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AAC'04 Agenda
AAC'04 Poster
Organizing Committee
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Laser Plasma Acceleration - Plasma Acceleration Subgroup
  We describe our studies on the generation of plasma wake fields by a relativistic electron bunch, and on phasing between the longitudinal- and transverse-fields in the wake. The leading edge of the electron bunch excites a high-amplitude plasma wake inside the over-dense plasma column, and the acceleration and focusing wake fields are probed by the bunch tail. By monitoring the dependence of the acceleration upon the plasma’s density, we approached the beam-matching condition and achieved an energy gain of 0.6 MeV over the 17-mm plasma length, corresponding to an average acceleration gradient of 35 MeV/m. Wake-induced modulation in energy and angular divergence of the electron bunch are mapped within a wide range of plasma density. We confirm a theoretical prediction about the phase offset between the accelerating and focusing components of plasma wake.
F. Grigsby & M. Fomotskyi : LWFA with Low Energy Raman Seeded Pulses
  Analytical and numerical calculations of plasma wakefield excitation and particle acceleration by Raman seeded laser pulse in self-modulation regime are presented. We derive energy threshold for self-modulation of diffraction-limited pulses. The parameter range where the Raman seeded amplitude plays an important role is investigated. We show that the seeded amplitude provides a coherent control mechanism for the phase of the wakefield wave. We show that with the use of Raman seed self-modulated wakefield acceleration is achievable for the pulses of intensities much lower than those typically used in the experiments. In particular, our 2D particle-in-cell simulations show that 30mJ pulse combined with Raman seeded pulse, which is 1% in intensity of the main pulse is capable of generating ~1nC of relativistic electrons.
Albert Reitsma: Laser wakefield acceleration - a fully self-consistent analysis
  In this talk, the interplay between pump depletion, electron dephasing and beam loading in the nonlinear regime of the resonant laser wakefield accelerator is discussed. A fully self-consistent one-dimensional model that includes laser pulse evolution and electron beam loading effects is presented. It is found that the laser pulse length is an important parameter that controls the efficiency. The energy spread can be minimized by exploiting the combined effects of beam loading and phase slippage. Simulation results of efficient acceleration up to 1 GeV in a single stage are shown.
Wayne Kimura: Laser Wakefield Acceleration Driven by ATF CO2 Laser
  A new experiment has begun that builds upon the successful STELLA experiment, which demonstrated high-trapping efficiency and narrow energy spread in a staged laser accelerator. STELLA was based upon inverse free electron lasers (IFELs); the new experiment, called STELLA-LW, is based upon laser wakefield acceleration (LWFA). The first phase of STELLA-LW will be to demonstrate LWFA in a capillary discharge driven by the BNL ATF terawatt CO2 laser beam. This will be the first time LWFA is conducted at 10.6-micron laser wavelength. It will also be operating in an interesting pseudo-resonant regime where the laser pulse length is too long for resonant LWFA, but too short for self-modulated LWFA. Analysis [1] has shown that in pseudo-resonant LWFA, pulse-steepening effects occur on the laser pulse that permits generation of strong wakefields. Various approaches are being explored for the capillary discharge tube including a polypropylene one [2] already demonstrated at the ATF and a hydrogen-filled one demonstrated at the University of Oxford [3]. Planned diagnostics for the experiment include usage of coherent Thomson scattering (CTS) to detect the wakefield generation. This will be one of the first times CTS is used on a capillary discharge. Details of the planned experiment and its model predictions will be presented.

[1] N. E. Andreev, et al., Phys. Rev. ST Accel. Beams 6, 041301 (2003).[2] D. Kaganovich, et al., Appl. Phys. Lett. 71, 2925 (1997).[3] A. Butler, et al., Phys. Rev. Lett. 89, 185003 (2002).

Serguei Kalmykov: Application of detuned plasma beatwave for generation of few-cycle electromagnetic pulses
  Two external laser beams with a near-resonant frequency detuning ($\Omega\approx\omega_{pe}$) drive a high-amplitude electron plasma wave (EPW) which modifies the refractive index of plasma so as to produce a periodic phase modulation of the pump field with a beatwave period. The pump then breaks up into a series of chirped pulses of a duration $2\pi/\Omega$. The chirp is positive (the longer-wavelength sidebands go ahead) when $\Omega<\omega_{pe}$ and negative otherwise. Finite group velocity dispersion (GVD) of radiation in a plasma can compress these positively chirped laser beamlets to a few-laser-cycle duration thus creating a chain of sharp electromagnetic spikes separated in time by $2\pi/\Omega$. The resonant interaction via the driven EPW can strongly reduce the GVD in the system of coupled laser sidebands. In this case, a separate plasma of higher density, where the laser spectral components become uncoupled, can be used for the pulse compression.
