Wakefield Electron Acceleration Research Articles

Electron bunch compression and acceleration in the laser wakefield

Abstract The compression and acceleration of an external electron bunch into the laser wakefield is studied using 3D modeling with the LAPLAC code and compared to analytical predictions. It is shown, for a laser propagating in a plasma channel, that the nonlinear laser pulse dynamics together with the finite laser spot size influence the electron bunch compression and acceleration due to the reduction of the laser pulse group velocity. The transverse bunch dynamics and loading effect determine the final bunch charge and density and restrict the compressed sizes of the trapped and accelerated electron bunch. The dynamics of the electron bunch are illustrated with a set of parameters where the accelerated bunch acquires an energy of the order of 2 GeV, and 1% energy spread with sub-micron sizes.

Femtosecond coherent pulses in the keV range from inner-shell transitions pumped by a betatron source

We present a new method to generate ultra-short X-ray laser pulses by using the recently demonstrated laser-driven betatron source to photo-pump inner-shell transitions. The proposed compact set-up will then open the route to a wide range of applications. The betatron spectrum and ion-population kinetics are modeled and the temporal evolution of the gain coefficient for the K-α transitions is assessed. Using measured values of divergence, duration, and number of photons per pulse of the betatron source as input parameters, local gain values close to 60 cm−1 are calculated for nitrogen at 3.2 nm. Significant gain values are also numerically obtained at shorter wavelengths (for neon at 1.5 nm) when the betatron energy distribution is optimized as suggested by recent laser wakefield electron acceleration experiments.

Numerical modelling of a 10-cm-long multi-GeV laser wakefield accelerator driven by a self-guided petawatt pulse

The use of a short-pulse petawatt (PW) laser (τL<200 fs, wavelength ≈1 μm) enables experimental realization of a self-guided, multi-centimetre-long multi-GeV laser wakefield electron accelerator. A comprehensive set of numerical simulations showed that a 150 fs, 1.33 PW pulse is self-guided over 10 cm of a static filling gaseous plasma of density 1–3×1017 cm−3 and is stable against relativistic filamentation. A fully broken electromagnetic wake (electron density ‘bubble’) is excited over the entire interaction length. Variations of bubble size and shape associated with nonlinear evolution of the driving pulse result in self-injection of background plasma electrons. Self-injection begins immediately after the first nonlinear laser focus, where pulse de-focusing forces the bubble to grow. Injection continues without interruption while the bubble expands, and ceases when the laser becomes self-guided and bubble evolution stabilizes. Self-injected electrons are accelerated to ∼7 GeV with less than 10% energy spread and ∼1.3 nC charge. Numerical modelling of the laser pulse dynamics over the entire plasma length is carried out using a time-averaged, fully relativistic, quasi-static three-dimensional (3D) axi-symmetric particle-in-cell (PIC) code, WAKE. The process of electron self-injection is explored by means of both test-particle modelling (WAKE) and 3D PIC simulations using the recently developed CALDER-Circ code in quasi-cylindrical geometry.

Compensating for frequency detuning in multibunch wakefield acceleration schemes

The wakefield electron acceleration scheme provides effective means of creating high accelerating field gradients in both dielectric-lined waveguide structures and plasma. We have studied the influence of errors in the structure of this scheme on the efficiency of energy transfer from the driving to witness (driven) electron bunches. Deviation of the wakefield frequency from the calculated optimum value leads to a misfit in the mutual positioning of bunches in the driving train and the corresponding decrease in the energy transformer ratio. It is suggested to compensate for the arising misfit by correcting distances between bunches in the driving train, with a resulting increase in the efficiency of energy transfer. Results of calculations are presented for a dielectric-filled 13-GHz structure with parameters of the Argonne Wakefield Accelerator.

