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 TO7: Laser-plasma Sources of Electromagnetic Radiation |
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Chair: John Luginsland, Air Force Office of Scientific Research Room: 556AB |
Thursday, November 1, 2012 9:30AM - 9:42AM |
TO7.00001: High energy Betatron x-ray production in the ionization-induced trapping regime Felicie Albert, Bradley Pollock, Jessica Shaw, Leigh Ann Kessler, Sarah Mills, Joseph Ralph, Arthur Pak, Kenneth Marsh, Christopher Clayton, Warren Mori, Chan Joshi, Siegfried Glenzer Betatron x-rays with photon energies larger than 20 keV have been observed from a GeV-class laser-plasma accelerator. The 250 TW Callisto laser at LLNL was used to produce and simultaneously observe GeV-class electron beams and keV Betatron x-rays. The laser was focused into various gas cells with sizes ranging from 3 to 8 mm, and containing mixed gases (He, N, O2) to accelerate large amounts of charge in the ionization induced trapping regime. Electron spectra were measured on large image plates with the two-screen method after being deflected by a large 0.4 Tesla magnet spectrometer. Betatron oscillations observed on the electron spectra were benchmarked against a simple analytical model (Runge-Kutta algorithm solving the equation of motion of an electron in the wakefield) as well as against PIC simulations using OSIRIS. This analysis allowed to retrieve the electron injection conditions into the wake. This analysis, combined with spectral and spatial measurements of the produced x-rays suggest an enhancement of the betatron mechanism, x-ray yield and photon energy in the ionization induced trapping regime of laser-wakefield acceleration. [Preview Abstract] |
Thursday, November 1, 2012 9:42AM - 9:54AM |
TO7.00002: Compton scattering gamma-ray source optimization Frederic Hartemann, Sheldon Wu, F\'elicie Albert, Chris Barty The interaction of a bright relativistic electron beam with an intense laser pulse via Compton scattering can generate tunable gamma-rays for precision nuclear photonics applications. The properties of the gamma-ray phase space will be outlined, in relation with the 6D electron bunch and 6D laser pulse phase space, along with collimation, nonlinear effects and other sources of spectral broadening. Optimization strategies will be outlines within the context of nuclear photonics applications. [Preview Abstract] |
Thursday, November 1, 2012 9:54AM - 10:06AM |
TO7.00003: Short-pulse, high-energy radiation generation using a laser wakefield accelerator W. Schumaker, M. Vargas, Z. He, V. Chvykov, B. Hou, V. Yanovsky, A. Maksimchuk, A.G.R. Thomas, K. Krushelnick, G. Sarri, B. Dromey, M. Zepf Recent experimental results of laser wakefield acceleration (LWFA) of electrons and their subsequent radiation generation driven by the HERCULES laser with up to 200TW are presented. In LWFA, the plasma ``bubble'' structure forces trapped, off-axis electrons to undergo transverse oscillatory motion during acceleration, resulting in synchrotron-like betatron radiation in the keV X-ray regime. Measurements indicate that the beam source size can be as small as 1 micron and that the radiation exhibits spatial coherence, allowing phase-contrast imaging. Data from Cu K-$\alpha$ generated using an identical geometry are presented to give yield and source size comparisons. Alternatively, the high energy ($>$200 MeV) electron beam can be subsequently converted via Bremsstrahlung into low-divergence beams of high-energy photons and positrons. These photons are spectrally resolved using a Compton scattering-based, high-energy (30-80 MeV) photon spectrometer. All of these subsequent beams are presumed to retain the short-pulse characteristic of the electron beam, resulting in high peak flux, making the source an excellent candidate for ultrafast pump-probe applications in the keV and MeV photon range. [Preview Abstract] |
Thursday, November 1, 2012 10:06AM - 10:18AM |
