Session UI2: Advances in Laser and Plasma Based Accelerators

GeV Laser Ion Acceleration from Ultrathin Targets: The Laser Break-Out Afterburner

A new laser-driven ion acceleration mechanism has been identified using particle-in-cell simulations. After a brief period of target normal sheath acceleration (TNSA) [S. P. Hatchett, et al., Phys. Plasmas, 7, 2076 (2000)], two distinct stages follow: first, a period of enhanced TNSA during which the cold electron background converts entirely to hot electrons, and second, the ``laser break-out afterburner'' (BOA) when the laser penetrates to the rear of the target and generates a large longitudinal electric field localized at the rear of the target with the location of the peak field co-moving with the ions. This mechanism allows ion acceleration to GeV energies at vastly reduced laser intensities compared with earlier acceleration schemes. The new mechanism enables the acceleration of carbon ions to greater than 2 GeV energy at a laser intensity of only $10^{21}$~W/cm$^2$, an intensity that has been realized in existing laser systems. Other techniques for achieving these energies in the literature [D. Habs et al., Progress in Particle and Nuclear Physics, 46, 375 (2001); T. Esirkepov et al., Phys. Rev. Lett. 92, 175003-1 (2004)] rely upon intensities of $10^{24}$~W/cm$^2$ or above, i.e., 2-3 orders of magnitude higher than any laser intensity that has been demonstrated to date. Also, the BOA mechanism attains higher energy and efficiency than TNSA where the scaling laws [Hegelich et al., Phys. Plasmas, 12, 056314 (2005)] predict carbon energies of 50 MeV/u for identical laser conditions. In the early stages of the BOA, the carbon ions accelerate as a quasi-monoenergetic bunch with median energy higher than that realized recently experimentally [Hegelich et al., Nature, 441, 439 (2006)].   

GeV electron beams from cm-scale laser driven plasma based accelerators.

GeV electron accelerators are essential to synchrotron radiation facilities and free electron lasers, and as modules for high-energy particle physics. Radiofrequency-based accelerators are limited to relatively low accelerating fields (10-50 MV/m) requiring tens to hundreds of metres to reach the multi-GeV beam energies needed to drive radiation sources, and many kilometres to generate particle energies of interest to high-energy physics. Laser-wakefield accelerators (LWFA) produce electric fields of order 10-100 GV/m enabling compact devices. Previously, the required laser intensity was not maintained over the distance needed to reach GeV energies, and hence acceleration was limited to the 100 MeV scale [1-3]. In this talk, results will be presented on the first demonstration of the generation of GeV-class beams using an intense laser beam. Laser pulses with peak power ranging from 10-50 TW were guided by a hydrogen filled capillary discharge waveguide [4]. Production of high-quality electron beams with 1 GeV energy by channelling a $\sim $40 TW peak power laser pulse in a 3.3 cm long gas-filled capillary discharge waveguide was observed [5]. Results will be discussed on the dependence of the electron beam characteristics on capillary properties, plasma density and laser parameters. \newline \newline [1] S.P.D. Mangles et al., \textit{Nature} \textbf{431}, 535-538 (2004). \newline [2] C.G.R. Geddes et al., \textit{Nature} \textbf{431}, 538-541 (2004). \newline [3] J. Faure et al., \textit{Nature} \textbf{431}, 541-544 (2004). \newline [4] D.J. Spence and S.M. Hooker, \textit{Phys. Rev. E} \textbf{63}, 015401 (2001).\newline [5] W.P. Leemans et al., submitted for publication.   

Laser-driven wavebreaking, electron trapping, and mono-energetic beam production

Recent breakthrough results reported in Nature\footnote{C.G.R. Geddes et al., Nature {\bf 431}, 538 (2004); S.P.D. Mangles et al., ibid., p. 535; J. Faure et al., ibid., p. 541.} demonstrate that laser-plasma accelerators can produce high quality (e.g., narrow energy spread) electron bunches at the 100 MeV level that may be useful for numerous applications. More recently, high quality electron beams at 1 GeV were produced in experiments at LBNL using 40 TW laser pulse interacting with a 3.3 cm plasma channel\footnote{W.P. Leemans et al., submitted.}. In these experiments, the accelerated electrons were self-trapped from the background plasma, often attributed to the process of wavebreaking. Using a warm fluid model, a general analytic theory of wavebreaking has been developed that is valid for all regimes of interest, i.e., arbitrary temperature and phase velocity\footnote{C.B. Schroeder et al., Phys. Rev. E {\ bf 72}, 055401 (2005).}. This theory indicates that the maximum electric field obtainable by a relativistic plasma wave is lower that previously calculated. The relation between wavebreaking and particle trapping is discussed, and various quantities, such as the fraction of electrons trapped (i.e., the dark current), are calculated\footnote{C.B. Schroeder et al., Phys. Plasmas {\bf 13}, 033103 (2006).}. A variety of methods for particle trapping relevant to present experiments, including 2D wavebreaking, density ramps, and laser injection, will be described\footnote{G. Fubiani et al., Phys. Rev. E {\bf 73}, 026402 (2006).}. Limitations from dephasing and pump depletion will be summarized. Also presented will be 2D and 3D simulations modeling the production high quality electron beams from laser-plasma accelerators.   

