Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session UO7: Wakefield Acceleration and Laser-plasma Electron Generation |
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Chair: Alec Thomas, University of Michigan Room: Governor's Square 12 |
Thursday, November 14, 2013 2:00PM - 2:12PM |
UO7.00001: Quasi-monoenergetic electron ring production from laser wakefield acceleration in the blowout regime Bradley Pollock, Felicie Albert, Joseph Ralph, Ken Marsh, Jessica Shaw, Paul Campbell, Nicholas Chavez, Alethia Barnwell, Arthur Pak, Chris Clayton, John Moody, Chan Joshi, Siegfried Glenzer We have observed quasi-monoenergetic rings of electrons accelerated to energies above 250 MeV during LWFA experiments in the blowout regime. These experiments utilize the 200 TW, 60 fs Ti:Sapphire Callisto laser system at LLNL and are performed using a He/N gas cell target. The results are compared with 2D OSIRIS simulations, where electrons trapped in the second bucket of the wake are observed to interact with on-axis electrons. In both the experiment and the simulation a ring of electrons is produced with a full-angle of $\sim$60 mrad and a narrow energy spread around the ring. Results will be shown for a range of electron densities and gas mixtures to determine the optimal conditions for producing this ring structure. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Thursday, November 14, 2013 2:12PM - 2:24PM |
UO7.00002: Laser wakefield acceleration of electrons with ionization injection in a pure N5$^{+}$ plasma channel Sung Jun Yoon, Andy Goers, George Hine, Jennifer Elle, Daniel Gordon, Howard Milchberg Preformed plasma channels have been successfully used in laser wakefield acceleration to accelerate electrons up to \textit{GeV} with modest laser intensity by eliminating the need for self-focusing. Here, we show that nitrogen is an excellent medium for ionization injection-based laser wakefield accelerators because of the extremely large ionization potential gap between the L-shell (98\textit{eV} to ionize N4$^{+}\to $N5$^{+})$ and K-shell electrons (552\textit{eV} to ionize N5$^{+}\to $N6$^{+})$. We have measured pure N5$^{+}$ plasma channels with a base density of $1\sim 5\times 10^{18}cm^{-3}$ and shock walls at $\sim 2\times 10^{19}cm^{-3}$ through hydrodynamic expansion of nitrogen cluster plasma. In this N5$^{+}$ plasma channel, we can decrease the laser intensity threshold for trapping and accelerating electrons by ionization injection and channel guiding. Particle-In-Cell simulations confirm trapping of electrons from N5$^{+}$ by tunneling ionization with initial laser intensity of $a_{0} =1$. Injection from the abundant N5$^{+}$ ionization source enables the space charge of the trapped electrons to stop further injection. The accelerated bunch can reach hundreds of \textit{pC} with energy gain of hundreds of \textit{MeV}. We will present preliminary results from corresponding acceleration experiments. [Preview Abstract] |
Thursday, November 14, 2013 2:24PM - 2:36PM |
UO7.00003: Imaging electron trajectories in a laser-wakefield accelerator by measuring the betatron x-ray spectrum angular dependence Felicie Albert, Bradley Pollock, Jessica Shaw, Alethia Barnwell, Paul Campbell, Nicholas Chavez, Ken Marsh, Yu Hsin Chen, David Alessi, Chris Clayton, Arthur Pak, Joseph Ralph, Sigfried Glenzer, Chan Joshi We have performed experiments using the 200 TW Callisto laser system at LLNL to produce GeV-class electron beams and keV Betatron x-rays. The laser was focused into various gas cells with sizes ranging from 3 to 10 mm that contained a mixure of gases (He, N, Ar). We demonstrate that it is possible to do a tomographic reconstruction of electron trajectories inside the channel of the laser-wakefield accelerator from the angular dependence of the Betatron x-ray spectrum, using an image plate-based spectrometer with differential filtering. Experimental results are benchmarked against a code that solves the equation of motion of electrons oscillating in the plasma wake and by calculating the corresponding x-ray radiation spectrum and profile. This combined single-shot, simultaneous spectral and spatial x-ray analysis allows for a 3D reconstruction of electron trajectories in the plasma with micrometer resolution. [Preview Abstract] |
Thursday, November 14, 2013 2:36PM - 2:48PM |
