Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 364 (2006) 611-622
The staged electron laser acceleration (STELLA) experiment demonstrated staging between two laser-driven devices, high trapping efficiency of microbunches within the accelerating field and narrow energy spread during laser acceleration. These are important for practical laser-driven accelerators. STELLA used inverse free electron lasers, which were chosen primarily for convenience. Nevertheless, the STELLA approach can be applied to other laser acceleration methods, in particular, laser-driven plasma accelerators. STELLA is now conducting experiments on laser wakefield acceleration (LWFA). Two novel LWFA approaches are being investigated. In the first one, called pseudo-resonant LWFA, a laser pulse enters a low-density plasma where nonlinear laser/plasma interactions cause the laser pulse shape to steepen, thereby creating strong wakefields. A witness e-beam pulse probes the wakefields. The second one, called seeded self-modulated LWFA, involves sending a seed e-beam pulse into the plasma to initiate wakefield formation. These wakefields are amplified by a laser pulse following shortly after the seed pulse. A second e-beam pulse (witness) follows the seed pulse to probe the wakefields. These LWFA experiments will also be the first ones driven by a CO2 laser beam. © 2006 The Royal Society.
P SOC PHOTO-OPT INS 6261 (2006) T2610-T2610
The past few years have seen a rapid growth in the development and exploitation of X-ray diffraction on ultra-fast time-scales. One area of physics which has benefited particularly from these advances is the the field of shock-waves. Whilst it has been known for many years that crystalline matter, subjected to uniaxial shock compression, can undergo plastic deformation and, for certain materials, polymorphic phase transformations, it has hitherto not been possible to observe the rearrangement of the atoms on the pertinent timescales. We have used laser-plasma generated X-rays to study how single crystals of metals (copper and iron) react to uniaxial shock compression, and observed rapid plastic flow (in the case of copper), and directly observed the famous alpha-epsilon transition in Iron. These studies have been complemented by large-scale multi-million atom molecular dynamics simulations, yielding significant information on the underlying physics.
JOURNAL OF QUANTITATIVE SPECTROSCOPY & RADIATIVE TRANSFER 99 (2006) 363-369
NATURE MATERIALS 5 (2006) 805-809
AIP Conference Proceedings 877 (2006) 534-540
The Staged Electron Laser Acceleration - Laser Wakefield (STELLA-LW) experiment is investigating two new methods for laser wakefield acceleration (LWFA) using the TW CO2 laser available at the Brookhaven National Laboratory Accelerator Test Facility. The first is seeded self-modulated LWFA where an ultrashort electron bunch (seed) precedes the laser pulse to generate a wakefield that the laser pulse subsequently amplifies. The second is pseudo-resonant LWFA where nonlinear pulse steepening of the laser pulse occurs in the plasma allowing the laser pulse to generate significant wakefields. The status of these experiments is reviewed. Evidence of wakefield generation caused by the seed bunches has been obtained as well as preliminary energy gain measurements of a witness bunch following the seeds. Comparison with a 1-D linear model for the wakefield generation appears to agree with the data. © 2006 American Institute of Physics.
23rd International Linear Accelerator Conference, LINAC 2006 - Proceedings (2006) 806-808
Experiments at the LOASIS laboratory of LBNL recently demonstrated production of 100 MeV electron beams with low energy spread and low divergence from laser wakefield acceleration. The radiation pressure of a 10 TW laser pulse guided over 10 diffraction ranges by a plasma density channel was used to drive an intense plasma wave (wakefield), producing acceleration gradients on the order of 100 GV/m in a mm-scale channel. Beam energy has now been increased from 100 to 1000 MeV by using a cm-scale guiding channel at lower density, driven by a 40 TW laser, demonstrating the anticipated scaling to higher beam energies. Particle simulations indicate that the low energy spread beams were produced from self trapped electrons through the interplay of trapping, loading, and dephasing. Other experiments and simulations are also underway to control injection of particles into the wake, and hence improve beam quality and stability further.
