Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session UO6: Laser-plasma Acceleration of Electrons |
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Chair: Alec Thomas, University of Michigan Room: 555AB |
Thursday, November 1, 2012 2:00PM - 2:12PM |
UO6.00001: Petawatt laser-driven acceleration of electrons to $>$ 2 GeV Neil Fazel, Xiaoming Wang, Rafal Zgadzaj, Watson Henderson, Yen-Yu Chang, Richard Korzekwa, Chi-Hao Pao, Hai-En Tsai, Zhengyan Li, Austin Yi, Vladimir Khudik, Hernan Quevedo, Gilliss Dyer, Erhard Gaul, Aaron Bernstein, Ted Borger, Michael Spinks, Mikael Martinez, Michael Donovan, Gennady Shvets, Todd Ditmire, Michael Downer We report self-injected laser wakefield acceleration of electrons beyond 2 GeV in a uniform, 6 cm long undoped helium plasma of density 3-5E17 cm-3, driven by 150 fs laser pulses of up to 120 J from the Texas Petawatt Laser. The highest energy beams to date contain $>$ 200 pC charge, with dN/dE peaking at as high as 2 GeV. At somewhat lower central energy (1.2 GeV), higher quality, dark-current-free beams with $<$ 0.25 mrad FWHM divergence, $\sim $10 pC charge and $\pm $ 25{\%} energy spread were obtained. Self-injected acceleration to $>$ 1 GeV was observed at plasma density as low as 1.7E17 cm-3. Electrons were accompanied by X-rays from the betatron motion of the accelerating electrons. Analysis of shadows cast by tungsten wire fiducials positioned precisely in the paths of the magnetically dispersed electrons and of the betatron X-rays enabled electron energy at 2 GeV to be determined with $\pm $ 10{\%} accuracy, without ambiguity due to electron launch angle variations. Simulations indicate that, with improvements in laser pulse focus quality, acceleration to 7 GeV is possible with the available pulse energy. [Preview Abstract] |
Thursday, November 1, 2012 2:12PM - 2:24PM |
UO6.00002: Laser Acceleration of Electrons in Shock Wave Enhanced Gas Jets Dmitri Kaganovich, Michael Helle, Daniel Gordon, Antonio Ting Controlling the gas density gradient and profile is important for electron and proton acceleration. Using an optimized gas density profile, we have demonstrated 40 times higher electron energy compared to a Gaussian gas jet without lost of charge or stability. Propagation of a shock wave through a gas jet can modify the gas density profile and create sharp density gradients [1, 2]. Using different shock waves energies and shock originating positions, we were able to modify the plasma density profile of a ``typical'' Gaussian gas jet into a variety of profiles, from thin (foil-like) structure to elongated profiles with fast rise and slow fall. We used a plasma bubble Cherenkov diagnostic [3, 4] to optimize the acceleration process. Accelerated electron energy and charge were cross-correlated with the second harmonic diagnostic signal. The optimized gas density profile generated stable 0.5 nC of 40 MeV electrons using a 10 TW laser. The shock wave modified gas jet can be used as a stand alone electron source or as an injector coupled to additional acceleration structures. We demonstrated stable injection of electrons from the shock wave modified gas jet into a lower density plasma. The results are also being studied with numerical simulations. \\[4pt] [1] D. Kaganovich et al., Physics of Plasmas \textbf{18}, 120701 (2011)\\[0pt] [2] D. Kaganovich et al., Applied Physics Letters \textbf{97}, 191501 (2010)\\[0pt] [3] D. F. Gordon, et al., Phys. Rev. Lett. \textbf{101}, 045004 (2008)\\[0pt] [4] M. H. Helle et al., Phys. Rev. Lett. \textbf{105}, 105001 (2010) [Preview Abstract] |
Thursday, November 1, 2012 2:24PM - 2:36PM |
UO6.00003: Low-emittance bunches from laser-plasma accelerators measured using X-ray spectroscopy Cameron G.R. Geddes, G.R. Plateau, D.B. Thorn, M. Chen, C. Benedetti, D.L. Bruhwiler, E. Cormier-Michel, E. Esarey, M.W. Fisher, A.J. Gonsalves, N.H. Matlis, K. Nakamura, S. Rykovanov, C.B. Schroeder, B. Shaw, S. Shiraishi, T. Sokollik, J. van Tilborg, Cs. Toth, S. Trotsenko, T.S. Kim, J.L. Vay, M. Battaglia, Th. Stoehlker, W.P. Leemans The presence of low emittance beams in laser-plasma accelerators is indicated by single-shot spectroscopic measurements of betatron X-rays. By matching the X-ray betatron spectra to analytical and numerical models of betatron radiation, the electron bunch radius inside the plasma is estimated to be $\sim$0.1 micron. The variation of beam radius with accelerator tuning is discussed. Photon-counting spectra and statistical fitting are used to establish confidence ranges. Combined with simultaneous electron spectrum and divergence measurements, the normalized transverse emittance is estimated to be as low as 0.1 mm mrad. Simulations show how such emittances can be formed by the self trapping process in laser-plasma accelerators. [Preview Abstract] |
