While the rare half of the plasma bubble is accelerating for electrons, the front half of it is decelerating. For positive ions it is just the opposite. Thus, ions trapped in the front of the bubble will observe an accelerating force. Trapping ions with smaller weight than that of the gas-atoms generating the plasma is easier, as heavier ions will secure a stable, less- mobile background, while lighter ions can move without risking the stability of the positive background. The pictures illustrate the acceleration of 1 H ions in a plasma formed from a 1:1.4 atom-ratio mixture of 1 H and 3 H by a very intense laser (of PW power): (a) electron density at t=320fs, (b), (c) and (d) are electron, proton and tritium densities at t=854fs. Energies of the accelerated protons reach as high as 10-15GeV. One practical problem related to One practical problem related to laser plasma accelerators of the bubble regime is the control of the moment of electron injection. The only practical, very recently (2006) demonstrated way [8] for this is the laser plasma accelerators of the bubble regime is the control of the moment of electron injection. The only practical, very recently (2006) demonstrated way [8] for this is the colliding pulse injection. Here we suggest another way of controlled injection: A nanowire is put in the way of the laser-pulse, and when the pulse hits the wire, electrons will be injected. The pictures illustrate steps of such a process in terms of electron- density contour plots: (a) t=0.67ps, just before the pulse hits the wire, (b) t=0.8ps right after the bubble passes over the wire, (c) t=4.67ps, when the injected electrons are still in the bubble, (d) t=5.2ps, after the bubble bursts and the accelerated electrons are released. colliding pulse injection. Here we suggest another way of controlled injection: A nanowire is put in the way of the laser-pulse, and when the pulse hits the wire, electrons will be injected. The pictures illustrate steps of such a process in terms of electron- density contour plots: (a) t=0.67ps, just before the pulse hits the wire, (b) t=0.8ps right after the bubble passes over the wire, (c) t=4.67ps, when the injected electrons are still in the bubble, (d) t=5.2ps, after the bubble bursts and the accelerated electrons are released. Large-scale laser-plasma wakefield computation Y. Li 1,2, K. Németh 1,2, B. Shen 1,2,3, J. Bailey 1,7, H. Shang 1,2, R. Soliday 1,2, R. Crowell 4, D. Gostola4, S. Chemerisov, W. Gai 2, 10 X.Y. Song 2, 9, K. C. Harkay 1,2, J. R. Cary 5,6, E. Frank 8, W. Gropp 8 1 Accelerator Systems Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA 2 Argonne Accelerator Institute, Argonne National Laboratory, Argonne, IL 60439, USA 3 State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, P.O. Box , Shanghai, China 4 Chemistry Division, Argonne National Laboratory, Argonne, IL 60439, USA 5 Center for Integrated Plasma Studies and Department of Physics, University of Colorado, Boulder, Colorado 80309, USA 6 Tech-X Corporation, Boulder, Colorado 80303, USA 7 University of Alabama, Huntsville, Alabama, 35899, USA 8 Mathematics and Computer Science Division, Argonne National Laboratory, Argonne, IL 60439, USA 9 Physics Department, Northern Illinois University, DeKalb, IL High Energy Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA A U.S. Department of Energy laboratory managed by The University of Chicago The Advanced Photon Source is funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences Advanced Photon Source 9700 S. Cass Ave. Argonne, IL USA Introduction: Laser-plasma accelerators [1] utilize the large Introduction: Laser-plasma accelerators [1] utilize the large electric fields generated in plasma by intense lasers to accelerate electrons. Two practical implementations involve the ‘bubble regime’ [2] and ‘plasma channel’ [3] ones. High energy, small energy spread and emittance (1 GeV energy with ~5% spread and 2mrad emittance) have been demonstrated [3]. Reproducibility of the demonstrated properties remains an issue. One of the areas we contributed to is the understanding of the origins of emittance in laser-plasma accelerators and providing controls to it [4]. Other areas we studied are: control of electron injection in laser-plasma accelerators by a nanowire [5], acceleration of ions in laser-plasma [6], dependence of energy spread on the injection circumstances [7]. electric fields generated in plasma by intense lasers to accelerate electrons. Two practical implementations involve the ‘bubble regime’ [2] and ‘plasma channel’ [3] ones. High energy, small energy spread and emittance (1 GeV energy with ~5% spread and 2mrad emittance) have been demonstrated [3]. Reproducibility of the demonstrated properties remains an issue. One of the areas we contributed to is the understanding of the origins of emittance in laser-plasma accelerators and providing controls to it [4]. Other areas we studied are: control of electron injection in laser-plasma accelerators by a nanowire [5], acceleration of ions in laser-plasma [6], dependence of energy spread on the injection