TO7.00004: Phase Contrast Imaging with Betatron Radiation from Laser Wakefield Acceleratred Electrons Michael Vargas, William Schumaker, Zhaohan He, Vladimir Chvykov, Victor Yanovsky, Anatoly Maksimchuk, Karl Krushelnick, Alec Thomas Laser wakefield acceleration in the bubble regime can be used to accelerate electrons to GeV energies while simultaneously wiggling them to produce a synchrotron like x-ray radiation called betatron radiation. Using HERCULES, a 100TW TiSapphire laser, 30fs pulses were focused onto a gas jet to accelerate electrons in the bubble regime. The spatially coherent betatron radiation produced by the transverse motion of the accelerated electrons was used for phase contrast imaging of custom fabricated samples. The fabricated samples were built to contain edges for phase contrast, while keeping the material thickness constant in order to eliminate signal variation from x-ray absorption. Two detectors were implemented to produce images at different x-ray energies. Direct detection on an x-ray CCD was used in the lower energy regime (1keV-15keV), while a fiber-coupled scintillator was used to image the higher energy x-rays (3keV- 60keV). Additionally, phase contrast imaging in both self-injection and ionization-induced injection cases was compared. [Preview Abstract] |
Thursday, November 1, 2012 10:18AM - 10:30AM |
TO7.00005: A Computational Investigation of Synchrotron Radiation Generation in Laser Wakefield Acceleration Experiments Paul Cummings, Alec Thomas A promising application of laser-wakefield acceleration (LWFA) technology is as a tunable source of x-ray and gamma radiation via synchrotron radiation. Such a source could have many potential applications, including microscale imaging of advanced composite materials. Consequently, the generation of synchrotron radiation in LWFA experiments is investigated computationally using the particle-in-cell simulation code OSIRIS 2.0. A novel computational model for explicitly simulating synchrotron radiation, involving the generation of particle-like ``macrophotons,'' is derived. Results from the validation of this model, using a simple particle-tracking code ``FXW,'' are discussed. Results from the integration of this model into OSIRIS 2.0 are presented and discussed. Preliminary results from simulations of recent LWFA experiments, which investigated the generation of synchrotron-like radiation, are presented and discussed. Finally, results from computational parameter sweeps over both the coma severity and electron plasma density, investigating the impact of these parameters on the production of synchrotron-like radiation in a LWFA experiment, are presented and discussed. [Preview Abstract] |
Thursday, November 1, 2012 10:30AM - 10:42AM |
TO7.00006: ABSTRACT WITHDRAWN |
Thursday, November 1, 2012 10:42AM - 10:54AM |
TO7.00007: ABSTRACT WITHDRAWN |
Thursday, November 1, 2012 10:54AM - 11:06AM |
TO7.00008: A method to generate isolated half-cycle XUV/x-ray pulses Hui-Chun Wu, Juergen Meyer-ter-Vehn In the recent paper [\textbf{Wu {\&} Meyer-ter-Vehn, Nature Photon. 6, 304 (2012)}], we show that single powerful ($\sim $terawatt) half-cycle XUV/x-ray pulses can be produced when irradiating a double foil target with intense few-cycle laser pulses. Focused onto an ultrathin foil, all electrons are blown out, forming a uniform sheet of relativistic electrons. A second layer, placed some distance behind, reflects the drive beam but lets electrons pass straight through. Under oblique incidence, light reflection provides the transverse current in the electron sheet, which emits intense half-cycle pulses. These half-cycle attosecond pulses can be used to control electron motion in materials and explore nonlinear XUV/x-ray optics. [Preview Abstract] |
Thursday, November 1, 2012 11:06AM - 11:18AM |
TO7.00009: Generation of narrow-band x- and $\gamma $-rays by inverse Compton scattering of chirped laser pulses Isaac Ghebregziabher Based on single particle tracking in the framework of classical Thomson scattering with incoherent superposition, we developed a fully relativistic, three dimensional numerical model that calculates and quantifies the characteristics of emitted radiation when a relativistic electron beam (EB) interacts with an intense laser focus [I. Ghebregziabher, B. Shadwick, {\&} D. Umstadter, \textit{arXiv}\textbf{ 1204.1068} (2012)]. Predictions of the model are in excellent agreement with theoretical analysis and our recent experimental measurements. For laser pulses of sufficient duration, we find that the scattered radiation spectrum is broadened due to interferences arising from the pulsed nature of the laser. We find that by appropriately chirping the scattering laser pulse, spectral broadening can be minimized, and the peak brightness of the emitted radiation is increased by a factor approximately five times. Furthermore, the effect of EB divergence on the spectral bandwidth of the scattered x-rays/$\gamma $-rays is investigated. We find that the bandwidth of the scattered radiation from an EB with divergence $\theta _{e }\approx $ 1/$\gamma $ (typical for EBs generated with conventional accelerators) may be obtained with$(\Delta \omega /\omega )_\gamma \approx \sqrt {4(\Delta \gamma /\gamma )^2+\gamma ^4\theta _e^4 /4} $. The use of this formula in the regime $\theta _{e }>$ 1/$\gamma $ (typical for EB's generated with laser-wakefield accelerators) overestimates the bandwidth of the scattered radiation. [Preview Abstract] |
