ±0.05 ◦ C. A threshold ionization process for the first Rb electron is used to turn the uniform neutral density into a uniform plasma density. The ionization potential is very low, Φ Rb = 4.177 eV, as is the intensity threshold for over the barrier
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AWAKE : a proton-driven plasma wakefield acceleration experiment at CERN

AWAKE : a proton-driven plasma wakefield acceleration experiment at CERN

The AWAKE Collaboration has been formed in order to demonstrate proton-driven plasma wakefield acceleration for the first time. This acceleration technique could lead to future colliders of high energy but of a much reduced length when compared to proposed linear accelerators. The CERN SPS proton beam in the CNGS facility will be injected into a 10 m plasma cell where the long proton bunches will be modulated into significantly shorter micro- bunches. These micro-bunches will then initiate a strong wakefield in the plasma with peak fields above 1 GV / m that will be harnessed to accelerate a bunch of electrons from about 20 MeV to the GeV scale within a few meters. The experimental program is based on detailed numerical simulations of beam and plasma interactions. The main accelerator components, the experimental area and infrastructure required as well as the plasma cell and the diagnostic equipment are discussed in detail. First protons to the experiment are expected at the end of 2016 and this will be followed by an initial three-four years experimental program. The experiment will inform future larger-scale tests of proton-driven plasma wakefield acceleration and applications to high energy colliders.
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AWAKE, the advanced proton driven plasma wakefield acceleration experiment at CERN

AWAKE, the advanced proton driven plasma wakefield acceleration experiment at CERN

The electron source is installed in an adjacent room 1.16 m below the level of the proton beam line. The electron transfer line consists of an achromatic dog-leg to raise the electron beam up to the level of the proton line and a part which bends the electron beam horizontally onto the proton beam axis. Details of the beam line design are described in [9]. Fig. 4 shows the layout of electron beam line, together with the proton and laser beam lines in the AWAKE facility. Directly after the accelerating structure of the electron source a quadrupole triplet matches the electron beam into the transfer line (see Fig. 3). Another quadrupole triplet just before the plasma is used to focus the beam into the plasma. Five additional quadrupoles are used to control the dispersion and the beta function. While the dispersion in the horizontal plane is almost zero along the part of the beam line downstream of the merging dipole (common line with the protons), the dispersion in the vertical plane is not closed due to the vertical kick given by the tilted dipole, which merges the electron beam onto the proton beam axis. However, the fi nal focusing system matches the beta functions and dispersion to the required 1 σ spot size of r 250 μ m at the focal point in both planes. Longitudinally the focal point is set at an iris (ori fi ce) with a free aperture of 10 mm about 0.5 m upstream of the plasma cell. The present optics provides the possibility to shift the focal point up to 0.8 m into the plasma cell without signi fi cant changes of the beam spot size. Ten kickers (correctors) along the electron beam line compensate systematic alignment and fi eld errors and a shot-to-shot stability of 7 100 μ m is predicted for a current fl uctuation of 0.01% in the power con- verters. In the common beam line upstream the plasma cell the proton, electron and laser beam are travelling coaxially. Studies on the proton induced wake fi elds on the beam pipe walls and their effect on the electrons show that the in fl uence of the proton beam wake fi elds on the electrons is negligible [10]. However, direct beam – beam effects show that the electron beam emittance blows Table 1
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AWAKE, The Advanced Proton Driven Plasma Wakefield Acceleration Experiment at CERN

AWAKE, The Advanced Proton Driven Plasma Wakefield Acceleration Experiment at CERN

The electron source is installed in an adjacent room 1.16 m below the level of the proton beam line. The electron transfer line consists of an achromatic dog-leg to raise the electron beam up to the level of the proton line and a part which bends the electron beam horizontally onto the proton beam axis. Details of the beam line design are described in [9]. Fig. 4 shows the layout of electron beam line, together with the proton and laser beam lines in the AWAKE facility. Directly after the accelerating structure of the electron source a quadrupole triplet matches the electron beam into the transfer line (see Fig. 3). Another quadrupole triplet just before the plasma is used to focus the beam into the plasma. Five additional quadrupoles are used to control the dispersion and the beta function. While the dispersion in the horizontal plane is almost zero along the part of the beam line downstream of the merging dipole (common line with the protons), the dispersion in the vertical plane is not closed due to the vertical kick given by the tilted dipole, which merges the electron beam onto the proton beam axis. However, the fi nal focusing system matches the beta functions and dispersion to the required 1 σ spot size of r 250 μ m at the focal point in both planes. Longitudinally the focal point is set at an iris (ori fi ce) with a free aperture of 10 mm about 0.5 m upstream of the plasma cell. The present optics provides the possibility to shift the focal point up to 0.8 m into the plasma cell without signi fi cant changes of the beam spot size. Ten kickers (correctors) along the electron beam line compensate systematic alignment and fi eld errors and a shot-to-shot stability of 7 100 μ m is predicted for a current fl uctuation of 0.01% in the power con- verters. In the common beam line upstream the plasma cell the proton, electron and laser beam are travelling coaxially. Studies on the proton induced wake fi elds on the beam pipe walls and their effect on the electrons show that the in fl uence of the proton beam wake fi elds on the electrons is negligible [10]. However, direct beam – beam effects show that the electron beam emittance blows Table 1
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Axionic suppression of plasma wakefield acceleration

