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Crossed beam energy transfer, CBET, in high-intensity laser–plasma interaction is investigated for the case of optically smoothed laser beams. In the two approaches to laser-driven inertial confinement fusion experiments, the direct-drive and the indirect-drive, CBET is of great importance because it governs the coupling of laser energy to the plasma. We use the two-dimensional wave-coupling code H armony to simulate the transfer between two laser beams with speckle structure that overlap in a plasma with an inhomogeneous flow profile. We compare the CBET dynamics for laser beams with spatial incoherence and with spatio-temporal incoherence; in particular we apply the smoothing techniques using random phase plates (RPPs) and smoothing by spectral dispersion (SSD), respectively. It is found that for laser beams (wavelength λ 0 ) with intensities ( I L ) above I L ∼ 2 × 10 15 W cm −2 ( λ 0 /0.35 µm) −2 ( T e /keV), both the so-called plasma-induced smoothing as well as self-focusing in intense laser speckles induce temporal incoherence; the latter affects the CBET and the angular distribution of the light transmitted behind the zone of beam overlap. For RPP-smoothed incident beams, the resulting band width of the transmitted light can already be of the same order as the effective band width of the SSD available at major laser facilities. We examine the conditions when spatio-temporal smoothing techniques become efficient for CBET. This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 1)’.

International audience; Crossed beam energy transfer, CBET, is investigated by taking into account the speckle structure of smoothed laser beams that overlap in a plasma with an inhomogeneous flow profile. Using the two-dimensional simulation code HARMONY, it is shown how ponderomotive self-focusing of laser speckles in crossing beams can significantly affect the transfer of energy from one beam to the other. The role of plasma flow on speckle self-focusing is investigated and revisited, in particular its consequences in terms of redirection and increasing angular spread of the laser beams due to beam bending and plasma-induced smoothing, respectively. At close-to-sonic flow the onset of self-focusing in the beam speckle structure occurs at considerably lower beam intensities than expected for the case without flow. CBET and speckle self-focusing can hence occur together when two crossed beams with equal frequency will resonantly exchange energy via their ponderomotively driven density perturbations flowing with sound speed. From the simulations it is found that consequences of ponderomotive self-focusing can be expected above an average intensity threshold scaling as I L ∼ 2 × 10 14 W cm −2 (λ 0 /1µm) −2 (T e /keV) with impact on the spatial and temporal coherence of the transmitted light. The density perturbations due to the ponderomotive force of the crossing beams can locally be enhanced in self-focusing speckles, partly leading to shock-like structures. These structures eventually increase the effect of plasma-induced smoothing and are at the origin of the stronger angular spread.

International audience; The current filamentation instability is a key phenomenon underpinning various processes in astrophysics, laboratory laser-plasma, and beam-plasma experiments. Here we show that the ultrafast dynamics of this instability can be explored in the context of relativistic laser-solid interactions through deflectometry by low-emittance, highly relativistic electron bunches from a laser wakefield accelerator. We present experimental measurements of the femtosecond timescale generation of strong magnetic-field fluctuations, with a measured line-integrated B field of 2.70 ± 0.39 kT μm. Three-dimensional, fully relativistic particle-in-cell simulations demonstrate that such fluctuations originate from the current filamentation instability arising at submicron scales around the irradiated target surface, and that they grow to amplitudes strong enough to broaden the angular distribution of the probe electron bunch a few tens of femtoseconds after the laser pulse maximum. Our results open a branch of physics experiments investigating the femtosecond dynamics of laser-driven plasma instabilities by means of synchronized, wakefield-accelerated electron beams.

This paper discusses the properties of electron beams formed in plasma wakefield accelerators through ionization injection. In particular, the potential for generating a beam composed of co-located multi-colour beamlets is demonstrated in the case where the ionization is initiated by the evolving charge field of the drive beam itself. The physics of the processes of ionization and injection are explored through OSIRIS simulations. Experimental evidence showing similar features are presented from the data obtained in the E217 experiment at the FACET facility of the SLAC National Laboratory. This article is part of the Theo Murphy meeting issue ‘Directions in particle beam-driven plasma wakefield acceleration’.

