Tunable Laser Plasma Acceleration With Ionization Injection

Laser plasma accelerators have produced high-quality electron beams with GeV energies from cm-scale devices and are being investigated as hyperspectral fs light sources producing THz to [gamma]-ray radiation and as drivers for future high-energy colliders. These applications require a high degree of stability, beam quality and tunability. Here we report on a technique to inject electrons into the accelerating field of a laser-driven plasma wave and coupling of this injector to a lower-density, separately tunable plasma for further acceleration. The technique relies on a single laser pulse powering a plasma structure with a tailored longitudinal density profile, to produce beams that can be tuned in the range of 100-400 MeV with percent-level stability, using laser pulses of less than 40 TW. The resulting device is a simple stand-alone accelerator or the front end for a multistage higher-energy accelerator.

This thesis establishes an exciting new beginning for Laser Plasma Accelerators (LPAs) to further develop toward the next generation of compact high energy accelerators. Design, installation and commissioning of a new experimental setup at LBNL played an important role and are detailed through three critical components: e-beam production, reflection of laser pulses with a plasma mirror and large wake excitation below electron injection threshold. Pulses from a 40 TW peak power laser system were split into a 25 TW pulse and a 15 TW pulse. The first pulse was used for e-beam production in the first module and the second pulse was used for wake excitation in the second module to post-accelerate the e-beam. As a result, reliable e-beam production and efficient wake excitation necessary for the staged acceleration were independently demonstrated. These experiments have laid the foundation for future staging experiments at the 40 TW peak power level.

Plasma-based acceleration, either driven by laser (LWFA) or driven by electron beam (PWFA) has the potential of accelerating electrons to GeV in a few cen- timeters. This allows construction of table-top accelerator which can be applied to build light source such as free electron laser (FEL) or high energy particle collider. The driver bunch loses energy to plasma when driving a wake. The following wit- ness bunch injected at correct phase will be accelerated. Here we report a novel injection scheme, laser-ionization injection where the witness bunch is formed by laser-ionizing higher-threshold gas such as He. Simulation and numerical calcula- tion is presented to evaluate the beam quality, the beam emittance is estimated to be 10 8 mrad. Experimental key issues such as timing synchronization of laser pulse and electron bunch and eliminate "dark current" are taken care of before the plasma acceleration experiment is carried out. Two beams are synchronized to 100-fs level via plasma radiation observation and Electro-Optic Sampling (EOS). "Dark current" is reduced to trivial level by tuning plasma density and driver bunch configuration. We observed 1 GeV gain of witness bunch with 5% energy spread.

Decoupling injection from acceleration is a key challenge to achieve compact, reliable, tunable laser-plasma accelerators (LPA). In colliding pulse injection the beat between multiple laser pulses can be used to control energy, energy spread, and emittance of the electron beam by injecting electrons in momentum and phase into the accelerating phase of the wake trailing the driver laser pulse. At LBNL, using automated control of spatiotemporal overlap of laser pulses, two-pulse experiments showed stable operation and reproducibility over hours of operation. Arrival time of the colliding beam was scanned, and the measured timing window and density of optimal operation agree with simulations. The accelerator length was mapped by scanning the collision point.

This thesis covers the few-cycle laser-driven acceleration of electrons in a laser-generated plasma. This process, known as laser wakefield acceleration (LWFA), relies on strongly driven plasma waves for the generation of accelerating gradients in the vicinity of several 100 GV/m, a value four orders of magnitude larger than that attainable by conventional accelerators. This thesis demonstrates that laser pulses with an ultrashort duration of 8 fs and a peak power of 6 TW allow the production of electron energies up to 50 MeV via LWFA. The special properties of laser accelerated electron pulses, namely the ultrashort pulse duration, the high brilliance, and the high charge density, open up new possibilities in many applications of these electron beams.

Recent scientific and technical advances have made it possible to create matter in the laboratory under conditions relevant to astrophysical systems such as supernovae and black holes. These advances will also benefit inertial confinement fusion research and the nation's nuclear weapon's program. The report describes the major research facilities on which such high energy density conditions can be achieved and lists a number of key scientific questions about high energy density physics that can be addressed by this research. Several recommendations are presented that would facilitate the development of a comprehensive strategy for realizing these research opportunities.

Plasma-based wakefield accelerator is attractive for generating quasi-monoenergetic electron beams using the bubble regime. The bubble is formed by an intense driver, which propagates through the plasma and expels all electrons transversely, creating a cavity free of cold plasma electrons that trailing behind the driver. Self-injection is applicable in the bubble regime, which can produce bunches of quasi-monoenergetic electrons. (1) Such electron bunching structure can be diagnosed with coherent transition radiation and may be exploited to generate powerful high frequency radiation [16].This thesis focuses on electron bunching phenomenon through WAKE simulations and theoretical analysis. The simulation is completed under laser-driven field ionization wakefield acceleration. The code is improved by taking into consideration the high frequency property of laser driver in wakefield acceleration. Finer grid size is introduced to the ionization injection part of WAKE, for increasing simulation accuracy without much sacrifice of programming efficiency. Various conditions for optimal bunching in the trapped electrons are explored computationally and analytically.

This book explores several key issues in beam phase space dynamics in plasma-based wakefield accelerators. It reveals the phase space dynamics of ionization-based injection methods by identifying two key phase mixing processes. Subsequently, the book proposes a two-color laser ionization injection scheme for generating high-quality beams, and assesses it using particle-in-cell (PIC) simulations. To eliminate emittance growth when the beam propagates between plasma accelerators and traditional accelerator components, a method using longitudinally tailored plasma structures as phase space matching components is proposed. Based on the aspects above, a preliminary design study on X-ray free-electron lasers driven by plasma accelerators is presented. Lastly, an important type of numerical noise—the numerical Cherenkov instabilities in particle-in-cell codes—is systematically studied.

Ultra-low emittance (tens of nm) beams can be generated in a plasma accelerator using ionization injection of electrons into a wakefield. An all-optical method of beam generation uses two laser pulses of different colors. A long-wavelength drive laser pulse (with a large ponderomotive force and small peak electric field) is used to excite a large wakefield without fully ionizing a gas, and a short-wavelength injection laser pulse (with a small ponderomotive force and large peak electric field), co-propagating and delayed with respect to the pump laser, to ionize a fraction of the remaining bound electrons at a trapped wake phase, generating an electron beam that is accelerated in the wake. The trapping condition, the ionized electron distribution, and the trapped bunch dynamics are discussed. Expressions for the beam transverse emittance, parallel and orthogonal to the ionization laser polarization, are presented. An example is shown using a 10-micron CO2 laser to drive the wake and a frequency-doubled Ti:Al2O3 laser for ionization injection.