Session CO2 - Laser Accelerators; Ultraintense Laser Applications.
ORAL session, Monday afternoon, November 15
Room 203, SCC

[CO2.001] First 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 >20 MeV are observed, implying an acceleration gradient of 20 GeV/m for the \sim1 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.

[CO2.002] 3D Simulations of Electron Acceleration and Beam Loading in Laser Wakefield Accelerators*

Laser wakefield accelerators (LWFA) have the promise of providing a very high electric field for particle acceleration. In LWFA, the ponderomotive potential of the laser excites a plasma wave, thus providing a longitudinal electric field suitable for particle acceleration. This plasma wave travels behind the laser pulse at the laser pulse group velocity. The energy in this plasma wave can be transferred to electrons, which then gain energy and accelerate. However, the electrons that are being accelerated also affect the plasma wake and tend to reduce its amplitude as they gain energy. Thus, only a finite number of electrons can be accelerated in a given wave. We present three-dimensional simulations of this process using the code QuickPIC. QuickPIC is a massively parallel code that treats both the plasma electrons and the accelerated electrons kinetically. In addition, it includes an extended paraxial treatment of the laser pulse envelope. The simulation results we present examine not only the maximum charge that can be accelerated but also the self-consistent evolution of the accelerating bunch, including effects such as beam hosing and beam misalignment.

[CO2.003] Mono-energetic electron beams from a laser-plasma accelerator

Laser-plasma electron acceleration experiments have previously produced relativistic electrons in large energy spread, quasi-thermal beams. Here we report the observation of mono-energetic features in the electron spectrum in a non-linear or forced laser wakefield experiment. The 340~mJ, 45~fs pulses were focused to a 25~\mum focal spot onto a gas jet. At high density the energy spectrum has a quasi-thermal distribution. At lower density multiple spikes appear in the spectrum with narrow energy spreads. With an increase in laser energy to 640~mJ it is possible to produce single energy beams of E \sim 60 - 80~MeV with spreads of < 5%. Simulations also produce such features and show that these beams are injected into the non-linear plasma wave when it breaks transversely. If the plasma density is high, such that the bunch has time to dephase from the plasma wave before it reaches the end of the interaction then quasi-thermal spectra are produced. If the density is lower these mono-energetic features can be propagated out of the plasma before they have dephased.

[CO2.004] Simulations of laser pulse coupling and transmission efficiency in plasma channels

The guiding of intense laser pulses in plasma channels is necessary to maximize the energy of electrons accelerated in a laser wakefield accelerator. A significant fraction of the energy in the laser pulse may be lost during and after coupling from vacuum into a channel. For example, imperfect coupling can lead to enhanced leakage of laser energy transversely through the channel walls. Tunneling ionization of neutral gas on the periphery of the gas jet could enhance refraction and hence reduce coupling efficiency. Ionization of neutral gas along the edges of a hydrodynamic channel by the transverse wings of the pulse might effect the confinement of laser energy inside the channel. We present 2D particle-in-cell (PIC) simulations, using the VORPAL code, to address these issues. We will discuss new power spectral diagnostics, based on simultaneous FFTs in space and time of the PIC data, which enable quantitative measurements of the scattered pulse energy and its spectra. Future work will be directed toward VORPAL parameter studies designed to optimize the amount of laser energy that couples into a channel.

[CO2.005] Efficient end-pumped plasma waveguide generation in clustered argon and hydrogen

Generation of plasma waveguides in clustered gases is a promising new method for generating low density plasma waveguides for applications like laser wakefield acceleration. Strong absorption and self-guiding of moderate energy femtosecond pulses by a clustered medium [1] allows end-pumping of the channel. The experimental setup is considerably simpler than in side-pumping schemes. Argon and hydrogen clusters were produced using liquid-nitrogen-cooled pulsed nozzles. Waveguides were generated using 100 fs pulses, and the evolution of the plasma channel was recorded using transverse interferometry. The extent of ionization produced by an intense guided pulse was studied by measuring its transmitted spectrum for both argon and hydrogen plasma waveguides. The guided intensity and coupling efficiency, measured using end-mode imaging, were found to be \sim3x10^17 Wcm-2 and \sim50% respectively.