Kazuhisa nakajima: Ultrahigh current electron acceleration in relativistic laser-plasma interactions
  Recently there is a great interest growing in advanced accelerator technologies based on laser and plasma acceleration mechanisms, which have a tremendous potential for applications to a wide range of sciences, not only basic sciences but also medical and industrial sciences. The novel particle acceleration technologies make it possible to produce unique high quality beams in a compact system, which are crucial for specific applications. In particular extraordinary evolution of ultraintense ultrashort pulse lasers can generate ultrahigh gradient accelerating fields of the order of 1 TeV/m in a plasma to accelerate electron beams well collimated with small emittance and femtosecond buch length up to 1 GeV in a 1 mm plasma. It is known that this mechanism of electron acceleration in plasma plays a role of accelerating protons and ions with tens of MeV energy as well as production of bright radiations. In the relativistic highly nonlinear regime, relativistic electron beams can be accelerated by more complex mechanisms based not only on wakefields and also on direct ponderomotive fields. We report the recent results of electron beam acceleration experiments made by using 20 TW, 20 fs laser pulses at JAERI-APRC. The experiments result in ultrahigh current relativistic electron beam acceleration of the order of Mega Ampere with energy spectrum characterized by a power law up to 40 MeV rather than a Maxwellian distribution. The talk will focus on the acceleration mechanism of ultrahigh current electron beams in plasma and their applications to femtosecond X-ray beam generation.
Sergey Gordienko: Electron acceleration in the Bubble regime: analytical theory and numerical simulations
  We present analytical theory and numerical simulations of laser-plasma electron acceleration in the Bubble regime that is able to generate a nearly monochromatic electron bunch [1]. This regime is achieved in underdense plasma provided that the laser pulse has an ultra-relativistic amplitude and its duration is shorter than the plasma period. We study equations of cold ultrarelativistic plasma hydrodynamics coupled with the Maxwell equations and find 3D solutions of these equations. Surprizingly, the efficient electron acceleration works in an almost linear regime that is free of a chaotic behavior inherent in non-linear dynamic systems. Particularly, there exists a solution in the form of a spherical cavity with radius R almost free from plasma electrons. The resonance interaction between the wave and the particles expels background plasma electrons from the cavity and we can relate the cavity radius R with the relativistic factor $\gamma_p$ of the cavity propagation: (2\pi R/\lambda_p)^2=2.6\gamma_p\sqrt{\ln(2\gamma_p)}$. The cavity rear side is unstable due to the electron density perturbations leading to electron trapping and acceleration. We find that the electrons can be accelerated in the Bubble up to the $\gamma_m$-factor $\gamma_m= 11\gamma_p^3\sqrt{ln(2\gamma_p)}$.p The development of the jet and cavity formation have also been investigated by 3D PIC simulations. [1] A. Pukhov, J. Meyer-ter-Vehn, Appl. Phys. B 74, 355 (2002).
Shouyuan Chen: Experimental Evidence of Ionization Blue Shift Seeded Forward Raman Scattering
  We report the results of spectroscopic experiments that wereconducted by focusing an intense ultra-short laser pulse into a heliumgas target. The scattered light from the interaction region wasmeasured spectrally and spatially from various directions as afunction of laser intensity and plasma density and correlated withthe accelerated electron beam. The experiment data showed thatforward Stimulated Raman Scattering (SRS) was sensitive to thefocus position of laser relative to the nozzle. Together with theplasma channel imaged by a CCD camera, the measurements indicatethat SRS is seeded by the ionization blue shifted light. The cross-phase modulation between the SRS and laser beam was also observedin the experiment.