Axionic suppression of plasma wakefield acceleration

A strength of the plasma wake fi eld mechanism, like the unipolar inductor, for cosmic acceleration is that it does not rely on ingredients outside of the standard model of particle physics. However, non-standard model effects may be relevant as a consequence of the ultra- strong electromagnetic fi eld strengths expected in the most extreme astrophysical environ- ments. In particular, it is conceivable that effects due to hypothetical particles with very weak coupling to light and matter may manifest in such environments. One of the most popular dark matter candidates, the axion, was proposed as an elegant solution to the strong CP problem in QCD [ 20 – 22 ] before its signi fi cance in the cosmological context was expounded [ 23 – 25 ] . Furthermore, in addition to the QCD axion, light pseudo-scalar particles are a generic consequence of type IIB string theory [ 26 ] ; hence, a range of experiments have been developed, or are under development, in an attempt to uncover the effects of axions and axion-like particles ( ALPs ) [ 27 – 33 ] . Although positive detection remains elusive in the laboratory, it is possible that ALPs play a signi fi cant role in the ultra-strong magnetic fi eld of a neutron star.
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Axionic suppression of plasma wakefield acceleration

Axionic suppression of plasma wakefield acceleration

A strength of the plasma wake fi eld mechanism, like the unipolar inductor, for cosmic acceleration is that it does not rely on ingredients outside of the standard model of particle physics. However, non-standard model effects may be relevant as a consequence of the ultra- strong electromagnetic fi eld strengths expected in the most extreme astrophysical environ- ments. In particular, it is conceivable that effects due to hypothetical particles with very weak coupling to light and matter may manifest in such environments. One of the most popular dark matter candidates, the axion, was proposed as an elegant solution to the strong CP problem in QCD [ 20 – 22 ] before its signi fi cance in the cosmological context was expounded [ 23 – 25 ] . Furthermore, in addition to the QCD axion, light pseudo-scalar particles are a generic consequence of type IIB string theory [ 26 ] ; hence, a range of experiments have been developed, or are under development, in an attempt to uncover the effects of axions and axion-like particles ( ALPs ) [ 27 – 33 ] . Although positive detection remains elusive in the laboratory, it is possible that ALPs play a signi fi cant role in the ultra-strong magnetic fi eld of a neutron star.
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Ultrahigh brightness bunches from hybrid plasma accelerators as drivers of 5th generation light sources

Ultrahigh brightness bunches from hybrid plasma accelerators as drivers of 5th generation light sources

Differences of LWFA and PWFA, and the advantages of using laser pulses and electron bunches, respectively, for plasma wakefield acceleration are discussed. In the context of the “Trojan Horse” underdense photocathode plasma wakefield acceleration, the advantages are combined in such a way that the long, dephasing-free acceleration lengths obtainable from high-current electron drive bunches set up the plasma blowout cavity, and that the short Rayleigh lengths of focused laser pulse are used to release ultracold photoelectrons inside the blowout. The electron bunch driver can come from conventional linacs, or from laser-plasma-accelerators. Bunches from conventional linacs have the advantage of stablity and high repetition rate, but the challenge of synchronization between the electron bunch, the rf phase, and the underdense photocathode laser pulse, and the challenge of producing sufficient current to trap Trojan electrons. Especially at lower electron energies, which may be attractive because then the linac can be shorter and the total footprint of the hybrid system is smaller, space charge effects may limit the obtainable peak current. It is proposed to explore whether a “PWFA-mode” of linacs can be developed, where the design goals would be on (temporary) peak current rather than on emittance and energy spread of the drive bunches, which play a secondary role in PWFA. Electron bunches from laser-plasma-accelerators typically have rather large energy spread and emittance, but also very high currents. Using these bunches as drivers for PWFA in general, and for Trojan Horse in particular, has the advantage of truly compact systems, and of inherently perfect synchronization between the electron bunch driver and the photocathode laser pulse. Drawbacks are the limited stability and repetition rate from today’s LWFA systems. Remedy may come from future high repetition rate laser systems.
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Acceleration of electrons in the plasma wakefield of a proton bunch