The role of laser speckle structure (hot spots) and its ponderomotive self-focusing (PSF), in crossed beam energy transfer (CBET), of smoothed laser beams is investigated in an inhomogeneous expanding plasma. Numerical simulations using the code harmony in two spatial dimensions, demonstrate how self-focusing of laser hot spots in crossed beams can significantly affect the transfer of energy from one beam to the other in addition to the stimulated Brillouin scattering (SBS) process. It is shown that for sufficiently intense laser beams, when the laser hot spots exceed the criterion for self-focusing in a plasma with flow, the angular spread of transmitted light beams increases considerably with the intensity, which arises in particular, in expanding plasma where significant beam deflection is observed. It is shown for the first time that besides SBS, the contribution of speckle structure, PSF, and deflections of the intense hot spots in multiple speckle beams to CBET, therefore matters.

The phenomenon of laser–plasma coupling is numerically studied by examining the nonlinear coupled temporal evolution of plasma wave amplitude in backward stimulated Raman scattering (BSRS). The theoretical model developed for numerical study takes into account the combined effects of relativistic and ponderomotive nonlinearities. The study shows that in BSRS, the phenomenon of nonlinear temporal growth and saturation of plasma waves strongly depends on laser radiation wavelength. This appears in the form of significant changes in saturation amplitude and saturation time of plasma waves excited due to long and short wavelength laser waves.

We gratefully acknowledge the support of the UK EPSRC (grant no. EP/J018171/1), the EU FP7 programmes: the Extreme Light Infrastructure (ELI) project, the Laserlab-Europe (no. 284464), and the EUCARD-2 project (no. 312453). The laser-plasma wakefield accelerator is a compact source of high brightness, ultra-short duration electron bunches. Self-injection occurs when electrons from the background plasma gain sufficient momentum at the back of the bubble-shaped accelerating structure to experience sustained acceleration. The shortest duration and highest brightness electron bunches result from self-injection close to the threshold for injection. Here we show that in this case injection is due to the localized charge density build-up in the sheath crossing region at the rear of the bubble, which has the effect of increasing the accelerating potential to above a critical value. Bunch duration is determined by the dwell time above this critical value, which explains why single or multiple ultra-short electron bunches with little dark current are formed in the first bubble. We confirm experimentally, using coherent optical transition radiation measurements, that single or multiple bunches with femtosecond duration and peak currents of several kiloAmpere, and femtosecond intervals between bunches, emerge from the accelerator. Publisher PDF

Here we explore ways of converting laser radiation into coherent electromagnetic radiation using laser-driven plasma waves. Several schemes are presented, including colliding laser pulses in magnetized plasma and utilizing ultra-short electron bunches from laser wakefield accelerators to produce intense single-cycle pulses through coherent transition radiation and few-cycle coherent synchrotron radiation in undulators and plasma channels. These sources rely on high current electron bunches with femtosecond durations, which can result in radiation over a broad range of frequencies from 1 to 10^5 THz.

The effects of relativistic nonlinearities on stimulated Raman scattering (SRS) of mildly relativistic laser radiation propagating through preionized underdense plasma is studied. The nonlinear mode coupling equations and linearized growth rates of Raman instabilities in presence of combined effects of relativistic and ponderomotive nonlinearities are derived. The temporal evolution of SRS process is then studied and analyzed using numerical techniques in presence of the two nonlinearities. It is observed that relativistic effects now taken into account significantly affect the growth rates and the nonlinear evolution of SRS process as compared to the case where they are neglected. The study shows that inclusion of relativistic nonlinearities along with the ponderomotive nonlinearities destabilizes SRS process

Nonparaxial, nonlinear propagation of a relativistic laser pulse in a preformed plasma channel having a parabolic density profile is analyzed. Nonlinearity arises due to relativistic motion of plasma electrons and their coupling with plasma channel. The effect of nonlinearities on betatron oscillations is analyzed and the nonlinear critical channel depth required for propagation of a matched laser pulse is evaluated. Mismatch due to nonlinear effects and additional condition for matched pulse propagation are obtained. Amplitude of the matched, as well as mismatched, beam is derived and finite pulse effects are studied.

The combined effect of relativistic and ponderomotive nonlinearities on the propagation characteristics and modulation instability of a laser beam propagating through partially stripped plasma is studied. Under the present approach it is seen that the ponderomotive nonlinearity tends to defocus the laser beam as against the nonlinear relativistic self-focusing phenomenon. Also density perturbation arising due to the ponderomotive nonlinearity when combined with relativistic nonlinearity tends to increase the modulation instability of the laser beam. The peak growth rate is enhanced in comparison to the case where ponderomotive nonlinearity is neglected.