Energy extraction from pulsed amplified stimulated emission lasers operating under conditions of strong saturation
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS 23 (2006) 1057-1067
PHYSICAL REVIEW LETTERS 96 (2006) ARTN 185002
IEEE Transactions on Plasma Science 33 (2005) 3-7
This paper describes an experiment to demonstrate, for the first time, laser wakefield acceleration (LWFA), driven by 10.6-μm light from a CO2 laser. This experiment is also noteworthy because it will operate in a pseudoresonant LWFA regime, in which the laser-pulse-length is too long for resonant LWFA, but too short for self-modulated LWFA. Nonetheless, high acceleration gradients are still possible. This experiment builds upon an earlier experiment called staged electron laser acceleration (STELLA), where efficient trapping and monoenergetic laser acceleration of electrons were demonstrated using inverse free electron lasers. The aim is to apply the STELLA approach of laser-driven microbunch formation followed by laser-driven trapping and acceleration to LWFA. These capabilities are important for a practical electron linear accelerator based upon LWFA. © 2005 IEEE.
Physical Review Letters 94 (2005) 125509 4pp-
Observation of structural anisotropy and the onset of liquidlike motion during the nonthermal melting of InSb
Physical Review Letters 95 (2005)
The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements observe the delayed onset of diffusive atomic motion, signaling the appearance of liquidlike dynamics. They also demonstrate that the root-mean-squared displacement in the  direction increases faster than in the  direction after the first 500 fs. This structural anisotropy indicates that the initially generated fluid differs significantly from the equilibrium liquid. © 2005 The American Physical Society.
Simulations of recombination lasing in Ar7+ driven by optical field ionization in a capillary discharge waveguide
Optics Communications 249 (2005) 501-513
We present calculations of the small-signal gain coefficient, gain length, and output energy of a recombination laser in Ar7+ driven by optical field ionization. Simulations are presented for both 400 and 800 nm pump radiation, and for two targets containing mixtures of argon and hydrogen: a gas cell, and a gas-filled capillary discharge waveguide. Extremely high values for the small-signal gain coefficient are calculated for the 4s-3p transition at 23.2 nm using a pump wavelength of 400 nm for both the gas cell and waveguide. Operation in the waveguide is predicted to greatly increase the XUV laser output owing to a large increase of the gain length. The calculations also show that use of the waveguide allows significant single-pass gain to be achieved even with pump radiation of 800 nm wavelength. © 2005 Elsevier B.V. All rights reserved.
Physical Review A - Atomic, Molecular, and Optical Physics 71 (2005)
We report a significant increase of the output of a 41.8-nm Xe 8+ laser achieved by means of multimode guiding of high-intensity femtosecond laser pulses in a gas-filled dielectric capillary tube. The optimized lasing signal from a 15-mm-long capillary was nearly an order of magnitude higher than that from a gas cell of the same length. Simulations of the propagation of the pump laser pulse in the capillary confirmed that this enhancement is due to reflections from the capillary wall, which increase the length of the Xe 8+ plasma column generated. The influence of gas pressure and focusing position on the lasing is also presented. © 2005 The American Physical Society.
LASER PART BEAMS 23 (2005) 351-356
We give on overview of recent advances in collisionally pumped optical field-ionization soft X-ray lasers developed at LOA. Saturated amplification has been achieved on the 5d-5p transition in Xe8+ at 41.8 nm, and on the 4d-4p transition in Kr8+ at 32.8 nm. We demonstrate a significant increase of the energy output from the Xe8+ laser driven within two types of wave-guide. Finally, we present results of a pioneering work aimed to set up and characterize the first true soft X-ray laser chain.
Solid State Communications 136 (2005) 181-185
It has recently been proposed that X-rays can be switched on sub-picosecond time-scales by using laser-generated coherent optical phonons: The so called phonon Bragg switch. We present here detailed simulations of the efficiency of such a switch by solving the time-dependent generalized Takagi-Taupin equations utilizing a perturbative approach. We explore the switching efficiency in diffraction from the (004) planes of GaAs as a function of both excited phonon wave vector and amplitude. © 2005 Elsevier Ltd. All rights reserved.
ASTROPHYSICAL JOURNAL 629 (2005) 1091-1101
Phys Rev E Stat Nonlin Soft Matter Phys 72 (2005) 036402-
A molecular dynamic (MD) code is used to compare the rates of heating by inverse Bremsstrahlung (IB) for circularly and linearly polarized radiation. For low intensities the heating rate is found to be independent of polarization. However, at higher intensities the variation of the heating rate with the radiation intensity is found to exhibit a sharper peak for circularly polarized than linearly polarized radiation. This difference is explained in terms of differences in the variation of the electron quiver speed during the optical cycle for linearly and circularly polarized radiation. An analytical expression--which includes a term which is nonlinear in the density of the plasma--for the rate of IB heating is fitted to the rates calculated by the MD code.