UO7.00004: High energy, low energy spread electron bunches produced via colliding pulse injection C.G.R. Geddes, N.H. Matlis, S. Steinke, D. Bruhwiler, M. Chen, E. Cormier-Michel, E. Esarey, K. Nakamura, G.R. Plateau, C.B. Schroeder, Cs. Toth, W.P. Leemans Injection into a high gradient laser-plasma accelerator can becontrolled by using the beat between ``colliding'' laser pulses to kick electrons at a specified location into a plasma wave which was operated below the threshold for self injection. The experiments used control over the laser optical mode and plasma profile to extend the acceleration distance in a gas jet target. This allowed acceleration of electrons to above 200 MeV using the 10 TW LOASIS laser. Colliding pulse injection into this high energy structure was used to control bunch quality, producing bunches with energy spreads below 1.5\% FWHM and divergences of 1.5 mrad FWHM. With injection location fixed by the colliding pulses, beam energy was controlled by plasma density and by target location with respect to the collision. Dependence of injector performance on laser and plasma parameters were characterized in coordination with simulations. Separate experiments recently demonstrated 0.1 mm-mrad emittance from self injected LPAs using betatron radiation, and comparison with present divergence and energy data indicate that these bunches may also be of very low emittance. [Preview Abstract] |
Thursday, November 14, 2013 2:48PM - 3:00PM |
UO7.00005: Quasi-phasematched laser wakefield acceleration J.P. Palastro, S.J. Yoon, D.F. Gordon, H.M. Milchberg A plasma wakefield driven by a laser pulse can accelerate electrons to GeV energies over centimeter length scales. The energy gain in laser wakefield acceleration (LWFA) is ultimately limited by dephasing, occurring when accelerated electrons outrun the accelerating phase of the wakefield. We apply quasi-phasematching, enabled by corrugated plasma channels, to overcome this limitation. An electrostatic wave driven in an axially periodic plasma is composed of spatial harmonics whose associated phase velocities can be tuned through the modulation period. By matching the modulation period to the dephasing length, a relativistic electron can remain in phase with a spatial harmonic and undergo linear energy gain over several dephasing lengths. Theory and simulations are presented showing that at weakly relativistic laser intensities of $\sim$10$^{17}$ W/cm$^{2}$ and modest pulse energies of $\sim$2 mJ quasi-phasematched LWFA results in energy gains in excess of 100 MeV larger than standard LWFA with identical parameters. [Preview Abstract] |
Thursday, November 14, 2013 3:00PM - 3:12PM |
UO7.00006: Origin and control of the picosecond pedestal in femtosecond laser systems and its effect on laser wakefield acceleration of electrons Dmitri Kaganovich, Joseph Penano, Daniel Gordon, Mike Helle, Bahman Hafizi, Antonio Ting The picosecond time scale pedestal of a multi-terawatt femtosecond laser pulse is investigated experimentally and analytically. The origin of the pedestal is related to the finite bandwidth of the laser system. Using a simple pulse splitter and delay line, we have produced significant (order of magnitude) improvement of sub-picosecond-scale laser contrast. We interpret this contrast enhancement as a result of linear interference among two chirped input pulses producing a modulated laser spectrum that is matched to the bandwidth of the optical system. Because we can work near points of constructive interference and the amplifier is run to saturation, the energy in the amplified pulse is preserved. This contrast enhancement is shown to increase the energy of laser wakefield accelerated electrons by creating favorable conditions for the accelerating plasma bubble. [Preview Abstract] |
Thursday, November 14, 2013 3:12PM - 3:24PM |
UO7.00007: Guiding of \textgreater~10 TW laser pulses in cm-scale cluster-based plasma channels Andrew Goers, Sung Yoon, George Hine, Jennifer Elle, Howard Milchberg Optical guiding of high intensity laser pulses over many Rayleigh lengths is critical for the realization of many high power laser physics applications, including laser wakefield acceleration of electrons. Clustered gases have been shown to be ideal targets for generating pre-formed plasma channels due to highly efficient absorption of laser pulse energy, even at relatively low densities. We demonstrate stable optical guiding over $\sim$ 1cm at intensities greater than 10$^{18}$ W/cm$^{2}$ in a plasma channel formed in an elongated cluster jet. Transverse interferometry of the channel allows experimental observation of channel formation dynamics and subsequent calculation of supported electromagnetic modes. Rapid hydrodynamic expansion of the laser heated cluster plasma creates highly curved plasma channels with shock wall electron densities exceeding 10$^{19}$ cm$^{-3}$ with simultaneous on-axis densities near 10$^{18}$ cm$^{-3}$. Interferometric measurements of the channel density after passage of a guided femtosecond laser pulse show that these highly curved channels improve the overall waveguide efficiency by decreasing tunneling of electromagnetic modes through the channel walls with peak guided energy throughputs exceeding 80{\%} of the incident laser energy. [Preview Abstract] |