Acceleration of electrons in the plasma wakefield of a proton bunch

High energy particle accelerators have been crucial in providing a deeper understanding of fundamental particles and the forces that govern their interactions. In order to increase the energy or reduce the size of the accelerator, new accel- eration schemes need to be developed. Plasma wakefield acceleration [1–5], in which the elec- trons in a plasma are excited, leading to strong electric fields, is one such promising novel accel- eration technique. Pioneering experiments have shown that an intense laser pulse [6–9] or elec- tron bunch [10, 11] traversing a plasma, drives electric fields of tens of giga-volts per metre and above. These values are well beyond those achieved in conventional radio frequency (RF) ac- celerators which are limited to about 0.1 giga-volt
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Optical plasma torch electron bunch generation in plasma wakefield accelerators

Optical plasma torch electron bunch generation in plasma wakefield accelerators

The use of plasmas for acceleration of electrons is an increasingly vivid topic, fueled by the fundamental advan- tage that the extremely high electric fields available super- sede those in conventional accelerators by many orders of magnitude. In recent years the ability to excite and control suitable plasma waves driven either by laser (laser wake- field acceleration, LWFA) [1 – 8] or electron beams (plasma wakefield acceleration, PWFA) [9 – 13] has increased sub- stantially. In both cases, injection of electron beams into the proper phase of the plasma wave is of paramount impor- tance to obtain high-quality witness bunches from the plasma. A multitude of injection methods has been con- ceived, among those hydrodynamics-based plasma density transition [14 – 20], injection by additional ionization [21 – 30] and Trojan Horse-type methods [31 – 38].
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Ionization induced electron injection in laser wakefield acceleration

Ionization induced electron injection in laser wakefield acceleration

Eq.1 to Eq. 4 give a full description of the wake excitation and particle dynamics for one dimensional wakefield acceleration. Ionization injection in one dimensional case can also be described by this theory. The ideal of ionization injection is firstly mentioned by Umstadter et al. in 1997 at the end of his paper on ponderomotive force injection, where he said that ”...Instead of using a fully ionized plasma (as above), one would use a medium with deeply bound inner shell electrons, which have an appearance intensity for tunneling ionization below that of the injection pulse but above that of the pump pulse ...” [8] The first detailed research work on ionization injection is done by Chen et al. in 2006 [9], where we used a double-pulse scheme. The first laser pulse works as a driver to excite a strong wakefield in a plasma, a second transversely propagated laser pulse ionizes the internal electrons of the high Z gas inside the wakefield at an appropriate time. In that paper, the contribution from ionization and ponderomotive force injections are compared in detail. Later on, the ionization injection scheme has been successfully generalized to a single laser pulse condition in which the wake excitation and ionization injection are performed by the same laser pulse. A lot of experimental studies have demonstrated this idea since 2010. Before that, ionization has already been used for electron injection in beam-driven plasma wakefield acceleration experiment.
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Tunable Electron Multibunch Production in Plasma Wakefield Accelerators

Tunable Electron Multibunch Production in Plasma Wakefield Accelerators

plasma wavelength, which is equal to the plasma cavity length. GeV-scale energies have been demonstrated both in laser wakefield accelerators (LWFAs) [3–5] and in beam-driven plasma wakefield accelerators (PWFAs) in cm to meter-scale [6] plasma sections. Witness bunch(es) to be accelerated in these systems can be injected from an external source or generated directly within the plasma itself. In this Letter, we show how ultra-high quality electron bunch trains can be simultaneously generated in one and the same plasma cavity and tuned independently over a large parameter range. Such highly tunable multibunches have been hitherto inaccessible but are highly important for various fields, such as driver- witness plasma wakefield acceleration, and light sources based on Compton scattering or (multi-color [7]) free-electron lasers (FELs).
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Plasma Wakefield Accelerator Research 2019 - 2040 : A community-driven UK roadmap compiled by the Plasma Wakefield Accelerator Steering Committee (PWASC)

Plasma Wakefield Accelerator Research 2019 - 2040 : A community-driven UK roadmap compiled by the Plasma Wakefield Accelerator Steering Committee (PWASC)

Achieving high luminosity is a challenge for plasma wakefield acceleration and given its current status, there has not been a great deal of discussion amongst the HEP community on the possible applications. The development of particle colliders driven by plasma accelerators is therefore a goal which must be considered to be a long-term objective. A significant effort over an extended period, with extensive international collaboration, will be required to meet these considerable challenges. Several international programmes are coordinating efforts in this area, such as the AWAKE, Helmholtz Virtual Institute for plasma wakefield acceleration, and EuPRAXIA projects; the ICFA panel on Advanced and Novel Accelerators (ANA) has hosted several workshops on developing roadmaps for plasma accelerator colliders and has formed the Advanced LinEar collider study GROup (ALEGRO), to co-ordinate the preparation of a proposal for an advanced linear collider in the multi-TeV energy range. The UK has strong representation in all these efforts, and in order to exploit its strength in this area it will be important that the UK groups are able to continue to contribute to these and other collaborative efforts. The US LWFA and PWFA communities have produced a similar roadmap with focus on developing both of these technologies towards a high-energy, high-luminosity linear e + e −
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Plasma Wakefield Accelerator Research 2019 - 2040 : a community-driven UK roadmap compiled by the Plasma Wakefield Accelerator Steering Committee (PWASC)