circumstances [7]. ANL’s laser plasma accelerators This is the type [2] of laser-plasma accelerators we focused our studies on. In this type of accelerator, a short (~30fs) and intense (~14TW) laser pulse hits a gas jet (of H 2 or He), instantaneously ionizes it and blows out the electrons from a spherical region (the ‘bubble’) with a diameter of ~ the plasma wavelength. The positive ions remain in the bubble due to their weight creating a very strong electric field (~50GeV/m). The bubble follows the motion of the laser pulse with a speed that equals with the group velocity of the laser pulse. Electrons flow around the bubble and collide at its base. Electrons may get injected into the bubble from its base, e.g. due to a counter-propagating weaker laser pulse [8]. The injected electrons will be accelerated due to the field of the ions in the bubble. Finally (typically in ~10ps) the laser pulse and the bubble hits through the gas jet, the bubble bursts and the accelerated electrons are released. TUHFF Laser Output 30fs, 0.6 J (20 10 Hz ANL CHM’s ANL CHM’s Terawatt Ultrafast High Field Facility. Magnet Scintillation Screen PMT Jet PMT Magnet Al foil Scint. screen Parabola mirror Laser beam Electron beam ANL/CHM’s radiolosyse experiment: Transverse e- beam profile measured at 27 mm after the jet average of 100 shots, single shot much better 6o6o 20 o Electron beam spectrum F(E) ~ exp(-E/ ) =2.3±0.3 MeV Electron pulse charge: 2-3 nC ± 15-30% Electron pulse duration: ~1-2 ps at the sample? Will be measured by EOS Dispersion is bad! 0.5 ps/cm ANL simulation: Electron injection by nanowire in the bubble regime B. Shen, Y. Li, K. Nemeth, H. Shang, Y.-C. Chae, R. Soliday, R. Crowell, E. Frank, W. Gropp, and J. Cary, Phys. Plasmas 14, (2007). ANL simulation: Electron injection by nanowire in the bubble regime B. Shen, Y. Li, K. Nemeth, H. Shang, Y.-C. Chae, R. Soliday, R. Crowell, E. Frank, W. Gropp, and J. Cary, Phys. Plasmas 14, (2007). ANL simulation: Bubble regime for ion acceleration B. Shen, Y. Li, and M. Y. Yu, and J. Cary, “Bubble regime for ion acceleration in a laser-driven plasma,” Phys. Rev. E 76, (R) (2007). ANL simulation: Energy spread vs. duration of injection pulse J. Bailey, Y. Li, K. Nemeth, and J. Cary, J. Undergraduate Research, submitted. In the colliding pulse injection, the properties of the accelerated electrons depend on the properties of the injection pulse as well. Interestingly, a longer injection pulse results in a much smaller energy spread than the shorter ones. The picture shows energy ditribution of accelerated electrons as a function of location within the bubble, at t=8ps (bubble close to burst), for injection pulses of 10, 30 and 50 fs duration. It can be seen that the energy spread greatly improves with the increase of the duration of the injection pulse. The explanation of this phenomenon is still subject of investigation. ANL simulation: Laser-driven betatron oscillation in a laser-wakefield cavity K. Nemeth, B. Shen, Y. Li, R. Crowell, K. C. Harkay, J. R. Cary, Phys. Rev. Lett., submitted. A plasma ‘bubble’, cut half slightly above the y=0 plane as of PIC simulation [9]. Light pulse and bubble propagate along x, light is polarized in z. Isodensity surfaces around the injected and accelerated electrons (red) and around the shell of the bubble (blue) are shown. ζ is the distance from the base of the bubble, along x. The injected electrons move in phase with the local electro- magnetic wave. We interpret this as a result of a driven oscillator. The fundamental oscillator is the betatron oscillation transverse to the optical axis. This is modulated by a periodic force due to the laser. Just as in classical mechanics, the resulting oscillation picks up the periodicity of the laser. As the laser’s phase is kξ, with ξ=x-v p t, transforming the x-t pairs of the PIC trajectories with the observed v p phase velocity, Coherence of the trajectories is observed. This is in accordance with driven oscillators again, as it looses memory of initial conditions. Numerical solution of a simple driven oscillator model reproduces the observed beam-shape and trajectories. Shortening the duration of the laser pulse reduces the emittance in the polarization plane significantly. Thus we explain the experimentally observed asymmetric beam profile [10], large betatron amplitude [11], and microbunching of beam [12]. We also provide a way to cure the large emittance. References: [1] T. Tajima and J. M. Dowson, PRL 43, 267 (1979) [2] A. Pukhov et.al., Appl. Phys. B 74, 355 (2002) [3] W. P. Leemans et.al. Nature Physics 2, 696 (2006) [4] K. Németh, B. Shen, Y. Li, H. Shang, K. C. Harkay, J. R. Cary, submitted to PRL J. R. Cary, submitted to PRL [5] B. Shen et.al., Phys. Plasm. 14, (2007) [6] B. Shen et.al., PRE 76, (R) (2007) [7] J. Bailey, summer internship works [8] J. Faure, et.al., Nature 444, 737 (2006). [9] VORPAL plasma simulation code, Tech-X Corp. [10] S. P. D. Mangles et.al., PRL 96, (2006). [11] K. T. Phuoc, et.al., PRL 97, (2006). [12] Y. Glinec et.al., PRL 98, (2007). [12] Y. Glinec et.al., PRL 98, (2007).