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Laser wakefield accelerators produce accelerating gradientsup to hundreds of GeV/m, and recently demonstrated 1-10 MeV energy spreadat energies up to 1 GeV using electrons self-trapped from the plasma. Controlled injection and staging may further improve beam quality bycircumventing tradeoffs between energy, stability, and energyspread/emittance. We present experiments demonstrating production of astable electron beam near 1 MeV with hundred-keV level energy spread andcentral energy stability by using the plasma density profile to controlselfinjection, and supporting simulations. Simulations indicate that suchbeams can be post accelerated to high energies, potentially reducingmomentum spread in laser acceleratorsby 100-fold or more.
Laser wakefield acceleration of electrons holds great promise for producing ultra-compact stages of GeV scale, high quality electron beams for applications such as x-ray free electron lasers and high energy colliders. Ultra-high intensity laser pulses can be self-guided by relativistic plasma waves over tens of vacuum diffraction lengths, to give>1 GeV energy in cm-scale low density plasma using ionization-induced injection to inject charge into the wake at low densities. This thesis describes a series of experiments which investigates the physics of LWFA in the self-guided blowout regime. Beginning with high density gas jet experiments the scaling of the LWFA-produced electron beam energy with plasma electron density is found to be in excellent agreement with both phenomenological theory and with 3-D PIC simulations. It is also determined that self-trapping of background electrons into the wake exhibits a threshold as a function of the electron density, and at the densities required to produce electron beams with energies exceeding 1 GeV a different mechanism is required to trap charge into low density wakes. By introducing small concentrations of high-Z gas to the nominal He background the ionization-induced injection mechanism is enabled. Electron trapping is observed at densities as low as 1.3x1018 cm−3 in a gas cell target, and 1.45 GeV electrons are demonstrated for the first time from LWFA. This is currently the highest electron energy ever produced from LWFA. The ionization-induced trapping mechanism is also shown to generate quasi-continuous electron beam energies, which is undesirable for accelerator applications. By limiting the region over which ionization-induced trapping occurs, the energy spread of the electron beams can be controlled. The development of a novel two-stage gas cell target provides the capability to tailor the gas composition in the longitudinal direction, and confine the trapping process to occur only in a limited, defined region. Using this technique a 460 MeV electron beam was produced with an energy spread of 5%. This technique is directly scalable to multi-GeV electron beam generation with sub-percent energy spreads.
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 Ic FWHM, respectively) and with central momenta of 0.76 +- 0.02 MeV Ic, 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 mu m 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 mu m capillary, a parameter regime with high energy bunches, up to 1 Ge V, was found. In this regime, peak electron energy was correlated with the amount of trapped charge. Simulations show that bunches produced on a down-ramn and iniected into a channel-guided LWFA can produce stable beams with 0.2 MeV Ic-class momentum spread at high energies.
In the pursuit of discovering the fundamental laws and particles of nature, physicists have been colliding particles at ever increasing energy for almost a century. Lepton (electrons and positrons) colliders rely on linear accelerators (LINACS) because leptons radiate copious amounts of energy when accelerated in a circular machine. The size and cost of a linear collider is mainly determined by the acceleration gradient. Modern linear accelerators have gradients limited to 20-100 MeV/m because of the breakdown of the walls of the accelerator. Plasma based acceleration is receiving much attention because a plasma wave with a phase velocity near the speed of light can support acceleration gradients at least three orders of magnitude larger than those in modern accelerators. There is no breakdown limit in a plasma since it is already ionized. Such a plasma wave can be excited by the radiation pressure of an intense short pulse laser. This is called laser wakefield acceleration (LWFA). Much progress has been made in LWFA research in the past 30 years. Particle-in-cell (PIC) simulations have played a major part in this progress. The physics inherent in LWFA is nonlinear and three-dimensional in nature. Three-dimensional PIC simulations are computationally intensive. In this dissertation, we present and describe in detail a new algorithm that was introduced into the Particle-In-Cell Simulation Framework. We subsequently use this new quasi three-dimensional algorithm to efficiently explore the parameter regimes of LWFA that are accessible for existing and near term lasers. This regimes cannot be explored using full three-dimensional simulations even on leadership class computing facilities. The simulations presented in this dissertation show that the nonlinear, self-guided regime of LWFA described through phenomenological scaling laws by Lu et al., in 2007 is still useful for accelerating electrons to energies greater than 10 GeV. Fortunately, in many situations the physics of LWFA is nearly azimuthally symmetric and the most salient three-dimensional physics is captured by the inclusion of only a few azimuthal harmonics. Recently, it was proposed by Lifschitz et al. [J. Comp. Phys. 228 (5) 2009] to model LWFA by expanding the fields and currents in azimuthal harmonics and truncating the expansion. The