James Cooley: Broad-Energy Electron Beam Injection and Loading in Laser Wakefield Accelerators
  Laser Wakefield Accelerators (LWFA), in the resonant regime, require use of an initial electron beam that can be injected into the accelerating phase of the wake. Ideally, the injection electron bunch would be mono-energetic with femto-second timing. Generating an injection electron bunch with this timing requirement will likely lead to use of an optical method to generate the electrons. Several methods for generating this electron bunch exist e.g., Laser Ionization and Ponderomotive Acceleration (LIPA) and Self-Modulated LWFA among others. Each of these schemes produces an electron bunch with a characteristic energy distribution. We examine the trapping characteristics for this type of injection source for resonant LWFA by modeling the distributions to have a broad energy spread that can be characterized using a Boltzmann distribution with an "effective temperature". We will present results of both analytic calculations and simulations which provide a methodology for optimizing the resulting accelerated electron bunch characteristics i.e., energy and energy spread, for a given LWFA configuration. We will discuss the required trade-offs between total energy gain and overall beam quality. These results will also include beam-loading effects and plasma wake disruption due to high charge density in the initial electron bunch.
Mike Downer: Femtosecond pump-probe studies of preformed plasma channels and clustered plasmas
  I will survey our recent pump-probe experiments in He plasma waveguides using sub-relativistic pump pulses of 0.2x10^18 W/cm^2. These experiments develop fs-time- and µm-space-resolved diagnostic techniques based on frequency-domain interferometry and photon acceleration that will be required to characterize channeled LWFA. I will update our current efforts to guide fully relativistic pulses. I will also report on our recent pump-probe experiments in clustered gases, an attractive medium for plasma channels and accelerators. We observe sharp, delayed resonant enhancements in third harmonic production from the expanding clusters. PIC simulations suggest that collective bulk electron oscillations are responsible for the enhanced nonlinear susceptibility.
Eric Esarey: Nonlinear group velocity, pump depletion, and electron dephasing inlaser wakefield accelerators
  The nonlinear evolution of sub-ps laser pulses in underdense plasmas is analyzed for arbitrary laser intensity. Some of the new results derived include expressions for the nonlinear group velocity of the laser pulse, the nonlinear phase velocity of the wakefield, thenonlinear frequency shift, and the laser pulse envelope distortion. New scalings are presented for the pump depletion length and the electron dephasing length. Analytical results are compared to numerical calculations based on fluid models. Implications for anoptimized design of a 1 GeV accelerator stage are discussed.

This work supported by DoE, DE-AC03-76SF0098.

Gwenael Fubiani: Beat wave injection of electrons into plasma waves using two interferinglaser pulses
  An electron injector concept that uses a single injection laser pulsecolliding with a pump laser pulse in a plasma is analyzed. The pumppulse generates a large amplitude laser wakefield (plasma wave). Thecounterpropagating injection pulse collides with the pump laser pulseto generate a beat wave with a slow phase velocity. The ponderomotiveforce of the slow beat wave is responsible for injecting plasmaelectrons into the wakefield near the back of the pump pulse. Testparticle simulations indicate that significant amounts of charge canbe trapped and accelerated (~ 10 pC). For higher charge, beamloading limits the validity of the simulations. The acceleratedbunches are ultrashort (~ 1 fs) with good beam quality (relativeenergy spread of a few percent at a mean energy of ~ 10 MeV and anormalized rms emittance on the order 0.4 mm.mrad). The effects ofinteraction angle and polarization are also explored, e.g., efficienttrapping can occur for near-collinear geometries. Beat wave injectionusing a single injection pulse has the advantages of simplicity, easeof experimental implementation, and requires modest laser intensityI ~ 8.8e17 W/cm2.