Laser wakefield electron acceleration on Texas petawatt facility: Towards multi-GeV electron energy in a single self-guided stage

Laser wakefield acceleration experiments with the 150 fs Texas Petawatt laser will be carried out in the unique regime with the shortest pulse duration among available petawatt facilities. Simulations via the time-averaged, fully relativistic, quasi-static 3D axi-symmetric particle-in-cell (PIC) code WAKE show that combination of nonlinear plasma wave focusing, relativistic self-focusing, and nonlinear temporal compression guide a 1.33 PW pulse over 10 cm of uniform plasmas. Self-guided TPW pulse has intensity above 10 19 W/cm 2 and drives the laser wakefield in the blowout regime in the plasma density range n 0 > 2 × 10 17 cm −3. Both test-particle (WAKE) and full 3D PIC modelling (using recently developed quasi-cylindrical CALDER-Circ code) demonstrate robust electron self-injection in the first plasma wave bucket (“bubble”) during the periods of laser defocusing and bubble growth. Injection terminates as soon as the laser evolution stabilizes. As a result, quasi-monoenergetic acceleration of ∼1.3 nC electron bunch to the 2.5–7 GeV is expected in 10-cm long plasmas.

Studies of laser wakefield structures and electron acceleration in underdense plasmas

Experiments on electron acceleration and optical diagnostics of laser wakes were performed on the HERCULES facility in a wide range of laser and plasma parameters. Using frequency domain holography we demonstrated single shot visualization of individual plasma waves, produced by 40TW, 30fs laser pulses focused to the intensity of 1019W∕cm2 onto a supersonic He gas jet with plasma densities ne&amp;lt;1019cm−3. These holographic “snapshots” capture the variation in shape of the plasma wave with distance behind the driver, and resolve wave front curvature seen previously only in simulations. High-energy quasimonoenergetic electron beams were generated using plasma density in the range 1.5×1019≤ne≤3.5×1019cm−3. These experiments demonstrated that the energy, charge, divergence, and pointing stability of the beam can be controlled by changing ne, and that higher electron energies and more stable beams are produced for lower densities. An optimized quasimonoenergetic beam of over 300MeV and 10mrad angular divergence is demonstrated at a plasma density of ne≃1.5×1019cm−3. The resultant relativistic electron beams have been used to perform photo-fission of U238 with a record high reaction yields of ∼3×105∕J. The results of initial experiments on electron acceleration at 70TW are discussed.

Ultra-intense Laser Plasma Interaction Studies at RRCAT, Indore

The Laser Plasma Division at the Raja Ramanna Centre for Advanced Technology (RRCAT), Indore has been engaged in research and development activities on laser plasma interaction using 10 TW, 45 fs, Ti:sapphire laser and a variety of in-house developed diagnostic systems for ultrashort laser pulses, plasma and x-ray radiation. Our recent studies include laser wakefield electron acceleration, high order harmonic generation, resonance enhancement of single harmonics, MeV energy bremsstrahlung radiation, absorption of ultrashort laser pulses in in-situ produced metal clusters, and ultrashort pulse K-shell x-ray line emission. This paper briefly describes these studies and important results obtained.

Cyclotron resonance effect on Wakefield excitation and electron acceleration

The acceleration mechanism of electrons in the presence of a strong axial magnetic field and Gaussian laser pulse is investigated by solving the dynamical equations numerically using a 2D relativistic code for particle acceleration. In the presence of a guide magnetic field, a large amplitude plasma wave of finite transverse extent and large phase velocity resonantly accelerates electrons when the Doppler shifted wave frequency equals electron–cyclotron frequency. After the electrons have gained high axial velocity, they may gain energy from the wave via Cerenkov resonance. The amplitude of the plasma wave increases when the strength of the magnetic field increases and hence the electron energy increases further.

