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A self-modulated laser wake-field accelerator configuration in which the laser pulse is optically guided by a plasma density channel is considered. Preliminary experiments on the generation of a plasma channel by a slow capillary discharge are described. It is shown that homogeneous channels with length L sub channel” lambda p can be produced, where lambda sub p is the plasma wavelength. Key issues are addressed, including phase detuning between the accelerated electron bunch and the wake field, beam-plasma and laser-plasma instabilities, and the effect of density variations that might occur over the length of the plasma channel. Numerical simulations, using present experimental parameters, show accelerating gradients in excess of 50 GV/m.
Particle accelerators enable scientists to study the fundamental structure of the universe, but have become the largest and most expensive of scientific instruments. In this project, we advanced the science and technology of laser-plasma accelerators, which are thousands of times smaller and less expensive than their conventional counterparts. In a laser-plasma accelerator, a powerful laser pulse exerts light pressure on an ionized gas, or plasma, thereby driving an electron density wave, which resembles the wake behind a boat. Electrostatic fields within this plasma wake reach tens of billions of volts per meter, fields far stronger than ordinary non-plasma matter (such as the matter that a conventional accelerator is made of) can withstand. Under the right conditions, stray electrons from the surrounding plasma become trapped within these "wake-fields", surf them, and acquire energy much faster than is possible in a conventional accelerator. Laser-plasma accelerators thus might herald a new generation of compact, low-cost accelerators for future particle physics, x-ray and medical research. In this project, we made two major advances in the science of laser-plasma accelerators. The first of these was to accelerate electrons beyond 1 gigaelectronvolt (1 GeV) for the first time. In experimental results reported in Nature Communications in 2013, about 1 billion electrons were captured from a tenuous plasma (about 1/100 of atmosphere density) and accelerated to 2 GeV within about one inch, while maintaining less than 5% energy spread, and spreading out less than 1/2 milliradian (i.e. 1/2 millimeter per meter of travel). Low energy spread and high beam collimation are important for applications of accelerators as coherent x-ray sources or particle colliders. This advance was made possible by exploiting unique properties of the Texas Petawatt Laser, a powerful laser at the University of Texas at Austin that produces pulses of 150 femtoseconds (1 femtosecond is 10-15 seconds) in duration and 150 Joules in energy (equivalent to the muzzle energy of a small pistol bullet). This duration was well matched to the natural electron density oscillation period of plasma of 1/100 atmospheric density, enabling efficient excitation of a plasma wake, while this energy was sufficient to drive a high-amplitude wake of the right shape to produce an energetic, collimated electron beam. Continuing research is aimed at increasing electron energy even further, increasing the number of electrons captured and accelerated, and developing applications of the compact, multi-GeV accelerator as a coherent, hard x-ray source for materials science, biomedical imaging and homeland security applications. The second major advance under this project was to develop new methods of visualizing the laser-driven plasma wake structures that underlie laser-plasma accelerators. Visualizing these structures is essential to understanding, optimizing and scaling laser-plasma accelerators. Yet prior to work under this project, computer simulations based on estimated initial conditions were the sole source of detailed knowledge of the complex, evolving internal structure of laser-driven plasma wakes. In this project we developed and demonstrated a suite of optical visualization methods based on well-known methods such as holography, streak cameras, and coherence tomography, but adapted to the ultrafast, light-speed, microscopic world of laser-driven plasma wakes. Our methods output images of laser-driven plasma structures in a single laser shot. We first reported snapshots of low-amplitude laser wakes in Nature Physics in 2006. We subsequently reported images of high-amplitude laser-driven plasma "bubbles", which are important for producing electron beams with low energy spread, in Physical Review Letters in 2010. More recently, we have figured out how to image laser-driven structures that change shape while propagating in a single laser shot. The latter techniques, which use t ...
Experiments conducted using a 200TW 60 fs laser have demonstrated up to 720 MeV electrons in the self-guided laser wakefield regime using pure Helium gas jet targets. Charge and energy of the accelerated electrons was measured using an electron spectrometer with a 0.5T magnet and charge callibrated image plates. The self-trapped charge in a helium plasma was shown to fall off with decreasing electron density with a threshold at 2.5 x 1018 (cm−3) below which no charge is trapped. Self-guiding however is shown to continue below this density limitation over distances of 14 mm with an exit spot size of 25[mu]m. Simulations show that injection of electrons at these densities can be assisted through ionization induced trapping in a mix of Helium with 3% Oxygen.
A new configuration of the laser wakefield accelerator is proposed in which enhanced acceleration is achieved via resonant self-modulation of the laser pulse. This requires laser power in excess of the critical power for relativistic guiding and a plasma wavelength short compared to the laser pulse- length. Relativistic and density wake effects strongly modulate the laser pulse at the plasma wavelength, resonantly exciting the plasma wave and leading to enhanced acceleration ... Particle acceleration, Laser, Nonlinear plasma waves.
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.
Plasma accelerators may be driven by the ponderomotive force of an intense laser or the space-charge force of a charged particle beam. The implications for accelerator design and the different physical mechanisms of laser-driven and beam-driven plasma acceleration are discussed. Driver propagation is examined, as well as the effects of the excited plasma wave phase velocity. The driver coupling to subsequent plasma accelerator stages for high-energy physics applications is addressed.