Channel-guided Laser Wakefield Accelerator Research Articles

Progress on laser plasma accelerator development using transversely and longitudinally shaped plasmas

A summary of progress at Lawrence Berkeley National Laboratory is given on: (1) experiments on down-ramp injection; (2) experiments on acceleration in capillary discharge plasma channels; and (3) simulations of a staged laser wakefield accelerator (LWFA). Control of trapping in a LWFA using plasma density down-ramps produced electron bunches with absolute longitudinal and transverse momentum spreads more than ten times lower than in previous experiments (0.17 and 0.02 MeV/c FWHM, respectively) and with central momenta of 0.76±0.02MeV/c, stable over a week of operation. Experiments were also carried out using a 40 TW laser interacting with a hydrogen-filled capillary discharge waveguide. For a 15 mm long, 200 μm diameter capillary, quasi-monoenergetic bunches up to 300 MeV were observed. By detuning discharge delay from optimum guiding performance, self-trapping was found to be stabilized. For a 33 mm long, 300 μm capillary, a parameter regime with high energy bunches, up to 1 GeV, was found. In this regime, peak electron energy was correlated with the amount of trapped charge. Simulations show that bunches produced on a down-ramp and injected into a channel-guided LWFA can produce stable beams with 0.2 MeV/c-class momentum spread at high energies. To cite this article: W.P. Leemans et al., C. R. Physique 10 (2009). On présente un résumé des progrès obtenus au Lawrence Berkeley National Laboratory sur : (1) les expériences sur l'injection dans une rampe de densité décroissante ; (2) les expériences sur l'accélération dans des canaux de plasmas de décharges capillaires ; et (3) les simulations d'accélérateur par champ de sillage laser (LWFA) en configuration multi-étages. Le contrôle du piégeage dans un LWFA à l'aide de rampes de densité décroissante produit des paquets d'électrons avec des dispersions de moment cinétique longitudinal et transversal plus de dix fois plus faibles que dans des expériences antérieures (respectivement 0,17 and 0,02 MeV/c FWHM) avec une valeur centrale de 0,76±0,02MeV/c, stable sur la durée de la semaine d'expérience. Des expériences ont également été faites à l'aide d'un laser de 40 TW interagissant avec un guide d'onde constitué par une décharge capillaire d'hydrogène. Pour un capillaire de longueur 15 mm et de diamètre 200 μm, des électrons quasi-monoénergétiques atteignant 300 MeV ont été observés. En jouant sur le délai du temps d'allumage de la décharge et en s'écartant ainsi des performances optimales de guidage, l'auto piégeage a pu être stabilisé. Pour un capillaire de longueur 33 mm et de diamètre 300 μm, un régime de paramètres avec des électrons atteignant 1 GeV a été trouvé. Dans ce régime, l'énergie maximum des électrons a été corrélée avec la quantité d'électrons piégés. Les simulations ont montré que des paquets d'électrons produits dans une rampe de densité décroissante et injectés dans un guide dans un LWFA peuvent constituer des faisceaux stables de haute énergie avec des dispersions en moment de l'ordre de 0,2 MeV/c. Pour citer cet article : W.P. Leemans et al., C. R. Physique 10 (2009).

Coherent and incoherent radiation from a channel-guided laser wakefield accelerator

Coherent and incoherent electromagnetic radiation emitted from a channel-guided laser wakefield accelerator (LWFA) is calculated based on the Lienard–Wiechert potentials. It is found that at wavelengths longer than the bunch length, the radiation is coherent. The coherent radiation, which typically lies in the infrared range, shows features that reveal details of the acceleration process and properties of the electron bunch, such as its duration, charge, energy, and offset with respect to the wakefield axis. It is found that the LWFA emits energy predominantly in the coherent range of frequencies. The incoherent range of the spectrum, which extends to the x-ray frequency range, consists of rather broad peaks caused by the acceleration. The radiated energy, power and the pulse duration are estimated.

GeV electron beams from a centimeter-scale channel guided laser wakefield accelerator

Laser wakefield accelerators can produce electric fields of order 10–100GV∕m, suitable for acceleration of electrons to relativistic energies. The wakefields are excited by a relativistically intense laser pulse propagating through a plasma and have a phase velocity determined by the group velocity of the light pulse. Two important effects that can limit the acceleration distance and hence the net energy gain obtained by an electron are diffraction of the drive laser pulse and particle-wake dephasing. Diffraction of a focused ultrashort laser pulse can be overcome by using preformed plasma channels. The dephasing limit can be increased by operating at a lower plasma density, since this results in an increase in the laser group velocity. Here we present detailed results on the generation of GeV-class electron beams using an intense femtosecond laser beam and a 3.3cm long preformed discharge-based plasma channel [W. P. Leemans et al., Nature Physics 2, 696 (2006)]. The use of a discharge-based waveguide permitted operation at an order of magnitude lower density and 15 times longer distance than in previous experiments that relied on laser preformed plasma channels. Laser pulses with peak power ranging from 10–40TW were guided over more than 20 Rayleigh ranges and high quality electron beams with energy up to 1GeV were obtained by channeling a 40TW peak power laser pulse. The dependence of the electron beam characteristics on capillary properties, plasma density, and laser parameters are discussed.