Thursday, November 1, 2012 2:36PM - 2:48PM |
UO6.00004: Single-shot characterization of high-quality laser-plasma-accelerated electron bunches using transition-radiation-based techniques Charlotte Palmer, Tobias Kleinwaechter, Lucas Schaper, Jens Osterhoff, Nicolas Bourgeois, James Cowley, Simon Hooker, Wolf Rittershofer, Shao-Wei Chou, Stefan Karsch, Konstantin Khrennikov, Antonia Popp, Matthias Burza, Martin Hansson, Olle Lundh, Anders Persson, Lovisa Senje, Claes-Goran Wahlstrom We report on the characterization of electron bunches, accelerated within a laser-driven plasma wakefield, using incoherent transition radiation (TR). TR is generated whenever a charged particle crosses an interface between different materials. Incoherent TR is often used within RF accelerators as a bunch diagnostic, although for short bunch-durations coherence effects restrict its usefulness. Usually this coherent TR has been used for the measurement of laser-plasma-accelerated bunch-durations. Instead incoherent TR allows simultaneous measurement of the transverse profile and charge of the bunch, as well as the bunch-duration by a different method than from coherent TR. Tailored gas targets were used with the high-power laser of the Lund Laser Centre (1J, 35fs) to generate electron bunches through ionization injection of electrons into a wakefield and their acceleration. Transition radiation diagnostics were employed for single-shot diagnosis of these bunches and the preliminary results are presented. [Preview Abstract] |
Thursday, November 1, 2012 2:48PM - 3:00PM |
UO6.00005: Characterization of the source size and transverse emittance of a laser wakefield accelerated electron beam by means of inverse-Compton-scattering cross-correlation N. Powers, S. Chen, I. Ghebregziabher, C. Maharjan, C. Liu, G. Golovin, S. Banerjee, J. Zhang, N. Cunningham, A. Moorti, S. Clarke, S. Pozzi, D. Umstadter A recent focus in the development of laser wakefield electron accelerators (LWFA) has been the control of electron injection by ionization of inner shell electrons directly inside the accelerating structure [1]. Since electrons that are injected near the threshold of ionization are injected on axis, the resulting electron beams are expected to have lower divergence, as has been observed experimentally [2-5]; and thus they should also have lower transverse emittance. We used a technique based on inverse-Compton-scattering cross-correlation to measure the source size and normalized transverse emittance of an ionization-injected, laser-wakefield-accelerated electron beam. This is to our knowledge the first time a LWFA electron beam has been characterized by this means. \\[4pt] [1] A. Pak, et al., Phys. Rev. Lett. \textbf{104}, 025003 (2010).\\[0pt] [2] C. McGuffey, et al., Phys. Rev. Lett.\textbf{ 104}, 025004 (2010).\\[0pt] [3] J. S.Liu, et al., Phys. Rev. Lett.\textbf{ 104}, 035001 (2011).\\[0pt] [4] Y.-C. Ho, et al., Phys. of Plasmas, \textbf{18},063102 (2011).\\[0pt] [5] M. Chen, et al., Phys. of Plasmas, \textbf{19}, 033101 (2012). [Preview Abstract] |
Thursday, November 1, 2012 3:00PM - 3:12PM |
UO6.00006: Staged Laser driven Electron Acceleration Thomas Sokollik, Satomi Shiraishi, Anthony Gonsalves, Kei Nakamura, Jeroen van Tilborg, Brian Shaw, Eric Esarey, Carl Schroeder, Carlo Benedetti, Csaba Toth, Wim Leemans Laser plasma accelerators have made tremendous progress over the last decade. Currently electron energies around 1 GeV [W. Leemans et al., Nature Physics 2, 696 (2006)] and above can be achieved. In the acceleration process, laser energy is transferred, via generation of a plasma wakefield by the laser pulse, to the electrons. The acceleration of electrons stops, when the laser energy is depleted. To increase the electron energy in current LPA schemes, laser systems with more pulse energy are needed, thus current laser plasma accelerators are limited by laser technology. Today, several projects are