Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session KI2: Particle Beams and Coherent Radiation |
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Chair: Farhat Beg, University of California, San Diego Room: Bissonet |
Tuesday, October 28, 2014 3:00PM - 3:30PM |
KI2.00001: Multi-GeV electron beams from capillary discharge guided sub-petawatt class laser pulses in the self-trapping regime Invited Speaker: Wim Leemans Laser plasma accelerators (LPAs) can produce acceleration gradients on the order of tens to hundreds of GV/m, making them attractive as compact particle accelerators. During the past decade, quasi-monochromatic electron beams at the 1 GeV energy level have been produced using laser pulses at the 40-50 TW peak power level. With the availability of petawatt class lasers, beams up to 2 GeV have been produced from 7 cm long gas cells at UT Austin using 150 J laser pulses and at the 1 GeV level with tails extending to 3 GeV at the GIST facility in Korea. In this talk we present experimental results using the 1 Hz petawatt class BELLA laser at LBNL of the generation of multi-GeV electron beams with center energy up to 4.2~GeV, 6~\% rms energy spread, charge approximately 10~pC and an rms divergence around 0.3 mrad. The beams were produced from 9 cm long capillary discharge waveguide structure with a plasma density of $\approx 7 \times 10^{17}\,\rm{cm}^{-3}$, powered by laser pulses with peak power up to 0.3~PW. Preformed plasma waveguides allow the use of lower laser power compared to unguided plasma structures to achieve the same beam energy. Detailed comparison between experiment and simulation indicates the importance of the near-field laser transverse mode quality on guiding and acceleration in the LPA. By tuning the plasma density, regimes were found where laser beams with a top hat near-field profile were guided well, and where high energy electron beams can be produced, with narrow divergence [$<$ 0.8~mrad (FWHM)], and relatively small integrated energy spread ($< 10\%$). Provided that the slice energy spread and emittance are sufficiently low, electron beams with this energy could power x-ray free electron lasers. Future experiments will aim at increasing the beam energy to the 10 GeV level. [Preview Abstract] |
Tuesday, October 28, 2014 3:30PM - 4:00PM |
KI2.00002: Applications of laser wakefield accelerators for biomedical imaging Invited Speaker: Zulfikar Najmudin Laser-wakefield accelerators driven by high-intensity short-pulse lasers are a proven compact source of high-energy electron beams, with energy gains of $\sim$ GeV energy in centimetres of plasma demonstrated. One of the main proposed applications for these accelerators is to drive synchrotron light sources, in particular for x-ray applications. It has also been shown that the same plasma accelerator can also act as a wigglers, capable of the production of high brightness and spatially coherent hard x-ray beams. In this latest work, we demonstrate the application of these unique light-sources for biological and medical applications. The experiments were performed with the Astra Gemini laser at the Rutherford Appleton Laboratory in the UK. Gemini produces laser pulses with energy exceeding 10 J in pulse lengths down to 40 fs. A long focal length parabola ($f/20$) is used to focus the laser down to a spot of size approximately $25\, \mu$m (fwhm) into a gas-cell of variable length. Electrons are accelerated to energies up to 1 GeV and a bright beam of x-rays is observed simultaneously with the accelerated beam. The length of the gas cell was optimised to produce high contrast x-ray images of radiographed test objects. This source was then used for imaging a number of interesting medical and biological samples. Full tomographic imaging of a human trabecular bone sample was made with resolution easily exceeding the $\sim 100\, \mu$m level required for CT applications. Phase-contrast imaging of human prostrate and mouse neonates at the micron level was also demonstrated. These studies indicate the usefulness of these sources in research and clinical applications. They also show that full 3D imaging can be made possible with this source in a fraction of the time that it would take with a corresponding x-ray tube. [Preview Abstract] |
Tuesday, October 28, 2014 4:00PM - 4:30PM |
KI2.00003: Coherent control of plasma dynamics Invited Speaker: Zhaohan He The concept of coherent control - precise measurement or determination of a process through control of the phase of an applied oscillating field - has been applied to numerous systems with great success. Here, we demonstrate the use of coherent control on plasma dynamics in a laser wakefield electron acceleration experiment. A tightly focused femtosecond laser pulse (10 mJ, 35 fs) was used to generate electron beams by plasma wakefield acceleration in the density down ramp. The technique is based on optimization of the electron beam using a deformable mirror adaptive optical system with an iterative evolutionary genetic algorithm. The image of the electrons on a scintillator screen was processed and used in a fitness function as direct feedback for the optimization algorithm. This coherent manipulation of the laser wavefront leads to orders of magnitude improvement to the electron beam properties such as the peak charge and beam divergence. The laser beam optimized to generate the best electron beam was not the one with the ``best'' focal spot. When a particular wavefront of laser light interacts with plasma, it can affect the plasma wave structures and trapping conditions of the electrons in a complex way. For example, Raman forward scattering, envelope self-modulation, relativistic self-focusing, and relativistic self-phase modulation and many other nonlinear interactions modify both the pulse envelope and phase as the pulse propagates, in a way that cannot be easily predicted and that subsequently dictates the formation of plasma waves. The optimal wavefront could be successfully determined via the heuristic search under laser-plasma conditions that were not known \textit{a priori}. Control and shaping of the electron energy distribution was found to be less effective, but was still possible. Particle-in-cell simulations were performed to show that the mode structure of the laser beam can affect the plasma wave structure and trapping conditions of electrons, which subsequently produces electron beams with a different divergence. The proof-of-principle demonstration of coherent control for plasmas opens new possibilities for future laser-based accelerators and their applications. This study should also enable a significantly improved understanding of the complex dynamics of laser plasma interactions. [Preview Abstract] |
Tuesday, October 28, 2014 4:30PM - 5:00PM |
KI2.00004: First Demonstration of Nonlinear Compton Scattering from LWFA Electrons Invited Speaker: Christopher Murphy In the next decade, several large-scale laser projects will become operational allowing focused intensities exceeding $10^{23}$ Wcm$^{-2}$ to be reached for the first time. Theoretical models predict the onset of classical radiation reaction and QED effects which will dramatically alter the plasma dynamics. Such a QED-plasma would enable the generation of conditions analogous to those which are believed to exist in extreme astrophysical environments such as pulsar magnetospheres. While such high fields are not yet observable in the lab frame, they are accessible in the rest frame of a relativistic electron beam, providing a preview of the physics available at next generation facilities. In particular it has been shown that non-linear inverse Compton scattering and quantum corrections play a crucial role as laser intensities exceed $10^{23}$Wcm$^{-2}$ yet inverse Compton scattering has not been experimentally observed in the relevant extremely non-linear and quantum regime. An ``all-optical collider'' setup, whereby a laser wakefield electron bunch collides with a high-intensity laser, will allow this regime to be reached for the first time. En route to these exotic regimes, the collider will provide a unique photon source complimentary to conventional facilities. We present experimental results demonstrating a low divergence gamma ray source ($>4$MeV) with short pulse duration ($<50$fs). We also demonstrate, for the first time, inverse-Compton scattering of a laser pulse in the very nonlinear regime and show that this scattering process contains much of the physics relevant to laser-solid interactions at $10^{23}$ Wcm$^{-2}$. We will outline the theory which supports an experimental campaign aimed at probing the quantum effects which will radically change our understanding of laser-plasma interactions at next generation facilities, aimed at reaching $10^{24}$Wcm$^{-2}$. [Preview Abstract] |
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