Raman amplification in plasma has been proposed to be a promising method of amplifying short radiation pulses. Here, we investigate chirped pulse Raman amplification (CPRA) where the pump pulse is chirped and leads to spatiotemporal distributed gain, which exhibits superradiant scaling in the linear regime, usually associated with the nonlinear pump depletion and Compton amplification regimes. CPRA has the potential to serve as a high-efficiency high-fidelity amplifier/compressor stage.

Modern accelerators and light sources subject bunches of charged particles to quasiperiodic motion in extremely high electric fields, under which they may emit a substantial fraction of their energy. To properly describe the motion of these particle bunches, we require a kinetic theory of radiation reaction. We develop such a theory based on the notorious Lorentz-Dirac equation, and explore how it reduces to the usual Vlasov theory in the appropriate limit. As a simple illustration of the theory, we explore the radiative damping of Langmuir waves.

The normalised transverse emittance is a measure of the quality of an electron beam from a particle accelerator. The brightness, parallelism and focusability are all functions of the emittance. Here we present a high-resolution single shot method of measuring the transverse emittance of a 125 ± 3 MeV electron beam generated from a laser wakefield accelerator (LWFA) using a pepper-pot mask. An average normalised emittance of erms,x,y = 2.2 ± 0.7, 2.3 ± 0.6 π-mmmrad was measured, which is comparable to that of a conventional linear accelerator. The best measured emittance was erms,x,=1.1 ± 0.1 π-mm-mrad, corresponding to the resolution limit of our system. The low emittance indicates that this accelerator is suitable for driving a compact free electron laser.

Progress in laser wakefield accelerators indicates their suitability as a driver of compact free-electron lasers (FELs). High brightness is defined by the normalized transverse emittance, which should be less than 1π mm mrad for an x-ray FEL. We report high-resolution measurements of the emittance of 125 MeV, monoenergetic beams from a wakefield accelerator. An emittance as low as 1.1±0.1π mm mrad is measured using a pepper-pot mask. This sets an upper limit on the emittance, which is comparable with conventional linear accelerators. A peak transverse brightness of 5×10¹⁵ A m⁻¹ rad⁻¹ makes it suitable for compact XUV FELs.

Raman backscattering (RBS) in plasma is an attractive source of intense, ultrashort laser pulses, which has the potential asa basic for a new generation of laser amplifiers.1 Taking advantage of plasma, which can withstand extremely high power densities and can offer high efficiencies over short distances, Raman amplification in plasma could lead to significant reductions in both size and cost of high power laser systems. Chirped laser pulse amplification through RBS could be an effective way to transfer energy from a long pump pulse to a resonant counter propagating short probe pulse. The probe pulse is spectrally broadened in a controlled manner through self-phase modulation. Mechanism of chirped pulse Raman amplification has been studied, and features of supperradiant growth associated with the nonlinear stage are observed in the linear regime. Gain measurements are briefly summarized. The experimental measurements are in qualitative agreement with simulations and theoretical predictions.

The Advanced Laser-Plasma High-Energy Accelerators towards X-rays (ALPHA-X) programme is developing laserplasma accelerators for the production of ultra-short electron bunches with subsequent generation of incoherent radiation pulses from plasma and coherent short-wavelength radiation pulses from a free-electron laser (FEL). The first quantitative measurements of the electron energy spectra have been made on the University of Strathclyde ALPHA-X wakefield acceleration beam line. A high peak power laser pulse (energy 900 mJ, duration 35 fs) is focused into a gas jet (nozzle length 2 mm) using an F/16 spherical mirror. Electrons from the laser-induced plasma are self-injected into the accelerating potential of the plasma density wake behind the laser pulse. Electron beams emitted from the plasma have been imaged downstream using a series of Lanex screens positioned along the beam line axis and the divergence of the electron beam has been measured to be typically in the range 1-3 mrad. Measurements of the electron energy spectrum, obtained using the ALPHA-X high resolution magnetic dipole spectrometer, are presented. The maximum central energy of the monoenergetic beam is 90 MeV and r.m.s. relative energy spreads as low as 0.8% are measured. The mean central energy is 82 MeV and mean relative energy spread is 1.1%. A theoretical analysis of this unexpectedly high electron beam quality is presented and the potential impact on the viability of FELs driven by electron beams from laser wakefield accelerators is examined.