Thursday, November 1, 2012 11:18AM - 11:30AM |
TO7.00010: Propagation Effects on THz Generation from Ionizing Two Color Laser Pulses Luke Johnson, Thomas Antonsen, John Palastro, Ki-Yong Kim Coherent mixing of an ultrashort laser pulse (800nm, 50fs) and its second harmonic in a nitrogen gas cell produces broadband THz radiation. Asymmetry in the time dependence around the peaks in the two color electric field results in a post ionization current that varies slowly in time driving the THz radiation. As a coherent process, the relative phase between the harmonics determines the rate of THz generation. Over several centimeters, the interplay between propagation effects such as gas and plasma dispersion, the nonlinear gas response $(P \propto \chi^{(3)}E^3)$, and diffraction, can contribute to the relative phase. To examine the role of these effects on THz yields, we model laser pulse propagation using a 2D scalar unidirectional propagation equation for the electric field spectral components [2]. Additionally, we examine the broadening of the THz spectrum and its far field pattern. Finally, we will discuss the possibility of enhancing the THz yield with the presence of a third harmonic.\\[4pt] [1] K. Y. Kim, Phys. Plasmas 16, 056706 (2009). \newline [2] M. Kolesik and J. Moloney, Phys. Rev. E 70, 036604 (2004). [Preview Abstract] |
Thursday, November 1, 2012 11:30AM - 11:42AM |
TO7.00011: Scalable THz generation in two-color laser-produced plasma Ki-Yong Kim, Yong Sing You, Taek Il Oh We report ultrafast, high-power terahertz (THz) generation in two-color laser-produced plasmas. For scalable THz generation, we have studied two schemes---long (one-dimensional) and fat (two-dimensional) plasma filamentation. In the case of long filament formation, we observe phase-matched THz generation, which occurs naturally due to off-axis constructive interference between locally produced THz waves. This emits conical THz radiation in the off-axis direction, peaked at 4$\sim $7 degrees depending on the radiation frequencies. In this case, the total THz yield increases almost linearly with the filament length. Because of this, one can effectively increase THz output energy by simply extending the filament length. This overcomes the saturation effect previously reported, mainly caused by overdense plasma creation and laser intensity clamping in filamentation. In addition, we observe THz polarization rotation and control along long plasma filaments. In the second scheme, a cylindrical lens is used to produce two-dimensional plasma sheets. This also provides a simple method for scalable THz generation with enhanced plasma volume and coherent THz field addition. [Preview Abstract] |
Thursday, November 1, 2012 11:42AM - 11:54AM |
TO7.00012: Thermal effects in Raman amplification of laser pulses in plasma R. Trines, F. Fiuza, R. Bingham, P. Norreys, R.A. Fonseca, L.O. Silva, R.A. Cairns Recent numerical studies into Raman amplification in plasma at high laser intensities and powers showed that multi-petawatt pulses of fs-ps duration and energies of 0.1-1 kJ can be produced via this process [Trines et al., Nature Physics (2011), Phys. Rev. Lett. (2011)]. In these studies, it was assumed that the plasma was collisionless and initially cold. In practice, however, plasma will necessarily have a temperature of at least a few eV, and collisions will play an important role for higher plasma densities ($10^{19} - 10^{20} \mathrm{\ cm}^{-3}$) or longer pump pulse durations ($> 100$ ps). In this paper, we will investigate the influence of thermal and collisional effects such as Landau damping, collisional damping/absorption, thermal filamentation and Bohm-Gross frequency shifts, through one-and two-dimensional particle-in-cell simulations. We will show how the inclusion of thermal effects affects the parameter window for optimal Raman amplification, compared to the cold-plasma case. This work was supported by STFC's CLF and CfFP, by EPSRC through grant EP/G04239X/1 and by FCT (Portugal) through grants PTDC/FIS/66823/2006 and SFRH/BD/38952/2007. [Preview Abstract] |