1. I. Alexeev, T. Antonsen, K. Y. Kim, and H. M. Milchberg, Phys. Rev. Lett. 90, 103402 (2003).

[CO2.006] Generation of short-pulse x-rays using laser-plasma accelerators

The production of ultrashort (tens of fs) pulses of x-rays from a laser wakefield accelerator (LWFA) is discussed. The high quality electron bunches recently obtained from LWFA experiments can be used to produce x-ray through several mechanisms: i) Betatron (synchrotron-like) radiation emitted as the accelerated electrons undergo transverse oscillations due to the radial focusing field of the wake; ii) Bremsstrahlung radiation that is produced by placing a solid target in the electron bunch path; and iii) Thomson scattering by using a second, counterpropagating laser pulse to intersect the electron bunch.. Although betatron radiation and Bremsstrahlung are inherently broadband, Thomson scattering can produce narrow bandwidth x-rays, provided that the electron bunch is monoenergetic. Recent theoretical and experimental results using the 10 TW l'OASIS laser system at Lawrence Berkeley National Laboratory will be presented. The comparison between the different x-ray generation mechanisms will be discussed. This work is supported by DoE, DE-AC03-76SF0098.

[CO2.007] Ultra-intense laser pulse propagation in engineered plasma channels

The injection of laser pulses into hydrodynamically preformed plasma channels is hindered by the neutral gas and down tapered channel wall at the entrance of the channel. To improve the coupling efficiency, we consider grafting a plasma funnel onto the channel using an auxiliary formation pulse. This eliminates the neutral gas near the channel entrance and provides a focusing element to funnel the high intensity laser into the channel. We generate channel and funnel profiles using our 1D hydrocode. These are then imported to the laser propagation code WAKE. Simulation results show that the averaged on-axis peak intensity inside the channel can reach 10^18 W/cm^2 for the optimal gas jet target assuming the channel is fully ionized. The pulse energy leaks when the channel is partially ionized.

[CO2.008] Signatures of Non-linear Resonance in Laser Irradiated Cluster Plasma*

Gases of atomic clusters interact non-linearly with intense laser pulses. Studies of the cluster dynamics using the particle-in-cell approach [1] reveal there is a size dependent intensity threshold above which the heating of cluster plasma is dominated by a nonlinear resonant absorption process. In this process, energetic electrons are created which form a halo surrounding the cluster. The response of the cluster to the applied field is characterized by a time dependent dipole moment that is significantly affected by the halo electrons. We use PIC simulation to find signatures in the dipole moment of the halo electrons. Specifically, we investigate the polarizability of the cluster and harmonic generation [2] due to the motion of the electrons in an anharmonic potential. *Supported by US Department of Energy and National Science Foundation. 1. T. Taguchi et. al. PRL v92, 205003 (2004) 2. B. Shim, G. Hays, M. Downer and T. Ditmire, Bull. Am. Phys. Soc. V48, 24 (2003)

[CO2.009] Dielectric Properties of Laser Exploded Clusters

When a laser irradiates a cluster, electrons are initially generated through field ionization and then by cascading collisional ionization as the cluster temperature rises. Once ionization has occurred, the laser field accelerates the electrons. This acceleration allows for a fraction of the electrons to escape the cluster, resulting in an overall charge imbalance. The remaining electrons are then bound by a cluster potential generated by the charge imbalance. After the pulse has passed, the cluster reaches a quasistatic equilibrium, with the electron dynamics dominated by the electrostatic potential of the cluster. Using the Vlasov equation for a duel-species plasma, we calculate the quasistatic cluster potential, and examine the electron dynamics in the potential. We then perturb the Vlasov equation to first order in field to calculate the polarization of the cluster. The cluster potential is then solved for self-consistently in both the adiabatic and non-adiabatic regimes. The effect of the laser pulse on the electron dynamics is explored. With these results, the polarizability of typical clusters has been calculated. Supported by the Department of Energy and the National Science Foundation.

[CO2.010] Temporal evolution of axicon-pumped plasma waveguides in clustered gases

We report generation of plasma waveguides in clustered gases using an axicon. The use of clustered media decouples pump-energy absorption from the mean gas density, and opens up a route to plasma densities lower than 10^18 cm^-3 in a hydrodynamic plasma waveguide. A 100 ps Nd:YAG laser was focused into clustered argon and nitrogen gas jets using an axicon to generate cm-long plasma waveguides. Side-pumping the channel avoids channel length limitations imposed by strong laser-energy absorption by the clustered medium; the length of the waveguides was limited by the size of the jet. Transverse interferometry was used to measure the evolution of the plasma density.