using or planning to use PW class laser systems to achieve electron energies up to 10 GeV [W. P. Leemans et al., AAC proceedings (2012)]. These laser systems represent the latest development in laser technology and are able to deliver the highest achievable laser intensities today. To overcome the electron energy limitation a staged acceleration concept is necessary. In this scheme multiple acceleration stages are placed in series, each driven by a separate laser pulse. Now the final electron energy is limited by the number of stages only. In a concept study a 1TeV electron-positron collider based on staged acceleration was envisioned in reference [W. P. Leemans and E. Esarey, Physics Today, 62, 44 (2009)]. We will present the latest results on a staged laser plasma experiment in which two stages and two laser pulses are used. [Preview Abstract] |
Thursday, November 1, 2012 3:12PM - 3:24PM |
UO6.00007: Simulations of multiple consecutive laser-plasma acceleration stages Jean-Luc Vay, Cameron Geddes, Eric Esarey, Carl Schroeder, Wim Leemans, Satomi Shiraishi, Thomas Sokollik Staging of multiple laser-plasma accelerators in series is important to increase peak energy while maintaining large average gradient. Such staging can circumvent the usual tradeoff wherein higher energies require lower plasma density and hence lower gradient. Simulations are being conducted that show acceleration to the maximum energy in a first stage, then coupling to and further acceleration in subsequent stages. Simulations have been performed with the Particle-In-Cell code Warp, using the boosted frame technique for higher efficiency. In front of each stage, the incoming laser is injected using the moving plane technique that was introduced in Warp. Between stages, the exiting laser is deflected by a plasma mirror that is modeled as a perfect conductor. Effects of beam coupling and control of the beam energy in the second stage and beyond are being characterized and will be discussed. [Preview Abstract] |
Thursday, November 1, 2012 3:24PM - 3:36PM |
UO6.00008: Injection of externally produced kinetic electrons into a self-guided laser wakefield accelerator Bradley Pollock, Joseph Ralph, Felicie Albert, Jessica Shaw, Christopher Clayton, Ken Marsh, Chan Joshi, Warren Mori, Leigh Kesler, Sarah Mills, Brian Severson, Alexandra Rigby, Siegfried Glenzer A two-stage laser wakefield accelerator is being developed at the Lawrence Livermore National Laboratory using the Callisto laser system. The first stage is a high density ($\sim $10$^{19}$ cm$^{-3})$, 5 mm He gas jet plasma which is driven by 30 TW of 800 nm laser light focused to an a$_{0} \sim $ 2. The $<$100 MeV electrons produced in this stage are deflected by a 0.5 T dipole magnet onto the axis of the second stage, which is a low density ($\sim $10$^{18}$ cm$^{-3})$, 15 mm He gas cell driven by 200 TW of 800 nm light also focused to an a$_{0} \sim $ 2; no additional electrons are trapped in this stage. Electrons injected into the second stage can then be further accelerated to higher energy without increasing the energy spread. Measurements of the transmitted laser profile and spectrum from the second stage indicate that the laser pulse is self-guided throughout the gas cell and that a strong wake is driven. These results compare well with particle-in-cell (PIC) simulations performed with the code OSIRIS. This work was performed under the auspices of the United States Department of Energy by the Lawrence Livermore National Laboratory under contract No. DE-AC52-07NA-27344. [Preview Abstract] |
Thursday, November 1, 2012 3:36PM - 3:48PM |
UO6.00009: Prospects of Hybrid Plasma- and Radiofrequency-Based Electron Acceleration at DESY Jens Osterhoff, Florian Gruener, Eckhard Elsen, Klaus Floettmann, Brian Foster, Reinhard Brinkmann, Bernhard Schmidt, Holger Schlarb, Frank Stephan The field of particle acceleration in plasma wakes has seen remarkable progress in recent years. Accelerating gradients of more~than 10 GV/m can now be readily achieved using either ultra-short intense laser pulses or particle beams as wake drivers. The demonstration~of the first GeV electron beams and a general trend towards improved reproducibility, beam quality and control over the involved plasma~processes has led to plasma-acceleration techniques beginning to draw considerable interest in the traditional accelerator