Thursday, November 14, 2013 3:24PM - 3:36PM |
UO7.00008: Pulsed Mid-infrared Radiation from Spectral Broadening in Laser Wakefield Simulations Wenxi Zhu, John Palastro, Thomas Antonsen Spectral red-shifting of high power laser pulses propagating through underdense plasma can be a source of ultrashort mid-infrared (MIR) radiation. During propagation, a high power laser pulse drives large amplitude plasma waves, depleting the pulse energy. At the same time, the large amplitude plasma wave provides a dynamic dielectric response that leads to spectral shifting. The loss of laser pulse energy and the approximate conservation of laser pulse action imply that spectral red-shifts accompany the depletion. We investigate, through simulation, the parametric dependence of MIR generation on pulse energy, initial pulse duration, and plasma density. [Preview Abstract] |
Thursday, November 14, 2013 3:36PM - 3:48PM |
UO7.00009: AWAKE, a Self-modulated, Proton-driven Plasma Wakefield Acceleration Experiment at CERN Patric Muggli Proton ($p^+$) bunches available today carry large amounts of energy ($kJ$). They are therefore potential drivers for plasma wakefield acceleration experiments aiming at large energy gain along a single long plasma. However, these $p^+$ bunches are also long ($\sim10~cm$). In dense plasmas, such that the plasma period is shorter than the bunch length, they are subject to the self-modulation instability (SMI) [Kumar, Phys. Rev. Lett. 104, 255003 (2010)]. The SMI forms a train of short bunches that can resonantly drive accelerating wakefields to the $GV/m$ level. Based on this scheme, the AWAKE experiment at CERN will use the $400~GeV$ bunch with $3\times10^{11}~p^+$ of the SPS. Numerical simulations show that over $10~m$ of plasma with a density in the $1-10\times10^{14}~cm^{-3}$ range the SMI can grow, and saturate when seeded. Seeding also allows for deterministic injection of the witness bunch. Externally injected $MeV$ electrons can reach $GeV$ energies in $\sim GeV/m$ accelerating gradient. Operating at low plasma density, i.e., larger accelerating structure, but with large average gradient eases the injection process and the bunches production and alignment. The physics program and the experimental set-up of the AWAKE experiment will be presented. [Preview Abstract] |
Thursday, November 14, 2013 3:48PM - 4:00PM |
UO7.00010: X-ray Radiation and Electron Injection from Beam Envelope Oscillations in Plasma Wakefield Accelerator Experiments at FACET K.A. Marsh, W. An, C.E. Clayton, C. Joshi, W. Lu, W.B. Mori, N. Vafaei-Najafabadi, C. Clarke, S. Corde, J.P. Delahaye, J. England, A. Fisher, J. Frederico, S. Gessner, M.J. Hogan, S. Li, M. Litos, D. Walz, Z. Wu, E. Adli PWFA experiments at FACET at the SLAC National Accelerator Laboratory have shown a correlation between ionization-injected electrons and the betatron x-ray yield. The PWFA experiments were carried out using a rubidium vapor heat pipe oven. The vapor density was 2.5x10$^{\mathrm{17}}$ cm$^{\mathrm{-3}}$ and was ionized by the 20 GeV electron beam via tunneling ionization. The injected charge and x-ray yield are attributed to the beam envelope oscillations where at the oscillation minima, the field of the beam is strong enough to ionize RbII, and at the electron oscillation maxima, the beam electrons radiate x-rays. In general the x-ray yield scales as r$^{\mathrm{2}}$n$^{\mathrm{2}}\gamma^{\mathrm{2}}$, but for a matched beam the x-ray yield is reduced and scales as r$^{\mathrm{3/2}}$n$^{\mathrm{3/2}}\varepsilon $. The FACET x-ray diagnostic can be used to tune the drive beam parameters for matched propagation by minimizing the x-ray yield. For a matched beam, there is no beam envelope oscillation, thus the x-ray yield and unwanted beam loading are greatly reduced. Injection of plasma electrons into the wake can limit the wake amplitude and deplete the accelerating gradient. Minimizing the x-ray yield should reduce unwanted beam loading. [Preview Abstract] |
Thursday, November 14, 2013 4:00PM - 4:12PM |
UO7.00011: Quantum Model for Atomic Response in Strong, Time Dependent Electric Fields T.C. Rensink, T.M. Antonsen, Jr., John Palastro Laser pulse propagation simulations typically involve simplified ionization models where plasma generation is treated via rate laws. These models neglect the fact that the bound electronic response of the atom, ionization, and ionization damping are a continuous process, and do not capture dynamics during the electronic transition from bound to free.~ We present a reduced 3D quantum model that treats the full time dynamics of the electronic response and compare it to current models. By replacing the Coulomb potential with a non-local binding potential, computation is reduced from 3$+$1D equation set to a 0$+$1D integral equation, offering a fast, continuous treatment of the electronic response for use in laser propagation simulations. [Preview Abstract] |