Plasma Wakefield Accelerator Research 2019 - 2040 : a community-driven UK roadmap compiled by the Plasma Wakefield Accelerator Steering Committee (PWASC)

Achieving high luminosity is a challenge for plasma wakefield acceleration and given its current status, there has not been a great deal of discussion amongst the HEP community on the possible applications. The development of particle colliders driven by plasma accelerators is therefore a goal which must be considered to be a long-term objective. A significant effort over an extended period, with extensive international collaboration, will be required to meet these considerable challenges. Several international programmes are coordinating efforts in this area, such as the AWAKE, Helmholtz Virtual Institute for plasma wakefield acceleration, and EuPRAXIA projects; the ICFA panel on Advanced and Novel Accelerators (ANA) has hosted several workshops on developing roadmaps for plasma accelerator colliders and has formed the Advanced LinEar collider study GROup (ALEGRO), to co-ordinate the preparation of a proposal for an advanced linear collider in the multi-TeV energy range. The UK has strong representation in all these efforts, and in order to exploit its strength in this area it will be important that the UK groups are able to continue to contribute to these and other collaborative efforts. The US LWFA and PWFA communities have produced a similar roadmap with focus on developing both of these technologies towards a high-energy, high-luminosity linear e + e −
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Radially polarized, half-cycle, attosecond pulses from laser wakefields through coherent synchrotronlike radiation

Radially polarized, half-cycle, attosecond pulses from laser wakefields through coherent synchrotronlike radiation

Different ways may, in principle, be used to induce injection of electron sheets into a wakefield [18]. A di- rect and promising approach was recently described using density-gradient injection in a controlled manner. The details of the scheme are documented in Ref. [19], where an up-ramp density profile followed by a plateau is employed. Wave breaking then occurs sharply at the density transition position, leading to a sudden longitudinal injection into the quasi-1D wakefield. This is in contrary to the transverse injection that normally occurs quasi-continuously in the bubble regime [12, 14]. Notice that longitudinal injection but in a gentle way has been observed in experiments [20]. The key elements of the present sharp longitudinal injection are summarized as follows. First, along the up-ramp, the first few wake wave periods trailing after the driving laser can travel at superluminal phase speeds for high enough density gradients. It is also valid for high wave nonlinearities, so that the density wave crests can be stably compressed into dense electron sheets without premature injection [19, 21]. The free of injection eventually terminates as the wake wave propagates into the following density plateau region, where the wave’s phase velocity falls below the light speed. As a result, a major part of the sheet electrons satisfying the injection threshold are trapped as whole in a very short time scale. They typically form an ultrathin (10s nanometers thick corresponding to attoseconds in duration) overcritical dense electron disk that accelerates in the wakefield.
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Undulator radiation driven by laser-wakefield accelerator electron beams

Undulator radiation driven by laser-wakefield accelerator electron beams

The laser wakefield accelerator (LWFA) is a very attractive alternative to radio frequency (RF) acceleration technology because of the roughly three orders of magnitude increase in the achievable accelerating gradient. 1 Intense femtosecond duration laser pulses are focused into gas or plasma targets resulting in the formation of highly relativistic electron bunches with enticing properties, given that the accelerator is so short (mm-cm scale): energy of 100s of MeV up to 4 GeV, 2 energy spread of less than 1%, 3 transverse normalised emittance less than 0.5 π mm mrad, 4 few femtosecond duration 5 and kiloAmpere peak currents. 3,5
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Influence of a Static Magnetic Field on Beam Emittance in Laser Wakefield Acceleration

Influence of a Static Magnetic Field on Beam Emittance in Laser Wakefield Acceleration

In Laser Wake-Field Acceleration (LWFA) [1-4], a laser creates a plasma wave wakefield with a phase velocity close to the speed of light (c). The acceleration gradi- ents in these wakefields can easily exceed 100 GeV/m, hence a cm-long plasma based accelerator can produce- GeV-energy electron beams. An electron injected in such a wave gains energy from the longitudinal component of the electric field, as long as the pump pulse is not de- pleted and the dephasing length is not reached. These wakefields have ideal properties for accelerating elec- trons. The transverse focusing field increases linearly with the radial distance and the accelerating longitudinal field is independent of the radial coordinate [5,6]. LWFA can be split into different options. The first corresponds to a plasma density e , a pulse length (cτ) matching half of a plasma period and a spot size (w 0 )
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