Julien FUCHS: Ultra-high brightness laser-accelerated proton source & novel acceleration schemes
  Laser-accelerated high energy protons have a number of remarkable characteristics that have triggered interest for potential applications of this source. Among these unique characteristics are its high degree of laminarity and extremely low source size, the capacity to manipulate the beam by playing with the laser intensity distribution or the target shape. We will present results on the measurement of the beam’s transverse emittance. We have used a technique that allows to directly image the proton-emitting surface by imprinting grooves on the target rear surface, and to fully reconstruct the transverse phase space. With this technique, we have determined experimentally that for protons of up to 10 MeV, the transverse emittance is as low as 0.004 mm.mrad (normalized, rms), i.e. 100-fold better than typical RF accelerators and at a substantially higher ion current (kA range). We have also shown that the removal of the co-moving electrons after 1 cm of the quasi-neutral (protons and electrons) beam expansion did not increase significantly the measured proton transverse emittance. The exceptionally low measured emittance stems from the extremely strong, transient acceleration that takes place from a cold, initially unperturbed surface and from the fact that during much of the acceleration the proton space charge is neutralized by the co-moving hot electrons. We will also present plans to inject the laser-accelerated protons into a simple induction linac cavity in order to post-accelerate them while keeping their transverse emittance. The cavity is planned to be driven by a pulsed power machine at U. of Nevada, Reno (1 MA, 2 MV, 1.9 ohm driver, with 80 ns rise time) that is being coupled to a 100 TW short-pulse laser that will be used for generating the protons. Our aim is to demonstrate the feasibility of injection and longitudinal phase space rotation of the laser source.
Nasr Hafz: Experiment on the Thomson Backscattering from LWFA e-beams
  In a collaboration between KERI and GIST in Korea, we are planning to use a 20 TW 40 fs Ti: sapphire laser system for generating ultrashort x-ray pulses by the Thomson backscattering from plasma-accelerated electron beams. A beam splitter will be used to split the main beam into two beams; one (high-power) will be focused on a gas jet for generating e-beam. The second (low power) beam will be focused on the e-beam after it is emitted from the plasma. The synchronization between the two beams can be achieved via an optical delay. The x-ray spectrum is expected to be of wide range form soft to hard x-rays as a result of the naturally large energy spread of the electron beam. Some details about this experiment are presented.
Tomonao Hosokai: Experimental study of fs electron bunch generation by laser plasma cathode
  Plasmas irradiated by intense short laser pulses can be an efficient, compact, and flexible source of multi-MeV electrons constituting a bunch with its duration in the femtosecond range. So we have studied a laser plasma cathode based on Laser Wake Field Acceleration (LWFA) with electron injection by wave-breaking, which is driven by a femtosecond intense laser pulse (Ti:Sapphire, max 12TW, lambda = 800nm , 50fs, 10Hz) tightly focused in supersonic helium gas-jet. The wave-breaking is the simplest way to inject energetic electrons into the wakes produced by an intense laser pulse. However, the wave-breaking requires a steep density plasma interface with order of the plasma wavelength. According to our previous experiment, this condition can be produced via generation of a shock-wave in the gas jet by proper laser prepulse irradiation. It shows ejection of a narrow-cone MeV electron beam from a gas jet depend strongly on the prepulse contrast ratio. Furthermore, in case of higher gas density, the effect of laser prepulse become more sophisticate.The laser pulse is refracted at the edge of the cavity produced by the prepulse and this effect limit the narrow-cone MeV electron beam production of LWFA. In addition, I will also mention a narrow energy spread e-spectrum obtained in our recent experiment, development of a shockwave-free supersonic gas jet and gas filled capillary discharge wave-guide.
Richard Hubbard: Trapping and Acceleration of Nonideal Injected Electron Bunches in Channel-Guided LWFAs
  The standard regime for the laser wakefield accelerator (LWFA) usually requires external injection of MeV electrons. Ideally, the injected electron bunch should be injected into the proper phase of the accelerating wake, have a bunch length that is small compared with the plasma wavelength, and a low emittance and energy spread. This paper reports simulation studies of several ‘nonideal’ injection schemes that demonstrate strong phase bunching and good accelerated beam quality in a channel-guided laser wakefield accelerator. For the case of monoenergetic, unphased (long bunch) injection, there is an optimum range of injection energies for which the LWFA can trap a significant fraction of the injected pulse while producing an ultrashort, high-quality accelerated pulse. These favorable results are due to a combination of pruning of particles at unfavorable phases, rapid acceleration, and strong phase bunching. In addition, the required minimum energy for injection may be substantially lower in a channel than in a uniform plasma because of a favorable shift in the size and phase of the portion of the wake that is both accelerating and focusing. Simulation agree well with the predictions of a simple Hamiltonian model using an ideal sinusoidal wake moving at the group velocity of the laser pulse. This long bunch case may apply to injection with a conventional or photocathode RF gun. Phased and unphased injection in a channel-guided LWFA with a broad injected energy spread has also been simulated. Although the trapping fraction is generally much smaller than in the monoenergetic case, some simulations exhibit final accelerated bunches with remarkably small energy spread. These results suggest that relatively poor quality injection pulses may still be useful in LWFA demonstration experiments. Implications for planned LWFA experiments at NRL are discussed.