community. As a~consequence,~DESY, Germany's leading accelerator center, has established a research program for~plasma-based novel acceleration techniques with the goal of exploiting the synergetic combination of conventional and new accelerator technology. Such a concept offers an attractive pathway to study many mechanisms occurring in plasma-based accelerators, for example electron-beam-emittance evolution, extreme bunch compression, the controlled emission of betatron radiation, and staging of accelerating units. In addition, it is assumed that bypassing the difficult-to-master process of particle self-injection, which is utilized in all current laser-plasma acceleration schemes, will greatly enhance the reliability of such machines compared to the state-of-the-art. [Preview Abstract] |
Thursday, November 1, 2012 3:48PM - 4:00PM |
UO6.00010: Studies of Spectral Modification and Extensions of the Paraxial Equation in Laser Wakefield Simulations Wenxi Zhu, John Palastro, Thomas Antonsen Ultrashort intense laser pulses propagating through underdense plasma can drive large amplitude plasma waves that deplete the laser pulse energy. The loss of laser pulse energy and the approximate conservation of laser pulse action imply that spectral redshifting accompanies the depletion. We investigate the spectral shifting of the laser pulse in the strongly depleted regime using WAKE with a recently implemented tenuous plasma, full wave equation. We consider laser and plasma parameters typical of the regime of total cavitation for which redshifting is particularly strong. We examine the scaling of the spectral shifting rate with pulse power, plasma density (including ramps and radial channels), and pulse length. We also consider the temporal and spectral properties of the modified laser pulse to determine the stability of the process for generating ultra short, ultra intense midinfrared pulses [1] for various applications. \\[4pt] [1] C.-H. Pai et al., Phys. Rev. A 82, 063 [Preview Abstract] |
Thursday, November 1, 2012 4:00PM - 4:12PM |
UO6.00011: Optical Spectra as a Wakefield Diagnostic for Laser-Plasma Accelerators Satomi Shiraishi, Carlo Benedetti, Anthony Gonsalves, Kei Nakamura, Brian Shaw, Thomas Sokollik, Jeroen van Tilborg, Cameron Geddes, Carl Schroeder, Csaba Toth, Eric Esarey, Wim Leemans Laser diffraction and pump depletion represent two fundamental limitations to the acceleration lengths of laser-plasma accelerators (LPAs). The diffraction can be mitigated using a capillary discharge waveguide to optically guide the laser. However, the laser pulse can oscillate transversely if it does not match the guiding condition. This mismatched guiding leads to inefficient coupling of laser energy into the plasma. The efficiency of the coupling can be estimated through optical spectra. As the laser pulse excites plasma waves, the spectrum is red-shifted and modulated. We present optical spectral analysis comparing experimental data with simulation. The spectral analysis is a non-destructive diagnostic of laser energy depletion and accelerating field. These measurements will be critical in staged LPAs. Measurement of laser energy depletion helps us determine an optimal length for each LPA module and the amplitudes of excited waves allows us to estimate the potential energy gain from the module for an externally injected electron beam. These studies contribute to improved control of LPAs and greater reliability. [Preview Abstract] |
Thursday, November 1, 2012 4:12PM - 4:24PM |
UO6.00012: Development and characterization of plasma targets for controlled injection of electrons into laser-driven wakefields Tobias Kleinwaechter, Lars Goldberg, Charlotte Palmer, Lucas Schaper, Jan-Patrick Schwinkendorf, Jens Osterhoff Laser-driven wakefield acceleration within capillary discharge waveguides has been used to generate high-quality electron bunches with GeV-scale energies. However, owing to fluctuations in laser and plasma conditions in combination with a difficult to control self-injection mechanism in the non-linear wakefield regime these bunches are often not reproducible and can feature large energy spreads. Specialized plasma targets with tailored density profiles offer the possibility to overcome these issues by controlling the injection and acceleration processes. This requires precise manipulation of the longitudinal density profile. Therefore our target concept is based on a capillary structure with multiple gas in- and outlets. Potential target designs are simulated using the fluid code OpenFOAM and those meeting the specified criteria are fabricated using femtosecond-laser machining of structures into sapphire plates. Density profiles are measured over a range of inlet pressures utilizing gas-density profilometry via Raman scattering and pressure calibration with longitudinal interferometry. In combination these allow absolute density mapping. Here we report the preliminary results. [Preview Abstract] |