Mono-energetic electrons from laser wakefield experiments: stability and future scaling

A great deal of interest in laser wakefield accelerators has been generated since the discovery that they can produce high quality (low emittance and low energy spread) ultra-short (less than 25 fs) relativistic electron beams. This talk will cover the ongoing research led by Imperial College at the Rutherford Appleton Laboratory and Lund Laser Centre. By controlling the laser parameters including contrast ratio, pulse duration and focusing geometry we can significantly improve the quality and stability of the electron beam produced in self-injected laser wakefield experiments. We will also discuss the scaling of laser wakefield accelerators to PW class lasers such as the Astra Gemini system at the Rutherford Appleton Laboratory.   

Developing high energy, stable laser wakefield accelerators: particle simulations and experiments

Laser driven wakefield accelerators produce accelerating fields thousands of times those achievable in conventional radiofrequency accelerators, and recent experiments have produced high energy electron bunches with low emittance and energy spread. Challenges now include control and reproducibility of the electron beam, further improvements in energy spread, and scaling to higher energies. We present large-scale particle in cell simulations together with recent experiments towards these goals. In LBNL experiments the relativistically intense drive pulse was guided over more than 10 diffraction ranges by plasma channels. Guiding beyond the diffraction range improved efficiency by allowing use of a smaller laser spot size (and hence higher intensities) over long propagation distances. At a drive pulse power of 9 TW, electrons were trapped from the plasma and beams of percent energy spread containing $>$ 200pC charge above 80 MeV with normalized emittance estimated at $<$ 2 $\pi$-mm-mrad were produced. Energies have now been scaled to 1 GeV using 40 TW of laser power. Particle simulations and data showed that the high quality bunch in recent experiments was formed when beam loading turned off injection after initial self trapping, creating a bunch of electrons isolated in phase space. A narrow energy spread beam was then obtained by extracting the bunch as it outran the accelerating phase of the wake. Large scale simulations coupled with experiments are now under way to better understand the optimization of such accelerators including production of reproducible electron beams and scaling to energies beyond a GeV. Numerical resolution and two and three dimensional effects are discussed as well as diagnostics for application of the simulations to experiments. Effects including injection and beam dynamics as well as pump laser depletion and reshaping will be described, with application to design of future experiments. Supported by DOE grant DE-AC02-05CH11231 and by an INCITE computational award.   

Application of the Finite-Element MICHELLE to RF Photoemission Modeling

RF photocathodes are difficult to model but continue to be at the forefront of solutions to many applications, especially as high power FEL sources. Modeling the photoemission process requires a high degree of computational mesh resolution to resolve geometrical and surface finish features, or simply fine spatial scale phenomena. The new Finite-Element (FE) MICHELLE [1] two-dimensional (2D) and three-dimensional (3D) steady-state and time-domain particle-in-cell (PIC) code has been employed successfully by industry, national laboratories, and academia and has been used to design and analyze a wide variety of beam sources and devices. In particular, the MICHELLE code has the ability to resolve small spatial scales, and is a good choice for photoemission modeling. To investigate the application of the Electrostatic time-domain model to emission properties of photocathodes, two code models are needed; an EM frequency-domain code and a PIC code. We use the STAR ANALYST [2] code for the Frequency Domain solutions and the NRL/SAIC MICHELLE code for the PIC solutions. The RF fields from ANALYST are imported into the MICHELLE code and clocked in time. MICHELLE adds the self fields and emits the beam according to an emission rule. For the photoemission process, we employ the NRL photoemission model [3], and can capture detailed spatial and temporal effects of the emission surface finish and beam development. In the talk, we will consider an example that investigates the effects of fine scale surface imperfections on the photoemission process. \newline \newline [1] John Petillo, et al., ``The MICHELLE Three-Dimensional Electron and Collector Modeling Tool: Theory and Design,'' IEEE Trans. Plasma Sci., vol. 30, no. 3, June 2002, pp. 1238-1264. \newline [2] Analyst is a commercial finite-element package for electromagnetic design. www.staarinc.com. \newline [3]K. Jensen, et al., ``The Quantum Efficiency of Dispenser Photocathode: Comparison of Theory to Experiment'' Applied Physics Lett. 85, 22, 5448, 2004.