Supported by the Department of Energy and Office of Naval Research.

Dino Jaroszynski: Progress on the UK Advanced Laser Plasma High-energy Accelerator ALPHA-X project
  One of the goals of developing laser-driven accelerator technology is to produce a compact coherent radiation source. The project, Advanced Laser Plasma High-energy Accelerators towards X-rays, ALPHA-X, aims to develop a laser driven wakefield (LWF) accelerator and establish the feasibility, in the longer term, of creating a compact free-electron laser. A consortium consisting of several UK universities and facilities, and collaborators from Europe and the US, has been set up to meet this challenge. To meet the stringent injection requirements, two different electron injection methods are being investigated. The first is a compact all optical method, which involves extracting electrons from plasma using an intense laser pulse and injecting them into the LWF accelerator. Encouraging recent results to create mono-energetic electron bunches for injection will be presented. In parallel we are developing a 2.5 cell 3 GHz RF photoinjector to produce electron bunches with durations a fraction of the plasma wake period. To characterise the ultra-short electron bunches new techniques based on electro-optic sampling of the Coulomb field have been developed. Recent results using these THz time domain techniques to measure a 300 fs electron bunch and the output from a THz free-electron laser will be presented. These demonstrate the feasibility of single shot measurements. A fully ionised hydrogen filled capillary has the dual purpose of acting as a waveguide for the driving pulse while providing the medium for the LWF accelerator. Recent results of the characterisation of a suitable plasma channel will be presented. These include measuring the plasma channel waveguide density profile using transverse interferometry and new techniques to measure the longitudinal density using Raman amplification. The latter has allowed measurements of the plasma density transition at the entrance to the channel, which is important for coupling the laser and electron beams into the channel.
Dmitri Kaganovich: Experimental Demonstration of a Staged Optical Injection and Laser Wakefield Acceleration
  A two stage experiment on optically injected Laser Wakefield Acceleration (LWFA) was performed at the Naval Research Laboratory. Two temporally and spatially synchronized laser beams at 2 TW and 10 TW respectively are each focused collinearly into two adjacent gas jets. Electrons at <0.5 MeV generated from the 2 TW laser beam are injected into the wakefields generated by the 10 TW laser beam. Accelerated electrons >10 MeV are observed, implying an acceleration gradient of 10 GeV/m for the ~1 mm acceleration distance. Apparent peaks and structures are observed in the energy spectrum. This is a clear demonstration of substantial laser wakefield acceleration of externally and optically produced injection electrons. Details of the experiment will be presented.
Kazuyoshi Koyama: Generation of Quasi-monoenergetic High-energy Electron Beam by Plasma Wave
  We have demonstrated an acceleration of a quasi-monoenergetic electron beam by trapping electrons in a plasma wave. Experiments were performed by focusing 2-TW (50 fs) laser pulses on supersonic gas jets of nitrogen or helium. An intensity in a focal spot diameter of $5\mu m$ was $5 \times 10^{18} W/cm^2$($a_0=1.5$). An electron density was estimated to be $1.3 \times 10^{20} cm^{-3}$ by the measured neutral gas density and the barrier suppression ionization model. The 1-D electron detuning length was around $60 \mu m$.Electron energy as well as an angular spread of the electron beam was measured by using an electron-spectrometer (ESM) with a 2-D sensor of the Imaging Plate (Fuji; BAS-SR). Besides a spectrum of a forward scattering, a side-scattering image of the pump laser pulse and an ultra short pulse shadowgraph of the plasma were simultaneously observed to study the laser interaction with plasma.The quasi-monoenergetic electron beam at 7 MeV was recorded at a high-contrast of 10:1. An energy spread of the peak of 2.5 MeV was limited by an enrgy resolution of the ESM. Background electrons had a Boltzmann-like energy spectrum with an effective temperature and a maximum energy of 6.5 MeV and 30MeV, respectively. The angular spread of the monoenergetic electron beam was $\pm 1.1degree$. An appearance of a Stokes Raman satellite in the forward scattering, which was a clear evidence of a plasma-wave excitation, well correlated with the quasi-monoenergetic electron beam. A frequency shift of the satellite coincided with a plasma frequency at the measured plasma density. Appearance of the Raman satellite coincided with appearances of a fishbone structure in a side-scattering image. Supposing the fishbone structure originated from the plasma wave, an acceleration length was estimated to be a few-hundred microns.\This work was supported by the Budget for Nuclear Research of the MEXT and the Advanced Compact Accelerator Department Program of the MEXT.