Controlled injection and acceleration of electrons in plasma wakefields by colliding laser pulses

In laser-plasma-based accelerators, an intense laser pulse drives a large electric field (the wakefield) which accelerates particles to high energies in distances much shorter than in conventional accelerators. These high acceleration gradients, of a few hundreds of gigavolts per metre, hold the promise of compact high-energy particle accelerators. Recently, several experiments have shown that laser-plasma accelerators can produce high-quality electron beams, with quasi-monoenergetic energy distributions at the 100 MeV level. However, these beams do not have the stability and reproducibility that are required for applications. This is because the mechanism responsible for injecting electrons into the wakefield is based on highly nonlinear phenomena, and is therefore hard to control. Here we demonstrate that the injection and subsequent acceleration of electrons can be controlled by using a second laser pulse. The collision of the two laser pulses provides a pre-acceleration stage which provokes the injection of electrons into the wakefield. The experimental results show that the electron beams obtained in this manner are collimated (5 mrad divergence), monoenergetic (with energy spread <10 per cent), tuneable (between 15 and 250 MeV) and, most importantly, stable. In addition, the experimental observations are compatible with electron bunch durations shorter than 10 fs. We anticipate that this stable and compact electron source will have a strong impact on applications requiring short bunches, such as the femtolysis of water, or high stability, such as radiotherapy with high-energy electrons or radiography for materials science.

Wake field excitation and electron acceleration by triangular and sawtooth laser pulses in a plasma: an analytical approach

The amplitude of wake fields generated by the finite-width laser pulse propagating in an underdense plasma are determined using weakly nonlinear equations (n′e≪n0) for triangular (TR) and two types of sawtooth (SD and SI, defined in section 2) pulses by making the pulse duration the same as the electron plasma period. In addition to density perturbations ne′, the potential ϕ and the wake field Ez behind the laser pulses, the expression for the change in relativistic factor γ or energy gain for an electron is obtained and discussed for all these types of laser pulses under the effect of laser frequency f, pulse duration τ (corresponding to plasma density n0) and laser intensity I0. The TR pulse is found to be most significant as in this case the electrons can be accelerated up to hundreds of MeV (471.6 MeV) when τ = 6 ps, I0 = 1.0×1012 W m−2 and f = 282 THz (corresponding to Nd:YAG laser of wavelength 1.064 μm); even more acceleration is possible with prudently selected parameters.

Bubble regime of wake field acceleration: similarity theory and optimal scalings

A similarity theory is developed for ultra-relativistic laser-plasmas. It is used to compare and optimize possible regimes of three-dimensional wake field acceleration. The optimal scalings for laser wake field electron acceleration are obtained analytically. The main message of the present work is that the bubble acceleration regime [see Pukhov A. & Meyer-ter-Vehn J. 2002. Appl. Phys. B74, 355] satisfies these optimal scalings.

Envelope equations and conservation laws describing wakefield generation and electron acceleration

Previous authors have proposed various envelope equations to describe the behavior of an electromagnetic pulse generating a wakefield. In general these retain second-order derivatives, the reason being that the eikonal contains the initial wave frequency. Here it is shown that if the evolution of the wave frequency is followed using ray-tracing equations, a first-order evolution equation is obtained. It can be shown with this formalism that wave action is conserved and the energy lost from the electromagnetic wave can be explicitly accounted for in terms of energy gained by the plasma. The energy balance equations suggest that an electron bunch which will extract energy efficiently from a wakefield can be at least as efficiently accelerated by direct interaction with the electromagnetic pulse.

Investigation of ultraintense femtosecond laser–plasma interactions through ω and 2ω imaging and spectroscopy

A laser–plasma interaction experiment was performed in order to match effective conditions for wake-field electron acceleration. A 30-fs pulse was made to interact with a preformed plasma generated via the exploding foil technique from a 1-μm-thick plastic film. The irradiance of the femtosecond pulse in the plasma was 1020 Wcm−2. The interaction conditions were investigated via imaging and spectroscopy at the fundamental and the second harmonic of the laser frequency, both forward and backward. Our data clearly show that conditions suitable for electron acceleration are achieved close to the propagation axis and can be easily reproduced from shot to shot. In contrast, significant growth of instabilities occurs at the boundaries of the interaction region. These observations are consistent with a preliminary evidence of forward acceleration of high-energy electrons. Optical, X-ray, and γ-ray data obtained for different positions of the foil target with respect to the laser focal plane further support this promising scenario.