Thursday, November 1, 2012 4:24PM - 4:36PM |
UO6.00013: Resolving Multiple Plasma Filaments using Single-shot Tomographic Imaging Nicholas Matlis, Andrew Axley, Wim Leemans Laser-induced plasmas can often exhibit structures that are heterogeneous or are composed of multiple filaments, making them difficult to measure. Current single-shot methods for measuring plasma structure require strong symmetry assumptions about the form of the plasma, leading to large errors in structure determination. Here we demonstrate a new technique capable of resolving such structure in a single shot without assumptions by using Spectrally-Multiplexed Tomography. [Preview Abstract] |
Thursday, November 1, 2012 4:36PM - 4:48PM |
UO6.00014: Density characterization of tapered super-sonic gas jet targets for laser wakefield acceleration Gregory Golovin, Emily Grace, Sudeep Banerjee, Chad Petersen, Kevin Brown, Jared Mills, Shouyuan Chen, Cheng Liu, Donald Umstadter Phase slippage between plasma wave and electron bunch limits maximum energy gain in laser-wakefield acceleration. Plasma-density spatial tailoring has been proposed as a way to overcome this dephasing problem [1]. In practice, such tailoring can be achieved in super-sonic gas jets by use of a nozzle with a tapered orifice. We have developed a 3-D temporally-resolved interferometric tomography technique to characterize dynamical density distribution of such gas jets. The SIRT (Simultaneous Iterative Reconstructive Technique) algorithm [2] was implemented. We also present preliminarily results on laser wakefield acceleration in the tailored gradient density profiles resulting from use of the characterized jets as targets. \\[4pt] [1] W. Rittershofer, C. B. Schroeder, E. Esarey, F. J. Gr\"uner, and W. P. Leemans, ``Tapered plasma channels to phase-lock accelerating and focusing forces in laser-plasma accelerators,'' \textit{Physics of Plasmas} \textbf{17}, 063104, (2010). \\[0pt] [2] P. Gilbert, ``Iterative methods for the three-dimensional reconstruction of an object from projections,'' \textit{Journal of Theoretical Biology} \textbf{36}, 105 (1972). [Preview Abstract] |
Thursday, November 1, 2012 4:48PM - 5:00PM |
UO6.00015: Effect of wire obstructions on the formation of modulated plasma waveguides Andrew Goers, Sung Yoon, George Hine, Jeff Magill, Howard Milchberg Modulated plasma waveguides have been proposed as a means of quasi-phase matching laser plasma interactions for applications including direct acceleration of electrons by a high intensity laser pulse. We have demonstrated a technique for axially modulating plasma waveguides by periodically obstructing gas flow out of a cluster jet using an array of wires. This technique is inherently simpler and more easily varied compared to demonstrated optical techniques which axially modulate laser intensity at the target. However, in the previous study [B. Layer, et. al., Opt. Exp. 17, 4263(2009)] the modulation period could not be made less than 200um due to an observed density drop in the plasma between the wires for unknown reasons. By obstructing gas flow with only two wires with variable separation, we examine the aforementioned issue. Since the gas flow out of the cluster jet is supersonic, we observe shock wave formation from the wires with transverse interferometry and shadowgraphy. We find that as we increase the mean cluster size in the gas flow the effect of the shock wave to decrease plasma density between the wires is diminished, representing a transition to a ballistic flow regime. By optimizing jet parameters (e.g. temperature and height of plasma from the wires) we have been able to achieve plasma guiding structures with modulation periods less than 200um. [Preview Abstract] |
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