Electron acceleration using plasma waves driven by ultra-short relativistic intensity laser pulses has undoubtedly excellent potential for driving a compact light source. However, for a wakefield accelerator to become a useful and reliable compact accelerator the beam properties need to meet a minimum standard. To demonstrate the feasibility of a wakefield based radiation source we have reliably produced electron beams with energies of 82±5 MeV, with 1±0.2% energy spread and 3 mrad r.m.s. divergence using a 0.9 J, 35 fs 800 nm laser. Reproducible beam pointing is essential for transporting the beam along the electron beam line. We find experimentally that electrons are accelerated close to the laser axis at low plasma densities. However, at plasma densities in excess of 1019 cm-3, electron beams have an elliptical beam profile with the major axis of the ellipse rotated with respect to the direction of polarization of the laser.

Stimulated Raman backscattering in plasma has been suggested as a way to amplify short laser pulses to intensities not limited by damage thresholds as in chirped pulse amplification using conventional media. Energy is transferred between two transverse electromagnetic waves, pump and probe, through the parametric interaction with a longitudinal Langmuir wave that is ponderomotively excited by their beat wave. The increase of the plasma temperature due to collisional absorption of the pump wave modifies the dispersion of the Langmuir wave: firstly, its resonance frequency rises (Bohm-Gross shift), and secondly, Landau damping sets in. The frequency shift acts in a similar way to a chirp of the pump frequency, or a density ramp: different spectral components of the probe satisfy the resonance condition at different times. This limits their growth, while increasing the bandwidth of the amplifier, thus leading to superradiant amplification. Landau damping may shorten the probe pulse, but reduces the amplification efficiency. We investigate these effects analytically and using numerical simulations in order to assess their importance in experimental demonstrations, and the possibility of applications.

The nonlinear regime of Raman amplification has been studied including the combined effects of relativistic and ponderomotive nonlinearities. The study is important for interaction of mildly relativistic pump and probe laser pulses. Nonlinear coupled temporal evolution of fields and density in Raman amplification is analyzed. It is shown that the saturation amplitude and time of the probe pulse in nonlinear regime depends upon the intensity of the electromagnetic waves and the density of the medium. Further in the nonlinear regime the probe laser pulse gain is severely affected by changes in both the electromagnetic wave amplitude and the plasma density.

An analytical study of significant photon acceleration (frequency up-shift) in a plasma density wake produced by two laser pulses in the mildly relativistic and linearized regime is presented. The wake amplitude is amplified and its phase controlled using two coaxially, co-propagating laser pulses, which are considered to be identical but separated by a fixed time. A third probe pulse, with a variable delay, is considered as "test particle" or quasi-photon propagating through the amplified density wake, which experiences significant photon acceleration because of the local temporal and spatial variation of the permittivity. The evolution of the "photon" is studied using Hamiltonian theory. The significant frequency up-shift is much larger than that produced by the wake of a single relativistic laser pulse in the highly relativistic nonlinear wake regime. Our study demonstrates that the inter-pulse separation between the "controlling" pulse and the "driver" pulse, producing the amplified density wake, can provide an additional degree of freedom for tuning the maximum up-shift of the probe photon frequency.

A detailed analytical theory is developed for studying the phenomenon of generation of high amplitude wakefield using a seed laser pulse and another coaxially propagating, trailing pulse. The two laser pulses are separated by a fixed distance between them and have the same polarization, frequency, length, intensity, and profile. The study shows that the maximum energy gained by the test electron significantly depends upon the distance between the two pulses. Phase space analysis shows that a test electron of significantly lower energy, injected behind the trailing laser pulse, will be trapped and accelerated to higher energy as compared to the single pulse case.

Raman backscattering (RBS) in plasma is the basis of plasma-based amplifiers and is important in laser-driven fusion experiments. We show that saturation can arise from nonlinearities due to coupling between the fundamental and harmonic plasma wave modes for sufficiently intense pump and seed pulses. We present a time-dependent analysis that shows that plasma wave phase shifts reach a maximum close to wavebreaking. The study contributes to a new understanding of RBS saturation for counter-propagating laser pulses.

1098-4402= A detailed analytical study of plasma wakefield generation in a wide parabolic plasma channel is reported. A perturbative technique involving orders of the incident laser beam and the effects of inhomogeneity of the plasma density is used to obtain explicit electric as well as magnetic wakefields. The axial and transverse forces acting on a test electron due to the wakefields have been evaluated.