Thursday, November 1, 2012 11:54AM - 12:06PM |
TO7.00013: Exploring the radiation reaction at 10$^{21 }$W/cm$^{2}$ Marija Vranic, Joana L. Martins, Jorge Vieira, Ricardo A. Fonseca, Luis O. Silva The experimental confirmation for the domain of validity of classical radiation reaction theory, as well as demonstration of the radiation reaction process is still missing. The thresholds for radiation reaction dominated regime are under a constant scientific debate. We present a theoretical and numerical study of the experimental conditions that would provide an answer to some of these questions in an all-optical configuration with laser systems available today (i.e. I$\sim $10$^{21 }$W/cm$^{2})$. We preformed 3-dimensional full-scale particle-in-cell (PIC) simulations of linear wakefield accelerators accounting for the radiation reaction, allowing the electron beam to interact with an intense laser after it leaves the plasma. We discuss measurable signatures of radiation reaction both in the electron spectra, and the backscattered laser photons, encompassing current and near-future laser technology. Electron energy loss of 40{\%} due to the radiation reaction was obtained using a laser of intensity 10$^{21 }$W/cm$^{2}$ and an LWFA electron beam of 1 GeV. The theoretical predictions for energy loss are in excellent agreement with the simulation results for various configurations presented.~ [Preview Abstract] |
Thursday, November 1, 2012 12:06PM - 12:18PM |
TO7.00014: Self-compression of few-millijoule laser pulses in non-linear plasma waves Zhaohan He, Bixue Hou, John Nees, Karl Krushelnick, Alexander Thomas Study of relativistic short laser pulse propagation in underdense plasma is of great interest in high field science, attosecond physics and applications such as plasma based accelerators. Temporal shortening of a laser pulse that drives nonlinear plasma wakefield has been observed using 100TW-class lasers interacting with millimeter scale plasma [1,2]. Here we report on experiments performed using the high repetition rate Lambda-cubed laser at the University of Michigan - a table-top sub-TW power laser systems operating at 500 Hz. The laser (pulse energy up to 8 mJ) was tightly focused (peak intensity $\sim $3$\times $10$^{18 }$W/cm$^{2})$ into a 100 $\mu $m scale length gas jet to generate a plasma with electron density $\sim $10$^{19 }$cm$^{-3}$. The temporal and spectral intensity and phase of the transmitted pulse was measured using second-harmonic-generation (SHG) frequency-resolved optical gating (FROG). The laser pulse was shortened from 37 fs to 16 fs with $>$90{\%} energy transmittance. The dependence on focusing condition, laser intensity and plasma density will be discussed.\\[4pt] [1] J. Faure, \textit{et al.}, Phys. Rev. Lett, \textbf{95}, 205003 (2005).\\[0pt] [2] J. Schreiber, \textit{et al.}, Phys. Rev. Lett. \textbf{105}, 235003 (2010). [Preview Abstract] |
Thursday, November 1, 2012 12:18PM - 12:30PM |
TO7.00015: Compression, spectral broadening, and collimation in multiple femtosecond pulse filamentation in atmosphere John Palastro, Thomas Antonsen, Howard Milchberg A sequence of femtosecond laser pulses propagating through atmosphere and delayed near the rotational recurrence period of N2 can resonantly drive molecular alignment. Through the polarization density, the molecular alignment provides an index of refraction contribution that acts as a lens copropagating with each laser pulse. Each pulse enhances this contribution to the index modifying the propagation of subsequent pulses. Here we present propagation simulations of femtosecond pulse sequences in which we have implemented a self consistent calculation of the rotational polarization density using linear density matrix theory. We find that a femtosecond pulse sequence can enhance pulse compression or collimation in atmosphere. In particular, when the pulses are delayed by exactly the rotational recurrence period, each subsequent pulse is increasingly compressed due to a combination of spectral broadening and negative dispersion. Alternatively, when the intensity peak of each pulse is centered on the maximum index generated by the proceeding pulses, each pulse is increasingly collimated. [Preview Abstract] |
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