[CO2.011] Ultra-low emittance proton beams from a laser virtual cathodeplasma accelerator

One key property of laser-accelerated MeV ion beams is their ultra-low transverse emittance. In recent experiments, it was found to be <0.004 mm mrad [Cowan et al., Phys.Rev.Lett.92, 204801 (2004)], at least 10^4 times smaller than the emittance of conventional thermal ion sources. We investigate the reasons for the low emittance of laser-accelerated proton beams by checking several candidates for emittance-generation mechanisms, apart from the geometrical effect that is caused by the roughness of the target surface; it is not considered here.

As our main tool we use 1D particle-in-cell simulations that have been modified to include binary collisions between particles [see Kemp et al., Phys. Plasmas, Sept 2004], and analytical work. Our restriction to 1D detains us from looking at geometrical effects, but it does allow to study the transverse width of the proton beam in momentum space, which is the relevant information for the beam emittance. We find that the lower emittance limit is mainly determined by electron-ion collisions during the initial acceleration phase from the cold rear target surface. Another possible mechanism responsible for the experimentally determined emittance is an ion-acoustic beam plasma instability. We find that it is not important for the fastest ions, while it could play a role for the lower energy ions.

This work was supported by DOE/NNSA under UNR grant DE-FC52-01NV14050.

[CO2.012] Essential aspects of the physics of laser-ion acceleration

The physics of laser driven ion acceleration has gained new interest in recent years due to the remarkable properties of these ion beams at high laser intensities and laser pulse energies. While important aspects of laser-ion acceleration have been investigated in recent years many remain unclear. In the presentation the heating and expansion of foils irradiated with sub-ps high intensity lasers is investigated. Particular focus is on the evolution of the electron distribution function during expansion. It is addressed how collisional heating and expansion related cooling of the plasma compete against each other and which energy partition between the electron and ion fluids has to be expected for a given target geometry. It is also addressed which role ``sheath acceleration'' plays during the plasma expansion process. Comparison with recent experiments and analytical models is given.

[CO2.013] PROTON BEAM DYNAMICS FROM THE FRONT AND BACK SURFACE OF DIELECTRIC AND CONDUCTIVE TARGETS

Recent experiments at the University to Michigan are helping to elucidate the dynamics of electrons and protons produced by high-intensity-laser interactions with thin-film solid targets by examining proton beam characteristics from increasing thicknesses of Mylar and aluminum. The proton beam energy and spatial profile are found to vary with target thickness as well as initial target conductivity. Half the peak proton energy is observed from Mylar targets as compared to aluminum targets as well as a much sharper reduction in proton energy with increasing target thickness. These differences originate from the strong inhibition of the hot-electron forward and return currents in the initially highly resistive material, which limits electron recirculation and thus proton energy from the target rear surface. In addition, evidence from energy and beam profile data supports the existence of target independent 5-MeV-maximum-energy beam from the target front surface. Other effects on beam profile due to target conductivity such as beam hollowing, originating from the electrothermal instability, are also presented.

[CO2.014] Relativistic Doppler effect: universal spectra and zeptosecond pulses.

We report on a numerical observation of the train of zeptosecond pulses produced by reflection of an relativistically intense femtosecond laser pulse from the oscillating boundary of an overdense plasma because of the Doppler effect. These pulses promise to become a unique experimental and technological tool since their length is of the order of the Bohr radius and the intensity is extremely high \propto 10^19~W/cm^2. We present the physical mechanism, analytical theory, and direct particle-in-cell simulations. We show that the harmonic spectrum is universal: the intensity of nth harmonic scales as 1/n^p for n < 4\gamma^2, where \gamma is the largest \gamma--factor of the electron fluid boundary, p=3 and p=5/2 for the broadband and quasimonochromatic laser pulses respectively.

[CO2.015] Femtosecond high-intensity laser-plasma harmonics, using practical double-plasma-mirror pulse cleaner

We describe high-contrast laser-plasma harmonic generation from 50fs pulses at intensities of a few times 10^19 W cm^-2, using a practical double plasma-mirror built into a f=10m null telescope. The plasma mirrors are situated near the far-field at the middle of the telescope, and can be configured to optimize the reflection efficiency and pulse contrast. The final focal spot contains more than 50% of the original pulse energy, is highly reproducible and nearly diffraction-limited. The pulse's leading edge is steepened and pulse contrast better than 10^10 is obtained. The harmonics produced contain regular fine structure which do not appear to originate in any preformed plasma.

Part C of program listing