Laser guiding for GeV laser–plasma accelerators

Guiding of relativistically intense laser beams in preformed plasma channels is discussed for development of GeV-class laser accelerators. Experiments using a channel guided laser wakefield accelerator at Lawrence Berkeley National Laboratory (LBNL) have demonstrated that near mono-energetic 100 MeV-class electron beams can be produced with a 10 TW laser system. Analysis, aided by particle-in-cell simulations, as well as experiments with various plasma lengths and densities, indicate that tailoring the length of the accelerator, together with loading of the accelerating structure with beam, is the key to production of mono-energetic electron beams. Increasing the energy towards a GeV and beyond will require reducing the plasma density and design criteria are discussed for an optimized accelerator module. The current progress and future directions are summarized through comparison with conventional accelerators, highlighting the unique short-term prospects for intense radiation sources based on laser-driven plasma accelerators.

Trapping and acceleration of nonideal injected electron bunches in laser Wakefield accelerators

Summary form only given. The standard regime for the laser wakefield accelerator (LWFA) usually requires external injection of MeV electrons. Ideally, the injected electron bunch should be injected into the proper phase of the accelerating wake, have a bunch length that is small compared with the plasma wavelength, and a low emittance and energy spread. There are several optical injection concepts that are theoretically capable of producing such precisely-timed (phased), high quality electron bunches, but the requirements are severe. This paper reports simulation studies of several nonideal injection schemes that demonstrate strong phase bunching and good accelerated beam quality in a channel-guided laser wakefield accelerator. A plasma channel is used to confine the laser pulse over a distance of many Rayleigh lengths. For the case of mono-energetic, unphased (long bunch) injection, the accelerated electrons have a large energy spread when the injection energy W/sub 0/ is high. However, there is an optimum range of injection energies for which the LWFA can trap a significant fraction of the injected pulse while producing an ultrashort, high-quality accelerated pulse. These favorable results are due to a combination of pruning of particles at unfavorable phases, rapid acceleration, and strong phase bunching. Simulation results for the trapping fraction, final average energy and energy spread, and the final bunch length agree well with the predictions of a simple Hamiltonian model using an ideal sinusoidal wake moving at the group velocity of the laser pulse. This long bunch case may apply to injection with a conventional or photocathode RF gun. The case of a channel-guided LWFA with a broad injected energy spread has also been simulated. Although the trapping fraction is generally much smaller than in the monoenergetic case, some simulations exhibit final accelerated bunches with remarkably small energy spread. These results suggest that relatively poor quality injection pulses may still be useful in LWFA demonstration experiments. The implications for planned LWFA experiments at NRL using various optical injection schemes will be discussed.

Quasimonoenergetic electrons from unphased injection into channel guided laser wakefield accelerators

A high-quality electron beam can be extracted from a channel guided laser wakefield accelerator without confining the injected particles to a small region of phase. By careful choice of the injection energy, a regime can be found where uniformly phased particles are quickly bunched by the accelerator itself and subsequently accelerated to high energy. The process is particularly effective in a plasma channel because of a favorable phase shift that occurs in the focusing fields. Furthermore, particle-in-cell simulations show that the self-fields of the injected bunches actually tend to reduce the energy spread on the final beam. The final beam characteristics can be calculated using a computationally inexpensive Hamiltonian formulation when beam-loading effects are minimal.

Simulation and design of stable channel-guided laser wakefield accelerators.

Most laser wakefield accelerator (LWFA) experiments to date have operated in the self-modulated (SM) regime and have been self-guided. A channel-guided LWFA operating in the standard or resonant regime is expected to offer the possibility of high electron energy gain and high accelerating gradients without the instabilities and poor electron beam quality associated with the SM regime. Plasma channels such as those produced by a capillary discharge have demonstrated guiding of intense laser pulses over distances of several centimeters. Optimizing the performance in a resonant LWFA constrains the on-axis plasma density in the channel to a relatively narrow range. A scaling model is presented that quantifies resonant LFWA performance in terms of the maximum accelerating gradient, dephasing length, and dephasing-limited energy gain. These performance quantities are expressed in terms of laser and channel experimental parameters, clearly illustrating some of the tradeoffs in the choice of parameters. The predicted energy gain in this model is generally lower than that indicated by simpler scaling models. Simulations agree well with the scaling model in both low and high plasma density regimes. Simulations of a channel-guided, self-modulated LWFA are also presented. Compared with the resonant LWFA regime, the requirements on laser and channel parameters in the SM regime are easier to achieve, and a channel-guided SM-LWFA is likely to be less unstable than a self-guided SM-LWFA.

Optically guided laser wake-field acceleration*

The laser wake-field acceleration concept is studied using a general axisymmetric formulation based on relativistic fluid equations. This formalism is valid for arbitrary laser intensities and allows the laser–plasma interaction to be simulated over long propagation distances (many Rayleigh lengths). Several methods for optically guiding the laser pulse are examined, including relativistic guiding, preformed plasma density channels and tailored pulse profiles. Self-modulation of the laser, which occurs when the pulse length is long compared to the plasma wavelength and the power exceeds the critical power, is also examined. Simulations of three possible laser wake-field accelerator (LWFA) configurations are performed and discussed: (i) a channel-guided LWFA, (ii) a tailored-pulse LWFA, and (iii) a self-modulated LWFA.