Thursday, November 14, 2013 4:12PM - 4:24PM |
UO7.00012: Fast electron production in intense laser-plasma interaction of multi-picosecond time scales A. Sorokovikova, B. Qiao, M.S. Wei, R.B. Stephens, P.K. Patel, H.S. McLean, F.N. Beg Intense (I$_{\mathrm{laser}}$\textgreater 10$^{\mathrm{18}}$ W/cm$^{\mathrm{2}})$ laser-plasma interaction offers a very efficient source of fast electrons at relativistic energies, which can be used for fast-ignition inertial confinement fusion, ultra-short x-ray sources and heating matter to warm dense states. We report theoretical and particle-in-cell simulation results for characterization of fast electron source produced from intense laser interaction with solid targets at the time scale of multi-picosecond and energy scale of kilojoule. A substantial increase in both fast electron average energy and laser-electron conversion efficiency has been observed when the laser pulse length was extended from 1 to 10 picoseconds. The enhanced electron acceleration is attributed to a significant thermal preplasma expansion on several picosecond time scale that forms a long flat ``shelf'' at near-critical (0.1n$_{\mathrm{c}}$ - n$_{\mathrm{c}})$ density region, and ponderomotive piling-up of electrons that leads to a sharp interface at relativistic critical density $\gamma $n$_{\mathrm{c}}$. Both of these eventually result in large amplitude increase and volume broadening of the electrostatic potential for electron acceleration. \\[4pt] [1] D. R. Welch et al., Phys. Plasmas 13, 063105 (2006). [Preview Abstract] |
Thursday, November 14, 2013 4:24PM - 4:36PM |
UO7.00013: Ionization-assisted Relativistic Electron Generation with Monoenergetic Features from Laser Thin Foil Interaction Artem Karpeev, Igor Glazyrin, Olga Kotova, Valery Bychenkov, Robert Fedosejevs, Wojciech Rozmus The concept of ionization-induced injection into the laser pulse to produce quasi-monoenergetic bunches of electrons from ultra-thin solid dense targets is analyzed. When the laser pulse propagates through semi-transparent foil the electrons from inner atom shells remain bound during the rise time of the laser pulse and are ionized by the laser intensity near its maximum amplitude, which satisfies the best injection condition for subsequent acceleration. The 2D3V PIC code PICNIC was used for simulation of a linearly polarized laser pulse with a wavelength $\lambda =1.053\mu m$ normally incident onto nano-sized DLC target. We performed simulations for 3~cases: (1)~5~nm carbon foil ionized due to field ionization (FI); (2)~the same, but already ionized foil, i.e. the foil in the form of a plasma slab with average charge \textit{\textless Z\textgreater} $=$ 3.4; (3)~42~nm carbon foil with FI. Comparison of the results obtained with different target models shows that a correct description of the interaction of a high contrast laser pulse with an ultra-thin solid dense target should include the FI effect. It was found that for the case (1) a bunch of quasimonoenergetic electrons from inner atom shells moves co-directionally with laser pulse and acquire energy $\sim m_{e} c^{2}a^{2}/2$. [Preview Abstract] |
Thursday, November 14, 2013 4:36PM - 4:48PM |
UO7.00014: Magneto-ionization laser wakefield assisted acceleration of GeV range quasimonoenergetic electron beams in He with CO2 impurity gas target Igor Glazyrin, Artem Karpeev, Olga Kotova, Valery Bychenkov, Robert Fedosejevs, Wojciech Rozmus Quasimonoenergetic electron beam with maximum energy of 1~GeV and several mrad divergence has been generated in He gas with CO2 impurity via wakefield acceleration with 80~TW, 30~fs laser pulse at the Advanced Laser Light Source (ALLS) installation. These measurements are supported by 3D3V PICNIC simulations with the model used for the tunnel ionization accounting. Numerical analysis has indicated the continuous injection and the acceleration of liberated electrons from different atom shells of all gases. Electrons from inner shells were ionized near the peak of the laser pulse and were injected into and trapped by the wake. This mechanism of electrons selection is weakly operating. CO2 impurity increases the stability of subsequent trapping of electrons. Through simulations it has found that laser-driven wakefield configurations oscillate periodically changing the number of bubbles with electron bunches from one to several (three on the average). It leads to transverse beams centroid motion which is likely head-tail instability. Focusing magnetic field for the case of CO2 impurity is assisted in stabilization of the instability. [Preview Abstract] |
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