Raman backscattering in plasma is subject to—collisional and collisionless—absorption of the interacting waves. A model for studying its role over a wide parameter range is developed by coupling the envelope equations for pump, probe, and plasma waves with those describing heating of the plasma. The latter is treated as a warm fluid, making the model useful for moderate temperatures and field amplitudes. The main effect is the time-dependent Bohm–Gross shift of the Langmuir resonance frequency, which can either enhance or suppress amplification; this can be further controlled by varying the frequency of the pump. Anisotropy in the collisional processes for longitudinal and transverse waves leads to temperature anisotropy at high field amplitudes. Direct Landau damping of the plasma wave, as well as its contribution to the frequency shift, can be neglected due to rapid saturation.

Photon acceleration of a laser pulse occurs in a medium with a space and time-varying permittivity. Using Hamiltonian formulation, a theoretical study of the frequency upshift of a probe laser pulse, which is considered as a "quasiphoton" or "test particle," propagating through an amplified plasma density wake of two copropagating laser pulses, is presented. The linear superposition of wakefields studied using an analytical model shows that the presence of a controlling pulse amplifies the wake of a driver pulse. The amplified wake amplitude can be controlled by varying the delay between the two pulses. Two-dimensional particle-in-cell simulations demonstrate wake superposition due to the two copropagating laser pulses. A phase space analysis shows that the probe photon can experience a significant frequency upshift in the amplified density wake. Furthermore, the range of photon frequencies trapped and accelerated is determined by the amplitude of the density wake.

In this paper, evolution of the spot size of an intense laser beam propagating in axially magnetized, cold, underdense plasma has been studied. The effect of longitudinal magnetization on the laser spot for a left as well as a right circularly polarized laser beam has been considered. Critical power for nonlinear self-focusing of the beam in magnetized plasma has been obtained.

The propagation of an intense laser beam, having a sinusoidal pulse profile, in underdense plasma, is analyzed. Applying variational technique and using a standard trial function for the laser pulse amplitude, simultaneous equations describing the evolution of pulse length and spot size are obtained. Relativistic nonlinearity, finite pulse length, and group velocity dispersion effects have been taken into account. Using numerical methods, a graphical analysis of simultaneous evolution of laser spot size and pulse length is presented.

When an intense laser pulse interacts with homogeneous plasma embedded in a transverse magnetic field, transverse current density oscillating with frequency twice that of the laser field is set up. This leads to generation of second harmonic radiation with significant conversion efficiency.

In this paper, evolution of spot size of an intense laser beam in cold, underdense, magnetized plasma has been studied. The plasma is embedded in a uniform magnetic field perpendicular to both, the direction of propagation and electric vector of the radiation field. Nonlinear current density is set up and the source dependent expansion method is used to determine the evolution of the spot size of a laser beam having a Gaussian profile. It is shown that transverse magnetization of plasma enhances the self-focusing property of the laser beam leading to reduction in critical power required to self-focus the beam.

Modulation instability of a laser pulse propagating through transversely magnetized underdense plasma is studied. It is observed that interaction of laser radiation with plasma in the presence of uniform magnetic field results in an additional perturbed transverse plasma current density along with the relativistic and ponderomotive nonlinear current densities, thus affecting the modulational interaction. In the plane wave limit it is observed that modulational interaction is more stable for magnetized plasma as compared to the unmagnetized case. The analysis shows that there is a significant reduction in the growth rate of modulation instability over a given range of unstable wave numbers due to magnetization of plasma.

Nature Communications 12(1), 2895 (2021). doi:10.1038/s41467-021-23000-7, Particle accelerators based on laser- or electron-driven plasma waves promise compact sources for relativistic electron bunches. Here, Kurz and Heinemann et al. demonstrate a hybrid two-stage configuration, combining the individual features of both accelerating schemes., Published by Nature Publishing Group UK, [London]

Beam-driven plasma wakefield acceleration (PWFA) has demonstrated significant progress during the past two decades of research. The new Facility for Advanced Accelerator Experimental Tests (FACET) II, currently under construction, will provide 10 GeV electron beams with unprecedented parameters for the next generation of PWFA experiments. In the context of the FACET II facility, we present simulation results on expected betatron radiation and its potential application to diagnose emittance preservation and hosing instability in the upcoming PWFA experiments. This article is part of the Theo Murphy meeting issue ‘Directions in particle beam-driven plasma wakefield acceleration’.