Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session FI2: Plasma-Based Acceleration and Light Sources |
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Chair: Chan Joshi, University of California, Los Angeles Room: Adam's Mark Hotel Plaza Ballroom EF |
Tuesday, October 25, 2005 9:30AM - 10:00AM |
FI2.00001: Plasma Wakefield Acceleration at SLAC: Results of the E-167 Experiment Invited Speaker: Plasma wakefield accelerators are emerging as one of the most promising schemes for future accelerators. Beam-driven plasma wakefield acceleration experiments are carried out in the FFTB beamline at SLAC. The sub-picosecond ultra-relativistic electron bunches field- ionize the lithium vapor. With a plasma density matched to the bunch length, an acceleration of more than one GeV in 10cm of plasma has been observed. A plasma wake can trap electrons and ions out of the plasma, accelerating them to relativistic energies. In previous experiments, signals that could be explained by trapped particles have been noticed. Therefore, we have set up time-resolved devices after the plasma. The bunch length of the trapped particles could be extremely short, and the present setup includes diagnostics to detect coherent emission of radiation at optical wavelengths. Furthermore, the energy gain appears to oscillate as a function of the length of the plasma channel. A lithium oven with a length that can be varied up to 30cm has been set up. After such a distance, numerical models predict that an incoming tilt in the electron beam could be amplified and a transverse instability could develop, leading to the break-up of the beam. In addition, we investigate the possibility to use the high flux of betatron/synchrotron X-Rays emitted in the plasma to create electron-positron pairs. Finally, we are making first steps towards a scheme where one bunch drives the plasma wake and a following bunch samples the accelerating field. To achieve this, a bunch from the SLC accelerator is split horizontally by a notch collimator. Due to an inherent position-time dependence of the particles, a twin bunch is created. In the long run, this concept presents a way to achieve mono-energetic particle bunches. [Preview Abstract] |
Tuesday, October 25, 2005 10:00AM - 10:30AM |
FI2.00002: Generation of Monoenergetic Electrons via LWFA in the Blowout Regime F.S. Tsung Recently, we reported the observation of low emittance, nearly monoenergetic electrons of approximately 240 MeV energy[1] in PIC simulations in which a 13TW, 50fs laser propagated through nearly 1CM of 3x10$^{18}$ cm$^{-3}$ preformed plasma channel. The simulations showed that self-injection occurs after the laser intensity increases due to a combination of photon deceleration, group velocity dispersion, and self-focusing. The monoenergetic beam is produced because the injection process is clamped by beam loading and the rotation in phase space that results as the beam dephases. Nearly simultaneously[2-4], three experimental groups from around the world reported the generation of near nanocoulomb of low emittance, high quality electron beams using similar laser parameters reported in our simulations. Although these simulations and experiments use a wide range of plasma parameters, laser powers and spot sizes, the mechanism by which the monoenergetic electrons are generated is universal. Using 3D PIC simulations with the code OSIRIS, we will describe how injection and acceleration occurs in these recent experiments and discuss how the energy and beam quality might be improved in the future. We will also show the difference between 2D and 3D simulations. \newline \newline [1] F. S. Tsung et al, \textit{Phys. Rev. Lett.,} \textbf{93, }185002 (2004). \newline [2] Mangles et al, \textit{Nature}, \textbf{431, }535 (2004). \newline [2] Geddes et al, \textit{Nature}, \textbf{431, }538 (2004). [2] Faure et al, \textit{Nature}, \textbf{431, }541 (2004). [Preview Abstract] |
Tuesday, October 25, 2005 10:30AM - 11:00AM |
FI2.00003: Differences and similarities between the wake excitation in the blowout regime for electron and laser drivers Invited Speaker: There has been much recent progress on accelerating electrons in plasma waves generated by short pulse lasers (LWFA) or electron/positron beam drivers (PWFA). For the laser case, simulations [1] and experiments [2,3,4] showed that 100$\sim $200 MeV mono-energetic electron beams can be produced when 10$\sim $30 TW lasers were sent through mm's of plasma. For the beam case, the E164x experiment at SLAC [5] showed more than 3Gev energy gain after a 28.5 Gev electron beam propagating through a 10 cm long plasma. In either case, the interaction between the driver and plasma is in the blowout regime where the driver pushes the plasma electrons outward forming an ion channel. We will present a nonlinear theory for how intense lasers or beam drivers create multi-dimensional plasma wakefields in this regime and also show the similarities and differences between wakes generated by two kinds of drivers. We will also show how this theory can be used to describe beam loading and how to optimize the transformer ratio for a beam driver. We will also discuss how to use this theory to design near term and futuristic PWFA and LWFA stages that approach the Tev energy range. Work supported by DOE grant nos. DE-FG03-92ER40727, DE-FC02-01ER41179, DE-FG02-ER54721and NSF Grant PHY-0321345. Simulations are done on at NERSC and Dawson cluster at UCLA. \newline \newline [1] F.S.Tsung et al \textit{PRL.}, \textbf{93}, 185002 (2004) \newline [2] Mangles et al, \textit{Nature}, \textbf{431}, 535 (2004) \newline [3] Geddes et al., \textit{Nature}, \textbf{431}, 538 (2004) \newline [4] Fauve et al., \textit{Nature}, \textbf{431}, 541 (2004) \newline [5] M. Hogan et al., \textit{PRL., to be published.} [Preview Abstract] |
Tuesday, October 25, 2005 11:00AM - 11:30AM |