complex amplitudes of the fundamental and first harmonic for the fields were solved on an r-z grid and a procedure for calculating the complex current amplitudes for each particle based on its motion in Cartesian geometry was presented using a Marder's correction to maintain the validity of Gauss's law. In this dissertation, we describe in detail the implementation of this algorithm into OSIRIS using a rigorous charge conserving current deposition method to maintain the validity of Gauss's law. We show that this algorithm is a hybrid method which uses a particles-in-cell description in r-z and a gridless description in phi (which we have subsequently coined the 'quasi-3D' method). We include the ability to keep an arbitrary number of harmonics and higher order particle shapes. Examples for laser wakefield acceleration, plasma wakefield acceleration, and beam loading are also presented. In almost all of the recent experiments progress on LWFA the plasma wave wake has been excited in the nonlinear blowout regime. A phenomenological description of this regime was given by Lu et al. [PRSTAB, 10 (061301) 2007]. This included matching conditions for the laser spot size and pulse length so that the laser evolution and wake excitation would be stable and the laser would self-guide. Scaling laws for the electron electron energy (self or externally injected) in terms of the laser and plasma parameters was also given. The parameters for the supporting simulations were limited due to the computational demands for such simulations particularly for higher electron energy. The recent implementation of the quasi-3D algorithm into OSIRIS including the charge conserving current deposit, now make it possible to study these scaling laws and examine how well they still hold for higher laser intensities and laser energies. We have studied in detail how well the nonlinear, self-guided regime works for existing and near term 15-100 Joule lasers. We demonstrate that the scaling laws do capture the key phenomenological characteristics LWFAs under a wide range of different laser and plasma parameters, but are not meant to give exact predictions for a choice of parameters. The simulations indicate that the self-injected particles reach slightly higher energies than estimated by the scaling laws, although the evolution of the maximum energy looks similar when scaled to the dephasing time. We also find that shape of the evolution of the energy, spot size, and wake amplitude scales if the normalized vector potential, and transverse and axial profile shapes remain fixed. If the normalized vector potential is changed then the scaling laws are still useful but the shape of energy evolution curve changes. We also used the scaling laws to optimize the energy gain for a fixed laser energy. We then use the quasi-3D OSIRIS code to study study in detail how to optimize the energy gain for fixed laser energy including how to optimize the axial laser profile. We find that shortening the pulse length and reducing the plasma density is effective in producing a higher energy beam with a low energy spread, given a fixed laser energy.
Laser wakefield acceleration of electrons holds great promise for producing ultra-compact stages of GeV scale, high quality electron beams for applications such as x-ray free electron lasers and high energy colliders. Ultra-high intensity laser pulses can be self-guided by relativistic plasma waves over tens of vacuum diffraction lengths, to give>1 GeV energy in cm-scale low density plasma using ionization-induced injection to inject charge into the wake at low densities. This thesis describes a series of experiments which investigates the physics of LWFA in the self-guided blowout regime. Beginning with high density gas jet experiments the scaling of the LWFA-produced electron beam energy with plasma electron density is found to be in excellent agreement with both phenomenological theory and with 3-D PIC simulations. It is also determined that self-trapping of background electrons into the wake exhibits a threshold as a function of the electron density, and at the densities required to produce electron beams with energies exceeding 1 GeV a different mechanism is required to trap charge into low density wakes. By introducing small concentrations of high-Z gas to the nominal He background the ionization-induced injection mechanism is enabled. Electron trapping is observed at densities as low as 1.3 x 1018 cm-3 in a gas cell target, and 1.45 GeV electrons are demonstrated for the first time from LWFA. This is currently the highest electron energy ever produced from LWFA. The ionization-induced trapping mechanism is also shown to generate quasi-continuous electron beam energies, which is undesirable for accelerator applications. By limiting the region over which ionization-induced trapping occurs, the energy spread of the electron beams can be controlled. The development of a novel two-stage gas cell target provides the capability to tailor the gas composition in the longitudinal direction, and confine the trapping process to occur only in a limited, defined region. Using this technique a 460 MeV electron beam was produced with an energy spread of 5%. This technique is directly scalable to multi-GeV electron beam generation with sub-percent energy spreads.
Plasma density gradients in a gas jet were used to control the wake phase velocity and trapping threshold in a laser wakefield accelerator, producing stable electron bunches with longitudinal and transverse momentum spreads more than 10 times lower than in previous experiments (0.17 and 0.02 MeV=c FWHM, respectively) and with central momenta of 0.76 +- 0.02 MeV=c. Transition radiation measurements combined with simulations indicated that the bunches can be used as a wakefieldaccelerator injector to produce stable beams with 0.2 MeV=c-class momentum spread at high energies.