Carl Schroeder: Trapping, wavebreaking, and dark current in nonlinear plasma waves
  The trapping of thermal plasma electrons in a nonlinear plasma wave of arbitrary phase velocity is investigated. The relation betweentrapping and wavebreaking is discussed. The threshold plasma wave amplitude for trapping is presented, thereby determining the fractiontrapped and the expected dark current in a plasma-based accelerator. Reduction of dark current in plasma-based accelerators is critical for producing high-quality electron beams. It is shown that the presence of a laser field (e.g., trapping in the self-modulated regime of the laser wakefield accelerator) increases the threshold for trapping plasma electrons. The threshold field for trapping reduces to previous wavebreaking calculations in the relevant limits. Implications for experimental and numerical laser-plasma studies are discussed.

This work supported by DoE, DE-AC03-76SF0098.

Hyyong Suk Suk: Generation of MeV-level high-energy electrons from the interaction of a 2 TW laser beam and a gas target
  We have an experimental program for laser plasma acceleration research at KERI (Korea Electrotechnology Research Institute). For the laser plasma accelerator research, we recently built and tested a table-top terawatt laser system, which can provide a 2 TW (1.4 J/700 fs) from the Ti:sapphire/Nd:glass hybrid-type system. As a first step in the laser accelerator research, we focused a 2 TW laser beam onto a He gas target, in which the density was on the order of 10^18 cm^-3. Its was observed that several MeV-level high-energy electrons with a charge of ~ 2 nC per bunch were produced from the self-modulated laser wakefield acceleration. The electron beam (especially outer part of the beam) is highly space-charge-dominated, so the beam was observed to expand very rapidly. In this talk, various beam measurement results are presented.

Antonio Ting: NRL Laser Injection Laser Wakefield Accelerator
  Optically generated injection electrons have the advantage of being tightly bunched and synchronized to the laser wakefield for phased injection and acceleration to final high energies. A high-density LIPA (Laser Ionization and Pondermotive Acceleration) injection mechanism is studied using the 10 TW laser beam of the NRL T3 laser and a gas jet of high density nitrogen. The highest energy electrons up to 7 MeV were produced in the 0 degree forward direction. The angular distribution peaked at the 30 degrees forward direction. Both observations agreed quite well with NRL simulations. These electrons are very desirable for injection into a second stage of channel-guided LWFA leading to ultra-high final energies. Setup and results of the experiments will be presented.
Mikhail Tushentsov: A study of undulator induced transparency of magnetized plasma in the linear propagation regime
  The propagation and mode structure of waves in magnetized plasma is investigated both numerically and analytically in the magnetically induced transparency regime at a linear intensity level.The magnetic undulator renders possible coupling of the transverse andlongitudinal electromagnetic waves in the plasma. This coupling yields ultra-slow hybrid electromagnetic waves with a group velocity which is substantially less than the speed of light in vacuum, and leads to an extreme energy compression in the plasma. The mode conversion problem is examined for different configurations of inhomogeneous external magnetic field and plasma density. Fluid and particle-in-cell codes are used for modelling this effect and a detailed comparison of the results with theory is performed.The proper selection of undulator wavelength combined with judicious choice of axial magnetic field profile enables microwaves to penetrate plasma with realistic (smooth) density profiles at moderate levels of the undulator field, suppressing the strong resonant absorption at the cyclotron frequency.
Jonathan Wurtele: Robust autoresonant excitation in the plasma beat-wave accelerator
  A modified version of the Plasma Beat-Wave Accelerator scheme is proposed,based on autoresonant phase-locking of the Langmuir wave to the slowlychirped beat frequency of the driving lasers by passage through resonance.Peak electric fields above standard detuning limits seem readilyattainable, and the plasma wave excitation is robust to large variationsin plasma density or chirp rate. This scheme might be implemented inexisting Chirped Pulse Amplification or $\mbox{CO}_{2}$ laser systems witha little technological effort.