FI2.00004: Characterization of femtosecond electron bunches from a laser-wakefield accelerator using THz radiation Invited Speaker: We report on the temporal characterization of laser-plasma-produced electron bunches, indicating ultra-short sub-50 fs charge structure. In the LOASIS laboratory at LBNL, the electron bunches are produced through the interaction of an intense ($>10^{19}$ Wcm$^{-2}$) laser pulse with an underdense ($\simeq 10^{19}$ cm$^{-3}$) Helium plasma. The femtosecond multi-nanoCoulomb bunches have relativistic energies, with a 100\% energy spread. As the bunch exits the plasma-vacuum interface, coherent transition radiation is emitted. Since the electron bunch is still dense and compact at the emission interface, the coherent spectrum of the intense radiation pulse covers the THz regime. Spectral and temporal measurements on the THz pulse are performed and correlated to the temporal properties of the electron bunch. Detection techniques such as Michelson interferometry, semiconductor switching, and electro-optic sampling are applied. The latter technique, where the THz electric field versus time is mapped out, provides detailed temporal structure of the radiation pulse, and by inference the electron bunch. The measurements indicate that THz radiation is emitted by a skewed bunch with a sub-50 fs rise time and a $\simeq$600 fs tail (half-width-at-half-maximum), which is consistent with ballistic debunching of 100\%-energy-spread beams during propagation. The electro-optic time resolution of the method was limited by the crystal properties. The Michelson interferometry and semiconductor switching experiments confirmed the femtosecond nature of the electron bunches. The electro-optic measurement also demonstrates shot-to-shot stability of the laser-wakefield accelerator (LWFA) as well as femtosecond synchronization between the electron bunch and the probe beam. This highlights the applicability of the LWFA in pump-probe experiments, where synchronized emission of x-rays, gamma rays, THz waves, NIR beams, and electron bunches is available. This work is supported by DoE under contract DE-AC02-05CH11231. [Preview Abstract] |
Tuesday, October 25, 2005 11:30AM - 12:00PM |
FI2.00005: Compression of laser radiation in plasmas using electromagnetic cascading Invited Speaker: We theoretically suggest an approach to generation of trains of few-femtosecond electromagnetic (EM) pulses in rarefied plasmas. The technique is based on the near-resonant laser beat wave excitation of electron plasma wave (EPW). The EPW modifies the refractive index of plasma thus inducing the periodic phase modulation of the driving laser (the modulation period being equal to the beat period). In spectral terms, the phase modulation is expressed as an EM cascading with the laser bandwidth proportional to the product of the plasma length, laser wavelength, and electron density perturbation in the EPW. In the case of beat wave downshifted from the Langmuir plasma frequency the longer-wavelength spectral components are advanced in time with respect to the shorter-wavelength ones near the center of each laser beat note. The anomalous group velocity dispersion of plasma compresses so chirped beat notes to a few-laser-pulse duration thus creating a train of sharp EM spikes with the beat wave periodicity. Depending on the plasma and laser parameters, chirping and compression can be implemented either concurrently in the same, or sequentially in different plasmas. Evolution of the laser beat wave and electron density perturbations is described in time and in two spatial dimensions (2D) in a weakly relativistic approximation. Using the compression effect, we demonstrate that the relativistic bi-stability regime of the EPW excitation [G. Shvets, Phys. Rev. Lett. 93, 195004 (2004)] can be achieved with the initially sub-threshold beat wave pulse. The effects of 2D evolution such as the relativistic self-focusing and cascade focusing are also addressed. We conjecture that this technique could be used for increasing the local power of sub- picosecond petawatt laser beams. [Preview Abstract] |
Tuesday, October 25, 2005 12:00PM - 12:30PM |
FI2.00006: Electron Beam Conditioning by High-Intensity Lasers and the Role of Off-Polarization Field Components Invited Speaker: Recent experimental results demonstrating the role of the longitudinal laser field components in direct laser-electron scattering at relativistic intensity are reported[1]. Electrons with E$\sim$100 keV obliquely incident on a relativistic intensity laser have been experimentally deflected by three degrees along the laser. This result is corroborated by simulations only with the longitudinal fields included. As this deflection is strongly dependent on the particle energy and laser intensity, this technique can be used to generate the shortest electron bunches ever produced with tunable energy and a small energy spread. Among the applications of these beams are ultrafast electron diffraction, laser-driven X-ray generation, and fast ignitor fusion. The requirements for the generation of a conditioned 1 MeV electron beam for fast ignitor fusion are derived. As the small off-polarization field components play a dominant role in ponderomotive scattering, all field components must be known accurately. An exact theoretical formalism is developed for arbitrary laser fields reflected from both on- and off-axis parabolas for all F/\#'s. Solutions are presented for Gaussian and super-Gaussian incident beams and compared to the standard TEM$_{00}$ mode. Large F/\# are in good agreement, but small F/\# off-axis parabolas produce significant deviations. Such high accuracy fields are required not only for beam conditioning but also for characterization of ultra-relativistic intensity lasers ($I\sim10^{23}$ W/cm$^{2}$), direct laser acceleration of electrons, and laser machining of sub-micron features. \newline \newline [1] S. Banerjee, S. Sepke \emph{et al.}, Phys. Rev. Lett. \textbf{95}, 035004 (2005) [Preview Abstract] |
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