Laser driven accelerators, in which particles are accelerated by the electric field of a plasma wave driven by an intense laser, have demonstrated accelerating electric fields of hundreds of GV/m. These fields are thousands of times those achievable in conventional radiofrequency (RF) accelerators, spurring interest in laser accelerators as compact next generation sources of energetic electrons and radiation. To date however, acceleration distances have been severely limited by lack of a controllable method for extending the propagation distance of the focused laser pulse. The ensuing short acceleration distance results in low energy beams with 100% electron energy spread, limiting applications. Here we demonstrate that a relativistically intense laser can be guided by a preformed plasma density channel and that the longer propagation distance can result in electron beams of percent energy spread with low emittance and increased energy, containing>109 electrons above 80 MeV. The preformed plasma channel technique forms the basis of a new class of accelerators, combining beam quality comparable to RF accelerators with the high gradients of laser accelerators to produce compact tunable high brightness electron and radiation sources.
This book contains the contributions to the Workshop on the Physics and Applications of High Brightness Electron Beams, held in July 2002 in Sardinia, Italy. This workshop had a broad international representation from the fields of intense electron sources, free-electron lasers, advanced accelerators, and ultra-fast laser-plasma, beam-plasma and laser-beam physics. The interdisciplinary participants were brought together to discuss advances in the creation and understanding of ultra-fast, ultra-high brightness electron beams, and the unique experimental opportunities in frontier high-energy-density and radiation-source physics which are offered by these scientific tools.The proceedings have been selected for coverage in:• Index to Scientific & Technical Proceedings® (ISTP® / ISI Proceedings)• Index to Scientific & Technical Proceedings (ISTP CDROM version / ISI Proceedings)
Density perturbations in a hydrogen-filled capillary discharge waveguide have been used to control the injection of electrons into a laser wakefield. This has allowed injection and acceleration in channels of lower density than previously possible, and the production of relativistic electron beams with improved stability. For parameters of optimum stability, the mean bunch energy was 300MeV +- 7 MeV rms, with divergence 1.3 mrad +- 0:1 mrad rms and pointing stability 0.8 mrad rms.
In this dissertation, a new method for producing ultra-bright electron beams in nonlinear plasma wave wakes driven by an electron beam driver is explored using particle-in-cell simulations and analytic theory. In order to understand this process an accurate description of a nonlinear wakefield is required. These nonlinear wakefields are excited by intense particle beams or lasers pushing plasma electrons radially outward, creating an ion bubble surrounded by a sheath of electrons characterized by the source term $S \equiv -\frac{1}{en_p}(\rho-J_z/c)$, where $e$ is the electron charge, $n_p$ is the plasma number density, $\rho$ is the charge density, and $J_z$ is the axial current density. Previously, the sheath source term was described phenomenologically with a positive-definite function thereby resulting in a positive definite wake potential. In reality, the wake potential is negative at the rear of the ion column, which is important for self-injection and accurate beam loading models. To account for this, in the first part of this dissertation a multi-sheath model in which the source term, $S$, of the plasma wake can be negative in regions outside the ion bubble is introduced. Using this model, a new expression for the wake potential and a modified differential equation for the bubble radius is obtained. Numerical results obtained from these equations are validated against particle-in-cell simulations for unloaded and loaded wakes. The new model provides accurate predictions of the shape and duration of trailing bunch current profiles that flatten plasma wakefields. It is also used to design a trailing bunch for a desired longitudinally varying loaded wakefield. The multi-sheath model is also applied to beam loading in laser wakefields. Areas where the multi-sheath model can be improved for laser drivers in future work are discussed. In the second part of this dissertation, a new method of controllable injection to generate high quality electron bunches in the nonlinear blowout regime driven by electron beams is proposed and demonstrated using particle-in-cell simulations. Injection is facilitated by decreasing the wake phase velocity through focusing the drive beam spot size. Two regimes are examined. In the first, the spot size is focused according to the vacuum Courant-Snyder (CS) beta function while, in the second, it is self-focused by the plasma ion column. The effects of the driver intensity and vacuum CS parameters on the wake velocity and injected beam parameters are examined via theory and simulations. For plasma densities of $\sim 10^{19} ~\centi\meter^{-3}$, particle-in-cell (PIC) simulations demonstrate that peak normalized brightnesses $\gtrsim 10^{20}~\ampere/\meter^2/\rad^2$ can be obtained with projected energy spreads of $\lesssim 1\%$ within the middle section of the injected beam and with normalized slice emittances as low as $\sim 10 ~\nano\meter$. In the last part of the dissertation, a predictive model for injection using the self-evolving driver method in the plasma focusing regime is developed. The model is used to characterize how the wake evolution and final injected beam parameters scale with the driver parameters. Parameter scans of PIC simulations using different drivers are performed and compared with the model predictions. In particular, the dependence of the injected beam parameters with the diffraction length, energy, intensity, spot size, and duration of the driver is examined. It is found that injection and optimal beam loading can be simultaneously achieved. The multi-sheath model is also used to study the beam loading effects from the injected bunch in this case. PIC simulation results indicate that the injected beam can be efficiently accelerated to $18.27$ GeV with a projected energy spread of $ 0.49\%$ and peak normalized brightess of $B_n \sim 10^{20}~\ampere/\meter^2/\rad^2$ for a plasma density of $\sim 10^{19} ~\centi\meter^{-3}$.