Experiments of high energy gain laser wakefield acceleration

The wakefield acceleration of electrons has a great potential for the future accelerator because of its high accelerating field gradient. We have obtained over 100 MeV acceleration gain by the wakefield generated by a 2 TW Ti : sapphire laser system. In the acceleration experiment, the 17 MeV electrons from a linac were used for the injection beam. The synchronization between the RF signal and the laser pulse was achieved within the time jitter of 3.7 ps. Due to the self-focusing and ionization, a long propagation length and high field gradient were realized. The self-focusing effect of the laser was confirmed by the laser spotsize measurement along the beam axis. The plasma density oscillation was measured by using the frequency domain interferometry. The acceleration gain expected from the plasma density measurement was consistent with the result of the acceleration experiments.

The laser wakefield acceleration experiment at Ecole Polytechnique

A laser wakefield electron acceleration experiment has been set-up at Ecole Polytechnique. An electron beam with 3 MeV total energy is injected in a plasma wave generated by laser wakefield using the new LULI CPA laser (400 fs [FWHM], I<10 17 W/cm 2). The first results show an effective acceleration of the order of 1 MeV, with a maximum when the electron density is close to the optimum value for which the laser pulse length is about half the plasma wavelength.

Plasma wakefield generation and electron acceleration in a self-modulated laser wakefield accelerator experiment

A self-modulated laser wakefield accelerator (SM-LWFA) experiment was performed at the Naval Research Laboratory. Large amplitude plasma wakefields produced by a sub-picosecond, high intensity laser pulse (7×1018 W/cm2) in an underdense plasma (ne≈1019 cm−3) were measured with a pump–probe coherent Thomson scattering (CTS) technique to last for less than 5 ps, consistent with the decay of large amplitude plasma waves due to the modulational instability. A plasma channel was observed to form in the wake of the pump laser pulse, and its evolution was measured with the pump–probe CTS diagnostic. The trailing probe laser pulse was observed to be guided by this channel for about 20 Rayleigh lengths. High energy electrons (up to 30 MeV) have been measured using an electro-magnetic spectrometer, with the energy spectra and divergence of lower energy (up to 4 MeV) electrons obtained using photographic films. Highly nonlinear plasma waves were also detected using forward Raman scattering diagnostics and were observed to correlate with the electron signals. Simulations of self-trapping of plasma electrons from the interaction of the laser wakefield with the slow plasma wave generated by Raman backscattering are also presented.

Nonlinear Optics in Relativistic Plasmas and Laser Wake Field Acceleration of Electrons

When a terawatt-peak-power laser beam is focused into a gas jet, an electron plasma wave, driven by forward Raman scattering, is observed to accelerate a naturally collimated beam of electrons to relativistic energies (up to 10(9) total electrons, with an energy distribution maximizing at 2 megaelectron volts, a transverse emittance as low as 1 millimeter-milliradian, and a field gradient of up to 2 gigaelectron volts per centimeter). Electron acceleration and the appearance of high-frequency modulations in the transmitted light spectrum were both found to have sharp thresholds in laser power and plasma density. A hole in the center of the electron beam may indicate that plasma electrons were expelled radially.

Controlled wake field acceleration via laser pulse shaping

We consider the interaction of high-intensity laser pulses with underdense plasmas and address the problem of the excitation of strong and stable wake plasma waves with regular electric fields to provide effective acceleration of charged particles over appreciably long distances. It is known that a relativistically strong laser pulse longer than the wavelength of plasma waves, propagating in a plasma is subject to self-modulation. This may result in a nonstationary behavior of the produced plasma wake field/particle dephasing, and reduced net acceleration. In this paper we present the results of 1(2/2)-D and 2(1/2)-D particle in cell (PIC) simulations which demonstrate that regular wake electric fields may be obtained by a properly shaped laser pulse (sharp steepening of its leading front). These results are relevant to the design of the 100 MeV laser wake field electron acceleration experiment that uses a terawatt picosecond CO/sub 2/ laser and is under construction at the Brookhaven Accelerator Test Facility.