Csaba Toth: A multi-beam, multi-terawatt Ti:sapphire laser system for laser wake-field acceleration of electrons and for bright THz radiation production
  The Lasers, Optical Accelerator Systems Integrated Studies (L’OASIS) Lab of LBNL operates a highly automated and remotely controlled Ti:sapphire chirped pulse amplification (CPA) laser system that provides synchronized beams of 2x1.0 TW, 12 TW, and 100 TW peak-power, in a unique, radiation shielded facility. The system has been specially designed for studying high field laser–plasma interactions and particularly aimed for the investigations of laser wake-field particle acceleration. It generates and recombines multiple beams having different pulse durations wavelengths, and pulse energies for various stages of plasma preparation, excitation, and diagnostics. The amplifier system is characterized and continuously monitored via local area network (LAN) from a radiation shielded control room by an array of diagnostics, including beam profile monitoring cameras, remote controlled alignment options, self-correcting beam-pointing stabilization loops, pulse measurement tools (single-shot autocorrelator for pulse duration and third-order correlator for contrast measurements, FROG for pulse shape studies). Detailed results of these diagnostics and experiments on laser wake-field acceleration will be presented.As an example application of the system, coherent THz radiation was produced from relativistic electrons accelerated via self-modulated laser wakefield acceleration (SM-LWFA) in a high density (10^19 cm^-3) pulsed gas jet. As the electrons exit the plasma, coherent transition radiation is generated at the plasma-vacuum boundary for wavelengths long compared to the bunch length. Radiation yield in the 0.3 to 19 THz range and at 94 GHz has been measured and found to depend quadratically on the bunch charge. Modeling indicates that optimization of this table-top source could provide more than 100 microJ/pulse THz energy. Together with intrinsic synchronization to the laser pulse, this will enable numerous applications requiring intense terahertz radiation.
Mori Warren: Near gev electron acceleration of self-injected electrons in cm scale plasma channels
  The -self-injection and laser wakefield acceleration of electrons is studied in three-dimensions using the particle-in-cell (PIC) code OSIRIS. The simulations model a 50fs, 13 TW, .8m m laser propagating through a leaky plasma channel with a minimum density of 3x1018 cm-3. The initial wake is not large enough to trap background electrons. However, the self-consistent evolution of the laser due to photon acceleration and deceleration, group velocity dispersion, and transverse self-focusing results in larger wake generated in the blowout regime. When the blowout is severe enough that electrons from the axis overtake those at the channel edge then those electrons on the wall are attracted to the axis ahead of the main group leading to self-injection. The resulting acceleration leads to a mono-energetic beam with central energy of 260 MeV and a ~20% energy spread. This first group of particles beam loads and eventually stretches the wake, leading to the trapping of a second bunch of electrons. The peak energy of the second bunch approaches 840MeV(with a more continuous distribution). We will discuss the evolution of the laser, the self-injection mechanism, the acceleration mechanism, the generation of mono-energetic bunch, and the difference between 2D and 3D simulations. The simulations followed 2.5x108 particles on 3600 x 256 x 256 grids for a distance of .94 centimeters.
Gennady Shvets: Plasma Beatwave Accelerator Based on a Nonlinear Bistability of Relativistic Plasma Waves
  Plasma beatwave is one of the originally proposed schemes for making a plasma-based accelerator. Its traditional implementation assumes that the plasma wave is a linear oscillator driven by the ponderomotive force of the two lasers detuned by the plasma frequency. In this talk we demonstrate how the off-resonance excitation of the plasma wave and the relativistic dependence of the plasma frequency on its amplitude can be used to observe the nonlinear bistability of the plasma wave. The relativistic threshold for observing the nonlinear bistability is derived and confirmed using particle in cell simulations. One of the surprising results is the realization that a very long laser beatwave pulse slightly detuned from the nonrelativistic plasma wave resonance can excite plasma waves behind the pulse if the peak pulse amplitude exceeds the threshold. Specific examples of laser and plasma parameters relevant to high-gradient plasma accelerators will given. Also, excitation of plasma wakes by a strongly modulated (for example, by an FEL interaction) electron beam is considered, and the possibility of developing a combined accelerator/fs-injector will be discussed.